The Bancroft Library
State Earthquake Investigation Commission
State Earthquake Investigation Commission
- Andrew C. Lawson
- A. O. Leuschner
- G. K. Gilbert
- George Davidson
- H. F. Reid
- Charles Burkhalter
- J. C. Branner
- W. W. Campbell
The reprinting of a sixty-year-old scientific publication, for scientific reasons and in response to a scientific demand, is a rare event indeed. Yet it is precisely for such reasons and in response to such a demand that the Carnegie Institution is presenting once again its Publication 87, The California Earthquake of April 18, 1906.
Dr. William W. Rubey, whose lifetime of investigation has influenced the course of modern geophysics so widely and deeply, and whose knowledge of the circumstances surrounding the 1906 earthquake is so unique, has honored this edition with an Introduction that provides in detail the setting in which the 1906 Report was made. It was Dr. Rubey who first sensed and emphasized the special timeliness of the Report today. We believe that geophysicists at large in coming years will share our gratitude to him.
The reprinting has been made possible by a grant of $15,000 from the Harry Oscar Wood Fund for this purpose. The late Dr. Wood, a noted seismologist and fifteen years a Research Associate of the Institution, was also, most appropriately, a Principal Contributor to this volume.
President, Carnegie Institution
Recommendations to republish this book have come from numerous sources—most influentially, from Dr. Ian Campbell. As State Geologist of the California Division of Mines and Geology, Dr. Campbell was very much aware of the current widespread interest, both popular and academic, in earthquakes and of the demand for this report at libraries throughout California and elsewhere.
In order to understand the place and impact of this report when it appeared and to view it in its historical perspective, it is of interest to review briefly the progress of the science of seismology internationally and in the United States up until the publication of this report in 1908 and 1910.
Earthquakes have always been a source of keen interest—and of terror—to man. In ancient and medieval times, they were variously attributed to volcanic action, to the collapse of rock caverns, to explosions or the rushing of winds underground, and to supernatural causes. With such a range of suggested explanations, it is not surprising that individual earthquakes that caused great devastation and loss of life should have become the object of detailed examination by persons of a curious and sceptical turn of mind and with a bent toward natural philosophy. It is in large part the result of a series of monographic studies by such investigators that seismology emerged as a scientific discipline independent of the unrestrained speculation of earlier writers.
The scientific investigation of earthquakes may be said to have begun with a study, published in 1761 by John Michell of Cambridge University, of the great Lisbon earthquake of 1755. Michell broke free from the interpretations of classical writers of the past and relied solely on the evidence of direct observations. A later milestone in the development of seismology was an investigation of a devastating earthquake at Naples in 1857. In 1862 Robert Mallet, a practical engineer of Dublin and later of London, published a classic monograph on this Neapolitan quake that stood without rival until R. D. Oldham's report in 1899 on the great earthquake of 1897 at Assam, India.
In the years following Mallet's work, seismology advanced significantly as a scientific discipline. Michele de Rossi, Guiseppe Mercalli, and associates in Italy; Francois Forel and Albert Heim in Switzerland; Eduard Seuss in Austria; Montessus de Ballore in San Salvador, France, and Chile; and others in Germany and Russia described individual earthquakes, compiled regional catalogs of their occurrences, and systemized the reporting of earthquake intensities. On May 2, 1877, an earthquake was felt widely throughout central Europe, and in the following year the Swiss Seismological Commission was founded. It survives today as the Swiss Earthquake Service and has served as the model for similar organizations in other countries.
Seismology emerged as a quantitative science in the late 1880's in Japan. On February 22, 1880, Yokohama was rocked by a destructive earthquake, which became the subject of a memoir published by John Milne, a British professor of geology and mining at the Imperial College of Engineering at Tokyo. The memoir was published in 1880 and later that year, on Milne's initiative, a group of British and Japanese teachers formed the Seismological Society of Japan. Designers and builders of the first effective seismographs, Milne and his co-workers investigated individual quakes, the nature of earthquake motion, and the distribution of quakes in space and time. Individuals in the group
The study of earthquakes advanced somewhat more slowly in North American than in Europe and Japan. J. D. Whitney, professor of geology at Harvard University, studied the Owens Valley, California, earthquake of 1872 and reported on it that same year. G. K. Gilbert, of the U. S. Geological Survey, in 1884 described numerous recent fault scarps bordering the Wasatch Mountains and elsewhere in the Great Basin and compared them to similar scarps produced by the Owens Valley quake. In 1889 C. E. Dutton, also of the U. S. Geological Survey, published a monograph on the great Charleston, South Carolina, earthquake of 1886. In a series of articles published from 1872 through 1886, C. G. Rockwood, professor of mathematics at Rutgers and then Princeton University, began the compilation of a catalog of earthquakes in the United States. Rockwood's catalog was continued for the earthquakes of California, Baja California, Oregon, and the Washington Territory from 1887 through 1898 by E. S. Holden, director of the Lick Observatory at Mount Hamilton, California, and two of his colleagues. These annual lists were published as Bulletins of the U. S. Geological Survey.
Holden installed the first seismographs in the United States at Lick Observatory and at the University of California at Berkeley in 1887. Probably the first seismic Station in North America to use a seismograph with continuous recording was at Toronto in 1896. By 1901, there were continuous recording Stations at Baltimore, Maryland; Philadelphia, Pennsylvania; Victoria, B.C.; and Mexico City, Mexico; and by 1904, there were Stations in Washington, D. C.; Cheltenham, Maryland; Sitka, Alaska; and Honolulu, Hawaii.
In 1901 the first conference to establish an international organization of seismologists met at Strasbourg. By the time of
the second conference in 1903, twenty countries were represented—double the number that had attended the first. H. F. Reid,
the author of Volume II of the present study, represented the United States and was also present at the first meeting of the
permanent commission in 1906, which meeting led to the founding of the International Seismological Association. That organization
met first at the Hague in 1907 and once again Reid was one of the delegates.  Gutenberg, Beno, Seismology, in Geology, 1888-1938, 50th Anniv. Vol. Geol. Soc. Amer., p. 466, 1941.
Much additional information about the history of seismology may be found in:
Geikie, Sir Archibald, The Founders of Geology, 2nd ed., Macmillan & Co., 1905. Reprinted, Dover Pubs., Inc., 486 pp., 1962.
Adams, F. D., The Birth and Development of the Geological Sciences, Dover Pubs., Inc., 1938. Reprinted, 506 pp., 1954.
Davison, Charles, The Founders of Seismology, Cambridge Univ. Press, 240 pp., 1927.
On April 18, 1906, shortly after 5:00 a.m., a great earthquake struck San Francisco and a long narrow band of towns, villages, and countryside to the north-northwest and south-southeast. Many buildings were wrecked; hundreds of people were killed; electric power lines and gas mains were broken. Fires broke out and burned wildly for days, utterly out of control because of severed water mains.
The ground had broken open for more than 270 miles along a great fault—the San Andreas rift. The country on the east side of the rift had moved southward relative to the country on the west side of the rift. The greatest displacement had been 21 feet about 30 miles northwest of San Francisco.
Nearly all the scientists in California began immediately to assemble observations on the results of the quake. Professor A. C. Lawson, chairman of the geology department at the University of California, took the first steps that led to Governor George C. Pardee's appointment, three days after the shock, of a State Earthquake Investigation Commission to unify the work of scientific investigations then under way. The members of this
At its first meeting three days after it was appointed, the Commission organized itself into two committees. One, chaired by Lawson, was to determine surface changes associated with the earthquake and to collect data on the intensity at different places. The second committee, with Leuschner as chairman, was assigned the collection of data on the time of arrival of the earthquake at different places. A few weeks later, when the main features of the quake had become known, a third committee, led by Reid, was appointed to consider problems of the geophysics of the earthquake. The three committees consisted of members of the Commission and 21 other scientists, many of whom—such as Fusakichi Omori, professor of seismology at the Imperial University of Tokyo and one of the greatest seismologists of Japan—were already well-known and many others who later became internationally known as leaders in geology, mathematics, meteorology, and other fields of science.
No State funds were available to defray the expenses of the Commissions investigation, but provision for this purpose was made by the Carnegie Institution of Washington.
The Commission submitted a preliminary report of twenty pages to Governor Pardee on May 31, 1906. In November of the same year, the constitution of the Seismological Society of America was adopted. The first president of the Society was George Davidson, followed in successive years by A. C. Lawson, J. C. Branner, and A. G. McAdie, a member of Committee II of the Earthquake Commission. The secretary of the Society for many years was S. D. Townley, also a member of Committee II.
In preparation of the final report, hundreds of people were interviewed and evidence was collected from every damaged area as well as from records from seismograph Stations throughout the world. Lawson spent the winter of 1906-1907 in Washington compiling and editing individual reports from more than twenty collaborating scientists. He prepared an introduction, a section on the geology of the Coast Ranges, and many explanatory statements and summaries to weld the work into one unified report. Volume I, parts I and II, and the accompanying folio atlas were published in 1908 by the Carnegie Institution of Washington. Volume II, The Mechanics of the Earthquake, by H. F. Reid, followed in 1910.
The exhaustive report was favorably received on its publication and it continues even today to be very highly regarded by seismologists, geologists, and engineers concerned with earthquake damage to buildings. Volume I, established a model of earthquake investigation and reporting that has been widely followed ever since. Furthermore, it affords an invaluable pictorial record against which tectonic and other geologic changes since 1906 can be compared. Reid's masterly presentation of the elastic rebound theory of earthquakes in Volume II, remains today an apparently satisfactory explanation of one of the more important mechanisms of seismic activity. In all, the report stands as a milestone in the development of an understanding of earthquake mode of action and origin.
The account of the California earthquake of April 18, 1906, contained in this report, exemplifies the spirit of coöperation which pervades the scientific work in our day. Immediately following the great shock not only was the necessity of a scientific inquiry generally perceived, but it was realized that the occasion afforded an exceptional opportunity for adding to our knowledge of earthquakes. The scientific men of the state, each on his own initiative, began the work of assembling observations; the more intelligent citizens became persistent in their inquiries as to the nature of the earthquake, its extent and intensity, and the causes in general of such terrible disasters; and the state, thru its then Governor, George C. Pardee, unified the work of scientific investigation by the appointment of a committee of eight to direct the work. This committee was appointed on April 21, 1906, and became known as the State Earthquake Investigation Commission. On May 31, 1906, the Commission submitted a "Preliminary Report" to the Governor, which was printed and very generally distributed. In this report the details of the organization of the Commission, the program of its work, and the results attained to that date are set forth. But while the Commission acted under the authority of the Governor of the State, no money was provided by the Government for the conduct of its work. The embarrassment arising from this lack of funds was relieved about June 1, 1906, by a subvention from the Carnegie Institution of Washington, which enabled the Commission to prosecute its work as it had been planned.
About the end of the year 1906, the greater part of the observational data having been collected, the work of sifting, coördinating, compiling, and editing the same devolved upon the Chairman of the Commission. The results of this work, including several special papers by various investigators, are contained in Volume I, parts I and II, of the report and in the twenty-five maps of the accompanying atlas. In this volume especial effort has been made to give due credit to every contributor, whether he be a scientific writer discussing some particular phase of the general problem, or a citizen assisting with local information. In all cases where there is no ascription of authorship the Chairman of the Commission is responsible for the statements made. The multiplicity of contributors has made it inconvenient to duplicate their names in the already lengthy table of contents.
In general, Volume I is a record of observations with quite subordinate discussion of the facts recorded. The effort to condense the record as far as possible has been tempered by the desire to omit no significant fact, so that the record may be as complete as possible for purposes of comparison with similar events which may occur in years to come. In the preparation of this volume the Chairman of the Commission gratefully acknowledges the kind advice and cordial assistance of Messrs. G. K. Gilbert and H. F. Reid. The Commission is also under great obligation to its Secretary, Mr. A. O. Leuschner, for his very efficient services.
Volume II is chiefly a discussion of instrumental records and of the data bearing upon the mechanics of earthquakes, by Mr. H. F. Reid, who also contributes a general
Chairman State Earthquake Investigation Commission.
On the morning of April 18, 1906, the coastal region of Middle California was shaken by an earthquake of unusual severity. The time of the shock and its duration varied slightly in different localities, depending upon their position with reference to the seat of the disturbance in the earth's crust; but in general the time of the occurrence may be stated to be 5h 12m A. M. Pacific standard time, or the time of the meridian of longitude 120° west of Greenwich; and the sensible duration of the shock was about one minute.
The shock was violent in the region about the Bay of San Francisco, and with few exceptions inspired all who felt it with alarm and consternation. In the cities many people were injured or killed, and in some cases persons became mentally deranged, as a result of the disasters which immediately ensued from the commotion of the earth. The manifestations of the earthquake were numerous and varied. It resulted in the general awakening of all people asleep, and many were thrown from their beds. In the zone of maximum disturbance persons who were awake and attending to their affairs were in many cases thrown to the ground. Many persons heard rumbling sounds immediately before feeling the shock. Some who were in the fields report having seen the violent swaying of trees so that their top branches seemed to touch the ground, and others saw the passage of undulations of the soil. Several cases are reported in which persons suffered from nausea as a result of the swaying of the ground. Many cattle were thrown to the ground, and in some instances horses with riders in the saddle were similarly thrown. Animals in general seem to have been affected with terror.
In the inanimate world the most common and characteristic effects were the rattling of windows, the swaying of doors, and the rocking and shaking of houses. Pendant fixtures were caused to swing to and fro or in more or less elliptical orbits. Pendulum clocks were stopt. Furniture and other loose objects in rooms were suddenly displaced. Brick chimneys fell very generally. Buildings were in many instances partially or completely wrecked; others were shifted on their foundations without being otherwise seriously damaged. Water or milk in vessels was very commonly caused to slop over or to be wholly thrown from the vessel. Many water-tanks were thrown to the ground. Springs were affected either temporarily or permanently, some being diminished, others increased in flow. Landslides were caused on steep slopes, and on the bottom lands of the streams the soft alluvium was in many places caused to crack and to lurch, producing often very considerable deformations of the surface. This deformation of the soil was an important cause of damage and wreckage of buildings situated in such tracts. Railway tracks were buckled and broken. In timbered areas in the zone of maximum disturbance many large trees were thrown to the ground and in some cases they were snapt off above the ground.
The most disastrous of the effects of the earthquake were the breaking out of fires and, at the same time, the destruction of the pipe systems which supplied the water necessary to combat them. Such fires caused the destruction of a large portion of San Francisco, as all the world knows; and they also intensified the calamity due to the earthquake at Santa Rosa and Fort Bragg. The degree of intensity with which the earthquake made itself felt by these various manifestations diminished with the distance from the seat of disturbance, and at the more remote points near the limits of its sensibility it was perceived only by a feeble vibration of buildings during a brief period.
The area over which the shock was perceptible to the senses extends from Coos Bay, Oregon, on the north, to Los Angeles on the south, a distance of about 730 miles; and easterly as far as Winnemucca, Nevada, a distance of about 300 miles from the coast. The territory thus affected has an extent, inland from the coast, of probably 175,000 square miles. If we assume that the sea-bottom to the west of the coast was similarly affected, which is very probably true, the total area which was caused to vibrate to such an extent as to be perceptible to the senses was 372,700 square miles. Beyond the limits at which the vibrations were sufficiently sharp to appeal to the senses, earth waves were propagated entirely around the globe and were recorded instrumentally at all the more important seismological Stations in civilized countries.
The various manifestations of the earthquake above cited, including the cracking and deformation of the soil and incoherent surface formations, were the results of the earth jar, or commotion in the earth's crust. The cause of the earthquake, as will be more fully set forth in the body of this report, was the sudden rupture of the earth's crust along a line or lines extending from the vicinity of Point Delgada to a point in San Benito County near San Juan; a distance, in a nearly straight course, of about 270 miles. For a distance of 190 miles from Point Arena to San Juan, the fissure formed by this rupture is known to be practically continuous. Beyond Point Arena it passes out to sea, so that its continuity with the similar crack near Point Delgada is open to doubt; and the latter may possibly be an independent, tho associated, rupture parallel to the main one south of Point Arena. It is most probable, however, that there is but one continuous rupture. The course of this fissure for the 190 miles thru which it has been followed is nearly straight, with a bearing of from N. 30° to 40° W., but with a slight general curvature, the concavity being toward the northeast, and minor local curvatures. The fissure for the extent indicated follows an old line of seismic disturbance which extends thru California from Humboldt County to San Benito County, and thence southerly obliquely across the Coast Ranges thru the Tejon Pass and the Cajon Pass into the Colorado Desert. This line is marked by features due to former earth movements and will be referred to in a general way as a rift, the term being adopted from the usage for analogous features in Palestine and Africa.
1 Roy. Geograph. Soc. vol. IV, 4, 1894. The Great Rift Valley, by J. W. Gregory, London, 1896.To distinguish it from other rifts of similar origin, it will be referred to more specifically as the San Andreas Rift, the name being taken from the San Andreas Valley on the peninsula of San Francisco, where it exhibits a strongly pronounced character and where its diastrophic origin was first recognized in literature.
The plane or zone on which the rupture took place is, so far as can be determined from a study of the surface phenomena, nearly vertical; and upon this vertical plane there occurred a horizontal displacement of the earth's crust or at least of its upper part. The displacement was such as to cause the country to the southwest of the rift line to be moved northwesterly relatively to the country on the northeast side of that line. The differential displacement in a horizontal direction was probably not less than 10 feet for the greater part of the Rift; in many places it measured over 15 feet, and in one place as much as 21 feet.
This differential displacement of the earth's crust along the plane of rupture constitutes a fault, and will be so referred to in the text of the report. It is named the San Andreas fault. The intersection of the fault plane or narrow zone with the surface of the ground is manifested by cracks, heaved sod, scarps, etc., and these manifestations are designated the fault-trace. As a result of this fault, all the fences, roads, railways, bridges, tunnels, dams, pipes, and other structures which crost its path were dislocated. All property lines and other survey lines which were intersected by it were offset. Inasmuch as the movement of the earth which caused the fault was not confined to its immediate vicinity, but was distributed over a considerable belt of country on either side of the trace of the rupture, the Latitudes and longitudes of a large portion of the Coast Ranges of California were changed, and the triangles established by the Coast and Geodetic Survey in its triangulation of the region were distorted.
In addition to the horizontal displacement there was, particularly toward the northern end of the fault, a vertical displacement probably nowhere exceeding 2 to 3 feet, whereby the country to the southwest was raised relatively to that to the northeast. In many places, however, particularly toward the southern end of the fault, no vertical displacement can be detected, and there is some indication that, if there was vertical displacement in this region, it was the reverse of that observed in the northern portion of the fault. This rupture of the earth's crust gave rise to certain manifestations at the surface which resemble those described above as a result of the vibratory commotion of the earth, due to the sudden displacement. The cracking and rending of the surface along the line of the fault is a direct expression of the rupture and displacement which originated the earthquake, whereas the cracks, fissures, and lurching of the soft bottom lands and the landslide cracks on the hillsides, whether near the fault line or remote from it, are referable to the oscillation of the crust. The two classes of phenomena must, therefore, be discriminated, particularly as there has been a tendency on the part of some observers to class the secondary phenomena with the primary and interpret the former as indicative of fault lines in the earth's crust, when in reality they are merely superficial phenomena.
While the shock was perceptible to the senses to the extent above indicated in California, Nevada, and Oregon, the distribution of the higher grades of intensity was remarkably linear and was definitely related to the fault line, and to the general trend of the coast of California. This may be brought out in a preliminary way by stating that a zone of destructive effects extends parallel to the Rift from Humboldt Bay, in Humboldt County, to the vicinity of King City in Monterey County, a distance of 350 miles. If we take the throw of brick chimneys and allied phenomena as indicating the limits of what may be called destructive effects, the width of this zone may be fairly approximated at about 70 miles, or about 35 miles on either side of the fault, or its prolongation where no actual fault is observable at the surface. The length of this zone of destruction is thus five times greater than its width, and the total area within which the shock was sufficiently severe to throw brick chimneys may be placed at something over 25,000 square miles; it being assumed that the severity to the southwest of the fault, beneath the waters of the Pacific, was equal to that on the land. If the fault near Point Delgada be regarded as distinct from that extending from Point Arena southeasterly, then the total area of these high intensities would be considerably larger in the direction of the Pacific.
Within this outer limit of destructive effects the intensity increased toward the fault. But proximity to the fault was not the only factor determining the degree of intensity. The soft, more or less incoherent, and water-saturated alluvial formations of the valleybottoms were much more severely shaken than the rocky slopes of the intervening ridges, and the structures upon them were consequently more commonly and more completely wrecked. It is not understood by this excessive damage on the valleybottoms
The California Earthquake of April 18, 1906
Volume I, Part I
Geology of the Coast System of Mountains
In common with many other mountainous tracts the world over, the Coast System has limits which are difficult of precise definition. The criteria which serve to discriminate one tract from another are various and have different values in different cases. Any attempt at precise definition must be more or less arbitrary. An outline of the extent and subdivisions of the system will, however, be presented in summary fashion.
On the north the Coast System extends to the northern end of Humboldt County, and in that county and in southern Trinity County the last typical ridge is South Fork Mountain. This is a remarkably linear ridge beginning near the coast and extending with a northwest-southeast course to the vicinity of North Yallo Bally Mountain. Beyond South Fork Mountain to the northeast lie the Klamath Mountains, a group more nearly allied in the history of its uplift and in its constituent rocks to the Sierra Nevada than to the Coast Range. On the south the Coast System is sometimes regarded as ending in Santa Barbara County; and the mountains of Southern California, thence east-southeast and south to the Mexican boundary, are regarded as a distinct system, being viewed as a northerly prolongation of the orographic axis of the peninsula of Lower California. The chief consideration favoring this distinction is the change in trend of the mountain ridges, which becomes apparent just north of the Santa Barbara Channel. Other facts favor this discrimination, such as the prevailing absence of the Franciscan formations in the mountains of southern California and the greater abundance of granitic rocks; but more especially the greater incisiveness of the structural lines, indicating, on the whole, more intense orogenic action. But these considerations are largely offset by the unmistakable continuity of the tectonic lines of the northern ranges into the mountains of southern California, and by the fact that the movements to which their larger features are due date from the close of the Tertiary. It would seem, therefore, that there is sufficient unity of character in these coastal mountains, in spite of their change of trend, to warrant their being classed as the Coast System from South Fork Mountain south to the Mexican boundary and beyond. That term may be used in a comprehensive sense, significant of the genetic and structural unity which runs thru them.
It will nevertheless be very convenient to recognize three subdivisions of the Coast System thus outlined. The first of these subdivisions extends from South Fork Mountain on the north to the Valley of the Cuyama River on the south, and may, in accordance with popular usage, be referred to simply as the Coast Ranges, the term "system" being used only when it is intended to express the more comprehensive view. The second subdivision is a broad chain extending from Santa Barbara County to the far side of the Colorado desert with a general trend of west northwest-east southeast, and including the San Rafael, Santa Ynez, Santa Susannah, Santa Monica, San Gabriel, and San Bernardino Ranges, and also, perhaps, the Chocolate Range. This chain is sometimes referred to as the Sierra Madre, tho the full application of the term in popular usage is not clear. The third subdivision embraces the mountainous country south and southeast of the valley of Southern California, the principal ranges of which are the Santa Ana and the San Jacinto. These have the northwest-southeast trend of the Coast Ranges and, in accordance with the suggestion of some of the earlier writers on Californian geology, may be referred to as the Peninsular chain.
The Coast Ranges of California have had a long and varied geological history. Their structure is complex and the sequence of formations differs at different points. Several of the more important groups of sedimentary rocks contain, so far as known, but few fossils or none at all. Only in recent years have the topographic maps necessary for an adequate study of the stratigraphy and structure of the region become available, and then only for limited areas. Nevertheless the general outlines of the geology of the Coast Ranges are known, and in some of the localities which have been topographically mapped, a considerable body of detailed information is at hand.
The oldest sedimentary rocks of the Coast Ranges are of unknown age. They comprize impure and somewhat magnesian limestone, quartzites, and various crystalline schists. The limestones are usually in the form of coarse marble varying in color from dark gray to white and containing frequently some graphite and less commonly lime silicate. The quartzites are thoroly indurated, as a rule, sometimes to the extent of being vitreous, and usually show well-marked stratification. The schists have as yet been little studied, and no adequate observations upon their character in detail have been put on record. They are known, however, to comprize both micaceous and hornblendic varieties.
These marbles, quartzites, and crystalline schists are known only in more or less fragmentary form, associated with considerable bodies of granitic rocks which have invaded them as batholiths. The most common occurrence of the marbles, quartzites, and schists is in the form of limited belts and isolated patches embedded in the granitic rocks, or in limited areas flanking the margins of the batholiths, and showing evidence of contact metamorphism. It is evident in most cases, and is probably generally true, that the granite of the Coast Ranges is of later date than the metamorphic sedimentary rocks associated with them. While the age of these pregranitic sedimentary formations is at present unknown, the age of the granite is suggested by its seeming identity with the granite of the Sierra Nevada. The latter is a vast batholith known to be intrusive in Paleozoic and Mesozoic strata as late as the Upper Jurassic. This granite has been followed thru the Sierra Nevada to Tehachapi and Tejon Pass, where the range curves sharply around and passes into the Coast Ranges. Passing northerly thru the Coast Ranges, granite identical in character with that of the Sierra Nevada, and carrying identical inclusions of older sedimentary rocks, is traceable in more or less extensive areas from the upper reaches of the Cuyama River to Bodega Head on the coast north of the Golden Gate. It thus seems probable that the granite of the Coast Ranges, like that of the Sierra Nevada, is of late Jurassic or post-Jurassic age. The granitic rocks of the Coast Ranges, together with the pregranitic rocks into which they are irruptive, constitute a complex which is thus the probable analogue of the Bedrock Complex of the Sierra Nevada.
This Cost Range Complex was subjected to vigorous erosion and then submerged to serve as the sea floor upon which the series of rocks known as the Franciscan was deposited. This series consists for the most part of medium coarse, dark gray or greenish-gray sandstone, strongly indurated, with subordinate shales and conglomerates. Intercalated with these sandstones are important horizons of foraminiferal limestone and radiolarian chert and admixtures of volcanic rocks, chiefly basaltic in character. In the vicinity of the Bay of San Francisco, where the series is best known, it falls into seven stratigraphic divisions. These are in ascending order:
(1) A group of arkose sandstones with some conglomerates and shales reposing unconformably upon the Montara granite and with an aggregate thickness of about 800 feet.
(2) A formation of light-gray, very compact and fine-textured forminiferal limestone ranging in thickness from about 60 to possibly a few hundred feet.
(3) Sandstones aggregating 2,000 feet in thickness.
(4) A formation of radiolarian cherts from 100 to 900 feet.
(5) Sandstone, 1,000 feet.
(6) Radiolarian cherts, 500 feet.
(7) Sandstone, 1,400 feet.
In this sequence of sedimentary strata, particularly toward its upper part, there are intercalated lavas at various horizons.
After their accumulation, but before the next higher series of rocks was deposited upon them, the Franciscan strata were invaded by intrusive rocks at points so numerous and so widespread thruout the Coast Ranges that these intrusive bodies constitute one of their most characteristic associations, in contrast to the series which succeed them. The intrusive rocks are of two general types. One is a highly magnesian rock, usually a peridotite, but with facies of pyroxenite and gabbro, the peridotite being generally almost completely serpentinized. The other is a basaltic rock grading into diabase and having in many of its occurrences the peculiar structure characteristic of the spheroidal basalts. In addition to the spheroidal structure on the gross scale, it is in some eases variolitic. Associated with both of these intrusives are areas, generally of limited extent and sporadic distribution, of glaucophane and other crystalline schists, which appear, where they have been most thoroly studied, to be the result of a peculiar kind of contact metamorphism.
The stratigraphic composition of the Franciscan series indicates an interesting to-and-fro migration of the shore line of that time, probably due to a vertical oscillation of the continental margin. The basal group of sandstones, shales, and conglomerates is clearly a terrigenous deposit laid down in proximity to the margin of the continental area from which the sediments were derived. The next succeeding formation, the foraminiferal limestone, on the contrary, is nonterrigenous. Its character as nearly pure carbonate of lime, except for the flinty lenses and nodules it contains, and the abundance of foraminifera, indicates that the sea-bottom over the present position of the San Francisco Peninsula was too remote from the shore to receive an admixture of sand or clay. That is to say, the conditions which favored the deposition of the limestone were inaugurated by a withdrawal of the shore line from the position which it occupied during the deposition of the underlying sandstones. And this lateral migration of the shore was doubtless the result of a sinking of the coast.
Above the foraminiferal limestone sandstones again occur, indicating a return of the shore to about its former position, doubtless due to an uplift of the sea-bottom and coast. These sandstones are in turn followed by a nonterrigenous formation of radiolarian cherts. These are for the most part flinty rocks containing abundant remains of radiolaria, marine organisms which secrete a siliceous test instead of a calcareous one, as in the case of the foraminifera. They contain no admixture of sand, and the shaly partings which separate the layers of chert are very doubtfully referable to land waste. Here again the sea bottom must have been deprest and the shore line caused to withdraw. These radiolarian cherts are followed again by sandstones, and these by a second formation of radiolarian cherts, the former as before indicating uplift of the sea-bottom and the latter depression. The last movement in Franciscan time was uplift, indicated by the sandstones, which rest upon the second horizon of radiolarian cherts and which constitute the topmost formation of the Franciscan series.
The age of the Franciscan is not positively known. Certain general considerations, however, contribute data upon which a tentative judgment as to this question may be based. Stratigraphically, the Franciscan lies upon the eroded surface of the Coast Range granites, the correlation of which with the post-Jurassic granites of the Sierra Nevada has been suggested. If such correlation be adopted, the age of the Franciscan must be
After the accumulation of the Franciscan strata as thus characterized, and perhaps in connection with the invasion of the series by peridotitic and basaltic intrusives, the region was folded and broken, and elevated within the zone of erosion. The elevatory movement was probably quite general. The Franciscan, while subjected to general denudation, was probably nowhere stript down to the underlying basal complex before it was submerged to receive the next succeeding sedimentary strata. These comprize the Knoxville formation, consisting wholly of shales and sandstones with quite subordinate layers and lenses of limestone, all in very regular and rather thin strata, significant of deposition in a shallow basin under fluctuating conditions of transportation. The Knoxville varies in volume from a few hundred to several thousand feet and is widely distributed over the Coast Ranges. It is succeeded in the vicinity of the Bay of San Francisco, and to a less marked degree in other parts of the Coast Ranges, by a formation of coarse conglomerate known as the Oakland conglomerate. This conglomerate attains a thickness of over 1,000 feet in places and follows the Knoxville shales in apparently conformable sequence. The change in the character of the deposits from shales to coarse conglomerates, without any interruption in the continuity of sedimentation, suggests an orogenic disturbance of the margins of the basin within which the Knoxville beds were accumulating, whereby the grades of the streams were greatly accentuated and the degradation of the continental region correspondingly accelerated.
The Oakland Conglomerate, or, where that is missing, the Knoxville shale, is directly followed by a formation of thick bedded sandstones and shales known as the Chico formation. It has a thickness in places of many thousands of feet. The entire volume of strata, from the base of the Knoxville to the top of the Chico, is usually referred to as the Shasta-Chico Series, the Shasta comprizing the Knoxville and Oakland formations, together with certain other paleontological subdivisions not here particularly mentioned. The series is remarkable for its great volume. In the northern Coast Ranges to the west of the Sacramento Valley, the thickness of the sedimentary section, comprizing practically only sandstones and shales, is as much as 29,000 feet. This vast accumulation of strata clearly signifies the development of a great geosyncline, or depression of the sea-bottom in that region in which deposition kept pace with subsidence thruout this portion of Cretaceous time. The Shasto-Chico series is usually regarded as comprizing the whole of the California Cretaceous, but the considerations cited above in regard to the Franciscan indicate that the latter may perhaps be included in the lower Cretaceous section of this region.
The movements which brought the Mesozoic to a close and inaugurated the Tertiary in the Coast Range region were not those of violent orogenic deformation such as characterize this period of geological time in many other parts of the world; but were rather of the nature of a partial elevation of the region, with quite gentle deformation, resulting in a notable restriction of the basin of deposition. The earliest Eocene strata show no marked structural discordance with the Chico. It is nevertheless very probable that a notable unconformity exists, since the abundant and characteristic Cretaceous fauna disappeared and was supplanted by an almost totally distinct assemblage of life forms. The Eocene of the California Coast Ranges falls into two paleontologically distinct groups which have been classed together as the Karquines series. The lower of these
The next succeeding group of rocks, belonging to the Oligocene division of the Tertiary, has been named the San Lorenzo Formation.
Arnold, U. S. G. S. Professional Paper No. 47, p. 16.It is known in Santa Cruz County, where it attains a thickness of 2,300 feet, made up chiefly of gray shales and fine sandstones. Its stratigraphic relations to the Tejon are not yet known, but its fauna is said by Arnold to contain many species which appear to be closely related to Tejon forms. It may thus be considered as following the Tejon conformably. It is in certain sections known to be unconformable beneath the oldest formation of the Miocene, known as the Vaqueros Sandstone, indicating that after the deposition of the San Lorenzo formation, the region of the Coast Ranges was disturbed and uplifted into the zone of erosion; and the following facts regarding the transgression of the Miocene Sea indicate that this uplift was a very extensive one. Such an uplift in time immediately preceding the Miocene is further indicative of a much closer relationship between the San Lorenzo and the Tejon than between the former and the Monterey.
Miocene time in the Coast Range region was characterized by a progressive subsidence with oscillations of the coast. The Miocene sea gradually transgrest the continental margin from the southwest, and as it did so spread a formation of arkose sands and conglomerates over the greater part of the southern Coast Ranges. This was followed, as the water deepened with progressive subsidence, by a remarkable deposit of bituminous shales. These shales are usually whitish or cream-colored, tho often of a purplish or other dark tint, and may be either of a soft chalky consistency, or opaline, or hard and flinty. It is thruout an essentially siliceous formation and is largely diatomaceous in character, tho more or less admixt with volcanic pumiceous ash. In some portions the ash is a prominent constituent, and in San Luis Obispo County there is a deposit aggregating about 1,000 feet in thickness of well-stratified volcanic tuff and agglomerate. In San Mateo County there are basalts which were erupted at this period. Interstratified with these siliceous shales, thin beds of more or less ferruginous and somewhat magnesian limestones are by no means uncommon. They are, however, lenticular or non-persistent, and are of a very compact texture and usually nonfossiliferous. There are also in some places thin but persistent beds of a peculiar, very hard, fine-grained, light-colored sandstone intercalated with the shales. In the southern portion of the Coast Ranges the bituminous shales accumulated to a thickness of several thousand feet, but in the middle Coast Ranges, in the vicinity of the Bay of San Francisco, the Miocene sea was characterized by an oscillatory or to-and-fro migration of its eastern shore line, due to alternate uplift and subsidence of the coast, quite analogous to that described for the Franciscan period. This gave rise to an alternation of shallow water in which sandstones were deposited, and deep water in which siliceous ooze accumulated with but little admixture of terrigenous material. We have thus in the territory between Mount Diablo and the Bay of San Francisco an alternation of four formations of bituminous
At the close of the Miocene, the Coast Range region was disturbed by orogenic movements and uplifted into the zone of erosion. It was then deprest irregularly so as to give rise to local basins of sedimentation in which accumulated great thicknesses of Pliocene beds, particularly about the Bay of San Francisco and southward. The oldest of these Pliocene formations is the San Pablo, which lies unconformably upon the Monterey strata. This is essentially a sandstone formation with a thickness of from 1,000 to 2,000 feet. It occurs on both sides of the Coast Ranges from the vicinity of the Bay of San Francisco southward and appears to have been laid down in two basins, separated by a barrier corresponding to the general axis of the present Coast Ranges. The formation on the east side of the range is characterized by a notable admixture of dark andesitie ash, which gives the unweathered exposures of the sandstones a distinctly blue color. This formation has a fauna of over 100 species, of which more than 40 per cent are living forms. This fact, and the unconformable superposition of the formation upon the Monterey, are warrant for placing it in the Pliocene. On the west side of the Coast Ranges, the San Pablo is best known in San Luis Obispo and Santa Barbara Counties, and is there free from volcanic admixtures, tho the basal beds are very commonly characterized by the presence of asphaltum, which cements the sand together and constitutes the well-known bituminous rock of the region. This asphaltum appears to have originated in part as a seepage from the upturned bituminous shale of the Monterey along the shores of the San Pablo sea, and molluscan remains of San Pablo age are often embedded in it.
Succeeding the San Pablo, but nowhere, so far as the writer is aware, reposing directly upon it, is the Merced series. The sediments composing this series were laid down in rather acute geosynclinal troughs, resulting from orogenic deformation of the coast in middle Pliocene time. Three of these troughs are known. The most northerly is that now occupied by the Valley of the lower Eel River in Humboldt County; the second is largely occupied by the Santa Rosa Valley in Sonoma County; the third is on the Peninsula of San Francisco, extending thence south to the coast of Santa Cruz County. The Merced strata in the Valley of the lower Eel River, and the typical Merced section near San Francisco, show each a thickness of something over a mile. In Sonoma County the marine Merced beds grade eastward into fluviatile conglomerates, admixed with volcanic ashes. The maximum thickness is about 3,500 feet. The lower part of the series is here characterized by a considerable volume of white volcanic pumiceous tuffs, which thinout rapidly to the westward. These were in part laid down directly on a land surface burying forests of huge sequoia, whole trees being now completely silicified.
For a description of the Merced beds of Sonoma County and the underlying pumiceous tuff, a paper by V. C. Osmont, Bull. Dept. Geol., Univ. Cal., vol. 4, No. 3, should be consulted.On the coast of Santa Cruz County, the series is represented by strata of lower stratigraphic horizons than nearer San Francisco, these lower beds having been called the Purissima formation, altho the sedimentation was continuous with that of the Merced. The
The accumulation of the Merced series to the great thickness above indicated in middle and northern California proves local depressions of the coast of over a mile below sea level in later Pliocene time. Similar orogenic deformation was in progress at the same time on the eastern side of the barrier corresponding to the then axis of the Coast Ranges. These movements gave rise to great troughs from which the sea was excluded, but which were occupied by fresh water, and filled with sediments equal in volume to those of the marine troughs to the west of the barrier. The greater part of these fresh-water beds are comprized in the Orindan formation, which may be the equivalent of the Cache Lake beds of the Clear Lake district
Described by G. F. Becker, U. S. G. S. Monograph XIII, pp. 219-221, 238-242.and of the Paso Robles in the southern Coast Ranges. They have an extensive distribution on the eastern side of the Coast Ranges, and in the vicinity of the Bay of San Francisco there intervenes between the base of the Orindan and the San Pablo a formation of white pumiceous tuff entirely similar to that at the base of the Merced series in Sonoma County, but containing here fresh-water fossils. This tuff attains a maximum thickness of about 1,000 feet and is known as the Pinole tuff. Thruout the Orindan there are occasional intercalated strata of volcanic tuff of moderate thickness. The Orindan lacustrine period was brought to a close in the region of the middle Coast Ranges by volcanic eruptions which resulted in extensive flows of lava and showers of ashes. Upon these lavas lake basins were later established and some hundreds of feet of fresh-water deposits (Siestan formation) accumulated in them, which were in turn buried by other lavas.
The accumulation of the Merced marine beds and the corresponding lacustrine and volcanic rocks was brought to a close by an acute and widespread deformation regarded as part of the general mountain-making movements which ushered in the Pleistocene in western North America. As a result of these movements, the Merced and Orindan basins were folded and faulted, and the basement upon which their contained strata had been laid down was lifted in part from a position over a mile below sea-level to one far above sea-level. The Pliocene formations were brought within the zone of active erosion and the evolution of the present geomorphic features of the Coast Ranges was inaugurated. When the degradation of the folded Orindan strata was well advanced, a lake basin was established across the edges of these strata and in it accumulated the various fresh-water beds and volcanic lavas and tuffs comprizing the Campan series. At a time within the Pleistocene when the geomorphic evolution of the coast had been well advanced to its present condition, the coastal belt was deprest 1,000 to 2,000 feet lower than it is at present, and then uplifted in stages marked by marine terraces along many parts of the coast. Since this there have been oscillations of the region about the Bay of San Francisco, the net result of which has been a depression allowing the sea to invade the valley-lands and thus make the magnificent harbor to which San Francisco owes its existence.
In the foregoing sketch of the formations of the Coast Ranges and their historical significance, it is desired to emphasize particularly the remarkable series of subsidences and uplifts which have affected the coastal region from the beginning of the Franciscan to the present. This record of oscillation is in marked contrast to the comparative stability of the Sierra Nevada. Except for a marginal strip of its foot-hill slopes, the region of the present Sierra Nevada has not been submerged beneath the sea. During the geological ages in which the Coast Range region has been repeatedly deprest to receive marine sediments, the sum of the maximal sections of which amounts to 65,000 feet of strata, the western edge of the Sierra Nevada region has probably never been
Between these two strongly contrasted provinces lies the great valley of California, one of the very notable geomorphic features of the continent. This valley is but one of a long series of similar depressions which lie along the western border of the North American continent, between the coastal uplands and the western edge of the continental plateau. In the north it has its probable analogues in Hecate Strait, the Gulf of Georgia, Puget Sound, the Willamette Valley, the Ashland Valley, and the depression between the Sierra Nevada and the Klamath Mountains. On the south we see its analogues in the Colorado Desert, the Gulf of California, and in the valley which lies between the southern border of the central plateau of Mexico and the Sierra Madre del Sur. In the Californian region we must interpret the axial line of this depression as a tectonic hinge, upon which the mobile coastal region has swung in a vertical sense upon the edge of the interior plateau, here represented by the Sierra Nevada. Whether this tectonic hinge is a more or less flexible zone upon which movement has taken place without rupture, or whether it represents a zone of dislocation, is not clear; but that differential movement has taken place along the valley line is one of the salient facts in the geological history of California.
A detailed account of the structure of the Coast System would involve a discrimination between features referable to the different orogenic movements which have affected the region at various periods of its history. Owing to this succession of movements, new structures have been superimposed upon older structures, or upon remnants of older structures, so often that the resultant effect is extremely complicated and not only difficult to unravel but difficult to state or describe in any simple way. In this summary review of the subject, no such detailed discrimination will be attempted. The only effort will be to call attention to the salient features, which are for the most part referable to the orogenic movements of later Tertiary and post-Tertiary time.
Marginal Features. — In a consideration of the structural features of the Coast System, its marginal lines on the east and west first claim attention. The eastern slope of the Coast Ranges rises from the floor of the Great Valley much more abruptly in general than does the western slope of the Sierra Nevada from the same valley floor. Turner
Am. Geologist, vol. XIII, p. 248.has suggested that the Great Valley east of the Coast Ranges is determined by a fault. There is some warrant for this view and it is certainly true in part. The very precipitous mountain front which rises from the valley at its southern end is without doubt a degraded fault-scrap, tho whether or not this fault or a series of similar faults can be followed along the edge of the mountains to their northern end is questionable. It is, however, safe to say that the eastern margin of the Coast Ranges represents a line of acute deformation, with the probability of that deformation having taken the form of faults in certain places. No one has yet made a sufficiently careful study of the question to make a more precise statement possible. In general, this line of acute deformation is not
On the seaward side the Coast System is usually regarded as being limited by the shore line. The precipitous coast rising to elevations of from 2,000 to 5,000 feet, extending from Cape Mendocino to Point Conception, and the popular notion that mountain ranges are confined to the land areas of the earth, are justification for this view. But in a more comprehensive view, embracing all inequalities of the earth's surface both above and below the sea-level, the western margin of the mountainous area, the familiar portions of which we call the Coast System, will have to be placed farther seaward. Off the coast of California the sea-bottom slopes down to the 3,000-foot submarine contour at a moderate angle and then plunges steeply to depths of over 12,000 feet. Beyond the foot of this steep slope the sea-bottom has very flat gradients and the 15,000-foot contour is far out to sea. From the Oregon line to Point Conception the 3,000-foot submarine contour, or the brink of the steep slope, lies off shore at a distance of from 15 to 35 miles; but at Cape Mendocino and at the Bay of Monterey this line is found much closer in. South of Point Conception this steep slope has the same general trend as to the north. That is to say, it shows no embayment in its course corresponding to that at the Santa Barbara channel and southward. This is particularly true of the course of the 6,000, 9,000, and 12,000-foot contours. The slope is by no means uniform for its entire length. From Point Arena to the latitude of the Golden Gate the grade is notably steep from the 3,000 foot to the 9,000-foot contour. This is also true off Point Conception. From the latter point southeastward the steep portion of the slope is from the 6,000-foot to the 12,000-foot contour; and the same statement holds for the slope off San Simeon Bay. In general, the steepest profile lies between the 6,000 and the 9,000-foot line.
This steep drop from the subcontinental platform to the broad floor of the Pacific must be regarded as the geomorphic expression of a rather acute deformation of the earth's crust, and those portions of the slope where the contours are crowded together, as for example between Point Arena and the latitude of the Golden Gate, off San Simeon Bay, off Point Conception, and off the platform of the Channel Islands, can scarcely be interpreted as other than fault-scarps. The slopes at the localities mentioned are quite comparable to the great fault-scarp which forms the eastern front of the Sierra Nevada. At the base of the slope off the Channel Island platform, the recent dredging operations of the Albatross brought up from a depth of 12,000 feet numerous fragments of rock similar to the bituminous shale of the Monterey series of the southern Coast Range. With this rock was found much asphaltum. This indicates that at the base of the slope there are talus accumulations of so recent a date that they have not yet been buried by oceanic sediments.
This line of acute deformation of the crust off the entire length of the coast of California can not be ignored in any consideration of the orographic features of the region. The slope referred to is doubtless devoid of those sculptural features characteristic of mountains within the zone of erosion, and which we are too apt to look upon as essential, but it constitutes nevertheless a notable mountain front rising from the floor of the Pacific. It is the natural western boundary of the mountainous tract which we call the
Having indicated the east and west boundaries of the Coast System as their dominant structural lines, we may now consider those features which pertain to the internal structure of the mountain tract. Here we must first take note of the coast line. The coastal slope of California characteristically rises abruptly from sea level to elevations of from 2,000 to 5,000 feet within a short distance from shore, from Cape Mendocino to Point Conception, with certain notable breaks in its continuity which are susceptible of special explanation. If along the shore line at the base of this abrupt slope we draw straight lines which are tangent to the headlands or chords to the minor embayments of the coast, these lines fall into two fairly constant orientations and clearly bring out the fact that the shore line has in reality a zigzag course, due apparently to the alternate control of two systems of structural lines, one of which is between N. 37° W. and N. 40° W., and the other between N. 10° W. and N. 15° W., thus intersecting at an angle of about 26°. Under this scheme of discrimination of the orientation of different portions of the coast line, the bearings of the following divisions may be thus listed:
|Localities.||Bearing of Mean Line.||Distance in Geographical Miles.|
|Cape Mendocino to Punta Gorda||N. 12° W.||14|
|Punta Gorda to Shelter Cove||N. 40° W.||25|
|Shelter Cove to Point Arena||N. 10° W.||64|
|Point Arena to Golden Gate, thru Tomales Bay||N. 40° W.||90|
|Golden Gate to Pigeon Point||N. 15° W.||40|
|Pigeon Point toward Santa Cruz||N. 40° W.||21|
|Point Pinos to Point Sur||N. 13° W.||19|
|Point Sur to Port Hartford||N. 37° W.||89|
|Port Hartford to Point Conception||N. 6° W.||44|
Now it is difficult to regard any considerable portion of the abrupt coastal slope of California between Cape Mendocino and Point Conception as other than a more or less degraded fault-scarp. If this view be accepted, it is clear that the trend of the coast and its geomorphic profile have been determined by two systems of faults meeting or intersecting at an angle of about 26° on their strike. Making some allowance for cliff recession, the base of both systems of scarps must lie some little distance off shore and be buried by the notable embankment of littoral sediments which conceals the true profile of the submarine rock surface.
Of the two systems of faults thus recognized as controlling the trend of the coast, one, viz. that which bears N. 37° W. to N. 40° W., conforms, as will be shown later, more or less closely with the prevailing structural lines, such as faults, folds, and belts of igneous rock found in the Coast Ranges; while the more meridional system is not a prominent feature of the Coast Ranges. It follows that since the mean trend of the California coast lies between the bearings of the two fault systems, the tectonic lines of the Coast System, if followed northwesterly, eventually emerge upon the coast. This obliquity of the
Am. Geologist, Vol. XI, Feb. 1893, p. 70.but the probable explanation of it has not heretofore been set forth.
The coastal scarp is interrupted at a number of points and in a variety of ways. The most notable and interesting interruption is that of the Bay of Monterey. This is not only an embayment of the coast, but is a depression in the Coast Ranges extending down over their submarine portion to the 12,000-foot contour below seal-level. It brings the 3,000-foot submarine contour well inside the general line of the coast. This submarine valley has been regarded by some writers as a submerged valley of subaerial erosion, but there is little warrant for this view and much that conflicts with it. The valley of the Bay of Monterey, subaerial and submarine, is a synclinal trough the axis of which is approximately normal to the trend of the coast and of the Coast Ranges as a belt. In the axis of the syncline, and probably parallel to it, is a fault seen in the canyon between Pajaro and Chittenden, which brings down the Tertiary rocks on the north side against the pre-Cretaceous granitic rocks of the Gavilan Range. Another interruption of the continuity of the coastal scarp is at the Golden Gate. Here the Coast Ranges have been locally deprest and the land valleys which were formerly drained by a trunk stream, where the Golden Gate now is, have been flooded by the waters of the ocean. The axis of this depression is, however, not well known. A third, apparent rather than real, interruption of the coastal scarp occurs at the place where the Point Reyes Peninsula projects out beyond the general line of the coast. Inside of the peninsula, however, there is a long narrow valley, the northern end of which is occupied by Tomales Bay and the southern end by Bolinas Lagoon, which separates it from the mainland proper; and to the east of this valley the coastal scarp rises with exceptional boldness.
The coastal scarp has had its profile modified in many places by wave-cut terraces formed during the uplift of the coast by stages in Pleistocene time, as previously stated. The relation of the coastal scarp to deformed basins of Merced (late Pliocene) strata indicate that it originated, in its essential features, at the period of orogenic activity which brought the Tertiary to a close. South of Point Conception the twofold system of faults which determines the configuration of the coast gives out and we enter upon a region of probably more complicated structure. The Santa Barbara channel appears to lie in a geosynclinal trough between the Santa Ynez range and the island chain from Anacapa to San Miguel. On the northeast side of San Clemente is a sharply defined fault-scarp, indicating that the island is a portion of an uplifted and tilted orographic block. The fault along which the scarp has been formed probably extends as far as the east side of Santa Barbara Island. San Clemente Island presents a magnificent series of wave-cut terraces up to an elevation of 1,500 feet. San Pedro Head is similarly uplifted and terraced, while the intervening island, Santa Catalina, shows no evidence of corresponding uplift, but has on the contrary been deprest. On the whole, the channel island platform between the edge of the subcontinental shelf and the coast presents the characters of a sunken mountainous tract, the inequalities of the surface of which are partly due to acute deformation and partly to erosional sculpture when the region was above sea-level. A more detailed interpretation of the structure of this region is rendered difficult by the absence of adequate soundings of the sea-bottom.
Granitic Rocks. — Coming now to the consideration of the more important structural features of the Coast System, in the territory between the coast and its eastern margin, it must be stated that even here our information is very scant. One of the most important features of the Coast System from a structural point of view is the occurrence of a belt of granitic rock having a very notable linear extent thruout the ranges. This granite, as has been already stated, appears, in the vicinity of Tejon Pass, both from
Bull. Dept. Geol. Univ. Cal., vol. 3, No. 1.and in the Perris plain.
To the northwest of Tejon Pass, this granite appears in a series of linearly disposed areas extending thru the ranges. It forms a notable feature of the Santa Lucia Range on the west of the Salinas Valley, and also of the Gavilan Range to the east of the same valley. The granite of the Santa Lucia Range runs out to sea at Point Pinos near Monterey, while that of the Gavilan Range extends into Santa Cruz County and appears on the coast at Point San Pedro, a few miles south of San Francisco. Farther north it is seen in the Farallon Islands, the Point Reyes Peninsula, and on Bodega Head. The Santa Lucia and the Gavilan thus expose two quite distinct lines of granitic outcrop, practically parallel, and both crossing the general trend of the Coast Ranges obliquely and reaching the coast. Indeed, the easterly limit of all the granite of the Coast Ranges crosses the entire breadth of the latter obliquely between the Tejon Pass and Bodega Head. This signifies, of course, that whatever manifestations of crustal deformation elevated these belts of granite, the lines or axes of such deformation were not coincident in direction with the mean trend of the Coast Ranges, or with the mean trend of either of the margins of the Coast Ranges. It is noteworthy, too, that all of the Coast Range granite as far south as the vicinity of Tejon Pass lies to the southwest of the Rift along which the movement occurred which generated the earthquake of April 18, 1906. It is further noteworthy that near the northern end of the granite belt at Tomales Bay and Bodega Head, the Rift actually follows the line of contact between the granite on the west and the sedimentary rocks which are faulted against it. These facts suggest that very probably the Rift is similarly situated in the more southern Coast Ranges with reference to a deeper-seated contact between granite and sedimentaries; in other words, that the eastern edge of the Coast Range batholith, whether that edge be an original feature of the batholith or a feature determined by faulting, is with some degree of probability the line which determines in part the course of the modern Rift. Southward from the vicinity of Tejon Pass, however, the Rift passes into the granitic terrane.
Folds. — The pre-Knoxville folds of the Coast Ranges are little known, owing partly to the burial of the Franciscan rocks by later deposits, and partly to the complexity of the structures where the rocks are exposed and the difficulty of discriminating the effects of the earlier and the later movements; but chiefly owing to the absence of adequate topographic maps, so necessary for such studies. The conspicuous folds of the Coast Ranges are those which have been imprest upon the Tertiary and older strata together. These are usually rather sharp and more or less symmetrical synclines and anticlines, involving usually many thousands of feet of strata. In some cases these are asymmetric and even overturned, as in the Mount Diablo region, but they are never so closely apprest as to induce general and important deformation of the internal structure of the rocks affected. The folding has been effected without flowage, except perhaps locally where soft clays or shales were involved, and there has been no development of slaty cleavage or schistosity. In general the axes of the folds have a northwest-southeast trend, but there are numerous deviations from this rule and the axes of the minor folds are usually more or less divergent, as is of course generally true. There is, however, a pronounced parallelism in the dominant synclines and anticlines, the axes of which extend for many miles. Several of these are more or less oblique to the mean trend of the Coast Range belt, and thus appear to be truncated on the coast line, or on the eastern margin of
Faults. — In the Coast Ranges there are numerous faults, but our knowledge of them is limited, owing to the small amount of geological mapping which has been done in the region. With the extension of cartographic work, many more than are now known will doubtless come to light. Of those at present known, the great majority have a general northwest-southeast strike, but there are several minor faults which trend transverse to the general strike. The faults of the Coast Ranges, as well as those of other parts of California, are indicated, as to position and extent, on Map No. 1. A summary reference to them is all that will be here attempted.
The most northerly fault of the Coast Ranges is one which Mr. O. H. Hershey calls Redwood Mountain fault. It is an overthrust, according to Mr. Hershey, heading to the northeast and having a throw of probably over 5,000 feet. It trends southeast along the southwest flank of South Fork Mountain for scores of miles, and doubtless determines the very straight trend of this great ridge. Parallel to it, on the southwest side of Redwood Creek, near Acorn, there is another fault having a throw of at least 1,000 feet, according to Mr. Hershey. Its extent is unknown. The precipitous southwest front of Mount St. Helena has been shown by Osmont
Bull. Dept. Geol., Univ. Cal., vol. 4, No. 3, p. 78.to be a degraded fault-scrap; and the downthrow on the southwest side of the fault is estimated by him to be not less than 2,500 feet. The western edge of the Sacramento Valley, from Benicia to Cordelia, is probably determined by a fault with an easterly downthrow.
In the Mount Diablo region, there is a pronounced overthrust fold which causes Miocene strata to rest upon Pliocene strata with a dip of 30° to 45° to the northeast. Louderback's work on the structure of Mount Diablo has shown that this over-tipt fold passes into a thrust fault whereby a considerable proportion of the mountain has been shoved to the southwest.
Results not yet published.The west side of San Ramon and Livermore Valleys is bounded for the most part by a steep mountain wall at the base of which, near Pleasanton, the Tertiary rocks are faulted down against the Franciscan. This fault extends southward thru Calaveras Valley and past Mount Hamilton. Its general course is about N. 35° W. It has an extent of at least 60 miles and may be very much longer. In the Berkeley Hills to the east of this there are many minor faults, both overthrust and normal, which will not be described in detail. In the Mount Hamilton Range, between the crest and the Santa Clara Valley, there are several faults, notably the Mission Creek, Mission Peak, Mount Hamilton, and Master's Hill faults, which have a more or less regular northwest-southeast trend; and there are several shorter faults transverse to these, and of variable strike.
The valley of the Bay of San Francisco and its prolongation southward in the Santa Clara Valley is bounded on the northeast side by a range of hills which presents a very even, straight, and on the whole, but little dissected, front to the southwest. This even front extends from near Point Pinole, on San Pablo Bay, to the vicinity of Hollister, a distance of about 100 miles, forming a very striking geomorphic feature of the Coast Ranges. At Berkeley and Oakland, and southeast of the latter, there is evidence that this even front represents a somewhat degraded fault-scrap, or series of scraps, and this interpretation may with very probable truth be placed upon it for its entire extent. Near Berkeley the slope of this degraded scrap is traversed by supplementary step faults, which are not improbably characteristic of it in other places; so that in regarding the feature as a fault-scarp it is not intended to apply that term too narrowly, but to include rather the idea of a zone of acute deformation traversed by step faults. This line has a course of about N. 35° W. North of San Pablo Bay, on the geographic prolongation of the line, a similar feature, tho by no means so straight, is found on the east side of the
An interesting and important fault traverses the peninsula of San Francisco, a little south of the city. The course of this fault can not be precisely determined, as its trace at the surface is obscured by Pleistocene and recent deposits. Its approximate position is at the southwest base of San Bruno Mountain, with a strike of about N. 43° W. By this fault the Merced strata, which are well exposed on the sea-cliffs south of Lake Merced to the thickness of over a mile, are dropt down against the Franciscan rocks, the throw being estimated at not less than 7,000 feet. To the northeast of the main fault, and close to the face of the mountain, is an auxiliary fault, and between these two faults there is a block of the Franciscan which has dropt only to a limited extent, and which is of the same character as the kernbuts of the Kern River.
Bull. Dept. Geol., Univ. Cal., vol. 3, No. 15.The bold and precipitous southwest face of San Bruno Mountain is thus a fault-scarp with two facets, one for the main fault and the other for the auxiliary, both being well exprest in the geomorphic profile of the mountain. This fault-scarp appears to be the southern prolongation of the scarp which forms the coastal steep slope to the north of the Golden Gate, and seems to converge upon the San Andreas fault, off the Golden Gate, making a very acute angle with it. It affords an excellent illustration of the general fact above alluded to, that the northwesterly members of the fault system controlling the configuration of the coast are prolongations of fault-lines within the Coast Ranges. Knowledge of the extent of this fault, altho its throw is so notable, is limited to the peninsula of San Francisco.
Outside of Fort Point, at the Golden Gate, and a little south of the point, is a very well exposed fault which appears to strike southeast across the city of San Francisco. The fault is nearly vertical and has a throw of at least some hundreds of feet, whereby the serpentine on the north has been dropt against a formation of radiolarian cherts.
The most interesting fault traversing the Peninsula of San Francisco is the San Andreas fault, on which movement was renewed on April 18, 1906, causing the earthquake. The extent and course of this fault are described in detail elsewhere. To the southwest of the San Andreas fault, on the Peninsula of San Francisco, and in the Santa Cruz Mountains, are several other faults of notable extent. Of these may be mentioned the Fifield, Pilarcitos, Castle Ridge, Butano, Boulder Creek, and San Gregorio faults, all of which are important features of the structure of the region.
On the southwest side of Montara Mountain is a very precipitous seaward slope, at the base of which strata of Miocene age are tilted at rather abrupt angles against the granite. The strata of arkose sandstone at the base still rest against the original floor of deposition, but it is difficult to see how such an acute uplift could take place in a granite massif without deformation of the granite. Such deformation might take the form of plastic flow if it were sufficiently deep-seated, or it might find its expression in a zone of faults; and as there is no evidence of plastic deformation, it is concluded that the uplift of Montara Mountain was effected by faulting within the granite, the same deformation appearing as flexure in the stratified rocks which flank the mountain on this side.
Northeast of the San Andreas fault are the Belmont and Black Mountain faults, the latter a branch from the San Andreas fault. In the gap between the Santa Cruz and Gavilan Ranges is a fault followed by the canyon of Pajaro River near Chittenden, which drops the Tertiary formations on the north against the pre-Franciscan granitic rocks of
South of the Bay of Monterey, one of the dominant structural lines of the Coast Ranges is the Santa Lucia fault, at the base of the Santa Lucia Range on the border of Salinas Valley. It is traceable from the vicinity of Bradley to the Bay of Monterey and it is probably the chief factor in determining the course of the Salinas-Valley and the steep easterly front of the Santa Lucia Range. Near its southern end, the Santa Lucia fault is paralleled on the southwest by another fault which probably determined to some extent the course of the valley of San Antonio River. Farther south a fault parallels the last two, between Dove and Templeton; and to the southwest of this lies the much longer fault which passes close to San Luis Obispo, extending from near San Simeon to the drainage of the Santa Ynez.
The northeastern flank of the San Emidio Range, at the southern end of the great valley, is with little question a fault-scarp. The same may be said of the north flank of the Santa Ynez Range and the south flank of the Santa Monica Range. The San Gabriel fault, which bounds the range of that name on the south, branches from the San Andreas fault near San Bernardino and follows the base of the range with an east-west trend. Beyond Pasadena it bends slightly to the north and extends thru to the coast in the vicinity of Carpenteria. Near Pasadena a branch fault leaves it, with a north-westerly strike, on the northeast side of the Verdugo Mountains. Southeast of Los Angeles, the most notable faults are the San Jacinto and Elsinore faults, both of which have very pronounced scarps. There are, however, several others. All the faults in this region have a northwest-southeast strike, and are thus in contrast to the system of faults extending from Point Conception to the Colorado Desert along the Sierra Madre, in which the dominant trend is east and west.
The foregoing summary enumeration of the more important faults at present known in the Coast System of mountains makes it clear that the San Andreas fault, upon which movement took place on April 18, 1906, is not a singular or unique feature of the structure of these mountains. It is only one of many faults, on all of which in time past there have occurred many differential movements, each productive of an earthquake. Map No. 1, upon which the above faults are represented, indicates other faults in California, Nevada and Oregon at present known to geologists.
In compiling the data for the representation of the faults of California, free use has been made of information kindly supplied by Messrs. H. W. Turner, W. Lindgren, W. C. Mendenhall, H. W. Fairbanks, J. S. Diller, F. M. Anderson, R. Arnold, J. C. Branner, G. D. Louderback, and O. H. Hershey.Perhaps the most interesting of these, from the present point of view, is the fault at the eastern base of the Sierra Nevada, upon a portion of which the movement took place that caused the earthquake of 1872. The map may be regarded as a preliminary attempt to bring together, in cartographic form, our knowledge of the position of faults in this region. A full discussion of these features, with references to the literature bearing upon them, would be out of place here, altho their occurrence suggests seismic possibilities.
Certain of the geomorphic features of the Coast Ranges, particularly as regards their margins, have necessarily been alluded to in the discussion of the structure. It is proposed here to describe quite briefly the salient characters of the relief, in their relation to the structure.
The Coast Ranges in general, between the coast and the Great Valley and north of Santa Barbara Channel, comprize a series of ridges and intervening valleys of mature aspect.
U. S. G. S. Bulletin, 196.
Between the headwaters of Eel River and the Bay of San Francisco the interpretation of the drainage as subsequent is not so certain, altho here the general geomorphic profile is even more mature than it is on the north, a fact referable to the softer character of certain geological formations which prevail. Here we have, as before, a system of stream valleys, notably Russian River Valley, Sonoma Valley, Napa Valley, and Berryessa, and Clear Lake Valleys, which are clearly evolved by stream erosion under the control of structure. The transverse connecting link from one longitudinal valley to another, which is so characteristic of subsequent drainage, is not apparent on the maps, but its absence may be more apparent than real. The lower stretch of Russian River, from Healdsburg to the sea, has the appearance of a transverse stream tapping a longitudinal valley of very mature character, and may be the remnant of an original consequent stream. This view, however, is open to the objection that the lower stretch of Russian River near the sea has a more youthful aspect than might reasonably be expected under the hypothesis. On account of the rather immature character of the transverse outlet of Russian River, it has been suggested that it is of later date than Russian River and represents a small stream which has cut its way back from the coast and captured the waters of the river, which formerly went to the Bay of San Francisco, the capture being
Somewhat similar features occur on the east side of the Coast Ranges. Cache Creek and Putah Creek, draining longitudinal valleys within the Coast Ranges, both emerge upon the Great Valley thru transverse gorges in the Blue Ridge, the most easterly of the Coast Ranges. These transverse gorges can scarcely be regarded as other than consequent trunks crossing a hard barrier within which, in softer formations, longitudinal or subsequent valleys have been evolved. The apparent absence of the transverse connecting links between Napa, Sonoma, and Petaluma Valleys is explained when it is recalled that while the streams draining these valleys flow directly to salt water, they nevertheless flow to a drowned valley. The trunk stream trench from which Petaluma, Sonoma, and Napa Creeks are subsequent branches lies below the waters of San Pablo Bay. In general, Santa Rosa Valley (lower part of Russian River Valley), Petaluma Valley, Sonoma Valley, and Napa Valley have been evolved by erosion along synclinal axes. This fact also tends to weaken their interpretation as due to subsequent development by headwater erosion; since, if the synclinal folds were exprest as troughs at the surface at the time of the folding, then the drainage would have been both consequent and parallel to the structure.
Coming farther south, the valley of the Bay of San Francisco and its extension in the Santa Clara Valley is a large feature in which deformation and erosion have probably played equal rôles. Its trend is strictly determined by the Haywards fault line previously described. Southward from Hollister, the valley loses its breadth and passes into the much more constricted valley of the San Benito River, draining the Coast Ranges to the east of the Gavilan Range. The Bay itself and its inland extensions afford a magnificent illustration of a drowned valley-land due to subsidence of the valley-bottoms below sea-level.
Livermore Valley, a few miles to the east of the Bay of San Francisco and separated from it by the ridge of the Berkeley Hills, is a very noteworthy feature. It is a broadly expansive alluviated valley, bounded on the west by the degraded fault-scarp which limits the Berkeley Hills to the east; on the east by the slopes of Mount Diablo; and on the south by the slopes of Mount Hamilton. On the north it is open by way of the wide and low San Ramon Valley to Suisun Bay, and the northern portion of the valley drains this way. The greater part of the waters which come to it from Mount Diablo and from Mount Hamilton, however, are carried off by Alameda Creek thru Niles Canyon, a narrow gorge which transects the bold ridge separating it from the Bay of San Francisco. Alameda Creek has a hydrographic basin of 600 square miles, and it is a remarkable fact that it finds its outlet across the strike of the range thru a bold ridge, instead of following the wide open valley leading directly to Suisun Bay with no barrier in its path. It is a fair inference that Livermore Valley is structural rather than erosional in its origin and that, anterior to the acute deformation of the region, the drainage was consequent in the path followed by Niles Canyon. The deformation involved the uplift of the Berkeley Hills and the complementary depression of the Livermore Valley tract, and this deformation proceeded at a rate which was sufficiently slow to permit the stream, by downward corrasion across the rising mass, to maintain its course. Alameda Creek in Niles Canyon is thus a remnant of the consequent drainage of the region and is antecedent to the uplift which gave rise to the Berkeley Hills.
In the Coast Ranges between the Bay of Monterey and the Santa Barbara Channel, the chief valleys are those of Salinas River and its tributary, the San Juan; the Carissa Valley, and the valleys of the Cuyama and Santa Ynez Rivers. Of these the Salinas Valley is the largest. It is a wide, terraced valley cut by the river out of rather soft Tertiary and later deposits, which appear to have been in part let down against the older rocks of the Santa Lucia Range by the Santa Lucia fault. In its lower part it lies between the Gavilan and Santa Lucia Ranges, and the trend thus established is maintained by the main stream as far as San Miguel. Beyond that the same general trend is continued up its tributary, the San Juan, and thence thru the Carissa Plains to a point close to the southern end of the Great Valley. The eastern side of the upper end of the valley, particularly the eastern side of the Carissa Plains, follows closely the line of the modern earthquake rift to be presently described; and there can be little doubt, not only that in so far as the valley is an erosional feature its erosion has been controlled by structural features, but also that deformational processes have had a considerable share in its evolution. The axis of the valley thus indicated is singularly straight and has a length of about 175 miles. Its upper part, the Carissa Plains, is an arid plain without drainage and contains a very saline lake. This plain is a surface of alluviation. The lower end of the valley opens widely on the Bay of Monterey and the fine stream terraces which flank its sides afford an excellent record of the recent uplift of the region.
The valley affords another striking illustration of the obliquity of the geomorphic as well as the structural features to the general trend of the Coast Range belt, and their constant tendency to emerge upon the coast. From the eastern margin of the valley at the south end of the Carissa Plains, one can look down upon the Great Valley, near Sunset, a few miles distant; and only a narrow ridge separates the two valleys, altho they differ greatly in altitude. From this point in its course of 175 miles, Salinas Valley crosses the entire width of the Coast Ranges. South of San Miguel, the Salinas River proper lies in a less open valley with north and south trend as far as Templeton, a distance of about 15 miles, and then opens out into a wider valley having a northwest-southeast trend for about 35 miles to the headwaters of the stream. Several of the minor tributaries of the Salinas show a marked tendency to the development of subsequent valleys. On the east side of the river, this is particularly marked on San Lorenzo Creek in Priest Valley, and on Chalome Creek in Chalome Valley. These comparatively large valleys may be referable in part, however, to deformation, inasmuch as they are on the line of the Rift. Their geomorphic history has not yet been studied. On the west side of Salinas Valley the two chief tributaries, the San Antonio and the Nacimiento, have developed well-defined subsequent valleys in the heart of the Santa Lucia Range.
In the valley of the Cuyama or Santa Maria River, the effect of a twofold structural control is apparent. The upper reaches of the river flow thru a broad valley with an alluviated bottom on the northeast side of the San Rafael Range. The general trend of the river in this part of its course is northwest-southeast, and it is separated from the Carissa Plains by a high mountain ridge with a very precipitous southwest front, which probably represents a fault-scarp. Below this expansive high valley, the stream enters a rather narrow canyon and shortly after this bends at right angles and flows southwest toward the coast, entering eventually on the broad Santa Maria Valley which is open to the sea. The contrast in the geomorphic character of the upper and lower reaches of the river, the greater age of the former, and the sudden change in the course of the stream where the two types of geomorphy meet, suggests that the high valley of the upper reaches was once connected with the Salinas drainage and that it has been captured from the latter, in comparatively recent time, by a stream cutting back from the coast at the northwest end of the San Rafael Range.
In the valley of the Santa Ynez, there is a marked departure from the northwest-southeast trend which characterizes the geomorphic features of the Coast Ranges in general, and a more striking instance than any yet cited of the obliquity of those features to the general trend of the Coast Range belt. The valley lies nearly east and west and its general slope is southward to the base of the precipitous northern face of the Santa Ynez Range. This face is, as has been indicated, a fault-scarp; and the course of the valley is thus seen to be in intimate relation to this dominant structural feature. To the west the valley opens widely to the sea, while to the east it loses its individuality in the headwater canyons of eastern Santa Barbara County and western Ventura County.
Between the Santa Ynez Valley and the upper Cuyama is the rugged and deeply dissected country culminating in the San Rafael Mountains on the northern side of the tract. This mountainous belt has a trend intermediate between the pronounced east-west trend of the Santa Ynez Range and the northwest-southeast trend of the Coast Range ridges and valleys to the north. For a portion of its length the belt is bounded on the south by the Santa Clara Valley, with a general east and west course; but across the head-waters of Santa Clara River the mountainous tract persists and finds its prolongation, with the same general trend, in the San Gabriel Range, and beyond Cajon Pass in the San Bernardino Range, both bold and lofty sierra. It may even be considered as extending, under the name of the Chocolate Mountains, to the Colorado River above Yuma. From Tejon Pass southeast to Cajon Pass, the northern side of this mountain tract presents a very abrupt front with a very straight course. At the base of the abrupt slope lies the San Andreas Rift. To the north of this, and between it and the southeast scarp of the southern Sierra Nevada, lies the Mojave Desert. To the south of the southeast end of the San Bernardino Range and west of the Chocolate Mountains lies the Colorado Desert. As has been already indicated, the south side of the San Gabriel Range is determined by a profound fault. Lying thus between two faults, the range is a magnificent example of a horst which has been thrust up between its bounding faults. It is the convergence of these two bounding faults which segregates the San Gabriel Range from the San Bernardino Range in the vicinity of Cajon Pass. The latter range is similarly bounded on the south by the same fault as that which determines the south front of the San Gabriel Range, but here it is coincident with the Rift. Between Los Angeles and Ventura lie the short ranges known as the Santa Monica and the Santa Susannah Mountains, inclosing San Fernando Valley. The Santa Monica Range is probably on the same line of orogenic uplift which finds its expression farther west in the Santa Cruz and Santa Rosa Islands.
South of the San Gabriel Range lies the fruitful valley of southern California, extending with an east-west course from the sea to San Bernardino. South of this valley, and between the Colorado Desert and a somewhat elevated coastal plain bordering the Pacific, is a mountainous tract, the ridges of which swing around into a more northwest-southeast trend, and so conform again with the prevailing trend of the ridges and valleys of the Coast Ranges north of the San Rafael Mountains. The valleys in this region are, however, less regular in their orientation than those of the northerly Coast Ranges, and the geomorphic features are less mature, if we except certain very old features which have survived from an earlier cycle of geomorphic evolution. The consequent character of the streams on the seaward slope is much more pronounced than in any part of the northern Coast Ranges, and on the whole the geomorphy of the region must be regarded as less advanced than to the northward, and more closely allied in its morphogeny with the Sierra Nevada than with that of the Coast Ranges of northern California.
The notable ranges of this region are the Santa Ana Mountains and the San Jacinto Mountains. The former present the features of a seaward sloping, tilted, orographic
Verbal communication from Dr. H. W. Fairbanks.To the northwest the range becomes subdued in the Puente Hills, where a broad anticlinal structure replaces in part the deformation by faulting. In two notable instances, and perhaps in others, the seaward streams of the Santa Ana Mountains cut entirely thru the range and drain the valley-land beyond its northeasterly scarp. These are the Santa Ana and the Santa Margarita Rivers. They are both probably antecedent to the more acute phases of the tilting of the region and have persisted in their course during the development of the fault-scarp.
The San Jacinto Mountains form an important feature of the region as a bold ridge with northwest-southeast trend lying between Perris Plain and the northern end of the Colorado Desert. Both sides of the range are precipitous and are probably determined by faults. On the southwest side there were notable ruptures of the ground in the earthquake of 1898, indicating that the fault on that side is still in active development.
The San Andreas Rift as a Geomorphic Feature
Extending thru the greater part of the Coast System of mountains from Humboldt County to the Colorado Desert, a distance of over 600 miles, is a line or narrow zone characterized by peculiar geomorphic features, referable either directly to the modern deformation of the surface of the ground or to erosion controlled by the lines upon which such deformation has taken place. This peculiar feature has been known, both to Californian geologists and to residents of the sections where its characters are most prominent, but its extent and importance were not fully appreciated until after the earthquake of April 18, 1906. It is commonly reported among the residents of the southern interior Coast Ranges, particularly in San Benito, Monterey, and San Luis Obispo Counties, that displacement of the ground occurred on this line in the earthquake of 1857 and in certain later earthquakes. The first reference in scientific literature to this feature appears to have been in the year 1893, in a paper entitled "The Post-Pliocene Diastrophism of the Coast of Southern California," by Andrew C. Lawson, which is quoted in the sequel. The next reference to this peculiar line is in the eighteenth annual report of the U. S. Geological Survey for 1896-1897, Part IV, in a paper by Schuyler on "Reservoirs for Irrigation," where, pp. 711-713, the significance of the line is fully recognized in the following words quoted in full:
This reservoir has especial interest, not only as the first one of any magnitude completed on the Mojave Desert or Antelope Valley side of the Sierra Madre in southern California, but because it lies directly in the line of what is known as "the great earthquake crack" of this region, which is marked by a series of similar basins behind a distinct ridge that appears to have been the result of the great seismic disturbance.
This remarkable line of fracture can be traced for nearly 200 miles thru San Bernardino, Los Angeles, Kern, and San Luis Obispo Counties, and deviates but slightly here and there from a direct course of about N. 60° to 65° W. There appears to have been a distinct "fault" along the line, the portion lying south of the line having sunken and that to the north of it being raised in a well-defined ridge. In many places along the great crack, ponds and springs make their appearance, and water can be had in wells at little depth anywhere on the south side of the ridge before mentioned. A tough, plastic, blue clay distinguishes the line of the break, in this portion of its course at least; and where the line crosses Little Rock Creek, the blue clay has formed a submerged dam, which has forced the underflow near the surface and created a "cienega" immediately above it. After crossing the line, the water of the creek drops quickly away into the deep gravel and sand of the wash. The same effect is noticeable at other streams, and it has been suggested as the probable cause of the very distinct rim marking the lower margin of the San Bernardino Valley artesian basin and confining its waters within well-defined limits, as this rim is nearly on a prolongation of the line that is traceable on the north side of the mountains — the break having crost the mountains thru the Cajon Pass on the line of Swartout Canyon.
In 1899 the essential features of the same line in the region north of the Golden Gate were recognized and discust by F. M. Anderson.
The Geology of the Point Reyes Peninsula, Bull. Dept. Geol., Univ. Cal., vol. 2, No. 5, p. 143 et seq. Anderson, however, supposed, as is indicated by the last paragraph of his paper, that the faulting antedates entirely the Pleistocene terrace formations.In later years Dr. H. W. Fairbanks has traced out the line in various field trips and has given several public lectures descriptive of its features and its significance, but has published no systematic account of his studies.
The fact that the earthquake of April 18, 1906, was caused by a rupture and displacement of the earth's crust along this line for a distance of about 190 miles, immediately focussed the attention of local geologists upon it. Among those engaged upon
A summary account will now be given of this rift line as a geomorphic feature.
The most northerly point in California at which geomorphic features directly referable to the violent rupture of the earth's crust have been observed are those noted by Mr. F. E. Matthes in the vicinity of Petrolia in Humboldt County. Here south of Petrolia, on high bare mountain spurs between Cooskie, Randall, and Spanish Creeks, he reports the occurrence of several small ponds and ridges such as have been familiar to those engaged in the field study of the earthquake phenomena as characteristic Rift features. Similar features are also found at the base of these spurs near the shore. These are in line with similar features found by the same observer between Telegraph Hill and Shelter Cove, a few miles to the southeast. Here, particularly in Wood Gulch (plate 1), is a narrow depression with ponds, ridges, and saddles, which appears to be essentially a feature due to deformation and to have determined the course of the drainage. The course of the depression is about N. 25° W. In this depression lies the trace of the fault upon which movement took place on April 18, 1906. Its course, if followed southward to the cliffs above Shelter Cove (plates 2A, 3A, B), heads out to sea with a trend nearly parallel to the coast. Great landslides occur along the coast in proximity to this line, and are in part on the Rift. The rocks traversed by the Rift in this part of Humboldt County appear to consist wholly of shales, sandstones, and conglomerates which are probably of Cretaceous age, altho since the geology of the region has not been studied, positive statements in this regard can not be made. The region is high and rugged, with a very precipitous descent to the sea, King Peak having an elevation of 4,090 feet at a distance of about 2 miles from the shore.
Point Arena to Fort Ross
From Shelter Cove to near Point Arena, the Rift, if continuous, lies beneath the waters of the Pacific. The continuity for this stretch is of course open to question, and in another place the considerations bearing upon this point will be presented. At the mouth of Alder Creek, 4.5 miles northwest from Point Arena, the Rift enters the coast from the sea and is thence traceable continuously to a point about 2 miles southeast of Fort Ross, a distance of about 43 miles, with a nearly but not quite straight course, being slightly curved with the convexity toward the ocean. (See map No. 2.) For our knowledge
With regard to the geology of the territory traversed by the Rift from the vicinity of Point Arena to Fort Ross, Dr. H. W. Fairbanks has kindly examined the ground and supplied the following note:
Except for a strip of sandstones (Walalla beds) of upper Cretaceous age extending along the coast north and south of the mouth of the Gualala River, and a triangular area of Monterey shale and sandstone underlying the coastal terraces in the vicinity of Point Arena, the rocks of almost the entire mountainous region between the upper Russian River Valley and the coast belong to the Franciscan. There seems to be but one fault in this region, and that is on the line followed by the Rift. The Walalla beds begin upon the coast a little south of Fort Ross and, extending inland, form the ridge between the Gualala River and the ocean. The formation thins out against the ridge bounding the Gualala Valley upon the northeast. The line of junction is an irregular one, for in places the soft sandstones reach quite to the top of the ridge referred to. These beds extend along the coast to the northwest for more than 30 miles, finally terminating 7 or 8 miles south of Point Arena, where they are overlain by the Monterey sandstones and shales. The Rift does not follow the contact between the Walalla and Franciscan formations and the vertical displacement does not appear to have been very great, as in only one place was it enough to bring up the underlying Franciscan rocks upon one of its walls. The Rift, for something more than a mile after emerging from the ocean southeast of Fort Ross, lies in the Franciscan formation, and the latter is greatly
― 28 ―crusht and broken along it. Back of Fort Ross, the surface rocks traversed by the Rift belong to the Walalla formation, and from this point for a number of miles to the northwest no other formation appears.
At the point where the road from Stewarts to Geyserville crosses the Gualala River, faulting and erosion have exposed the underlying Franciscan formation. This appears upon the northeast side, showing that the opposite side, that toward the ocean, has dropt. The Franciscan occupies but a narrow strip and is replaced for some distance up the ridge upon the northeast, by Walalla sandstones. These relations are shown in the cross-section sketch shown in fig. 1. Near the mouth of the Walalla River the formation upon the coast side of the Rift still appears to be the Walalla sandstones; the rocks upon the opposite side are buried under the alluvium of the valley. After leaving the valley of the Garcia River, the Rift lies wholly within the Franciscan formation until it disappears in the ocean. The Monterey shales with sandstones at their base form nearly the whole of the coastal terraced plain in the vicinity of Point Arena. They rest unconformably upon the Franciscan rocks and dip at a steep angle to the southwest. The Monterey formation nowhere appears to come in contact with the fault.
Bodega Head to Bolinas Bay
General Note. — From the point 2 miles south of Fort Ross where the Rift in its southeasterly course leaves the shore, it passes beneath the Pacific for a distance of 12 or 13 miles. Its observed course to the northwest of Fort Ross, if projected southeasterly with a slight curvature, would strike the shore again at Bodega Head; and here it is found on the low ground of the isthmus that connects the head with the mainland. The Rift here coincides in position with a fault described by Osmont,
Bull. Dept. Geol., Univ. Cal., vol. 4, No. 3.whereby the Franciscan rocks to the east are dropt down against the pre-Franciscan dioritic rocks of the headland. Immediately to the east of the fault-trace is a marsh. Across the mouth of the bay formed by the headland is a sandspit and the fault-trace should cross the spit near its abutment upon the shore line, but the drifting sands preclude its finding an expression here in geomorphic forms.
To the south of Bodega Head the Rift follows Tomales Bay (plate 6A) to its head near Point Reyes Station. This is a remarkably linear inlet of the ocean lying between Point Reyes Peninsula and the mainland, having a length of about 15 miles and not exceeding a mile in width. It has generally been regarded as a feature determined by a fault,
Cf. Anderson, Geology of Point Reyes Peninsula, Bull. Dept. Geol., Univ. Cal., vol. 2, No. 5.the same as that noted by Osmont at Bodega Head, whereby the Franciscan rocks of the mainland were brought against the pre-Franciscan granitic and dioritic rocks of the peninsula. The bay is quite shallow, but both of the slopes above the shore line are rather precipitous, and the ridge crests on either side attain elevations of over 1,000 feet. On the mainland side of the bay there are some rather vaguely defined terraces, both in the form of wave-cut benches and delta embankments. On the same side of the bay there are marine deposits of late Pleistocene age, containing abundant molluscan remains which have been elevated to about 25 feet above sea-level, and which are the equivalent of similar deposits at a similar elevation on the east side of San Pablo Bay.
To the south of Tomales Bay the Rift lies in a remarkable defile with abnormal and ill-adjusted longitudinal drainage, which extends thru to Bolinas Bay, a distance of about 14 miles. On the east side of the defile is the steep coastal slope of the mainland, rising to a ridge crest from 1,000 to 1,700 feet in height. The transverse gullies in this slope are shallow, and detract but little from the general effect of a fairly regular but uneven steep slope. On the west is an even steeper but more incised and rugged slope, which forms the eastern edge of the peninsular land mass. This slope culminates in crests having an altitude of about 1,500 feet. The most striking geomorphic feature of the bottom of the defile is the presence of low ridges with intervening ravines or gullies elongated parallel to the general axis of the depression. More or less hummocky surfaces, with hillocks and hollows having no regular orientation, also occur. In the hollows ponds are fairly common features. The chief drainage is to Tomales Bay by Olema Creek, which heads within 2.5 miles of Bolinas Lagoon; and the divide between this stream and the parallel one which flows to the southeast has an altitude of about 400 feet above sea-level. The southeast end of the depression is submerged beneath sea-level, and is cut off from Bolinas Bay by a sandspit. The very shoal water inside of the sandspit is known as Bolinas Lagoon. (See plate 6B.)
The rocks on the east side of the defile belong wholly to the Franciscan series. On the west side, at the north end, we have chiefly the granitic and dioritic rocks of the peninsula with limited masses of crystalline limestone into which these rocks are intrusive. Farther south the granitic rocks are overlain by the shales of the Monterey series, and these rocks form the west side of the defile for several miles. The shales have inconstant and often very high dips. Still farther south the sandstones of the Merced series lie unconformably upon the Monterey shales, and near the town of Bolinas dip uniformly at moderately low angles toward the axis of the defile. It is thus apparent that the axis of the defile crosses more or less obliquely or transversely the contact between the Monterey and the granitic rocks, and also the contact between the Merced and the Monterey. It is also a remarkable fact that altho on the east side of the defile the Franciscan rocks constitute the mountain mass to a thickness of several thousand feet, this entire series, together with the Knoxville, Chico, Martinez, and Tejon, is almost entirely absent between the Monterey and the granitic rocks on the peninsula in the immediate vicinity. This indicates clearly that in pre-Monterey time the peninsular mass had been uplifted on a fault along the present coastal scarp, so that the granite was brought against the Franciscan and denuded of its unconformable mantle of sedimentary strata before it was submerged to receive the deposits of Monterey time. It is also clear that inasmuch as there is a great volume of Monterey shales on the peninsular or seaward side of this fault line, and no trace of the same formation on the mainland to the east of the fault line, one of two things must have happened. Either the submergence which permitted the deposition of the Monterey shales was confined to the peninsula and was effected by a downthrow of that block on the same fault as that upon which it had earlier been upthrust, so that there was no sea over the territory east of the fault; or, if the regions on both sides of the fault were submerged together, then in post-Monterey time the east side of the fault was lifted into the zone of erosion and denuded of its covering of Monterey shales so thoroly that no trace of them now remains. There is no escape from one or the other of these conclusions, and each of them involves a movement on the fault with relative downthrow on the southwest side, or the reverse of that which occurred in earlier, pre-Monterey time. From this interpretation it follows that the defile extending from Tomales Bay to Bolinas Bay lies along the trace of a fault which dates from pre-Miocene time, and that upon this fault there have been large movements in opposite directions so far as the vertical component of such movements is concerned. The trace of this ancient fault is also the line of the modern Rift.
The dip of the Merced beds at Bolinas toward the Franciscan rocks of the mainland is quite analogous to the dip of the same beds toward the Franciscan of San Bruno Mountain on the San Francisco Peninsula,
Cf. A Sketch of the Geology of the San Francisco Peninsula. U. S. G. S., 15th annual report.and has the same significance, viz., that the Merced beds have been relatively downthrown on the west against the older rocks. The fault in the Tomales-Bolinas defile has usually been regarded as identical with and a continuation of the San Bruno fault of San Francisco Peninsula, and there seems to be no good reason for changing this judgment, altho, as will appear shortly, the modern Rift to the south of the Golden Gate does not coincide with the trace of the San Bruno fault, but leaves it at a small angle and pursues a course nearly parallel, but to the southwest of it. It is noteworthy, also, that while on the Point Reyes Peninsula, particularly in the vicinity of Bolinas, there is a magnificent wave-cut terrace at an altitude of about 300 feet, with a width of 1 to 1.5 miles between the base of its sea-cliff and the brink of the present sea-cliff, no such feature is to be found on the landward side of the fault-line on the coastal scarp between Bolinas Lagoon and the Golden Gate.
Characteristics of the Rift (G. K. Gilbert, pp. 30-35). — In a broad sense the structural trough in which lie the two bays is a feature of the great Rift. In a narrower sense the Rift follows the lowest line of the trough, controlling the topography of a belt averaging 0.75 mile in width. The physiographic habit of the trough is that of a depression occasioned by faulting. It is remarkably straight. One wall, the southwestern, is comparatively steep; the other is comparatively gentle. The gentler slope is an inclined plateau with incised drainage. Viewing the trough from any commanding eminence, the physiographer readily frames a working hypothesis of faulting and tilting. He sees in the southwestern wall a fault-scarp of moderate freshness, and in the northwestern wall a slope originally less steep, in which erosion has been stimulated by uplift and tilting. The general facts of the geology of the district, as worked out by Anderson,
Geology of Point Reyes Peninsula, by F. M. Anderson. Bull. Dept. Geol. Univ. Cal., vol. 2, No. 5.agree with this theory. The axial line of the valley is recognized by him as the locus of a fault, or fault-zone, and the rocks of the southwest wall are everywhere older than those which adjoin them at the base of the opposite slope. The gentler slope is well shown by plate 7A. Plates 8B and 41B also show something of the gentler slope, and plate 7B of the bolder.
In a general way the two slopes are drained by streams which descend to the axis of the valley, and are there gathered in two longitudinal trunk streams which flow severally to Tomales Bay and Bolinas Lagoon; but in a central belt following the lowest part of the trough the details of drainage are comparatively complex, and their complexity is associated with peculiarities of the relief which serve to distinguish the central belt from the bordering slopes. In the bordering slopes the subordinate ridges conform in normal manner to the drainage, having evidently been developed by the erosion of the canyons which separate them. In the axial belt the ridges are evidently independent of the drainage, often running athwart the courses which would normally be followed by the drainage. In part the ridges divert or control the drainage; in part the drainage traverses and interrupts the ridges.
The influence of the ridges on the drainage is illustrated by the accompanying diagrams. Fig. 2 shows the actual drainage system; fig. 3 the system which would be developed if there were no special conditions along the axial zone. The small ridges of the axial zone trend parallel to the axis, and their interference gives parallel courses to various streams which would otherwise unite. The influence of the drainage on the ridges is illustrated by fig. 4, which shows a small ridge resting on the side slope of a larger ridge. The drainage of the larger ridge breaks thru the smaller, making gaps. Plate 7B shows the slope of a greater ridge at the right; and at the left two bushy hills
Similar relations between ridges and drainage lines are found in regions of steeply inclined strata, each ridge being determined by the outcrop of a resistant formation, or at least all of the preceding description might apply to the topography of such a region; but other characters remain to be mentioned, and these serve for discrimination. Where a steep-sided ridge is determined by the presence of a resistant formation, the determining rock follows and usually outcrops along its crest; but in the ridges under consideration there are few rock outcrops, and such as occur are not systematically related to the crest lines. The formation of the crest is not always the same thru the whole length of the ridge, and it is not always a rock of such character as to resist erosion. Between the ridges are linear valleys, and many of these are occupied by streams, but in a number of instances they are crost by the drainage. Often they include local depressions, with ponds or small swamps, this character being so pronounced that forty-seven such ponds were seen between Papermill Creek and Bolinas Lagoon, a distance of 11
In view of these characters, and especially of the abundance of ponds, it is evident that these little valleys are not products of stream erosion; and that in so far as they are occupied by streams the streams are adventitious. Their true explanation is suggested by their relation to certain of the earthquake phenomena of April, 1906. As will presently be described in detail, the trace of the earthquake fault thru the greater part of its course in the larger valley follows the edge of one or another of these small valleys; and in places where the fault movement included vertical dislocation, such dislocation nearly always tended to increase the depth of the valley. (See plate 10B and fig. 6.) Of the numerous minor or secondary cracks developed by the earthquake in the immediate vicinity of the main fault, a considerable proportion occurred at the edges of the little valleys, following more or Iess closely the line along which the bottom meets the side; and with these cracks also there was usually a little vertical dislocation, the ground
Collectively these ridges and valleys occupy a belt from 0.5 to 1 mile in width, and constitute the local development of the Rift, using that term in its narrower sense. They make up the entire surface of the belt, except where overpowered by some vigorous creek. The individual ridges are not of great length, being 2 or 3 miles at the most, and usually much less. Some of them end by wedging out, others by dropping down until replaced in the same line of trend by valleys. Their greatest height above base, except where the adjacent valleys have been deepened by erosion, is about 150 feet. The narrower have straight, acute crests; the broader have undulating backs with more diversity of form than is shown by the associated valleys. Some are crost by curved or straight depressions, and these depressions have all the characters of the parallel valleys, including the association of earthquake cracks.
In the remainder of this report the term Rift will be applied only to the narrow belt just described. Regarding it as the surface expression of a great shear zone or compound fault, the ridges are the tops of minor earth-blocks, and the valleys are in part the tops of relatively deprest blocks and in part depressions resulting from the weathering of crusht rock. Considering the Rift as a physiographic type, I find it convenient to have a specific name for one of its elements, the small valley; and in some of the descriptions which follow I shall speak of it as a fault-sag. (See plates 7B, 8A, and 11.)
The general relation of the Rift to the greater valley is illustrated by the cross-profile in fig. 7. Along its northeastern side it everywhere lies lower than the adjacent slope of the greater valley, the produced profile of the valley slope passing above the fault-ridges as well as the fault-sags. Along its southwestern side some of the fault-ridges appear to project above the restored profile of the greater valley, while the fault-sags lie below. If I interpret the structure correctly, the great compound fault concerned in the making of the valley trough — a fault of which the vertical dislocation amounts to several thousand feet — includes a certain amount of step-faulting, which
The limits of the Rift are not definite. The boundaries drawn in fig. 2 serve to indicate the belt in which the Rift structure dominates the topography, but do not indicate the limits of the Rift structure. Within the belt the dislocations have been so recent and of such amount as to keep ahead of weathering and erosion, so that their expression has been little dimmed by the processes of aqueous sculpture. Outside the belt the evidences of recent dislocation are less striking, but nevertheless exist. The inter-stream ridges of the northeastern slope are here and there indented and creased in such a way as to indicate recent faults of small amount trending parallel to the Rift. In the vicinity
Mussel Rock to Pajaro River
From Bolinas to the vicinity of Mussel Rock, about 8 miles south of the Golden Gate, the course of the Rift is beneath the waters of the Pacific, across the bar in front of the entrance to the harbor. Near Mussel Rock it intersects the shore at a great landslide (plate 12A) in rocks of the Merced series. At Mussel Rock, the basal beds of the Merced series rest directly upon an old land surface of worn-down Mesozoic rocks, and the basal bed contains abundant cones of Pinus insignis resting upon cemented alluvium. The cone-bearing bed immediately underlies marine strata and numerous fossils occur near the base of the series at the top of the ridge. The Merced strata here have a dip of about 15° to the northeast. The contact between the Merced and the older rocks trends southeast across the peninsula; and for some miles the Rift is approximately coincident with the trace of the contact and, for some portions of this distance, exactly so. From the shore line the course of the Rift is the same as that of the steep cliffs which rise at the back of the Mussel Rock slide to an altitude of over 700 feet. From the top of these cliffs, at an elevation of about 500 feet above sea-level, the course of the Rift as far as San Andreas Lake is marked by a line of shallow longitudinal depressions, ponds, and low scarps. (See plate 12B, 13, and 14.) There are eight ponds in this stretch of about 4.5 miles. This portion of the modern Rift was recognized as such in 1893.
"The line of demarkation between the Pliocene and the Mesozoic rocks, which extends from Mussel Rock southeastward, is in part also the trace of a post-Pliocene fault. The great slide on the north side of Mussel Rock is near the land terminus of this fault-zone, where it intersects the shore line. Movement on this fault-zone is still in progress. A series of depressions or sinks, occupied by ponds, marks its course. Modern fault-scarps in the Pliocene terrane are features of the country traversed by it." The Post-Pliocene Diastrophism of the Coast of Southern California, by Andrew C. Lawson, Bull. Dept. Geol., Univ. Cal., vol. 1, No. 4, pp. 150-151.
At a point about 4 miles from the Mussel Rock slide, the longitudinal depression which marks the course of the Rift becomes much more pronounced and passes into a remarkably straight and deeply trenched valley, the greater part of which has been converted by large dams into the San Andreas and Crystal Springs Lakes, used as reservoirs by the Spring Valley Water Company as water supply for the city of San Francisco. This straight valley (see plate No. 15) has an extent of 15 miles with a steady course of S. 34° E. to a flat divide southwest of Redwood City, whereby one passes over into the end of a similar but less pronounced valley, in which are situated Woodside and Portola. The San Andreas and Crystal Springs Lakes valley is almost wholly in the Franciscan terrane and the axis of the valley is discordant with the structural lines and contact planes of its constituent formations and intrusive masses. At the upper end of San Andreas Lake, however, the southwest edge of the Merced terrane forms in part the boundary of the valley on the northeast side for a short distance. The valley as a geomorphic feature (plate 16A) dates back fairly well into the Pleistocene. It is drained
To the southeast of Crystal Springs Lake, the valley followed thus far bifurcates about 2 miles beyond the lake, on either side of a median ridge. The two branches are nearly parallel. The east branch rises to a wide and rather flat divide, with streams heading in it from both sides. The other branch, altho it is more incisive, has no well-defined stream, but has a small swamp at its lower end. It rises to a sharper divide, from which there is a descent into the narrow straight canyon of West Union Creek. It is this western depression that the Rift follows. Near Woodside the canyon of West Union Creek expands into a more open valley, with steep mountains on the southwest and lower hills on the northeast. The Rift follows this straight valley (plate 16B) to its southeastern end, and then ascends to the saddle which separates Black Mountain from the mountains to the west. From this saddle it descends to the narrow canyon of Stevens Creek. It crosses the canyon at a small angle near its upper end and parallels the creek on the southwestern side, at an elevation of about 500 feet above it. It then passes thru the saddle between Stevens Creek Canyon and Congress Springs, and keeps well up on the slopes to the west of Congress Springs behind a series of shoulders and knolls to a reservoir on a saddle thru which it passes. From this saddle southeastward the line of the Rift again lies along the southwest side of a longitudinal valley and so continues on a line independent of the present drainage to the pronounced notch in the crest line of the range at Wright Station.
In this stretch of the Rift from Crystal Springs to Wright, the coincidence of the Rift with the major geomorphic features is very striking for the first half of the distance. In the second half, if we judge by the fault-trace, it appears to be quite independent of, tho parallel to, the canyons; and its only manifest relationship to the geomorphic features is its coincidence with a series of saddles or windgaps in the transverse spurs of the mountains. Its general parallelism with, and proximity to, the crest of the range thruout the entire stretch is pronounced. In the notch at Wright, the Rift intersects the crest line and passes from the northeastern flank of the range to the southwestern.
The general features of the Rift from Wright to Chittenden are described by Mr. E. S. Larsen in the following note:
From the hills above Wright Station to the village of Burrell, a distance of about 2 miles, the Rift follows along the ridge above Los Gatos Creek, which drains to the east. The drainage of the western slope of the ridge is to the Pacific. For most of this distance the Rift is a short distance on the Los Gatos Creek side. It usually occupies a small, trough-like depression; or, where it cuts just above the heads of the small gullies, there are low, rounded knolls between the gullies. These knolls are seldom over 30 feet higher than the trough. Just southeast of Burrell, the Rift traverses the ridge and follows a gully into Burrell Creek, which it crosses. It continues in a southeasterly direction, parallel to the creek and about halfway up the ridge to the southwest of it. The elevation of the ridge is only about 400 or 500 feet above the creek bed, and the top is rounded, with a steep slope below this to the Rift, and a gentle slope below the Rift
Following the Rift to the southeast, it passes at the divide into the head of Eureka Canyon, rises on the northeast bank, and slowly gets farther away from the creek, cutting across the tributary creeks and rarely following one of the smaller gulches for a short distance. The typical section here gives a steep slope on the high hills to the northeast, then about 0.25 mile of gently sloping, rolling hills, and finally the steep slope to the creek itself. The Rift is on the gentle slope, generally at some distance from either of the changes in slope. This continues for about a distance of 2 miles on to Grizzly Flat. Here the high steep hills to the northeast are separated from the lower hills to the southwest by a flat about 500 feet across. The Rift is on this flat near its center, and usually marks the northeast boundary of a series of low knolls. It continues on the flat for about 0.5 mile to where the hills close together and leave a rather steep-walled gulch. The Rift follows up this gulch for about a mile, and then crosses into the head of another creek, which it follows down for about 3 miles, where the stream turns sharply to the north. For the upper mile the gulch is rather sharp and deep, but at Hazel Dell the hills on both sides are low and rolling, while the lower mile is again rather steep, opening at the turn to a rather flat country. At Hazel Dell and other points, the Rift occupies a small but distinct trough very near the southwest bank of the creek. From here to Chittenden, a distance of about 8 miles, it follows parallel to Pajaro Valley, well up on the hills, and cuts across the canyons at almost right angles.
The typical section up one of these ridges gives a gentle slope from the valley to an elevation of about 1,000 feet; a steep slope for about 50 feet in the opposite direction, which marks the Rift; a very gentle slope for about 1,000 feet across; and finally, the steep upper slope of the hills. Over this area the Rift is nearly always marked by a trough, which often gives rise to a small lake perched on a ridge between two steep canyons. At a few points, especially about a mile northwest of Chittenden, small streams and gullies tend to follow the Rift, and they then make a sharp turn where they leave it. At Chittenden the Rift again passes thru a pronounced notch in the crest of the range occupied by the canyon of Pajaro River (plate 17A), from the western flank of the dominant ridge of the Santa Cruz Range to the eastern flank of the Gavilan Range.
The rocks traversed by the Rift from Mussel Rock to Pajaro River are, so far as known, almost wholly confined to the Franciscan and Monterey series, the former prevailing in the northern part and the latter occurring only in the southern. At Pajaro River the Rift encounters the granitic rocks of the Gavilan Range, but these lie wholly on its western side.
Pajaro River to the North End of the Colorado Desert
By H. W. Fairbanks
The earthquake of April 18, 1906, opened and displaced the walls of the old fault along the Rift as far south as the town of San Juan in San Benito County. The fault-trace passes directly under the western span of the Southern Pacific Railroad bridge across San Juan River, as shown by the displacement of the piers at the end of the bridge, a distance of 3.5 feet. For a distance of nearly half a mile on either side of the bridge, the river has established itself in the Rift. To the northwest the steep slopes of Mount Pajaro facing the canyon do not show any regular fissure. This does not, however, indicate any discontinuity in the fault, for the surface of the whole mountain is more or less broken by auxiliary cracks, secondary fissures, and slides.
Southeast over the hills from the point where the Rift leaves the river, the characteristic features of the Rift make their appearance. It is marked by a small pond (plate 17B), springs, and a more or less continuous ridge with its steeper face toward the southwest. The fissure of the recent earthquake follows this series of features (plate 18A), and, at a point halfway between the bridge and San Juan, there is shown in a broken fence a horizontal displacement of 4 feet. A mile before reaching San Juan, granitic rocks are exposed upon the southwest side of the Rift. Shortly beyond this point the Rift leaves the hills and traverses the western edge of the valley of the San Benito River. The ridge which we have been following is now lost in the level floor of the valley, but as far as traceable its course is directly toward the low bluff upon the eastern edge of the town of San Juan. The fissure of the recent earthquake is to be seen where it crosses the road 0.5 mile northwest of San Juan, but has not been noted farther along the old Rift line. It appears to bend more easterly, and this probably connects it with the disturbances of the earth between Hollister and San Juan. Mr. Abbe, of San Juan, states that the earthquake of 1890 opened the old Rift and that the displacement of the walls, tho small, was in the same direction as in the recent earthquake.
The town of San Juan stands upon a bench of gravel which dips gently in a south-westerly direction, but upon its northeastern side presents a steep face which, near the old mission, has a height of about 50 feet. This bluff is marked thruout its length of 0.5 mile by several springs; and there can be little doubt that it owes its existence to a fault movement uplifting and tilting toward the southwest a portion of the floor of the valley, and that it thus originated in the same way as other similar features which we shall find to be characteristic of the Rift. The Rift leaves the valley southeast of San Juan and gradually rises along the eastern slope of the Gavilan Range. It intersects the head of San Juan Canyon, and has here given rise to an interesting modification of the drainage. San Juan Canyon is long and narrow and is formed by the union of several small streams which, rising upon the higher slopes of the range, pursue a normal course toward the San Benito Valley, until reaching the Rift, they turn northwest and slightly away from the fracture line, giving rise to San Juan Canyon. At the point where the Rift intersects the canyon, the narrow ridge between the canyon and the valley has been broken thru, and the whole drainage passes directly down the mountain, abandoning the canyon, which is now filling with débris fan material.
For about 10 miles southeast from the head of San Juan Canyon, the Rift follows the eastern slope of the Gavilan Range. It is marked by small valleys and gulches, by hollows and ridges upon whose sides oak trees are growing; and it is followed almost continuously by a wagon road. One of the most interesting features along this portion of the Rift is Green Valley, a broad cienega due to the filling up with gravels and silt of a valley lying close under the steeper portion of the Gavilan Range. There are two fault-lines below the valley and about 0.25 mile apart. The cienega is due to vertical
The Rift comes out upon the San Benito River 4 miles above Paicenes P.O. For several miles up the river from this point, the Rift line is masked by the recent flood plain. Above Mulberry P.O., and just before coming to the bridge across the river, a most striking and interesting feature appears. Upon the east side of the river, and separated from it by a ridge, is a narrow depression half a mile long and 75 feet deep, without any external drainage. The ridge between it and the river extends a mile northwest of the sink, and presents a steep face to the northeast. The road passes along the eastern base of the ridge and opposite the sink makes use of its even crest. The river makes a sharp bend at the bridge, and the Rift crosses to the west side. Faulting has here brought to the surface, upon the west side of the Rift, limestones associated with the crystalline schists and granitic rocks of the Gavilan Range.
In order to follow the Rift beyond the mouth of Willow Creek, we leave the San Benito River road at the mouth of the creek and follow to its head a long narrow canyon which has evidently been eroded on the line of fracture. At the head of the canyon we come out upon a bit of open rolling country which, but for a low ridge, would drain into the San Benito. This ridge has evidently been raised along the Rift, diverting a stream which would naturally be tributary to the San Benito, so that now it forms the head of Bear Creek and flows down past the Chelone peaks into the Salinas River. Several undrained hollows (plate 18B) mark the Rift as it follows the ridge between Bear Valley and San Benito River. The formation of both walls is probably of Tertiary age up to a point near San Benito P.O., where the Franciscan series constitutes the southwest side and the Tertiary the northeast. South of San Benito P.O., there is a considerable area where the surface has been much changed as a result of some one of the movements along the old Rift. A fertile valley, perhaps 0.5 mile long, appears to have been formed thru subsidence, while on the southwest is an abrupt ridge 200 feet high and fully a mile long. The ridge without doubt has been produced by faulting. Its abrupt northeastern face and long, gentle, southwesterly slope suggest in a remarkable manner the great fault blocks of the west, such as the Sierra Nevada Range. The ridge gradually sinks in a southeasterly direction, blending with Dry Lake Valley. The latter is about 2 miles across and has no external drainage. The fault-scarp already mentioned extends as a low ridge part way across the valley and is utilized by the road.
Looking southeast across the valley in the direction which the Rift pursues, a mountain is seen which seems to have been sharply cut off. Descending a narrow valley to the southeast of Dry Lake Valley, we reach the foot of a steep escarpment (plate 19A) where there are apparent two, and possibly three, lines of displacement. The middle one passes at the foot of the main cliff, which is between 400 and 500 feet high. It can not be said with certainty that the whole cliff is the result of faulting, altho it is certainly so in part. The formation in the cliff is sandstone of either Tertiary or Cretaceous age. About 5 miles northwest of Bitterwater there is an interesting valley which has been so disturbed that it has no external drainage, while thru its center passes a ridge formed along the Rift. The ridge forms a fine roadbed. Descending toward Bitterwater Valley and P.O., another ridge appears which is as even and regular as a railroad grade. Bitterwater Valley is occupied during the wet season by a marshy lake.
Southeast of Bitterwater, the Rift leaves the younger formation, and at Lewis Creek both walls are in the Franciscan rocks. For 20 or 25 miles now, the peculiar features of the Rift by which we have followed it are almost absent. The Franciscan series, including old sedimentary rocks, serpentines, and other basic igneous rocks, does not lend itself well to the preservation of such records, but appears to be greatly broken and crusht and marked by enormous landslides in the vicinity of the Rift. The Rift crosses Lewis Creek about 2 miles above its mouth and then passes up over a high ridge lying between Lewis Creek and San Lorenzo Creek. On the north side of Lewis Creek there is an enormous landslide, which has nearly blocked the valley. The slide is undoubtedly hundreds of years old. The ridge on to which the Rift passes after leaving Lewis Creek is crost by it at such a small angle that it does not reach the southern base until we get to the head of Peach Tree Valley, a distance of 20 miles. The ridge its whole length is shattered and broken, and, as before said, marked by innumerable rockslides. The rather steep slopes appear to move every wet season. The headwaters of the San Lorenzo Creek (Peach Tree Valley) have been robbed by Gaviota Creek, possibly as a result of some movement connected with the Rift. Just above where the stream has been diverted, there is another great landslide which the road crosses to reach Slack Canyon.
At the mouth of Slack Canyon, the Rift leaves the Franciscan series, and coincides again with an ancient fault in which the Miocene sandstones are thrown down upon the southwest against the older formation just referred to. Passing from Slack Canyon over a divide, we come to the headwaters of Indian Creek and Nelson Canyon. As the Rift occupies steep slopes much of this distance, it is distinguished chiefly by landslides and rapid gullying of the surface. In Nelson Canyon the Rift follows an old fault in which the Miocene formation has been thrown down upon the southwest side, and the northeast wall so raised that the granite on which the Franciscan series rests is exposed. Ascending the divide toward the head of Nelson Canyon, a long, nearly straight ridge of Miocene clays divides the drainage and appears to be due to some one of the movements along the Rift.
The Rift can be traced thru the hills at the head of the Cholame Valley by its characteristic features, as well as by bluffs which are undergoing rapid erosion. It crosses the road a mile west of Parkfield and exhibits here a regularly rounded ridge 200 feet wide and 20 feet high at the most elevated point. (Plate 19B.) That the ridge must be hundreds of years old is shown by the great oak trees that are growing upon it. One white oak is fully 8 feet thru. Large springs mark the fissure at this point, and are found along it the whole length of the Cholame Valley. According to a resident, the Rift opened along the ridge in the earthquake of 1901, the opening being distinctly traceable for several miles. Southeasterly from the point just described thru the Cholame Valley, there appears no very prominent ridge or escarpment, altho springs and cienegas, marking a gentle swell in the flat open surface of the valley, indicate the line of the Rift.
The region about Parkfield, in the upper Cholame Valley, has been subjected to more frequent and violent disturbances than almost any other portion of the entire Rift. An auxiliary fissure begins near the main Rift a little west of Parkfield, and extends in a more easterly direction along the east side of Cholame Creek. (See plate 20.) The once flat, open valley has been broken along this line, and a bluff nearly 200 feet high formed facing the Creek. This bluff, now deeply eroded, must have been formed during one of the oldest disturbances. The lowland between this bluff and Cholame Creek shows the effect of great disturbance over a considerable area. Innumerable hollows interlace and extend in all directions. They resemble nearly obliterated creek beds except that they have no outlets. Parallel with the front of the dissected bluff, but a little back from
The people living along the Rift for 150 miles southeastward from the Cholame Valley tell wonderful stories of openings made in the earth by the earthquake of 1857. The first settler in Cholame Valley was erecting his cabin at that time, and it was shaken down. The surface was changed and springs broke out where there had been none before. In 1901 a fissure opened in the road which crosses the branch fault just described. After each successive shake it is reported that the fissure opened anew, so that the road had to be repaired again in order to be passable.
Upon the western side of the Cholame Valley, near its southern end, the main Rift again exhibits an interesting bluff which cuts off the débris fans of the back-lying hills. This bluff faces northeasterly. Where the Rift crosses the creek as it passes out of the Cholame Valley, a low escarpment was formed upon the west side which must for a time have dammed the creek and given rise to a lake. From the outlet of the Cholame Valley the Rift line can be seen as it rises along the low rolling hills, and disappears over their tops. It is marked by a distinctly steeper slope facing northwesterly, showing that an uplift of 30 to 50 feet took place upon the west side. The region traversed thru the Cholame Valley southeast to the Carissa Plain and for some miles beyond, exhibits no older formation than the Miocene Tertiary, the effects of older faulting, if such has occurred here, being masked by recent deposits. Continuing the examination toward the southeast, the writer came upon the Rift at the northern end of the Carissa Plain, 4 miles northeast of Simmler P.O., and in direct line with its course where last seen. Here the width of the broken country is much greater than usual, being nearly a mile. A number of lines of displacement can be distinguished; some nearly obliterated, others comparatively fresh. This is a region of light rainfall and of gentle, grass-covered slopes, presenting just such conditions as would preserve for hundreds of years the effects of moderate displacements.
The Rift zone continues to be traceable along the western base of the Temblor Range, finally passing out on to the gently rolling surface of the eastern edge of Carissa Plain. Broken and irregular slopes, cut-off ridges, blocked ravines, and hollows which are white with alkaline deposits from standing water mark the Rift. Carissa Plain has a length of about 30 miles. About halfway the Rift begins to be marked by a low and nearly obliterated bluff upon its northeastern wall. This is at first little more than a succession of ridges or hills cut off on the side next to the level plains. These detached ridges finally become connected in a regular line of hills with a steep but deeply dissected slope toward the southwest and long gentle slopes toward the northeast. This ridge is clearly a fault block, and now separates the southeastern arm of Carissa Plain from Elkhorn Plain. It probably originated during some one of the earlier movements along the Rift; in fact, it is reasonable to suppose that it is of the same age as other important scarps which mark the Rift thruout its whole course, and which came into existence as a result of some mighty movement opening the earth for several hundred miles.
Except for one slight bend, the ridge which we have been describing follows a straight course toward the southeast for a distance of nearly 20 miles, finally blending in a much larger mountain-like elevation. This has a height of perhaps 500 feet above the sink at its southern base. Its deeply dissected front is in line with the front of the ridge already described and the two appear to have originated together. The steeper face is deeply sculptured into gullies and sharp ridges, while the back slopes off gently toward the southern end of Elkhorn Plain. Plainly visible along the steep front of the line of hills described are the lesser ridges and hollows produced during the last violent earthquake in this region, probably in 1857. (See plate 21A, B, C.)
A gentle divide separates the southern end of the Carissa Plain from a long narrow sink extending along the Rift line toward the southeast. (Plate 21D, E, F.) This sink includes an area 6 miles long and in places its drainage is fully 3 miles wide. Several deprest alkali flats, covered with water during the wet season, receive the scanty run-off of this dry region. These depressions are several hundred feet wide and are bordered upon opposite sides by quite sharp bluffs, in some places 100 feet high. The phenomena suggest the sinking of long narrow blocks between two walls. This reach of 6 miles between the ranches of Job and Emerson is one of the most interesting areas examined. The larger scarps belong to some ancient disturbance, while the last one, probably dating from 1857, is marked by features comparatively insignificant.
As we ascend the long grade from the sinks just described, to Emerson's place, near Pattiway P.O., the Rift features become smaller and less regular, altho easily followed. (See plate 23A.) At Emerson's the Rift passes thru a sag in the hills and across the head of Bitter Creek. It then rises and crosses a flat-topt hill between this creek and the west fork of Santiago Canyon; and descending to the east fork keeps along the steep mountain slope upon the south until it finally crosses the divide between San Emedio Mountain and Sawmill Mountain. Thru this section the Rift gradually bends toward the east, and in Cuddy Canyon, farther east, it has an east and west direction for a few miles.
The Rift itself is scarcely distinguishable in Bitter Creek and Santiago Canyons, owing to steep slopes and rapid erosion, as well as numerous landslides. Santiago is one of the deepest and narrowest canyons in this portion of the mountains. Its whole southern slope, that traversed by the Rift, has been more or less affected by slides producing many little basins along the edge of the flat-topt divide between the drainage into the San Joaquin Valley and Cuyama River. Huge masses of earth and rock are still moving, as shown by fresh cracks and leaning trees. In one place the edge of the divide has split away in such a manner as to produce long narrow ridges with depressions behind them, closely imitating the real Rift features. Santiago Canyon marks a great fault of earlier times. Soft Tertiary formations are faulted down thousands of feet upon the south side of the canyon, while upon the north appear the steep granitic slopes of the western spur of San Emedio Mountain.
The Rift appears upon the north side of the pass which leads from Santiago Canyon to San Emedio Canyon. Two lines of disturbance are here plainly visible. Going down the west branch of San Emedio Canyon, the Rift zone is plainly traceable, but nowhere does it form important features. Passing to the east fork of the canyon, we continue on the line of the Rift to the divide leading over to Cuddy Valley. (See plate 22A.) Beginning upon the divide, a broad rounded ridge, fully 50 feet high upon its southern side, extends down the slope in a direction a little south of east. Cutting thru the center of this ridge longitudinally is a deep V-shaped depression, as tho a movement later than that which formed the ridge had opened a fissure thru its center. On the sides of the ridge, as well as the slopes of the fissure in it, large pine trees are growing in an undisturbed condition. Continuing down the ridge, we find that in the course of a mile it gives place to an escarpment facing northerly. A valley 4 miles long and 0.5 mile wide lies below the escarpment and contains meadows and a small lake without any outlet. Springs mark the Rift line. The escarpment has been much eroded, but toward its eastern end it has a height of nearly 300 feet and is covered with a growth of pine trees among which are stumps of large dead trees in an undisturbed condition. The valley and the bluff are doubtless the product of the earliest movement in the epoch of which we are treating. The last movement left a comparatively small ridge traceable here and there along the base of the greater.
Continuing on the line of the Rift, we enter and pass for 10 miles down Cuddy Canyon.
At Gorman Station, several miles below Gorman, there is a wonderfully regular ridge forming a marsh. In this vicinity the earthquake of 1857 is reported to have done much damage, shaking down an adobe house and breaking up the road. The little lake upon the divide halfway between Gorman Station and Neenach P.O. is due to débris brought down from the hills upon the south thru which the Rift zone passes. The Rift follows a very regular and straight course, a little south of east, along the mountain slopes south of Antelope Valley. Thru the most of the distance, as far as Palmdale, it occupies a series of valleys shut off by considerable elevations from the open slopes of Antelope Valley. After traversing the northern slopes of Libre and Sawmill Mountains, the Rift crosses the head of Oak Grove Canyon, then another small canyon with branches eastward and westward along the break, and eastward of this a long canyon opening out to the fertile valleys about Lake Elizabeth.
Lake Elizabeth and Lower Lake (plate 24B) are both due to the blocking of the drainage of two valleys extending along the Rift. These valleys lie on the slope of the range toward the desert (Antelope Valley), but their outlet is southward by a narrow canyon thru the heart of the mountains lying between the desert and Santa Clara River. A low escarpment along the southern side of the valley in which Lake Elizabeth lies, and eastward, is replaced by a lofty rounded ridge which appears to be due to some one of the movements along the old fault. For several miles east of the lake (plate 25A) the distinctive and characteristic features of the Rift are not as easily made out, altho the ridge just mentioned is full of springs and exhibits a widespread landslide topography. Toward the eastern end of this ridge small hollows and a low, indistinct escarpment again appear. The ridge separates Leones Valley, a fertile and well-watered district 5 or 6 miles long, from the open Mojave desert on the north.
From Leones Valley to and beyond the point where the Rift zone crosses the Southern Pacific Railway, a constant succession of cienegas is found on the upper side; that is, on the side toward the mountains. Movements have evidently been so often repeated and so intense along the Rift as to grind up the rocks and produce an impervious clayey stratum, bringing to the surface the water percolating downward thru the gravels of the waste slopes. A mile west of Alpine Station on the Southern Pacific Railway, there begins another escarpment with its abrupt face toward the south. This extends to and across the railroad. South of the escarpment the surface has sunk so as to form a basin. (Plates 25B and 26A.) This has been artificially enlarged and used as a reservoir for irrigation about Palmdale. The main escarpment is 40 to 50 feet high in places, and where the railroad crosses it there appear to be two, and older and a younger one. From the summit of the ridge marking the Rift west of Alpine, an extensive view eastward is obtained. The long desert waste plain leading up to the foot of the mountains on the south (San Gabriel Range) exhibits a strikingly interesting feature. It is not continuous across the line of the Rift, but shows a break with the uplift upon the lower side. The amount of displacement appears to be between 200 and 300 feet.
An extended study would be necessary to determine in detail the geology of the Rift from Gorman Station eastward. Near Gorman a dike of basaltic or andesitic lava extends parallel with it for some distance. Granitic rocks often form one side, while soft Tertiary beds of a light or reddish color frequently appear in the raised ridges. Between Palmdale and Big Rock Creek, low discontinuous ridges, springs, and cienegas point out the line of the Rift, altho there are stretches of several miles at a time where either the original displacement was not great or erosion has removed its effects. Four miles west of Big Rock Creek there is one fine escarpment 0.333 mile long and 40 feet high, facing the mountains on the south. (Plate 27A.) In this section there are indications of at least two movements. (See plate 26B.) The Rift passes just below Big Rock P.O. east of Big Rock; a trail on the northern slope of the mountains and a wagon road on the southern side of the divide follow the Rift continuously to a point near the mouth of Cajon Canyon. On the north side of the mountains (San Gabriel Range) there is no important depression on the Rift between Big Rock Creek and Swartout Valley; nevertheless the comparatively recent movements have been of sufficient magnitude to produce ridges and hollows giving a continuous and easy route for the trail along the slope of the mountains.
Before reaching the divide leading over to Swartout Valley, we encounter some striking features. Near the head of Mescal Canyon a ridge has been split away from the mountain, diverting the little streams from above and making two drainages where one would normally appear. In places this ridge (plate 27B) is as sharp and as perfect as tho formed but yesterday; but the great pine trees, growing upon its top and sides (the altitude here being nearly 7,000 feet), tell us that it must be hundreds of years old. At the head of the canyon the trail leads thru a sharp V-shaped cut where the bare sliding surfaces make it appear as if movement had recently taken place in the Rift. (See plate 28A.)
Passing over a sag in the mountains to Swartout Valley, the Rift is less prominent as a topographic feature, but a line of springs marks its course. Lone Pine Canyon is remarkable for its length and straightness. The Rift passes down its whole length but it is not very prominent. Springs appear at several points, also small cienegas with a slight escarpment below them. At the mouth of Lone Pine Canyon and a little above its junction with the Cajon Canyon (plate 28B) are more interesting features. Two lines of displacement appear here, and between them a long, narrow sunken block with a small lake in its lowest portion. (See plate 29A.)
The line of disturbance now crosses Cajon Canyon, giving rise to broken and sliding cliffs; and then, passing over a spur of the San Bernardino Range, comes out at its foot before reaching Cable Canyon. From this point the Rift continues on southeasterly at or near the junction of the gravel slopes of the San Bernardino Valley and the steep mountain slopes of crystalline rocks. The uniformly straight course which the Rift exhibits in this portion of its length takes it diagonally across the mountains from the northern and desert side of the San Gabriel Range to the southern side of the San Bernardino Range.
The torrential streams emerging from the San Bernardino Range upon the gravel slopes of the broad valley at its base have cut wide flood plains in the ancient gravels which accumulated along the foot of the mountains. The remaining portions of this old slope lying between the stream plains are called mesas. Back of Devore Heights there appears a rounded ridge formed out of the mesa gravels. As we continue toward Cable Creek, springs and cienegas are found to be numerous just above it. East of Cable Creek the ridge becomes an escarpment facing the valley, and in places shows a height of about 75 feet. Viewed in profile, this escarpment breaks the uniform slope of the mesa gravels, almost reversing their slope on the upper side. On the west side of Devil Canyon there is a double escarpment in the gravels, both apparently being due to movements along the Rift. (See plate 29B.) Back of the Muscupiabe Indian reservation and north of the
East of City Creek begins a huge rounded ridge formed in the mesa gravels, and this can be traced nearly to Plunge Creek. This ridge is 150 feet wide and steeper upon its upper side, where the greatest displacement shown is about 40 feet. The structure and shape of the gravel ridge make it appear likely that faulting and folding were both concerned in its making. Above this ridge and at the highest point where it crosses the mesa, water is obtained in abundance for irrigation at a depth of 18 to 20 feet, while in the mesa below the ridge no water is found at a depth of 200 feet.
The Santa Ana River has cut out a wide stretch of the mesa gravels, and has exposed upon its eastern bank a good section of these gravels. The gravels at their upper edge do not lap over the crystalline rocks but appear faulted down against them. A 0.25 mile below the fault is the mouth of Morton Canyon, the stream issuing thru a long, narrow canyon eroded in the mesa gravels. Morton Canyon emerges from the steep mountains about 2 miles to the southeast and has taken this peculiar course thru the gravels to the Santa Ana River, instead of flowing directly down across them, as do all the other streams. The explanation of the turning to the northwest of this canyon at the point where it meets the gravels is found in the peculiar appearance of the gravel slope when viewed in profile. This, instead of rising with normal slope, becomes steeper toward the upper edge, and then descends abruptly to Morton Canyon. The movement on the Rift has broken and lifted up the gravels to such an extent that the waters of Morton Canyon were diverted and turned down to the Santa Ana River along the upper side of the ridge. Since this displacement took place, they have had time to cut the canyon in which they are now flowing. Southeast of the point where the Rift crosses Mill Creek, the peculiar topographic features which have characterized it for so many miles become very indistinct. It was at first thought that the Rift terminated in this vicinity but closer examination made it clear that such is not the case.
The southern portion of the San Bernardino Range lying between Mill Creek and the Conchilla Desert appears to have undergone great disturbance at a recent date. As a consequence, erosion has been rapid and extensive, and surface features which farther north made the Rift easy to follow have in this region been almost completely obliterated. Potato Canyon extends along the line of the Rift to the southeast of Mill Creek. Its features indicate that the history of the fault is a complex one. The canyon originated thru erosion upon the fault contact between the crystalline rocks of the San Bernardino Range and the older Pleistocene deposits along its base. Following this period of erosion was one in which gravels were again deposited and this was succeeded by the present period in which erosion is active. Potato Canyon is the last of the longitudinal depressions of any size marking the line of the Rift. Between its head and the desert to the southeast the main drainage features pay little attention to the structural conditions, because of the steep grades of the stream channels and consequent rapid erosion. Nevertheless small lateral canyons have been formed along the fault contact of the gravels with the crystalline rocks of the higher portion of the San Bernardino Range, so that from the proper viewpoint the fault line can generally be traced in the topography. The drainage of Potato Canyon is clearly influenced by the fault, for instead of there being one stream course in it, there are two — one upon each side.
A mile southeast of Oak Glen, which is at the head of Potato Canyon, there are large springs which issue upon the line of the fault. Near this point a depression appears upon a gravel ridge, where it meets the crystalline rocks. The depression is in line with the course of the fault, and may with reason be attributed to dislocations similar to those so
To the east of the San Gorgonio River, the topography as shown upon the San Gorgonio quadrangle gives little indication of the presence of an important fault-line. However, an examination of Potrero Creek shows small transverse canyons and one broad, grassy flat with springs upon the line of the fault. In Stubby Canyon and other small canyons north of Cabazon Station, the fault is finely shown. Here, as at the point where the Santa Ana River issues from the mountains, the older Pleistocene gravels have been faulted down against the crystalline rocks. Rapid erosion of both the Pleistocene deposits and the crystalline rocks has given rise to steep and precipitous slopes in this section, and upon these the fault is clearly shown. The schists and gneisses thru a width of hundreds of feet adjoining the fault have been so crusht by pressure and movement that they quickly crumble upon exposure. Dark clay marks the plane of movement which inclines to the north at an angle of about 80 degrees. Later than the period of main faulting has come an elevation of the range as a whole, giving rise to rapid erosion upon both sides of the line of fracture. Remnants of gravel mesas and mature topographic forms appear in places. A notable example of an area of old topographic features now being destroyed by the modern canyons is shown to the west of Stubby Canyon and 1,000 feet above it.
There are traces here and there of recent displacements along the Rift. These are of the nature of little sags without outlets and low ridges or escarpments not easily explainable as a product of ordinary erosion. These may have arisen as the product of landslides, but the landslides themselves are doubtless related to fault movements. The great débris fans built up along the north side of San Gorgonio Pass indicate rapid removal of a vast amount of rock material from the adjoining slopes of the San Bernardino Range consequent upon recent uplift.
Before investigating this region it was thought that the Rift, if it continued on south-easterly, would be found crossing the San Gorgonio Pass in the neighborhood of Cabazon and skirting the eastern base of the San Jacinto Range; but this proved not to be the case. Instead, it was found to turn more and more easterly and finally to extend parallel with the pass without reaching it. The course of the Rift, then, instead of being in the direction of the Salton Sink, is toward the Conchilla Desert north of Palm Spring Station.
Looking east from a point near the mouth of Stubby Canyon, the gravel mesa thru which the Whitewater River issues from the mountains, appears to be faulted upward, giving rise to a well-defined escarpment facing north toward the crystalline rocks. This northward facing escarpment accords in relative position with the traces of escarpments farther north near Oak Glen, and shows that the latest displacement has been the reverse of the earlier. The last seen of the Rift is in the sides of the Whitewater Canyon, where the gravels are faulted down against the crystallines. East of the Whitewater one enters upon the Conchilla Desert over which has been spread the wash of Mission Creek. For a distance of 6 or 8 miles, and perhaps much more, the bedrock is completely buried by the recent accumulations.
The San Bernardino Range rapidly decreases in height to the southeast of Mission Creek, but appears to be continuous with the desert range lying north of the Salton Basin. The latter range of crystalline rocks appears to be separated from the lowlands of the basin by a more or less continous line of barren yellow hills formed of soft late Tertiary rocks. Judging from a cursory examination, these yellow hills are separated from the higher mountains behind by a structural break indicated by a series of longitudinal valleys. A prolongation in a northwesterly direction of the supposed fault line indicated by these
An examination of the northerly and easterly base of San Jacinto shows conditions opposite to those characterizing the southern slope of the San Bernardino Range. Erosion is generally slow upon the slopes of San Jacinto, while the rapid erosion from the opposite side of San Gorgonio Pass has crowded the stream channels close to the base of the former range. In fact, the base of the San Jacinto Range appears to be deeply buried by the stream deposits. The desert face of San Jacinto has long been free from disturbances. Long, jagged ridges project out into the desert, while the intervening canyons, instead of furnishing material for extensive débris fans, are floored by accumulations characteristic of the desert as a whole.
Toward the southern end of that spur of the San Jacinto Mountains which projects into the Colorado Desert and is known as the Santa Rosa Mountains, the débris fans are larger and remains of gravel deposits appear high up on the sides of the mountains. The only suggestion that a fault traverses the Salton Basin in the direction of the mouth of the Colorado is the presence of mud volcanoes and several small pumiceous eruptions near the center of the basin. These are, however, so far removed from any known fractures in the crust that their evidence is of little value. Besides, it is entirely possible that the mud volcanoes may be due to chemical action in the deeply buried sediments of the Colorado delta.
It may be reasonably assumed, then, from our best knowledge, that the southern end of the great Rift is to be traced for an unknown distance along the base of the mountains bordering the Salton Basin upon the northeast, in all probability gradually dying out.
San Jacinto Fault
The San Jacinto fault (plate 30), with which there has been associated at least one severe earthquake since the region has been known, has a length of at least 75 miles. The course of the fault is northwest and southeast, and it is marked by canyons or steep mountain scarps nearly its whole length. The fault first appears upon the south in the form of a regular mountain wall inclosing the north end of Borego Valley. The latter is a western arm of the Colorado Desert lying between the Santa Rosa Mountains and the main watershed of the Peninsular Range. At the northern end of Borego Valley beds of late Tertiary age appear faulted down upon the southwest side of the mountain wall referred to. The peculiar topographic features of this fault-block ridge, and the presence of gravels along portions of its summit, make it appear of recent origin. Northwest of Borego Valley the canyons entering Coyote Creek have brought down immense quantities of rock débris, a fact which indicates recent disturbance along their headwaters. Terwilliger Valley includes a broad expanse of country of low relief upon the summit of the range between San Jacinto and Borego Valley. A portion of the valley is scarcely drained at present, having apparently undergone some subsidence next to the fault-line which forms the southern face of Mount Thomas.
In a northwesterly direction, the fault can be traced in continuous mountain scarp or canyon until within about 8 miles of the town of San Jacinto. A broad valley intervenes until we get north of the town, when a mountain wall commences again, and extends for many miles in the direction of Colton. Reports state that the San Jacinto earthquake of 1899 was most severe along the line of the fault thus traced. Great masses of rock are
The regular mountain wall facing southwest and extending northwest from San Jacinto appears to be older than that toward the southern end of the fault. Mineral springs occur near or on this line, and the marshy area at the point where the San Jacinto River ceases following this fault-scarp and turns toward the southwest suggests very strongly a subsidence.
Review of Salient Features
It will be of advantage briefly to review the salient features of the San Andreas Rift, in the light of the facts presented in the foregoing detailed description of its extent and character, and of other facts to which attention will be directed.
The San Andreas Rift has been traced with three interruptions from a point in Humboldt County, between Point Delgada and Punta Gorda, to the north end of the Colorado Desert, a distance of over 600 miles. These three interruptions are: The stretch between Shelter Cove and the mouth of Alder Creek, where for a distance of about 72 statute miles it traverses the bottom of the Pacific Ocean; the stretch from the vicinity of Fort Ross to Bodega Head, where for 13 miles it is similarly on the ocean bottom; and the stretch from Bolinas Lagoon to Mussel Rock, where it lies beneath the Gulf of the Farallones for about 19 miles. Of these interruptions only the first involves any doubt as to the continuity of the feature, and this doubt is in large measure removed by the evidence cited hereafter as to the position of the trace of the fault of April 18, 1906.
Thruout its extent the Rift presents a variable relation to the major geomorphic features of the region traversed by it. In Humboldt County it lies within the mountainous tract inland from the coast but to the seaward side of the higher land. From Shelter Cove to Alder Creek it lies to the west of a steep, terraced, coastal slope. From Alder Creek to Fort Ross, it finds its expression in a series of rectilinear, sharply incised valleys, the alinement of which converges upon the coast line to the south at a very acute angle. But near Fort Ross the Rift, without deviation of its general trend, crosses the divide to the coastal side of the ridge which separates these valleys from the ocean, and traverses the terraced coastal slope. Beyond Fort Ross it again lies to the west of a steep coastal slope. From Bodega Head to Bolinas Lagoon the Rift is a remarkably pronounced depression, lying between the main coastal slope and the rather high and precipitous easterly side of the Point Reyes Peninsula. About 0.6 of this depression is below sealevel, forming Tomales and Bodega Bays. This defile is one of the most remarkable and interesting phases of the Rift. It has been a line of repeated faulting in past geological time, and evidently separates a well-marked and probably relatively mobile crustal block from the main continental land mass.
South of Mussel Rock the Rift traverses for a few miles a rolling upland, marked by ponds and old scarps, but with no very marked contrast in relief, and then passes into the very marked and rectilinear San Andreas Valley, along the base of the northeast flank of the Santa Cruz Range. From here to the gap at Wright Station it lies along the base of the range at a distance nowhere greater than 2 miles from the crest. Passing thru the gap at Wright, it crosses from the northeast flank of the range to the southwest flank. Similarly passing thru the gap between the Santa Cruz and Gavilan Ranges at Chittenden, it is again found on the northeast flank of the latter. In effecting this last-mentioned change of position relatively to the mountain crests, a distinct deviation in the trend of the Rift is observable (see map No. 5) as if the path of the Rift accommodated itself to
The Rift as a whole, when plotted upon a general map of the state on a scale of about [Mathematical Equation], appears as a sensibly even line with marked curvature, convex toward the Pacific. This curvature is for the most part due to change in the course of the Rift between the southern end of Carissa Plain and Tejon Pass. In this segment of its course its trend changes from about S. 40° E., along the edge of Carissa Plain to S. 65° to 70° E., along the southern edge of the Mojave Desert, the change being gradual and distributed over an arc about 40 miles in length. The general curvature is also accentuated by the change in course between Point Arena and Shelter Cove, on the assumption of continuity between these points. If, however, we take the segment of the Rift between Point Arena and the south end of Carissa Plain, the curvature is very much less marked; and its path on the small scale map referred to approximates a straight line. The curvature is distinct, however, and, as in the general case, is convex to the Pacific. The chord of the arc found by stretching a line from the south end of Carissa Plain to the mouth of Alder Creek has a bearing of about N. 40° W. and a length of about 360 miles. The point on the arc most distant from this chord is on a normal to the latter thru San Jose, the distance being about 15 miles.
When the Rift is plotted on larger scale maps (see maps Nos. 2, 4, 5, 21, and 23), it becomes apparent that the course of the Rift is not a smooth uniform curve, but is characterized by several minor curvatures in opposing directions. In locating these curves, advantage is taken of the fault-trace, as far as it extends, as a sharp line within the Rift indicating its mean trend at any point on its course. These curvatures are most interesting features on a line of diastrophic movement, where that movement may be, as it was on April 18, 1906, essentially horizontal on a nearly vertical plane or zone.
The most northerly curvature susceptible of measurement is that shown by the segment of the Rift between the mouth of Alder Creek and Fort Ross. The line connecting the two ends of this segment, at the points where it intersects the shore line, is a little more than 43 miles in length, and has a bearing N. 37° W. The Rift, as located for this purpose by the fault-trace, lies wholly on the southwest side of this chord. The bearing of the fault-trace at the mouth of Alder Creek, where it converges upon the chord, is N. 30° W., and at Fort Ross its bearing is N. 40° W. The fault-trace is at its maximum distance from the chord about the middle of this segment, the distance being about 0.75 mile, and here the bearing of the Rift is sensibly the same as that of the chord, N. 37° W. Between Fort Ross and Bodega Head, where the Rift passes under the Pacific, there is probably a slight reversal of this curvature; since, if the course of the fault-trace at Fort Ross were continued, even as a straight line, it would pass to the eastward of the point where it actually intersects the neck of the headland. This slight concavity to the southwest probably extends as far as the mouth of Tomales Bay. From Bodega Head south thru Tomales Bay to Bolinas Bay, the course of the Rift as a large geomorphic feature is
Between Mussel Rock and San Andreas Dam the fault-trace has a slight concavity to the southwest. The projection of its course seaward from Mussel Rock would not meet the southward projection of the fault-trace from Bolinas. There can be little question as to the continuity of the fault-trace across the Gulf of the Farallones and its path on the bottom of the Gulf must, therefore take the form of a very flat sigmoid curve with a slight concavity to the southwest in the Bolinas moiety of the submarine segment and a corresponding convexity at the Mussel Rock end. Between San Andreas dam and Chittenden, the fault-trace indicates a pronounced curvature in the general trend of the Rift. The chord between these two points is about 55 miles in length and bears N. 44° W. The fault-trace lies wholly to the southwest of this line with convexity toward the Pacific. The point on the curve most distant from the chord is about its middle part the distance being about 2.25 miles. On this segment of the Rift there is locally a rather abrupt reversal of the curve, south of Black Mountain, which is best seen on map No. 22.
Between Chittenden and a point near Priest Valley there is another pronounced curvature in the general course of the Rift, where it passes over to the northeast flank of the Gavilan Range. Here the curvature is concave toward the southwest. The chord is 60 miles long, and bears, as before, N. 44° W. and the Rift lies wholly on the northeast side. The point on it most distant from the chord is near the middle of the segment and the distance is 2.4 miles. From Priest Valley to the south end of Carissa Plain, the Rift is nearly straight, but with minor curvatures which can not be more particularly defined owing to the absence of good maps. The general bearing for this segment is about N. 40° W.
The marked curvature between the south end of Carissa Plain and Tejon Pass has already been noted. From the latter place to the north end of the Colorado Desert beyond which the Rift has not been traced, there are numerous curvatures in the course of the Rift; but since the Rift for this segment is indicated on maps Nos. 6 to 15 on a scale of 1 or 2 miles to the inch, it will be unnecessary to do more than refer to these maps for their characterization. The general course of the Rift in this region is a flat curve concave to the south-southwest.
It thus appears that the Rift, as a whole, has a curved course convex to the Pacific and that this general curvature is characterized by a succession of slightly curved rather than straight, segments. The amount of the curvature, as it appears upon the maps is determined to some slight extent by the character of the projection. But the general conclusion above reached without quantitative expression is independent of the projection adopted for the maps.
A most interesting general feature of the Rift is in relation to the granitic rocks of the Coast Ranges. The granites of the southern Sierra Nevada pass into the Coast Ranges in the vicinity of Tejon Pass, and extend thence in a series of more or less elongated but discrete areas thru the Santa Lucia, Gavilan, and Santa Cruz Ranges, and beyond the Golden Gate to Point Reyes Peninsula and Bodega Head. From the southern end of Carissa Plain to Bodega Head, this granite lies wholly to the southwest of the Rift. At one point in the Rift, however, in Nelson Canyon, Fairbanks has found the granite exposed on the northeast side of an old fault having a downthrow on the southwest. Southward it passes into a region where granitic rocks prevail on both sides of the Rift. The Rift in the Coast Ranges thus appears to serve as a line of demarkation between two
In a discussion of the rift as a geomorphic feature, it becomes a matter of interest to determine the relative importance of diastrophism and erosion in its evolution. There can be no boubt that where the rift is coincident with pronounced longitudinal valleys, the latter, altho controlled as to orientation by the faulting along the Rift, owe their features in a large measure to erosion. This is true, for example, of the valleys of the Garcia and Gualala Rivers and the San Andreas Valley. It is not so clear, however, as regards the depression between Point Reyes Peninsula and the mainland. It has been pointed out that in past geological time there has been a recurrence of faults with large vertical displacement on this portion of the Rift, dating back to pre-Miocene time and possibly to the Cretaceous and it may be that here the depression is essentially diastrophic in origin and only modified to a minor degree by erosion; similarly with some of the valleys along the Rift, and extending from it in the Southern Coast Ranges. The Cholame Valley and the valley of Carissa Plain may be essentially diastrophic in origin, modified by erosional degradation on their sides and by aggradation of their bottoms. The depressions which constitute the Rift along the southern margin of Mojave Desert appear to be practically wholly diastrophic, altho somewhat aggraded. Where the Rift hugs the $steep northwest flank of the Santa Cruz Range as far as Wright Station, and the similarly steep southwest flank of the same range from Wright to Chittenden, it is difficult to avoid the conclusion that these steep mountain flanks are in reality degraded fault-scarps, and are,therefore the walls proper of a great asymmetric Rift valley. The same conclusion applies to the steep north flank of the San Rafael and San Gabriel Ranges, on the south side of Mojave Desert. The complete discrimination of effects of diastrophism and erosion in the larger features of the relief associated with the Rift will require many years of patient field work.
With regard to the minor feature which characterize the Rift in detail, thruout its extent the discrimination is less difficult chiefly because the diastrophic effects are of comparatively recent date and their distinctive features are not yet obliterated by the ravages of erosion. These consist chiefly of scarps, low ridges, and sinks or ponds. In many cases it is apparent that both erosion and aggradation are controlled by these minor features, and that the latter tend to become obliterated. These minor scarps, ridges, and sinks are not referable to any single earth-movement, but are referable to a recurrence of movement on the same general line. In the southern Coast Ranges the observations of Fairbanks show that one of these movements was of exceptional importance. By it displacements and disturbances of the surface were effected which dwarf all similar events in historic times. For miles at a stretch the earth, upon one side or the other of the fault line, sank, giving rise to basins and to cliffs 300 to 400 feet high. These features, in the opinion of Fairbanks, who personally examined them, were the result of one movement. This displacement probably occurred not less than 1,000 years ago. It is certainly older than the great trees growing upon the ridges and hollows formed by it. Since then it is probable that numerous displacements of less extent have taken place, but the geomorphic effects of the smaller movements have, in some considerable measure, been effaced. Since the settlement of the state, the strain in the earth's
It remains to call attention, in a word, to the alinement of the Rift with certain of the larger continental features. The Rift is known from Humboldt County to the north end of the Colorado Desert. As a line of small displacements it has not been traced farther; and in the usage of the term it has been understood as terminating at the point where it eluded field observation. But it is by no means certain that, as a larger feature, it does not extend far to the south. The Colorado Desert and its continuation in the Gulf of California are certainly diastrophic depressions, and may with much plausibility be regarded as a great Rift valley of even greater magnitude than the now famous African prototype first recognized by Suess. This great depression lies between the Peninsula of Lower California and the Mexican Plateau. All three of these features find their counterpart in southern Mexico. The Sierra Madre del Sur is the analogue of the peninsular ridge; it lies on the line of its prolongation, and is similarly constituted geologically. Inside of this range, and between it and the edge of the Mexican Plateau, is a pronounced valley system which is the analogue of the Gulf of California.
On this valley-line lies the deprest region about Salina Cruz, well known to be subject to repeated seismic disturbances. On the same general line lies Chilpancingo, the seat of the recent disastrous Mexican earthquake. Following these great structural lines southward, they take on a more and more latitudinal trend; and beyond Salina Cruz the geological structure indicates that this seismic belt crosses the state of Chiapas and Guatemala, to the Atlantic side of Central America with an east and west trend, and falls into alinement with Jamaica. It thus seems not improbable that the three great earthquakes of California, Chilpancingo, and Jamaica may be on the same seismic line which is known in California as the San Andreas Rift.
The Earth Movement on the Fault of April 18, 1906
The successive movements which in the past have given rise to the peculiar geomorphic features of the Rift, either directly or by control of erosion, have with little question been attended in every case by an earthquake of greater or less violence. The earthquake of April 18, 1906, was due to a recurrence of movement along this line. The movement on that day was of the nature of a horizontal displacement on an approximately vertical fault plane or zone, whereby the country on the southwest side was moved to the northwest and the country on the northeast side to the southeast. This displacement was manifested at the surface by the dislocation and offsetting of fences, roads, dams, bridges, railways, tunnels, pipes, and other structures which crost the line of the fault. The surface of the ground was torn and heaved in furrow-like ridges. Where the surface consisted of grass sward, this was usually found to be traversed by a network of rupture lines diagonal in their orientation to the general trend of the fault. Small streams flowing transverse to the line of the fault had their trenches dislocated so that their waters became impounded. These and similar phenomena of disruption constitute the fault-trace.
The width of the zone of surface rupturing varied usually from a few feet up to 50 feet or more. Not uncommonly there were auxiliary cracks either branching from the main fault-trace obliquely for a few hundred feet or yards, or lying subparallel to it and not, so far as disturbance of the soil indicated, directly connected with it. Where these auxiliary cracks were features of the fault-trace, the zone of surface disturbance which included them frequently had a width of several hundred feet. The displacement appears thus not always to have been confined to a single line of rupture, but to have been distributed over a zone of varying width. Generally, however, the greater part of the dislocation within this zone was confined to the main line of rupture, usually marked by a narrow ridge of heaved and torn sod.
The amount of the horizontal displacement, as measured on dislocated fences, roads, etc., at numerous points along the fault-trace, was commonly from 8 to 15 feet. In some places it exceeded this and at one place it was as much as 21 feet. Toward the south end of the fault the amount of displacement was notably less and finally became inappreciable. Nearly all attempts at the measurement of the displacement were concerned with horizontal offsets on fences, roads, and other surface structures at the point of their intersection by the principal rupture plane, and ignore for the most part any displacement that may be distributed on either side of this in the zone of movement. The figures thus obtained may, therefore, in general be considered as representing a minimum for the amount of differential movement. In one or two cases, however, when the displacement has been measured on soft ground subject to slumping, and the measured offset is higher than usual, the results may be in excess of the true crustal displacement.
Besides this horizontal displacement of the crust, there was also, particularly in the region north of the Golden Gate, a distinct uplift of the country to the southwest of the Rift, relatively to that on the northeast. This differential vertical movement was made
The differential displacement of the earth's crust above indicated occurred only on the northern portion of the Rift. South of San Juan, in Benito County, there is no indication along the Rift in the shape of rupture of the soil, or the dislocation of transverse structures, which points to the displacement of the underlying formations. It is not, however, to be certainly inferred from this that there was no deep-seated rupture south of that point. Many earthquakes are known which are referable to sudden slips in the earth's crust for which there is no corresponding rupture at the surface. It is probable that the slip, which is so manifest as a surface rupture to the north of San Juan, was continued as a subsurface movement for many miles south of that point.
North of San Juan the displacement on the fault-trace has been followed practically continuously to a point on the northern coast of California a little beyond Point Arena, a distance of 190 miles. At this point the fault-trace as a continuous feature passes out to sea, and the evidence of displacement is lost. At Shelter Cove, in southern Humboldt County, however, where as previously stated the Rift features appear again, evidence of displacement due to movement on April 18 is also found. The doubt as to whether the Rift in Humboldt County is continuous with that which leaves the coast near Point Arena, of course also applies to the question of the continuity of the rupture on the day of the earthquake. If we assume that the line of rupture is continuous thruout, then its full extent from San Juan to Telegraph Hill is about 270 miles.
Beginning with southern Humboldt County, a somewhat detailed account will now be given of the phenomena of the displacement which occurred on April 18, 1906.
We are indebted to the observations of Mr. F. E. Matthes for our knowledge of the facts concerning the portion of the coast from Shelter Cove northward. The low headland north of Shelter Cove, known as Point Delgada, is traversed by several fissures trending roughly parallel with the general sweep of the coast and presenting essentially the same surface appearance as the fault fractures observed in Sonoma and Mendocino Counties. While it has been found impracticable to demonstrate by actual measurement the existence of a horizontal displacement along any of these new fissures — in the absence of fences or other objects of sufficiently defined outline — yet it has seemed warranted to regard them as true fault or shear fractures, to be classed in the same
The effects of a horizontal shear on thick grass sod in open country, as observed in a number of localities along the zone of faulting in Sonoma and Mendocino Counties, are as follows: On fairly level ground, where conditions are simplest and no vertical movement is evident, the sod is torn and broken into irregular flakes, twisted out of place and often thrust up against or over each other. The surface is thus disturbed over a narrow belt, whose width apparently varies with the magnitude of the displacement. Along the main fault, where the throw amounts to 10 feet or more, a width of 5 or 6 feet is not uncommon; on the secondary fractures, where the throw does not exceed a foot, the belt is generally only a foot wide. Whatever the width of the belt, the sod within it, as well as the unconsolidated material underneath, appears loosened up and not compact. It consequently takes up more space than before it was disturbed, and the surface of the belt is therefore slightly raised above the level of the ground, from an inch to a foot or more, according to the magnitude of the disturbance. Within such a belt there is seldom, if ever, a well-defined, continuous, longitudinal crack, the toughness of the sod precluding a clean shear fracture. Rather, there is a marked predominance of diagonal fractures resulting from tensile stresses.
To sum up, a horizontal displacement produces and may therefore be recognized in grassy country by a fault-trace showing:
- The disturbance of the sod over a narrow belt.
- The generally raised surface of this belt.
- The absence of a single continuous, longitudinal crack.
- The tearing of the sod along numerous diagonal fractures.
- The twisting and thrusting of sod flakes against and over each other.
It was mainly by the aid of these criteria that the fault lines in the vicinity of Shelter Cove were determined as fault or shear fractures, distinct from the innumerable cracks due to the settling of earth masses consequent upon the jar of the disturbance. In practically all of these the sod had been ruptured by mere tension, or tension accompanied by more or less vertical shearing. Furthermore, as will presently appear, the location of the fault fractures was in many instances facilitated by their association with the characteristic fault topography observed all along the line.
What appears to be the main fault-trace was first observed in the bottom of Wood Gulch, where it runs immediately along the wagon road for a hundred feet or more. It was thence traced south to its southern terminus on the beach of Shelter Cove, and then north across Humboldt Creek up Telegraph Hill. Subsequently several apparently detached lines of a similar character were discovered in the neighborhood of the main fault, as shown on the sketch map. Beginning at the south end, this line may be traced as follows:
On the beach of Shelter Cove, 100 yards west of the frame hut of Snider (at the mouth of Deadman Gulch), the fault passes thru the bluffs obscured by dislocated masses of dark conglomerates. From the top of the escarpment, however, it is easily traced for some distance down. The approximate contour map of the fault (fig. 10) sufficiently describes the topography here. A notable feature is a small elongated pond on the steep hillside, walled in by a small ridge. Thru this the fault-trace passes longitudinally. Continuing north, the line remains east of the upper valley of Wood Gulch until it joins the wagon road in the bottom. The line is by no means straight, as the bearings on the map indicate. A pronounced angle in its course exists at A (fig. 10), and the coincidence in this change of azimuth with the abrupt topographic change at this point is strongly suggestive of a hade to the west. Near the loop in the road at B the fault is easily recognized, except where the road has been repaired. The fault-trace here passes thru a
Of the auxiliary cracks, the first one, C (see fig. 11), is a less pronounced disturbance than the main fault-trace, passing thru a depression bordered on its east side by a low scarp due to former faulting. A small pond encircled by the road lies on this fault-trace. Its bearing (not measured) is such as to make the line converge toward the main fault-trace and intersect the same in the vicinity of the pond in the bottom of Wood Gulch. The horizontal displacement along this line is probably small, much like that on the
In search of the north end of the fault, the following itinerary was made: From Telegraph Hill northeast to the old ridge road, to Albert Boots' ranch, thence up King's
Finally, an excursion up the coast to Cooskie Creek and then south along the beach to Shelter Cove served to encompass the entire area under investigation. A short side trip was made up the creek flowing from King's Peak, but nothing definite could be learned regarding the location of the fault. South from Hadley's ranch at Big Flat, the precipitous mountain slopes have been destroyed by extensive and high landslides, the dislocated materials of which have frequently advanced out upon the beach in the form of glacier-like tongues. The waves at high tide have since nipt these protruding masses and truncated them at their ends. Many of the slides occurred apparently on the sites of older ones. Their continuity and extent made the discovery of the fault in this neighborhood impracticable. The prevalence of great slides along the coast, back inland, seems to suggest the possibility of the fault curving along the coast, and gradually leaving it south of the Big Flat Ranch. In the belief that this might be the case, and that the fault might continue closely along the coast for some distance, to reënter farther north, a visit was made to the great slide at Cape Fortunas — the most extensive slide along the north coast. No trace of the fault could be discovered here, however. No visit was made to Cape Mendocino nor to Needle Rock, a small promontory south of Shelter Cove. As seen from the cove, this rock has a pronounced saddle suggestive of faulting. Should the fault-trace run thru it, it would have a very strongly curved course, parallel with the coast.
Mr. Matthes' account of the conditions in the vicinity of Shelter Cove may be supplemented by the following note by Professor A. S. Eakle:
Shelter Cove appears as a broad slope spreading out and forming a circular coast line of about 2 miles in length, with a flat plain 6 to 10 feet above the sea. The ocean is constantly wearing away the land and no beach surrounds it. Half a mile from the ocean the land begins to rise in grassy hills which are abruptly cut off from the high mountains behind by a deep canyon. The formation of the cove indicates that it has been broken off from the hills above by a huge landslide, perhaps by a former earthquake. The gorge which separates it from the mainland is on a line with the general coast. On the south side of the cove there are three parallel deep gorges which extend a short distance into the hills; and their continuation over the hills is shown by slight depressions which appear to have been clefts which have become almost filled with the wash of the hills. Along all these lines of weakness fissures were opened and the ground subsided 2 to 3 feet. Cross fissures running from one depression to another are also present. The trend of the main fissures followed the coast, which is northwest-southeast. On the high crests of the Cooskie and King Mountains, which border the coast north of the town, fissures and landslides were reported by ranchers looking for cattle, but this region was not visited. In the range south of the cove landslides were also reported and a photograph of a large one was taken. The rocks of the coast are sandstones and black shales, and the hills and plain of the cove were composed of blue and yellow sandy clay, evidently derived from the decomposition of the shales.
Point Arena to Fort Ross
For the course of the fault and the phenomena of earth movement along it for the stretch of 43 miles between the mouth of Alder Creek and the point on the shore south of Fort Ross where it passes beneath the Pacific, we are again indebted to Mr. F. E. Matthes, who, on behalf of the Commission, made an examination of this territory. In the vicinity of Fort Ross, however, several observers contributed notes as to the phenomena there. For this entire distance, the rupture of the ground and its differential displacement are strongly marked and, except for the occasional local obscuration of the phenomena by brush and timber, are easily traced. The fault-trace enters the shore less than half a mile north of the mouth of Alder Creek and crosses with a course of S. 28° E. the bench-land, or wave-cut terrace, to the banks of the creek about 500 feet in from its mouth (fig. 12). Over the surface of the bench it is marked by characteristic rending and heaving of the sod. At the point where it reaches Alder Creek, the stream bank is rocky and steep, and the course of the crack can be traced down the rocky bluff, tho somewhat obscured by talus. The face of the bluff is shown in plate 32A. On the edge of the bench above the stream cliff (B, fig. 12), there is a rocky knob projecting above the general level. The earth crack passes close to the southwest side of this knob. The hade of the crack on the face of the bluff for a height of about 50 feet is very nearly vertical, but its deviation, if any, from the vertical could not be accurately measured on account of the ragged character of the bluff and the loose rock upon its face. On the northeast side of the rocky knob above referred to, there is evidence of a less well-marked parallel crack, as indicated on the sketch (fig. 12). This also appears on the rocky bluff of the stream cliff, but is less distinct than the main crack.
Southeast of this point, the fault-trace follows the broad stream bed of Alder Creek for nearly a mile, passing beneath a bridge, the wreck of which is shown in plate 32B. In this view, the horizontal offset of the bridge along the fault-line is well shown. It is apparent that this offset is not less than the width of the bridge. On the southwest side of the stream, near the bridge (A, fig. 12), the fault-trace is flanked by peculiar, isolated, rocky knobs similar to that on the northeast side. It is not clear, however, that these rocky knobs have more than an accidental relation to the fault, since they may possibly be residual sea-stacks upon the uplifted wave-cut terrace.
After leaving Alder Creek, the phenomena of surface rupture and displacement were traced thru a series of ranches to the divide passing over to Brush Creek, and down to the vicinity of Manchester. Along nearly this entire distance between Alder Creek and Brush Creek, the line passes thru a series of depressions, swamps, and ponds, the majority of which are not connected with the neighboring streams. Offsets due to the displacement were measured on two fences of Mr. E. E. Fitch's ranch, and the amount of movement was found in each case to be 16 feet, the southwest side having moved relatively toward the northwest. The vertical displacement was, as a rule, quite small; only in a few places did it amount to a foot, presenting a low scarp of that height facing the northeast. To the north of Manchester, an east-west fence line was offset in three places,
Southeasterly from Manchester the line of dislocation passes over the dividing ridge between Brush Creek and Garcia River, presenting the same general features. The upthrow is distinctly on the southwest side, but amounts, as a rule to only a few inches. The horizontal displacement was measured on a line fence south of the divide. The fence is offset in two places. The principal displacement amounts to 13 feet; while on the minor offset, a little to the east the displacement is 2.5 feet. The relative movement in both offsets is in the same direction, making the northwesterly displacement of the southwest side 15.5 feet. This fence is shown in plate 33A. South of this divide the main fissure passes thru a depression immediately east of a prominent knob projecting south from the divide; while a subordinate fissure traverses featureless hillsides from 100 to 150 feet farther east.
For some distance up the Garcia River from the point where the Rift intersects if the line of dislocation traverses the flat alluvial bottom land, crossing and recrossing the stream bed. At David Jones' ranch it leaves the bottom and ascends obliquely the side of the valley; and from this point to its head waters it remains on the western side of the valley. Its path is thru a belt of ridges and swamps. Part of the way there are two sets of ridges, the lower or eastern of which coincide with the new line of rupture. Looking along the valley, the more prominent of these ridges appear as notable features of the transverse profile. Opposite Hutton's ranch the line is found about 500 feet west of the river, and about 60 feet up on the valley slone. It crosses a road and fence here, producing offsets of 10 feet in both in the same sense as before noted. At the head of the Garcia River, the fault-trace passes thru a low saddle into the valley of the Little North Fork of the Gualala River.
Down the Little North Fork, the fault-trace follows the axis of the valley on its west side; at a point 1.5 miles north of its junction with the North Fork it runs lengthwise for over 100 yards with the grade of an abandoned logging railroad, tearing the same to pieces. Interesting evidence of the condensation or shortening of the ground in this vicinity is afforded by the buckling of the rails as seen in plate 33D. Here the main line of dislocation lies about 100 feet to the east of the track in the stream bed. The effect of the movement was to shorten the steel rails either by buckling or telescoping after the snapping off of the fish plates. The small trestle in the distance is traversed at an acute angle by an auxiliary line of dislocation and is similarly shortened. At the locality shown in plate 33C, the buckling in the foreground resulted in the breaking of the rails. Similar instances of the shortening of the steel are shown in the distance. Here the main line of dislocation of the earth lies about 50 feet to the east of the track and parallel with it. Plate 33B is a nearer view of the trestle above referred to. Below this point the fault-trace lies in the stream bed for some distance crossing the North Fork at a point 200 feet east of its junction with the Little North Fork. Two lines of faulting appear here, both of which caused considerable damage to the railroad track; but the latter having been repaired before the date of Mr. Matthes' visit no measurements of offsets were obtainable.
From this point southeasterly, evidence of dislocation along the line of the Rift, in its course up the valley of the South Fork of the Gualala, is obscured by the dense brush to
The upthrow on the fault east of Stewart's Point is on the west side; but farther up stream, where the fault runs along the steep west side of the valley below Casey's ranch, the upthrow is apparently on the east side. The foot trail from Casey's ranch to the river follows a marked longitudinal depression in the steep slope for 100 feet, and it is along the abrupt west side of the small ridge flanking the hollow (see fig. 14) that the fault-trace is located. The upthrow measures fully 2 feet, while the height of the ridge above the hollow varies from a few feet to more than 10 feet. The depression pitches to the north and is drained by a tiny brook. The fault-trace happens to coincide with the latter at a point where the trail crosses the watercourse over a rough wooden bridge. The horizontal movement along the fault practically destroyed the bridge. No measure of the displacement could be obtained here, but the indications are such as to warrant the belief that it did not amount to 15 feet, and that probably some of the horizontal shear had been distributed over minor lines of displacement higher up the slope, and marked by landslides. These landslides above the depression in which the fault-trace lies are an important factor in the interpretation of the phenomena. It is easily possible that the scarp looking southwesterly over the depression referred to does not represent the real movement on the fault plane, but is a landslide effect. In any event, the proximity of the landslide weakens very much any judgment that might be formed, implying a reversal of the vertical movement which normally prevails along the line of the fault.
From Casey's ranch southeast, detailed observations were found to be impracticable owing to the dense tangle of brush and fallen timber. The ridge between the upper stretch of the river and the coast is crost by the fault-trace thru a swampy saddle above Plantation House, and the fault-trace traverses the swamp. Plantation House stands practically on the line of disturbance, about midway in a zone 270 feet wide traversed by six roughly parallel lines of rupture. The general bearing of the principal line was found to be N. 38° W. Southward the main fault passes thru a series of swampy hollows along an abandoned road, now impassable because of the cracks thru it. The line was traced south for 2 miles, its general appearance being found to remain the same thruout. There is a marked upthrow along its west side, not exceeding a foot at any place. Where
At Buttermore's ranch, about a mile east of Timber Cove, the displacement is distributed over three fissures, the principal one running 30 feet west of the dwelling. It intersects three fences, all of which show offsets of about 8 feet. The original crookedness of the fences and the repairs made since the earthquake make the accurate determination of the displacement impossible. The fault-trace was followed for some distance south and north from the ranch thru the forest, and found to follow the swampy depressions most of the way with low scarps or ridges to the west. The ranch and its fields lie for the most part in a broad swampy saddle. The upthrow in this neighborhood is on the west side, not exceeding 15 inches anywhere.
North and south of Fort Ross, the phenomena of displacement are well displayed, both on the open-terraced coastal slope and in the timber. The rupture follows for the most part a single well-defined line in the path of the old Rift, coinciding in many places with ancient scarps and the slopes of low ridges. (See plates 35A and 36D.) The fault-trace is commonly marked by a ridge of heaved sod with diagonal cracks as illustrated in plates 35B and 36B. New scarps occur as shown in plates 36C, and 38A, B, as well as accentuations of old scarps. There are, however, several subparallel cracks. Two of these, having each a length of about 150 feet, lie to the west of the main line at a point 1,250 yards northwest of Doda's ranch-house, one 50 and the other 100 feet distant from the main crack and disposed en échelon. Within 300 yards to the southeast of this are two short cracks still closer to the main one, and springing from it, at about 450 yards northwest of Doda's ranch-house, is a parallel crack 440 feet in length and 60 feet from the main line. In this case the scarp appears upon the auxiliary crack, and not upon the main line of rupture. Between the short discontinuous crack and the main line is a swampy depression. On the southeast side of the ravine, southeast of Doda's house, the main crack is paralleled by two subordinate cracks, one on each side. That on the southwest side is about 250 feet long and 50 feet from the main line. It has a low scarp facing northeast, but not so pronounced as that on the main line of rupture. The crack on the northeast side of the main line has a length of about 1,125 feet and converges upon the latter toward the northeast. At its northwest end it is 190 feet from the main crack and at its southeast end only 50 feet distant. It has a low, discontinuous scarp facing northeast.
In a distance of 7,250 feet measured along the line of the fault, there are twelve stretches of scarp ranging in length from 125 feet to 1,000 feet, counted both on the main and on the auxiliary cracks and aggregating 3,000 feet in length. Of these eight face northeast and four southwest. The eight scarps facing northeast aggregate 2,250 feet in length, while the four facing southwest aggregate 750 feet. Two of the southwesterly facing scarps, however, aggregating 375 feet in length, are on the descent to the ravine southeast of Doda's house, where there is considerable sliding of the ground, and they may possibly be accounted for as secondary features due to landslides. The other two scarps facing the southwest are unexplained. They are abnormal and are so exceptional that they scarcely weaken the general conclusion that the vertical component of the movement on the fault was upward on the southwest side. The amount of this vertical movement in the vicinity of Fort Ross probably does not exceed 3 feet. In the first hasty examination of the ground, it appeared as if the amount of vertical movement might have been as much as 4 feet. This impression was due to the fact that in places preëxisting scarps were closely followed by the fault-trace, and a sufficiently careful discrimination was not made between the proportion of the scarp due to the new displacement, and that due to
The distribution of the line of faulting for a typical stretch of the Rift near Fort Ross, the auxiliary cracks, the disposition of the scarps upon these, and the relation of the whole to the old Rift features, are well shown on map No. 3 by Mr. F. E. Matthes. The horizontal displacement is also indicated on the map, but this needs more detailed statement.
On the line of the fault, about 300 yards northwest of the road from Sea View to Fort Ross, a steel water-pipe was dislocated by the earth movement, and found to be offset 8 feet, the southwest portion having moved northwesterly. This of course affords only a minimum measure of the relative movement. Where the road just mentioned intersects the fault-trace, both the road and the bordering fences were offset about 7.5 feet, with a slight sag on the northeast side. The zone of shearing here was from 10 to 20 feet wide. A wagon road on the Call ranch, south of the one above referred to, was offset 12 feet 3 inches, the line of dislocation being marked by an open fissure in the soil a few feet deep, and several short diagonal cracks, as shown in plate 36C. Another offset fence is shown in plate 36A, the displacement being here 8 feet at the fault-trace. The effects of the earth movement in the timber to the south of this are well shown in plate 34A. Several large trees standing on the fault line were split or torn asunder. The offset of the south line fence of the Call ranch was carefully surveyed by Mr. E. S. Larsen, and the results of his survey are shown in fig. 15. The bearing of the fence is N. 36° E. He reports that for the first 1,000 feet from the southwest end of the fence the greatest error in alinement was about 1 inch, and that practically there was no deformation in this stretch. In the next 125 feet going northeast there was found a deviation from this alinement of 4 inches to the southeast. In the next 50 feet the deviation in the same direction was 7 inches more. In the next 140 feet the deviation in the same direction was 3 feet 7 inches more. Then came the fault-trace with an abrupt displacement of the fence of 7 feet 5.1 inches. Northeast of the fault-trace the fence retained its line very well. In 100 feet it was out only 1 inch. Beyond this it enters the timber and its course is somewhat influenced by the larger trees, but maintains its line within a few inches. These measurements give a total horizontal displacement of 12 feet distributed
Bodega Head to Tomales Bay
The location of the fault across the neck of land which connects Bodega Head with the mainland was determined by Prof. J. N. LeConte. He reports that on the south side of this neck the main earthquake fissure was found passing about 50 yards west of a house occupied by Mr. Johnson. It could be traced as a multitude of small cracks in the swampy land from the bay to the road, then as a well-defined fissure up the small depression west of the house for 200 yards to where it disappeared in the sand dunes. No trace of it could be detected in the sand dunes, which reach from this point entirely across the peninsula. Only one fence crosses the fissure and this had been repaired so that no measurement of the displacement was possible. The movement was evidently northward on the west side as was shown by the direction in which the bushes were bent. The vertical movement was about 18 inches, the uplift being on the west side. The sand spit which closes the bay on the south was examined for evidence of movement, but nothing could be detected in the drifting sand.
At the mouth of Tomales Bay there are two points projecting westward from the east shore, and both of these, according to the observations of Prof. R. S. Holway, are crost by the fault-trace. The first is a long, flat sand-spit extending well across the mouth of the Bay just south of Dillon's. The line of the fault was still visible in the sand on June 11, 1906, in spite of the obliterating action of the wind and the recent rains. The line lies near the base of the spit and has a northwest-southeast course. On each side of the crack are crater-like depressions, some of them being double or overlapping. Mr. Keegan, the owner of Dillon's Beach, reported that these craterlets were numerous and distinct. In some instances a great deal of sand and water had been ejected. Others are reported on the southwest side of the fault-trace, from which the belt containing them extends some 70 feet. The craterlets vary in size up to 6 feet in diameter and it is reported that on the day after the earthquake the water which stood in them could not be bottomed by a fishing pole.
About 1.5 miles southeast of this spit is a promontory about 100 feet high projecting into the bay. Some 400 yards from the end of this promontory on top of the ridge is a line of depression with two or three small ponds. The main fault fissure here divides into two cracks, one each side of this depression, which is about 150 feet in width. Standing on this ridge, the line of the fault can be traced at low tide for nearly 1.5 miles across the bottom of the bay to the sand-spit to the northwest, its course in general being parallel to the axis of Tomales Bay. (See plate 38C.) The horizontal displacement where the fault crosses the promontory is about 8 feet, as determined by the line of tall grass at the edge of the little ponds, the westerly side having shifted to the northwest.
Tomales Bay to Bolinas Lagoon
By G. K. Gilbert
The Fault-trace.—The trace traverses the zone of the Rift. Its general course is N. 35° W. and it nowhere departs more than a few hundred feet from the straight line connecting its extreme points. For considerable distances it is a single line of rupture; elsewhere it is divided into parts which separate and reunite; and in yet other portions it is composed of unconnected parts arranged en échelon. There are no vertical sections exhibiting hade, but the relation of the trace to sloping surfaces indicates that the fault-plane is approximately vertical.
For considerably more than half its length the surface expression is a ridge from 3 to 10 feet wide and ranging from a few inches to about 1.5 feet high. (See plates 37B and 40A.) The ground constituting the ridge is in fragments, loosely aggregated, so that there are considerable voids. Where pasture lands are crost the turf is torn into blocks, and these, in conjunction with the cracks which separate them, make up a pattern. This pattern is always irregular and sometimes gives no evidence of system, but usually its lines have a dominant direction, traversing the ridge obliquely, the northern ends of the cracks pointing toward the eastern boundary of the ridge, and the southern ends toward the western boundary. The cracks have resulted from stresses connected with the horizontal faulting, in which the southwest block moved northwest with reference to the northeast block. (See plate 39.)
In other places, and usually for short distances, the surface expression is a shallow trench (plates 40B and 46B), with ragged vertical sides from 2 to 5 or 6 feet apart, and occupied by loosely aggregated fragments of the ground, the pattern of the fragments and interstices being similar to that observed in the case of the ridges. This phase suggests that just below the surface the fault may be somewhat open, so that there has been an opportunity for fragments to drop into it.
In a third phase the ground is not notably elevated nor deprest but is traversed by a system of cracks obscurely parallel one to another and making an angle of about 45° with the general direction of the trace. Their orientation is such that they run nearly north and south. The cracks do not meet, but leave the intervening strips of ground in full connection with the undisturbed ground outside the trace. This phase occurs chiefly in wet alluvium.
There are a few spots where for short distances the surface expression is a simple straight fracture along which horizontal motion took place.
In the detailed descriptions which follow, the first three phases described above will be spoken of as the ridge phase, the trench phase, and the echelon phase.
The most southerly observation of the fault-trace was on the spit separating Bolinas Lagoon from the ocean. Near the west end of the spit its surface is covered by small dunes, and among these the trace was seen in its echelon phase. After a lapse of nine months the drifting of the sand had obliterated most of the cracks, but a few were still visible. Inside the spit lie a number of islands, the largest of which, Pepper Island, has a nucleus of sand (the vestige of an ancient spit), but superficially consists mainly of a fine tidal deposit. In the earlier field excursions the fault-trace was here overlookt, the echelon cracks by which it is represented being mistaken for secondary cracks, but at the present time (spring of 1907) it is easily traced, even from a distance, because the vegetation on the two sides of it has acquired different colors. Unfortunately the camera does not discriminate these colors. (Plate 41A.) The echelon phase here dominates, but the ground east of the trace is deprest about a foot, and this depression has so changed the relation of certain plants to the tides that they now find the conditions of life unfavorable and are dying out. This matter will be considered more fully in another connection.
In the U. S. Geological Survey map of this region the islands are not represented. In fig. 28 they are represented as they appear at half-tide, or, more strictly, the parts shown are those covered by vegetation. This figure also shows the corresponding part of the delta of Pine Gulch Creek. After crossing Pepper Island and a smaller island immediately adjacent, the fault-trace disappears under the water of the lagoon and it was next seen on the mainland of the southwest shore near the head of the lagoon. In the interval it probably crosses the delta of Pine Gulch Creek between the lines of high and low tide, but this tract was not examined until after the floods of March, 1907, which overspread it with alluvium. A disconnected group of cracks opening in the alluvial plain of the creek about 400 yards to the west (plate 39A) probably marked the position of a divergent branch of the fault. This line of disturbance crost the creek and road near the bridge in the northern settlement of Bolinas, trending approximately north and south and fading out in both directions.
The trend of the fault-trace on Pepper Island is about N. 34° W., and if continued would bring the trace to the shore at the head of the lagoon, but its actual position on the mainland is farther west, indicating that there is either a swerving or an offset in the part not seen. Near the shore the fault occasioned a number of landslides which obstructed the road until removed; and beyond the confusion occasioned by the landslides the trace consists of a number of subparallel cracks occupying a belt several yards in width. There is also a nearly parallel branch of the trace in a fault-sag lying a little farther west, but this could be followed only a short distance, and has since been largely obliterated by plowing. Mr. Nunes, who cultivated this sag, states that it once contained a pond or marsh, and this he had drained, but the water stood there again after the earthquake, showing that the earthquake had caused a depression of the bottom of the sag.
The diffused cracks on the main line soon gather into a narrow belt and descend into a narrow sag, containing the barn and other farm buildings of the Steele place. After following the sag for a short distance, the trace gradually rises on its eastern wall, crosses obliquely an intervening ridge, and enters a parallel sag toward the east. In this sag, which also is narrow, the trace intersects one of the roads leading from Bolinas to Woodville and immediately begins to ascend the narrow ridge bounding the sag on the east. Crossing this ridge obliquely, it skirts for 0.25 mile the western border of the much broader sag in which the water of Pine Gulch Creek gathers before it enters the canyon from which it is named. This wall it descends obliquely, and, just before reaching the bottom of the sag, intersects and offsets a line of eucalyptus trees marking a property and township boundary. The ridge phase dominates in this region (plate 37B), and near the line of eucalyptus trees the trace itself has a small offset to the west. (See fig. 18.)
Now for nearly half a mile the trace follows a valley-bottom, being divided on the way between two or three branches. The ridge phase obtains, but there are several places in flat alluvial ground where the ordinary group of cracks is replaced by a single crack with clean shear. On Mr. Strain's place two fences were crost which afforded measurement of horizontal displacement. Beyond them the fault-trace becomes once more single, and, after passing a group of very small ridges and sags, begins to climb the eastern wall of a larger sag, which here contains Pine Gulch Creek. (See plate 41B.) Along its line there soon develop a small sag and ridge constituting a sort of shelf or notch on the wall of the deeper sag (plate 42A), and in this small sag are several ponds. (Plates 10A, 54A, and 55A.) The sag first rises for a distance and then gradually descends. The fault-trace exhibits here in alternation the ridge and trench phases, and at many points there is an apparent vertical displacement with throw of 1 or 2 feet toward the northeast. (Plates 10B and 48B.) Near Bondietti's house the individuality of the sag is lost, and the fault-trace swerves somewhat to the east. A parallel trace develops west of it, and the two come together near Beisler's place. Northwest of Beisler's is a relatively high fault
Thence the trace descends obliquely to the canyon of Olema Creek 150 feet below. Where the creek makes a decided bend toward the west the trace crosses it twice, and then follows near its west bank for several miles. Not far from the second crossing it is subject to a series of offsets, giving to the trace as a whole the same echelon character commonly observed in the arrangement of its details. It is noteworthy that where these offsets occur the trace swerves somewhat toward the right and the new line begins at the left, so that the arrangement is essentially a magnification of the arrangement of cracks in what I have called the echelon phase of the fault-trace. There is this difference, however, that the elements of the larger echelon make a comparatively small angle with the general course of the trace. At several points in this part of its course the trace follows steep slopes from which the timber has not been cleared. On these slopes, which face the northeast, its course sometimes coincides with that of a very narrow sag occupied by marshy ground. Elsewhere it crosses an upland to which a series of sags gives gentle undulation and here it touches or passes near a number of ponds. (Plate 43.) The route of the fault-trace in this region and thence north to Papermill Creek is shown by fig. 16, a compilation based on data from several sources, including a few original measurements.
A mile south of the village of Olema the trace enters a sag which is followed continuously for nearly 3 miles. At first the sag is narrow and is attached to the northeast slope of a ridge, but approaching the Shafter place the ridge crest sinks and a broad sag replaces it in the line of trend. (Plate 42B.) In following the eastern edge of this sag from the Shafter place to Papermill Creek the fault-trace also follows the western base of a line of hills. The hills are peculiar in that their western, or more strictly southwestern, base, being determined by faulting, is nearly straight (plate 42B); while their northeastern base, modified by the erosive action of Olema Creek, is scalloped. In this region the ridge phase of the fault-trace dominates, being occasionally replaced by the trench phase. (See
Just south of the head of Tomales Bay, Papermill Creek enters the valley from the east, crosses to the southwest side of the valley and then turns toward the bay, in which it has built a delta. The delta occupies the whole width of the bay and is about 3 miles long, the greater part of it being submerged at high tide. At the head of the delta Olema Creek joins Papermill, bringing its tribute of detritus; and on the opposite side of the valley Papermill Creek receives the water of Bear Valley Creek, which brings no sediment but filters for some distance through a marsh. At the head of the delta a road crosses the valley, resting partly on the delta and partly on the marsh just mentioned, and furnished with an embankment to lift it above the floods. Just before reaching this road the fault-trace enters the marsh, where it quickly expands to a width of nearly 60 feet and exhibits the trench phase. Not only was the road offset by the fault and earthquake, but the portion between the walls of the trench was dropt down, the embankment sinking into the soft earth until nearly flush with the marsh. In restoring the embankment about 3.5 feet of earth were added. Close to the road Papermill Creek was crost, with offsetting of banks, and thence the fault was continued thru the delta to its end. (See plate 46A.) Its course is nearly straight and of such direction as to pass just outside the end of the cape near Millerton, the bearing being N. 35° W. At several points it is margined on the northeast by a lane of water (plate 45A), indicating that a narrow tract on that side is deprest, but no evidence was found of a general depression of land on one side of the fault as in Bolinas Lagoon. The echelon phase is dominant; the ridge phase does not appear. The trench phase obtains for short distances, and is combined for larger distances with the echelon. Where the trench phase occurs, it coincides with the zone of abundant cracks and is thus distinguished from the sag holding the lane of water.
The general relation of the sag to the fault-trace is shown in fig. 17. It occurs only on the northeast side, but is so persistent that, from a commanding position, the fault can be traced out by means of the water lanes. The depression will probably average more than 50 feet wide, but it eludes measurement because it fades out gradually on the side away from the fault. The greatest noted depth is 17 inches, but the average is probably less than a foot. In attempting to interpret this feature I assume that beneath the smooth plane of the delta, and buried by its soft deposits, is a variegated topography of the rift type; and the hypothesis I advance is that the new-made sag on the delta plain is the surface echo of a fault-sag of the buried topography which was made deeper by the event of April, 1906. It has already been pointed out (page 67) that the sags of the Rift which were touched by the new fault were apparently deepened; and if the true explanation of the delta-sag has been suggested, we have in that feature an indication that the deepening was not only apparent but real.
At the northern edge of the Inverness settlement is an outlying or branch fault-trace about half a mile long. (Plates 45B and 47A.) Starting in what is called the "Second Valley," it ascends to a mesa and then descends toward the "Third Valley," its course being about N. 20° W. In crossing the upland it is associated with a fault-sag and there exhibits the trench phase with horizontal displacement of 2.5 feet. Two shorter traces, trending northward, occur on the slope between the "First Mesa" and the "Second Valley."
Measurements of throw.—At all points where horizontal throw was observed, the ground at the southwest, as compared to ground at the northeast, moved northwestward. On Papper Island in Bolinas Lagoon a horizontal displacement was shown by jogs in the directions of the south coast, of the limit of vegetation at the north, and of a well-defined change of flora dependent on the relation of land levels to tide. These various features are too indefinite to give value to measurement of offset, but the general indication is that the amount of throw is somewhat larger on the island than at the nearest points of measurement on the mainland.
A mile northwest of the head of Bolinas Lagoon the fault-trace intersects a row of eucalyptus trees which had been set to mark a property line, the boundary between lands of S. S. Southworth and S. McCurdy. The row is now both dislocated and curved, and as there is reason to believe it was originally alined with care, its present condition shows the distortion of the ground at the time of the earthquake. The fault-trace, as shown by the accompanying map (fig. 18), is here offset en échelon, and the row of trees is not only crost by one section of the trace but approached by the other. At the point of crossing the dislocation is 10 feet. On the northwest side of the fault are six trees, all in line. On the southwest side are a dozen or more trees of which all but three are in line. If the line of either straight division be projected across the fault (broken lines in map) it passes 13.5 feet to the left of the line of the other division. The three trees nearest the fault on the southwest side follow a gently curving line. The indication is that about three-fourths of the whole displacement occurred on the main plane of the fracture, and the remainder was diffused thru the ground adjoining on the southwest. A closely related condition exists at the southern limit of the same field, where the fault-trace intersects a fence at right angles. The offset is 7 feet 8 inches, and this is accompanied by a change of direction. (Fig. 19.) Unfortunately the fence is too short to indicate in full the changes of the ground, but the suggestion is that in addition to the visible offset, there is a diffused shear affecting the ground southwest of the fault so that the entire displacement is greater than the amount shown by the offset. Assuming the fence to have been originally straight, the total displacement here was more than 12 feet.
On Mr. E. R. Strain's place, west of Woodville, measurements are afforded by the disturbance of two fences. The more southerly (fig. 20) is crost by two visible branches of the fault, and there is probably more or less diffused shear in the intervening ground. The fence, said by Mr. Strain to have been originally straight, has now two straight portions, AB and CD, and the distance from AB to E, on the line of CD produced, is 15 feet. The second fence, standing a little farther north, is intersected by one visible fault-trace, the continuation of the trace which crosses the first fence near B. On this line the fence is broken and offset 8.5 feet. The remnant of fence to the southwest is straight, but swerves in approaching the fault-trace, as indicated in fig. 21 and in plate 49A. The total displacement of the straight portions of the fence is about 11 feet.
The four localities last mentioned are included in the space of 0.5 mile. Their several indications of the total displacement, in the order of position from south to north, are 12 +, 13.5, 15, and 11 feet. The range of these determinations is 4 feet and their approximate mean 13 feet. At each locality the indicated displacement consists partly of definite faulting along one or two planes of fracture, and partly of diffused shear, distributed thru a belt of rock, or at least a belt of soil. At each locality the indicated shear is all in one direction. At each locality the measurement depends for its authority on the assumption that the disturbed fence or row of trees was originally straight.
Eight miles farther north, at Mr. W. D. Skinner's place, near Olema, the entire fault is apparently concentrated in a single narrow zone, and the several measurements made are in close accord. The fence south of his barn (fig. 22) was offset 15.5 feet. The barn, beneath which the fault-trace past, remained attached to the foundation on the southwest side, but was broken from it on the northwest side and dragged 16 feet. A path in the garden, originally opposite steps leading to the porch, was offset 15 feet. (Plate 38D.) A row of raspberry bushes in the garden was offset 14.5 feet. The mean of these four measurements is 15.25 feet, and their range is 1.5 feet.
The road running southwest from Point Reyes Station and crossing the valley at the head of Papermill Creek delta was offset 20 feet. (Fig. 23 and plate 47B.) As the fault-trace at this point was between 50 and 60 feet wide, and as the embankment of the road for that distance was broken into several pieces, it was not possible to make certain that the dissevered remnants of the road had originally been in exact alinement. It is probable, however, that the road was approximately straight before the earthquake, and that the exceptionally great offset at this point is to be explained as the result of a horizontal shifting of the surface materials. The embankment of the road rested on marshy ground so soft that a portion of the embankment sank into it, and material of this character was in other localities demonstrably shifted.
A number of other measurements of displacement were made, but these, for various reasons, do not seem worthy of record, altho some of them were noted in an earlier report. Several were connected with the dislocation of trails, but in every such instance the trail made only a small angle with the strike of the fault and part of it was broken up along with the fractured turf. The endeavor to find more favorable angles of intersection drew attention to the fact that because the dominant trend of hills and valleys in the Rift is northwest-southeast, the lines of easy travel, minor as well as main, are largely parallel to the fault-trace. Other measurements were connected with the offset of fences, and, altho definite in themselves, have little value because there is reason to believe they represent only a part of the local displacement. The part represented by them is in every case less than 10 feet. It is noteworthy in this connection that most farm fences which were intersected by the fault-trace either terminated within a few yards of it or changed direction at about that place. Like the trails, they were adjusted to topographic peculiarities created by earlier faulting along the same line.
The phenomena of vertical displacement are in general so irregular as to indicate that they were determined chiefly by surface conditions. Where the ground sloped toward the northwest the horizontal throw caused an apparent vertical downthrow to the northeast. (Plate 48A.) Where the ground sloped toward the southeast an apparent vertical throw to the southwest was produced. Where the fault-trace followed a narrow sag interrupting the side slope of a ridge, the apparent vertical throw was on the side toward the ridge, as indicated in the diagram, fig. 6. (See also plates 10B and 48B.) The only unqualified record of vertical displacement is on Pepper Island in Bolinas Lagoon, where the mean of seven measurements shows a downthrow of 12 inches on the northeast side. The question whether the faulting along the plane of rupture was accompanied by the elevation or depression of large areas will be discust in another place.
Movement normal to the fault-plane. — Where the fault-trace is a trench, imperfectly filled by fragments of soil and rock, it is clear that the walls of the fault stand farther apart than before the earthquake. Where the fault-trace has the echelon phase and consists of a system of cracks, not accompanied by visible elevation of the surface, it is also evident that the walls stand farther apart. Where the fault-trace is a ridge, composed of fragments of soil, with more or less interstitial void, it may be assumed that the voids are at least equivalent to the ridge in volume. As the fault-trace is made up almost wholly of these three phases, it follows that in the visible part of the fault its walls did not approach as a result of the faulting but receded a little.
In this connection mention may be made of the fact that at the Shafter ranch a fault crevice was momentarily so wide as to admit a cow, which fell in head first and was thus entombed. The closure which immediately followed left only the tail visible. At this point the fault-trace was a trench 6 or 8 feet wide, and the general level of the soil blocks within it was 1 or 2 feet below that of the adjacent undisturbed ground.
One suggestion in connection with the recession of the fault walls near the surface of the ground is that temporary stresses incidental to the faulting caused permanent compression of the adjacent terranes. It is a fact familiar to engineers that most superficial formations, while in their natural, undisturbed condition, have a structure involving voids, and that they may be comprest by overpowering this structure. But, if I understand the matter, such formations are not compressible (except elastically) when their voids are full of water, so that accommodation for dilatation of the fault-zone could have been made in this way only so far as the ground was dry. As the ground was full of water in many places — including,
Another suggestion is that the surface phenomena are essentially representative of what occurred at greater depths — that is, that in depth, as well as superficially, the faulting left the fault walls farther apart than they were before. Fissure veins show that voids have often resulted from subterranean faulting. Unless the surface along which the movement occurred is mathematically plane — or conforms to some equally difficult geometric condition — the two fault walls should not accurately fit together after the movement, but should tend to maintain contact thru only a part of their extent. If thru a part of their extent they are separated, the walls are on the average farther apart than before.
There would necessarily be some adjustment thru changes within the rock masses on the two sides of the fault. Compressive strains would be locally increased and reduced, and there would be subordinate movements among the minor earth blocks of the great shear zone whose surface features appear in the Rift. We have evidence of such adjustments, in fact, in branches of the fault-trace and in a system of bedrock cracks presently to be described; as well as in the subsidence of the bottoms of sags in the immediate vicinity of the fault. Interpreting other sag phenomena in the light of the long sag of the Papermill Creek delta, the fault of 1906 appears to have permitted a very considerable volume of material to sink into its fissure.
The general tendency of this discussion falls in line with a generalization as to the Rift, which in the Bolinas-Tomales region appears to show distinctly more local subsidence than local elevation.
Earlier fault-traces.—Because the future is to be judged by the past, there is much interest in the question of the frequency and recency of fault movements along the Rift previous to 1906. In my later studies of the Rift belt, I have had in mind the possibility of discovering fault-traces similar to that of 1906 but less fresh in appearance. In the little bluffs at the edges of sags, and in the ponds and marshes, there is abundant evidence of early faulting, but it is essentially geologic and does not necessarily pertain to occurrences of the past century or two. The fault-trace, however, is a relatively perishable and transient phenomenon, and its preservation might have comparatively definite meaning.
At two localities I thought I discovered old "traces" of the ridge type. In each case the features occur on a hill slope where the trace made in 1906 appears in several divisions or branches; and what I took to be old traces are distinguished chiefly by the absence of cracks. The localities are close together, about 0.5 mile south of the Shafter ranch, and may be identified by means of plate 43B. The features occur on the slope at the left, but are too indefinite to be recognized in the view. If these old traces have been properly identified, they are of very moderate antiquity. I should suppose that the ridges of the recent trace would lapse to such a condition in four or five years and that they might persist, under pasture conditions, for two or three decades. The history of the recent trace shows that a single plowing means effacement, but the general appearance of the field in which the old traces occur indicates that it was never plowed.
Cracks. — In preliminary reports I have classified the earthquake cracks as primary and secondary, the primary being occasioned by strains which existed before the earthquake, and the secondary being caused by the earthquake. With the multiplication of observations this classification has become increasingly difficult, and I now find it more convenient to group the cracks as superficial and deep, or superficial and bedrock.
Many of the superficial cracks are in alluvium. In the field excursions of April and May, 1906, they were seen in all alluvial formations within the Rift belt and for some distance on each side. The greater number appeared to be merely partings without
It is noteworthy that the neighboring road crossing a marshy portion of the Papermill delta was much less cracked, and the difference is probably to be ascribed to the difference in height and strength of the two embankments. The thinner one suffered the more.
The localities enumerated are merely those which came under observation. Within the zone of high intensity no marshes and no bottom lands were seen which did not exhibit cracks, and I regard their cracking as a general phenomenon. The elaborate cracking of a roadway across one of the marshes seems specially significant. In the adjacent soft marsh close attention was necessary to discover cracks. To a large extent they were concealed by the vegetation, and it is probable also that many which were opened during the earthquake agitation immediately closed again and were practically obliterated by the welding of the mud. But the road embankment, being free from vegetation and composed of comparatively rigid and brittle material, retained all the cracks made during the agitation, and thus served to record the thoro shattering of an unconsolidated formation when subjected to strong vibration. (Plate 50.)
Another class of superficial cracks affected hillsides, penetrating only the coating of loose material — decomposed rock and taius. the conspicuous individuals of this type are those that follow contours. Along these there was often a notable width of crack, accompanied by a settling on the down-hill side, and many cracks of this type are still visible. They are in effect the heads of incipient landslides and might with equal propriety be described under another caption. They are numerous thruout the Rift belt and fairly abundant on steep hillsides for more than a mile to the west. East of the Rift they are inconspicuous and believed to be rare. Some of the best examples are on the northeastern slope of Mount Whittenberg, about a mile from the fault-trace, the locality being favorable for observation because of the absence of forest.
Superficial cracks of a third type are connected with side-hill roads. (See plate 51.) In such roads there is usually a notch cut in the hillside and the excavated material is thrown outward so as to make an embankment. The roadbed thus consists in part of the natural formation and in part of an artificial and relatively loose embankment. In the loose material, and frequently along the line separating it from the firmer ground, cracks were extensively developed, often accompanied by evident settling of the outer bank. Their magnitude depended in part on the character of the material, but in large part also on the intensity of the earthquake. Where they were of such magnitude as to injure the roadway they were soon obliterated by road repairers, and elsewhere they tended to disappear in consequence of the traffic; but while they lasted they constituted an excellent gage of intensity, and much use was made of them in districts where there were few buildings.
Bedrock cracks occurred at many points within the Rift, usually appearing as branches from the faults. They were seen also at a number of points west of the Rift, their distribution reaching to the ocean in the vicinity of Point Reyes, ten miles from the faulttrace. At the more remote points they were quite small, often barely discernible, and no system of arrangement was discovered. They are peculiarly prominent along the summit of the ridge constituting the southwestern rim of the main Bolinas-Tomales trough. This summit was visited on four lines of road, and at each locality conspicuous cracks were found. On the road from Inverness to Point Reyes Post Office, about a mile in a direct line from Tomales Bay, a crack was traced for more than 800 feet. Its general trend is east and west, but its course is not straight and it has a branch diverging at 45°. Along this crack there is a horizontal throw of from 2 to 6 inches, the south side having moved westward with reference to the north side.
On the next road to the southward a group of cracks was seen at a point a mile from the shore of Tomales Bay. These cracks occur on a crest trending northwest and southeast, and their trend makes a small angle with that of the crest. The arrangement of the cracks suggests horizontal shear, but no definite obervation was made on this point. They extend for several hundred feet at least, but were not traced out.
On Mount Whittenberg there are two bedrock cracks. One of these crosses the northeastern spur of the peak near its junction with the main crest. Its trend is approximately northwest and southeast and at one point it margins a fault-sag. As it assumes in one place the ridge phase of the fault-crop, I infer that it has horizontal displacement. On the opposite side of the main crest is a crack which was traced for about 1,000 feet. Its general course is northwest-southeast, but it is not straight and exhibits a vertical throw of 1 or 2 feet to the southwest. At one point it touches a fault-sag. Between these two long cracks a group of short cracks occurred, with similar trend, on a knob constituting a portion of the main divide.
About 6 miles farther south, at the head of Pine Gulch Creek, another road crosses the range, and in following this a group of cracks was seen. A short distance west of the divide, and about a mile in a direct line from the fault-trace, is a fault-sag trending northwest-southeast. On each side of it a crack was seen, the eastern crack being the wider and showing a small throw to the southwest. This crack was traced for about 0.75 mile and found to curve thru an arc of nearly 90° from southeast to southwest. At its southwest end, or at least the southwestern limit of tracing, it is on a ridge, and it there expands into, or else is replaced by, a group of cracks diverging fan-wise. On each member of the group faulting took place, the downthrow being toward the northwest except in the case of two apparently short cracks with downthrow to the southeast. On four of these cracks the throw was greater than 1 foot, and at one place it was about 5 feet. Each crack was associated with a preëxistent bluff or scarp, indicating that earlier movements have occurred at the same place. The field in which the principal phenomena occur is cultivated with the exception of the steeper scarps, whose faces retain a bushy growth. (See plates 52A and 53A.)
A tract lying between this locality and the coast, and extending several miles in each direction, exhibits a peculiar topography intermediate in type between that of the Rift and that commonly associated with landslides. Near the coast are a number of basins with ponds or lakes of much larger size than those along the Rift, and in association with these are seen a number of sags similar to the fault sags of the Rift. On several lines which were thought from the physiography to represent partings between dislocated blocks, earthquake cracks were seen, and on one of these near the coast there was a vertical displacement of 3 feet, the downthrow being to the southwest.
All thru the Rift there is association of earthquake cracks with fault-sags; probably half of the sags were bordered by such cracks on one side or the other, the crack usually
Springs.—The general testimony of residents is that the flow of springs was modified all thru the peninsula west of the Rift. As it was practically impossible to get quantitative data, I made few records of specific instances, but every farm owner or farm tenant of that region with whom I talked told me of some spring whose flaw had been increased, diminished, or stopt at the time of the earthquake, the change being either temporary or permanent. Several lakes of the group near the coast (known as Seven Lakes) experienced changes, the greater number having their levels lowered. A pond known as Mud Lake, on the divide at the head of Pine Gulch Creek and about a mile from the fault-trace, suddenly and permanetly lost its water at the time of the earthquake. At the same time a small spring on the east side of the ridge and about 0.75 mile in a direct line from the pond, was suddenly enlarged, a torrent of water gushing from it for several hours and then gradually diminishing. It is suggested with much plausibility by residents that these two phenomena were connected, the earthquake opening a subterranean course thru which the water of the pond was conveyed to the hillside spring. I heard of no changes in spring east of the fault-trace, altho a number of inquiries were made.
Interpretation of bedrock cracks and springs.—The changes in springs are of course the results of changes in the conditions of underground circulation, and in a general way may be ascribed to the influence of newly-formed cracks. The spring phenomena and the visible cracks may be grouped together as indications of bedrock fracturing, and their distribution indicates the regions in which the rocky foundation of the land was more or less shattered. That region includes the Rift and extends from it to the ocean. The phenomena diminish somewhat with distance from the Rift, but the fracturing appears to have been important and general thru a belt 4 or 5 miles broad.
Landslides.—The earthquake started a number of landslides. A few of these were on the line of the fault, especially where its trace intersected a cliff facing Bolinas Lagoon. Others were from cliffs of earth or weak rock bordering the ocean, one of the bays, or a creek. None were seen of unusual type or of great importance, except from the obstructions to roads which they occasioned. South of Willow Camp a road overlooking the sea had been cut in the face of previous landslides, and the renewed movement put it out of commission. In the same manner roadways were obstructed at the entrance to Bolinas Lagoon, at two points near the head of the lagoon on the west side, and on the coast of Tomales Bay at Inverness.
There were many landslides of the dry type on hillsides, masses of earth and rock breaking away on steep slopes and tumbling to the bottom. The largest seen were on the high ridge west of Tomales Bay, in the vicinity of Sunshine Ranch. Closely related to these were small falls of earth and rock from the low cliffs created in the construction of side-hill roads. (Plate 53B.) They occurred at a few places within the Rift and
On the west side of the main ridge west of the head of Tomales Bay there occurred two wet slides. In one case a hillside bog was loosened from the slope on which it rested and descended as a flow of mud to a canyon bottom 100 or 200 feet below. In the other case the earth beneath a wet meadow in a rather steep canyon flowed down the canyon for about 0.5 mile, overpowering trees on its way and leaving a deposit 15 or 20 feet deep in places. This was the largest individual slide observed.
In all the cases mentioned the conditions were such that slides would have taken place at some time had the earthquake not occurred. But this statement may not properly apply to the cases about to be mentioned.
On the steep southern face of Mount Tamalpais a number of rocks were loosened and rolled down the slope, some of them being large enough to cut swaths thru the thicket which were visible for months afterward. Similar swaths were seen under a crag in the vicinity of Willow Camp. In the bottom-lands of creeks it happened at many places that a slice of the alluvium was separated by a crack parallel to the bank and slid into or toward the stream. In some cases alluvium lying with a gentle slope adjacent to a marsh slid toward the marsh, opening a crack along its upper edge.
Mention has already been made of numerous hillside cracks which marked incipient landslides. In such cases the downward motion apparently began during the earth-quake agitation, but the momentum acquired was not sufficient to continue the motion after the earthquake stopt. In a very large number of these localities motion was resumed and landslides occurred during a period of excessive rainfall in the spring of 1907. (Plates 54A and 55A.) So far as my observation goes, all of the landslides having this history were wet, the material usually flowing freely down the slope as a thin mud. The probable explanation is that the cracks made in April, 1906, served to admit the water flowing over the surface during the rains of 1907, so that the material which was too dry to flow in 1906 acquired the proper consistency and continued its course the following year. The number of landslides which the earthquake induced in this indirect way is possibly as large as the number which were an immediate consequence of the shock.
The phenomena of landslides bring to attention certain conditions of flow which affect a variety of earthquake features. Consolidated formations hold steep slopes by virtue of cohesion. Incoherent formations maintain the "slope of repose" — 30° to 35° — by virtue of the resistance to sliding, or the static friction, of their particles. Certain formations, including some clays and clay mixtures, become coherent by drying and incoherent by wetting. Incoherent formations, as a rule, have a less coefficient of friction when wet than when dry. For these reasons the addition of water is the ordinary immediate cause of a landslide; it overcomes cohesion, or else it reduces the frictional resistance, and slipping or flowing is the result. During a strong earthquake, agitation overcomes the cohesion of feebly-coherent formations and suspends the operation of static friction between the particles of incoherent formations, thus affecting the materials somewhat as water affects them. In the case of landslides, it may enable an incoherent dry formation to flow as if wet; and it may temporarily give to an incoherent formation, wet or dry, a condition of quasi-liquidity.
Ridging and shifting of tide lands.—The general width of Tomales Bay near its head is about a mile, tho it is constricted at one point by a promontory jutting out from the east shore. (Fig. 24.) Papermill Creek, entering at its head, has built a delta which slopes so gently toward the deeper water that the tides range over it for a distance of about 3 miles. The upper half of the slope is covered by vegetation of various kinds and the lower half is of bare mud. In the region of vegetation the soil has sand as well as mud, and the bed of the stream is of sand and gravel. As the delta deposit has been
A large portion of the delta was thrown by the earthquake into gentle undulations, the difference in height between the swells and hollows being usually less than a foot. The chief evidence of this is found in the distribution of pools at low tide, and where vegetation is present the evidence from pools is supplemented by that from the condition of the plants. The undulations were not elongate and were not found to have a systematic relation to the fault.
When the tidal mud was first seen after the earthquake, it was observed to be covered with ridges and troughs. (Plate 54B.) This corrugation was gradually smoothed out by the action of the waves (plates 55B and 56A), so that at the expiration of a year its expression was largely lost, tho a few of the larger ridges could still be traced, and much of the plain retained a pattern imprest on it by the ridging. It is probable that the entire tract of tidal mud was thus affected, altho the ridges were not seen on the area lying nearest to the east shore. That area did not come under observation until after the spring floods
Without going deeply into the question of interpretation, it would seem that in the production of this ridging the tidal mud must have behaved as a quasi-liquid, being thrown into waves by the agitation to which it was subjected. When the agitation ceased it became once more a quasi-solid, and preserved the form it had at the moment of change.
There was also a horizontal shifting of mud over a considerable area. Residents familiar with depths of water in the vicinity of Inverness stated that the earthquake caused a decided shoaling along the coast, but that the relation of water levels to firm ground was unchanged. It was also stated that a channel which had existed parallel to the west shore of the bay, and to which piers had been run, was obliterated by the earthquake. The shoaling might have been caused either by an uplift of the bottom or by a shifting of the mud of which it is composed toward the shore. That the second of these explanations is correct seems to be shown by the following facts.
At various places along the shore, from Inverness to a point 1.5 miles southward, the tidal mud seemed to be crowded against the firmer ground at the shore, being pushed up in a ridge, as shown in the accompanying photograph. (Plate 55B.) Two piers at Inverness, light wooden structures, resting on piles and extending out several hundred feet from the shore, were telescoped. (Figs. 26 and 27.) In the case of Martinelli's pier the telescoping was shown by the inclination given to piles at the landward and bayward ends, from which it appears that the ground in which the piles were set was crowded together, so that the foundation of the pier was shortened, while the superstructure resisted shortening. The resistance was temporary only, for before the agitation ceased the pier was broken in two; and the inclination of the piles is supposed to have been given during the early stages of the tremor. Coincident with the movement of the
In the case of Bailey's pier, which is beyond the delta, the most important telescoping, as shown by the slanting of piles (fig. 27), was close to the shore, and nearly the whole structure was transported by the shifting mud. It also sagged more than a foot just beyond the middle, and the attitudes of the associated piles suggest that the sag corresponds to a hollow made in the surface of the mud. The pier was so badly broken as to require extensive repairs, and in making these repairs Mr. Bailey used the old material for flooring, but found that he had enough lumber remaining for 12 feet of flooring, so that he inferred a shortening of 12 feet. The whole pier was shifted to the northwest, being given a curved form (plates 57B and 58), and the maximum amount of shifting in that direction was at least 25 feet, altho the circumstances did not admit of accurate measurement. Combining the movement toward the shore with the offset parallel to the shore, it is probable that the direction and the maximum amount of shifting were about the same as in the case of the Martinelli pier.
It is a notable feature of this displacement that the disturbed material moved up the slope instead of down, so that the transfer was not only independent of gravity but opposed to it. The phenomenon, therefore, does not fall in the same category with landslides, and if properly interpreted it may throw light on the mechanics of the earthquake pulses.
The area thru which the shifting of the mud took place is indeterminate. It affected a shoal parallel to the west shore of the bay and more than a mile long. At the piers the width of the affected region was at least 400 feet and may have been much more. The reported closing of the channel suggests 700 or 800 feet as a minimum estimate, but the outer margin of the affected area was probably beneath the water of the bay and outside the range of observation. The firmer part of the Papermill delta appeared not to be included in the movement. All of the area known to be affected lies southwest of the fault-trace, which in that neighborhood is about 2,000 feet from the shore.
The Question of Local Elevation and Depression of Land
Introductory. — Dr. C. Hart Merriam was told by an Indian living near Marshall, on the northeast shore of Tomales Bay, that since the earthquake the clam belt on that shore had been less accessible. The tides also came higher than formerly, the highest tides surrounding his cabin, whereas formerly they did not reach it. Mr. C. J. Pease, of Olema, also stated that the clam industry on the northeast shore of Tomales Bay had been much injured by changes due to the earthquake. Thru President D. S. Jordan I was put in communication with Dr. S. S. Southworth, of Bolinas, who reported various phenomena indicating a lowering of the land on the east side of the fault, and a lifting on the west side. On September 27 and October 15, 1906, being in Bolinas and its vicinity, I made a preliminary examination of some of the features described by Dr. Southworth. They were of such a character that it seemed desirable to enlist the aid of zoölogists and botanists, and to this end a conference was soon afterward called in
About Bolinas Lagoon. — In presenting the evidence as to land-movements in the vicinity of Bolinas Lagoon, first place will be given to testimony of residents, and this again will be classified according to locality, beginning with the features west of the fault-trace.
Dr. Southworth has lived in Bolinas several years, and his activities during that period have led him into almost continuous observation of the coast and the tide. There is a clam patch on the ocean front between Bolinas and Duxbury reef (see fig. 28 and plate 56B), to which he has frequently resorted at suitable stages of the tide. It has been his custom regularly to consult the tide tables to ascertain whether the water stage would expose the patch. He reports that before the earthquake there were ordinarily about four low tides in the month, occurring by daylight, during which clams might be obtained, and that since the earthquake twenty or more days are available. He infers that the land was lifted at least a foot, possibly more, at the time of the earthquake. He states also that about 5 miles to the northwest there is a tract, exposed only at low tide, where abalones are abundant, and that people living near there have found them much more accessible since the earthquake than before. In Bolinas Lagoon a channel between Pepper Island and the mainland is not now navigable at certain tide stages which formerly made it entirely navigable.
Dr. Gleason, owner and master of a vessel plying between Bolinas and San Francisco, states that formerly it was his custom to turn his vessel in the channel between Pepper Island and the west end of the sand-spit, but that after the earthquake he found the place too shoal, so that, after a number of trials in which his vessel was grounded, he has adopted the practice of entering the lagoon stern first, to avoid the turn.
The following observations pertain to localities east of the fault. A road which skirts the northeast shore of the lagoon is not altogether on the mainland, but in places follows the strand between high and low water, and if it is used at high water the horses must ford. Dr. Southworth states that since the earthquake these fords have become more difficult, so that to pass them safely or comfortably they must be reached when the tide, as indicated by the tables, is lower than was formerly necessary. Mr. B. C. Morse, however, who lives on the mainland east of McKennan Island, and who ordinarily crosses the lagoon to Bolinas every day, states that he has noticed no change in the relation of
It will be observed that all this testimony, with the single exception of Mr. Morse's observation of water-levels near his house, tends to show a general sinking of the land east of the fault, and a general rising of that to west of it.
Professor Kofoid, in seeking evidence from the distribution of marine life, found the barnacle the most available form. It is abundant at many places; its shell remains as a witness after the death of the animal, and its upward limit bears, at many places, a definite relation to the line of high tide. The best places found for observation were certain groups of piles at Bolinas and along the northeast shore of the lagoon. From a study of these localities it appeared that in the upper part of the barnacle zone the percentage of dead shells is notably greater on the west side of the fault than on the east side, but there is not a well-marked zone of dead barnacles on the west side, nor is there a zone of exclusive young barnacles on the east side. The evidence thus gives a qualified support to the theory of elevation and subsidence. Outside the lagoon, on the open sea-front, the upper limit of barnacles is too indefinite and irregular to be available for a study of this character.
Visiting Pepper Island in company with Professor Holway, I found the position of the fault-trace clearly indicated by a difference in the color of the vegetation. The island is low, only a narrow strip at the south remaining above water at ordinary high tide, and from this strip there is a gentle slope toward the north and northwest. The vegetation on the highest part is somewhat varied, but the lower slopes are occupied almost wholly by a single species of Salicornia (pickle-weed). This is locally the lowest lying of the shore forms, and it descends the slope to a somewhat definite line beyond which the mud is bare. It is evident, therefore, that its lower limit is determined purely by physical conditions and not at all by the competition of other plants. It is thus peculiarly sensitive to changes in the relation of land to water. West of the fault a broad area covered by this plant presented, at the time of the visit, a brownish-green color, while the adjacent areas east of the fault had a dull brown color. The contrast was so strong that the eye could readily trace the line of the fault. We found also that the ground east of the fault was, in general, lower than the ground at the west, and I afterward made a series of measurements showing the average difference in elevation to be 12 inches.
Pepper Island was subsequently examined by Professor Jepson, who not only traced the brown color of the Salicornia to an abundance of dead and dying plants, but found considerable corroborative evidence in the condition of other species living at slightly higher levels. On McKennan Island a similar condition was found. The island is girt by a zone of Salicornia, the outer or lower belt of which was found to be brown. A single measurement of the vertical range of dead and dying plants gave 10 inches.
The northeastern shore of the lagoon was examined for evidence of similar character, but the result was less satisfactory. The lowest plant growth is not everywhere the same and the local conditions are materially different. The slope is less gradual, the soil is more gravelly, and there is deposition of detritus eroded from the land by streams and waves. At some points a belt of plants at the extreme limit appeared to be suffering
In the discussion of these data, the first point to be noted is that the killing of Salicornia thru the lower part of its zone definitely indicates a lowering of the ground on which it stands. The plant normally travels down the slope as far as it can tolerate the tidal submergence and there stops; and its inability to sustain itself in a well-defined belt constituting the lower part of its former range shows that the submergence in the belt has become intolerable. The amount of submergence is shown by observation on McKennan Island to be at least 10 inches, and if allowance is made for a certain amount of lag in the response of the plant to change of condition, the lowering of the land may have been several inches more than this. If McKennan Island and the eastern part of Pepper Island subsided the same amount, it is probable that the only change on Pepper Island was a subsidence of its eastern part, the western part remaining at its former level. In that case the amount appears sufficient to account for the overwashing of the spit, altho no measurement is there practicable.
The tract of land, whose subsidence appears to be demonstrated by the botanic evidence and the overtopping of the spit by waves, is bounded on the southwest by the fault, but its other limits are not known. In the immediate vicinity of the fault it may reach the head of the lagoon; that it does not extend beyond is rendered probable by the fact that there is no vertical dislocation in the fault-trace at a point about one mile northwest of the lagoon where the trace is favorably exposed on flat ground. It may be possible that the area of subsidence is limited on the northeast along an old line of dislocation which coincides approximately with the northeast side of the lagoon. This dislocation has not been determined by a study of the geologic structure, but is indicated by the physiography, and was presumably concerned in the making of the basin occupied by the lagoon.
The evidence of elevation west of the fault is less coherent. Dr. Southworth's observations on the clam patch give a presumption in favor of elevation, but they are not well supported by the evidence from barnacles and plants. The botanic evidence indicates that the entire dislocation shown by the measurement on Pepper Island is a subsidence on the east and does not include elevation on the west. The evidence from the barnacles suggests, without proving, a slight elevation at the Bolinas wharfs, but by no means indicates so great an elevation as would be necessary to account for the increased facility in reaching the clam patch. Dr. Gleason's report of the shoaling of water in a channel near Pepper Island undoubtedly shows a local change, but such a change may have been produced by a horizontal shifting of unconsolidated material such as occurred in Tomales Bay. On the other hand, it is not possible to explain the phenomena of the clam patch bya hypothesis of shifting bottom, for the sand in which the clams live is contained in shallow basins of visible bedrock, and any change in the relation of surface to tide at that point is a bedrock change. As the Rift belt with its numerous earlier dislocations extends nearly to the clam patch, it is not impossible that there were differential movements west of the fault-line and that the ground occupied by the clam patch and the abalone patch rose independently of the western division of Pepper Island.
About Tomales Bay. — Professors Kofoid, Torrey, and Holway examined practically the whole shore of Tomales Bay, and also visited the outer or ocean side of Tomales Point. Their attention was directed especially to the condition of barnacles at the upper limit of the zone of marine life, and the evidence they found does not show any change in level, either by elevation or subsidence. It is their opinion that the injury to the clam industry along the northeastern shore of the bay is referable to other causes, including at some points the exceptional inwash of detrital material fromthe shore, and at others the shifting of loose material toward the center of the bay.
After the discovery of the vertical dislocation in Pepper Island, I visited the Papermill delta at the head of Tomales Bay in search of similar evidence of displacement, but failed to discover it. There are on the delta several tracts on which water stands after the fall of the tide, and the plant growth, especially Salicornia, shows deterioration in these areas; but the areas are not systematically related to the fault-trace. They occur on both sides and at least one of them is intersected by the trace. They constitute part of the evidence of a gentle, broad undulation of the delta surface, which appears to have been occasioned by the earthquake. A tentative theory to account for this undulation is that lenticular bodies of soft clay, included in the delta deposit, experienced a certain amount of flow during the earthquake period. The lane of water following the fault-trace, and described on an earlier page, is an independent phenomenon closely associated with the fault, and the depression causing it does not extend indefinitely toward the east. In a general way, the half of the delta east of the fault stands as high as the half at the west. On the lower slope of the delta, beyond the region of plant growth, there is a tract east of the fault which received the principal deposit of sediment brought by the floods of 1907. The localization of this deposit suggests that the transporting current may have been guided by a depression of the surface, but if so the depression was not bounded on the one side by the fault line; its southwestern boundary is a distributary of the creek. As the tract on which this deposit took place is opposite a portion of the tract in which mud was shifted toward the southwest shore, it seems possible that the area of shifting here included practically the whole width of the bay, and that the resulting elevation of the bottom toward the west was accompanied by a lowering of the bottom toward the east. In that case, the apparent lowering of the clam zone at various points on the northeast shore may be correlated with the phenomena near the head of the bay, and the whole ascribed to a general shifting of loose material in the bottom of the bay toward the west.
Bodega Bay. — As the title of Bodega Bay is variously applied on different maps, it is proper to specify that the body of water here intended is the land-lockt lagoon east of Bodega Head, and not the open roadstead farther south. Professor Ritter examined this with care, studying especially the distribution of barnacles, and found no evidence of absolute or differential change of level.
Summary. — At Bolinas Lagoon, subsidence occurred east of the fault, its vertical amount being approximately a foot. The subsided tract included the greater part of the area of the lagoon, and may have had its eastern limit along the eastern shore of the lagoon. The subsidence was possibly a continuation of the local movement of dislocation by which the basin containing the lagoon was created. There may have been local elevation of a tract extending from Bolinas westward and northwestward. The evidence is not demonstrative, but leaves a presumption in favor of such elevation.
About Tomales Bay and Bodega Head there was probably no appreciable change in the general elevation of the land, most facts which tend to show such change being explained by assuming that in Tomales Bay there was a general shifting of mud and other incoherent material toward the west. Such shifting had been fully demonstrated in the vicinity of Inverness.
Postscript. — Since the preceding paragraphs were written, some additional data have been gathered bearing on the question of the elevation of land between Bolinas and Point Reyes. As the most satisfactory biological evidence with reference to changes in level had been found in the response of the plant Salicornia, it occurred to me that pertinent information might be obtained by examining the lower limit of land vegetation at Limantour Bay. That bay is an extensive, ramifying, drowned valley lying east of Point Reyes promontory. It is separated from the ocean by a spit, past the western end of which a channel is maintained by tidal currents, just as in the case of Bolinas Lagoon. The eastern end of the bay is at the western base of the ridge bounding the Bolinas-Tomales trough, and is 8 or 10 miles northwest of the abalone locality.
If the land in this locality was raised at the time of the earthquake, the height of the tide at all stages, with reference to the land, would be lower; and the lower limit of Salicornia, being dependent on the relation of the land to tide water, would descend the slope in response to the change in level. The feature, therefore, to be lookt for was a new growth of Salicornia at a lower level than the older growth. Such new growth was actually found (June 5, 1907), not in a continuous belt, but in numerous patches having certain common characteristics.
At the points visited the tide marsh characteristically ends in a little step or bluff about 8 inches high. Above this step the gentle slope is covered by a mat of vegetation in healthy condition, the dominant plant near the step being Salicornia. Below the step is a mud surface, which usually inclines more rapidly than the platform above. If I understand the origin of this topography, the step has arisen from the gradual bayward extension of the platform, which, by reason of its vegetal covering, is enabled to arrest mud suspended in the water. There is also doubtless an accumulation of the roots and stems of the Salicornia. In places there are outlying platforms of the nature of islands and similarly covered by Salicornia. On the other hand, the broader platforms are interrupted by channels thru which the tidal waters escape, and there are also lakelike hollows abruptly margined by steps a few inches high. The slope below the step is ordinarily of bare mud, but on it are numerous patches of young Salicornia, and there are similar tracts in tidal channels and in some of the lake-like hollows inside the platform. The vertical range of this young growth is quite definite, its lower limit being from 13 to 16 inches below the outer edge of the platform. In some cases the young growth is straggling, but usually it makes a mat as close and complete as on the platform above, and the height is nearly as great. It is distinguished from the older growth chiefly by a slight difference in color. Whether such a luxuriant and dense growth of Salicornia could be produced in the period of 13.5 months, I am not prepared to say. Except for that doubt, however, the phenomena are just such as would be expected to follow an elevation of the land.
In an estuary at the edge of the bay were two fence stakes on which barnacles were set. At the upper limit of the barnacles, I examined a dozen individuals, finding them all alive, and I saw none of the adherent plates which remain after the death of the old barnacles. I did not learn the history of these stakes. If placed after the earthquake, the evidence of the barnacles would not be in point. If placed before the earthquake, the evidence of the barnacles, so far as it goes, is opposed to that of the new colonies of Salicornia.
Second Postscript, added to proof sheets in November, 1907. — Early in October, 1907, Dr. S. S. Southworth reported that the clam patch near Bolinas had again become less accessible, its relation to tides being practically as before the earthquake. The apparent change was not associated with any precise date, but it had been suspected since the middle of summer. On October 17 I visited the locality, selecting a time when the predicted sea-level at low tide was approximately the same as on November 25, 1906, when I had taken the photographs reproduced in plate 56B and plate 56C.
- At low-water on the afternoon of Nov. 25, 1906, the tide-staff reading was: 6.10 Feet
- At low-water on the afternoon of Oct. 21, 1907, the tide-staff reading was: 5.61 Feet
- In the hour following low-water the computed rise of the tide was 0.3 ft.; and this gives as the height of water at the time of the observation at the clam patch: 5.91 Feet
- Difference: .19 Feet
Thus the discrepancy of 1.5 feet, deduced from a consideration of the predicted tides, is reduced by a comparison of the observed tides to about 0.2 foot, a quantity so small as to be referable to errors of observation.
Before tide-gage records were obtained I had revisited Pepper Island and Limantour Bay. On Pepper Island the vertical dislocation was remeasured and found to be unchanged. At Limantour Bay the subject of examination was the condition of the new growth of Salicornia. If the land had subsided since the preceding June, the colonies of Salicornia which had invaded the mud flat (plate 56D) would have been subjected to unfavorable conditions, and might be expected to show the influence of those conditions. All the colonies that had previously been observed were reëxamined and they were found, without exception, to have deteriorated. The green heads, which had formerly testified to their lusty growth, had become much less numerous and were modified in color; their stems were blackened on the surface and had become somewhat curled, and in general they appeared less healthy than the plants of the same species growing at higher levels. Where the slope was continuous, there was a fairly sharp line of separation between the healthy and unhealthy plants, and two measurements indicated the zone of impairment to have a vertical range of 10 inches.
The comparative observations of water-stages show that the land at the locality of the clam patch has not recently undergone the suspected depression, as compared to land at the tidal station near San Francisco. If an important change has taken place at one locality, it has affected the other also. On the other hand, it is noteworthy that Dr. Southworth's observations at the clam patch (first of its increased accessibility, and after of its decreased accessibility) led to two predictions as to the condition of Salicornia in Limantour Bay, both of which were verified. Their success in prediction gives assurance that they record an actual change of some sort — a change not restricted to the locality of the clam patch. The two lines of evidence taken together — the leveling by water-plane from the tide-gage to Bolinas, and the observation of shore conditions at Bolinas and Limantour Bay — suggest the possibility of a general change in the relation of land to sea, affecting the whole coast from San Francisco to Limantour Bay. So far as the observations go, such a change might pertain to either land or sea. In the line of this suggestion it is to be noted that November 25, 1906, falls within a period of exceptionally low tides at San Francisco entrance. For 21 low tides, from November 20 to November 30, the mean of the observed heights was 1.08 feet below the mean of the predicted heights. From October 17 to October 21, 1907, on the other hand, the mean of 10 observed tides was only 0.32 foot below the corresponding mean of predicted tides. The subject appears to deserve further investigation and discussion than is practicable while these pages are in press.
The observations which occasioned this postscript, while suggesting lines of enquiry which may profitably be followed, do not materially affect the conclusion already summarized as to local changes in the elevation of the land. A tract, including the east part of Pepper Island and much of the area of Bolinas Lagoon, subsided at the time of the earthquake, the amount of subsidence at the point of most satisfactory measurement being 12 inches. The region west of the fault, including the ocean coast from a point near Bolinas to Limantour Bay, may or may not have been uplifted at the same time, and may or may not have subsequently subsided. The evidence is incomplete and apparently somewhat conflicting.
Special reports on the biologic evidence follow.
Report on a Biological Reconnaissance of Bodega Bay Region
By William E. Ritter
Accompanied by Mr. E. L. Michael, I examined the Bodega Bay region on October 26-30, for evidences of a faunal modification resulting from the earthquake of April 18, 1906.
My first effort was to secure information from residents of the district bearing on the question. A number of families living on the shores of Bodega Bay have their dwellings close to the water's edge. Since the bay is small, closely land-lockt, and hence especially free from surf, and since these families spend much time on the water with their small boats, which they beach on the gradually shelving shores or tie to their little private piers, it seemed that their testimony would be peculiarly reliable. It appeared that any appreciable change of level of the water along the shore or any noticeable effects on the shore life would hardly escape detection. I talked with five persons of this sort, each by himself. All were unequivocal in affirming that neither the level of the water nor the animal life of the bay were in any wise altered by the earthquake.
The earthquake fault at the only point at which it has been located here, passes thru the sand-dunes at the head of the bay; and from its general course and the place where next observed to the south, must have past nearly parallel with the eastern shore of the bay and either have followed the shore or have been to the landward of the shore. In other words, nearly if not the whole of the bay, together with the peninsula of which Bodega Head is a part, is on the west or seaward side of the fault. All the facts we were able to gather by direct observation pertain to the rocky shore of the bay side of the peninsula. Since the rock here is a firm granite, and since in some localities the walls are nearly perpendicular, are even-faced, and are washt by the waters thruout the day excepting at extreme low tide, they are very favorable for furnishing testimony of the kind sought. The question to be answered was: Do the organisms that live immovably fixt to the rocks show evidence of having either extended or withdrawn the upper limit of their vertical distribution within recent time? The organisms that would be available as testimony would be those that are most permanent in structure, and extend up to the very limit of the high tide. Of first importance are the barnacles, two species, Balanus balanoides and Chthamalus stellatus. A species of Mytilus, and perhaps one or two species of marine algæ, are also more or less available. Our attention was given to the barnacles chiefly, but somewhat to the mussels also. Neither of us was sufficiently familiar with the algæ to make much use of them.
We could get no evidence that any of these organisms had either extended or withdrawn their limits of distribution. In the absence of accurate knowledge on the rate with which barnacles develop, there might be some uncertainty as to whether the limits had been extended; since, however, the individuals at the upper limit were not found to be in general smaller than those farther down; and further, and still more importantly, since the remains of dead individuals were quite as numerous proportionally in the upper zone of distribution as in the lower, we could but conclude that there was an absence of evidence of extension. In other words, there was no evidence of subsidence of the shore.
As to the question of whether the shore has been elevated at this point, the evidence I think is more positive. Not only is there lack of proof that elevation has occurred, but there is ample proof that it has not. This is furnished by the barnacles chiefly. On the vertical granite walls above mentioned, these organisms almost completely cover the surface up to about 7 feet above mean low water. As already stated, the remains of dead individuals are uniformly distributed thruout the area; or, to speak more accurately, they are not more numerous proportionally in the upper limit of distribution than in any other portion, as would surely be the case had the upper limit been lifted above the former high-water mark. It should have been pointed out that the 7 feet to which the barnacles extend must be very near, if not quite the limit, of high tide.
The character of the remains of dead animals is such as to preclude, I believe, being misled by the facts. In addition to the heavy calcareous wall which characterizes the superstructure of the animal, there is a well-defined continuous platform closely fused to the substratum to which the animal adheres. After death the superstructure of the shell falls away, leaving the platform as a smooth, hard, calcareous scab clinging to the rock. This is very durable, as one can see by observing old piles that have been taken from the water and to which these barnacle remains cling. Had any appreciable elevation of this shore occurred, there would surely be a zone of dead barnacle shells at the upper range of the distribution. The testimony of the mussels, so far as it goes, is confirmatory of that furnished by the barnacles.
Report on A Biological Reconnaissance of Tomales Bay Region
By Charles A. Kofoid.
On October 26-28, 1906, in company with Profs. H. B. Torrey and R. S. Holway, I made an examination of the shore of Tomales Bay to obtain evidence of faunal modification resulting from the earthquake of April 18, 1906. The places specially examined were as follows: the northeast shore from Millerton to Preston Point; Hog Island near the mouth of the bay; the southwest shore from near Tomales Point to the region opposite Marshall, and from Inverness to the head of the bay. The outer face of Tomales Point was also explored for a short distance near "Shell Beach."
Search was made for evidence of a change in level in the two sides of the bay and especially for evidence of depression of the northeast shore and elevation of the southwest shore. For this purpose critical examination was made of barnacles in situ on rock in place along the shores between tide levels. The fauna of the bay includes no generally distributed organisms attached to rock within tide levels except the barnacles (Balanus sp.). Mussels are rare and there are very few attached seaweeds far from the mouth of the bay.
The barnacles are, however, sufficiently abundant and widely distributed to afford an excellent index to any recent change in levels. If the northwest shore line, about 0.5 to 1.5 miles from the main earthquake trace, had been deprest even a few inches we might expect to find young barnacles, the young of the year which are easily distinguished by their brownish-gray color, softer texture of the shell, and certain structural features, invading the new territory above the old to an extent equivalent to the depression. If the southwest shore had been elevated, we should expect to find a number of dead barnacles in the region above the old barnacle limit and a relative absence of young in the upper levels. The upper limit of the growth of barnacles lies below the level of highest tides, and is more or less distinctly marked, according to the exposure to prevalent currents and wind and to exposure to the sun; and it is also modified by the slope and texture of the substratum.
The two shores present strong contrasts in the matter of exposure to prevalent, winds, to the sunshine and in the texture of the substratum, the rocks of the northeast shore belonging to the Franciscan, more or less metamorphosed, and those of the southwest shore being of a granitic nature. These contrasts produce considerable modifications in the distribution of the barnacles.
A critical examination of the data reveals no conclusive evidence of any recent change in the distribution of barnacles that can be attributed to a change in the levels of rocks in place. There is no sharp and uniform contrast between the two sides of the bay in the matter of the distribution of these organisms. There is no uniform or extensive invasion of higher levels by young barnacles on the northeastern shore and no marked destruction of old barnacles and absence of the young at high levels on the southwestern shore. The conclusion is reasonably certain that there has been no appreciable change in levels of either shore as a whole.
Especial care was taken with the examination of the rocky shores of Preston Point which is crost by the main fault, but even here there is no biological evidence of a change in levels on the two sides. In many regions barnacles have been killed in great numbers, apparently by silt in the waters. In other cases barnacle-coated substrata have been shifted with the mud, sand or gravel in or on which they lie, but such changes are of a local or superficial character. Hog Island, which lies very near the line of the fault but is not crost by it, shows no uniform change in its barnacle fauna. The outer sea-cliff of Tomales Point, tho very much shattered and with considerable talus from rock falls resulting from the earthquake, shows no disturbances in its fauna traceable to seismic movement. Local testimony of dealers in fish, of fishermen and of clam diggers indicates a great falling off in shipment of clams since the earthquake, traceable to departure of clam diggers, destruction of clams in places, by shifting of clam beds or their burial with detritus from cliffs. No change of levels which might not be traceable to shifting of loose deposits was noted.
There was local testimony of increased wash along the railroad embankment skirting the northeastern shore, or sinking or rising of known shoals in the bay, and of a depression of the gravel spit on which the fishing village stands. Probably all of these phenomena are explicable as the results of local loosening up of the railroad embankment and shifting of loose deposits, rather than as a result of a general movement of the earth's crust.
Report on a Biological Examination of the Bolinas Lagoon Region, November 24-25, 1906
By Charles A. Kofoid
Bolinas Lagoon. — The distribution of barnacles along the shore, on the piles, etc., was examined with reference to possible changes in level, near Bolinas wharf on the western side of the lagoon and on Morse's wharf on the eastern side, the principal locations on which barnacles occur about the bay. In neither case was there evidence or local testimony of disturbance in levels of the ground on which the barnacle-bearing substrata were located. The possibility of local slumping of soil is not, however, entirely excluded. No barnacles on rock in place were observed in the bay.
There is no evidence from the distribution of barnacles of any change in level of the eastern side of the bay. There is neither any marked destruction of old or absence of young in the upper levels such as would follow elevation, nor any marked recent occupation of an upper belt by young barnacles such as would follow depression. On the western side of the bay, on the piles of the warehouses at the landing, there was a faintly defined zone 6 to 8 inches wide in which the proportion of dead barnacles was unusually large. The percentage of dead in the uppermost levels on Morse's pier on the eastern side of the bay varied from 2 to 35 per cent with predominant range of 10 to 20 per cent. On the piles on the western side at similar levels, the proportion of dead was predominantly 40 to 60 per cent and not infrequently ran above these figures. Below this upper belt there was frequently less destruction and a relatively greater number of young than was found in the uppermost levels. It may be that this destruction was due to elevation, tho the uppermost belt of barnacles is still just submerged at a 5.4-foot tide. It might also be due to the considerable increase of silt attendant upon the large amount of talus shaken into the bay and along the adjacent shore line by the earthquake. The fact that barnacles attached themselves to and throve on buildings thrown into the bay not far from the piles in question, would indicate that destruction by silt was confined to the time of the earthquake or that destruction did not take place as a result of silt.
The "studio building" at Bolinas, which was thrown into the bay by the earthquake and raised some months later, was well covered on submerged portions by barnacles, mainly half or two-thirds grown. This fact makes it certain that the young barnacles have been attached in large numbers since the earthquake, and that their distribution, therefore, affords critical evidence of change in levels.
The Sea Coast Line. — The evidence here from the distribution of barnacles is inconclusive, owing to the great range of movement of water in the breakers and the relative scarcity and small size of the barnacles present. There was no evidence of any change of levels, but their numbers are probably too small to afford evidence of a movement of a few feet only.
The Clam Patch. — The evidence of elevation here is entirely in the nature of testimony. The barnacles on rock in place in this region are too near the low-tide level to afford a satisfactory criterion. A few rocks in place near the upper levels show no trace of extensive destruction of barnacles such as might follow an elevation of 1.2 feet which Dr. Southworth believes to have taken place. But here again the biological evidence is too incomplete to have much weight. There is no doubt that there were clams in a shallow gravel bed resting on rock in place and abundantly exposed at a 2.1-foot tide.
In my opinion, from the evidence in hand, there was no depression of the eastern margin of Bolinas Lagoon as the result of the earthquake of April 18, 1906. Dr. Southworth's testimony, taken in conjunction with the destruction of barnacles in the upper levels on the western side of the bay, suggests the possibility of a small elevation on that side.
Extract From Report on a Reconnaissance of Tomales Bay Region
By R. S. Holway
Below is a copy of the few notes made by me during the trip to Tomales Bay, October 26-28, 1906. The object of the trip was to examine the shore lines of the bay for indications of recent changes in level as shown by the effects on animal life. Drs. Kofoid and Torrey, the biologists of the party, recorded observations in detail and I have merely the general note as follows:
"The upper limit of barnacles was found to be a quite sharply defined line on the rocky shores of the bay. Any recent change in level of a foot or more would have been easily detected in my judgment. No evidence of such change was found. . . ."
Report on an Examination of Plants on Pepper Island, Bolinas Lagoon,
April 9, 1907
By Willis L. Jepson.
Salicornia ambigua Michx. Pickle-weed. — This is the most abundant species and forms extensive colonies on both sides of the fault-trace. The difference in color of the areas on the two sides of the trace at once strikes the eye, the east area being dull or dead brown, the area west a livelier or greenish brown. This difference in color was found to be correlated with a difference in health. The plants west of the fault are in normal condition; the plants east of it are either dead or dying. Dead plants still standing show wasted or shrunken black stems. Dying plants show shrunken main axes bearing above a few short joints of green which are very much thicker than the main axis. In the normal plant the joints are no thicker or scarcely thicker than the main axis. A broad and very marked zone of dead or dying Salicornia surrounds McKennan Island which lies east of the fault.
Statice Limonium L. var. california Gray. Sea Lavender. — Rather common in small areas on both sides of fault-trace. West of the fault plants are in normal condition, with large bright green leaves. East of the fault plants are dead or unhappy. Dead plants consist of nothing but caudices or short branching stems which form miniature forests of black stumps in the lowest places. Unhappy plants are those struggling to maintain existence and showing only a small tuft of small leaves. Similar colonies of dead plants were found on McKennan Island.
Grindelia cuneifolia Nutt. Marsh Grindelia. — The majority of the plants east of the fault are dead. Many plants west of the fault are dead, especially immediately west of the fault. The dying out is, in the main, doubtless due to old age in the colony.
Mesembryanthemum œquilaterale Haworth. Sea Fig. — Plants immediately west of the fault were healthy. One plant was found immediately east of the fault; this was killed completely.
Distichlis spicata (L.). Salt-grass. — Plants west of fault were thriving more than plants east in adjacent areas. (This species ranges to 600 feet above the sea.)
Frankenia grandifolia C. and S. Yerba Reuma. — Similar slight differences as in the preceding case.
Triglochin maritima L. Arrow-grass. — Coming up freely like young blades of grass west of the fault. Not appearing at all or reluctantly on east side.
Jaumea carnosa Gray. Fleshy Jaumea. — Less readily found on the east side of the fault. Plants on the west side were in somewhat better condition.
Populus species. Planted. — All individuals on east side of fault were dead.
Summary. — The difference in the health of the plants east and west of fault-trace indicates comparatively recent changes in conditions and would be explained by the assumption that there had been a change of level east of the fault. If there has been no such change it would be difficult to say why the affected areas should conform closely to the fault-trace. The plants on McKennan Island were also examined. The argument in favor of assuming a depression for Pepper Island east of the fault would hold good for McKennan Island. On the other hand, I should be strongly against the opinion that the condition of shore-line plants indicated a change in level on the east shore of Bolinas Lagoon.
Mussel Rock to Crystal Springs Lake
Course of the fault-trace. — The point at which the fault-trace intersects the shore, on emerging from the ocean on the south side of the Golden Gate, is only approximately known. About 0.875 mile to the southeast of Mussel Rock, it has been located with precision at its intersection with the wagon road on the west side of the coastal ridge a little below its crest, and thence followed continuously for many miles. Projecting its course, there determined, in a northwesterly direction, it would pass out to sea in the midst of the large landslide which scars the coast immediately to the north of Mussel Rock, where the basal beds of the Merced series rest upon the older rocks. At the time of the earthquake there was an extensive movement of the landslide, and a tongue of landslide material, about 50 feet high and about 200 feet wide, was projected into the ocean across the narrow strip of beach.
On February 27, 1907, according to the observations of Mr. H. O. Wood, this projecting tongue of landslide had been entirely removed by the action of the waves, and alinement of the beach and sea-cliff had been reëstablished.This movement naturally obscured all evidence of the position of the fault-trace, which was doubtless overridden by the slide. All about the crest to the east of the landslide, and on its south side, the ground was greatly disturbed by fresh landslide cracks, scarps, and fissures, extending well back from the edge of its encircling cliffs. It appears to be probable that not only did the movement of the landslide obscure the evidence of the fault-trace, but also that the latter was here diffuse and scattered, and that the displacement was superficially taken up by the plasticity of the landslide material.
From the point southeast of the Mussel Rock slide where the fault-trace resumes its definite and easily recognizable character, to Crystal Springs Lake, our information regarding the course of the fault-trace and the earth movement on the fault is in part from observations made by Mr. Robert Anderson, and in part from observations recorded in a paper by Herman Schussler,
The Water Supply of San Francisco before, during, and after the Earthquake of April 18, 1906, and the Subsequent Conflagration. New York, 1906.supplemented by the observations of Mr. H. O. Wood, Andrew C. Lawson, and others.
South of the road, at a point 0.875 mile southeast of Mussel Rock, begins the furrow which marks the surface path of the fault. The furrow as such does not cross the road to the north of this point. The side-hill slope, however, is very much fissured by landslide movements both above and below the road, and scarps are seen. From this point, the furrow runs uninterruptedly southeastward to the east side of the north end of San Andreas Lake, where, with a course of about S. 33° E., it passes beneath the waters of that reservoir. As it approaches the lake, the trace of the fault does not lie in the axis of the valley, but runs along its eastern side. It thence passes thru the lake on the northeast side, crossing a number of small promontories, to the east end of San Andreas dam; thence, with a course of S. 37° E., it traverses the east side of the valley between this dam and Lower Crystal Springs Lake, passes thru the latter and intersects the old dam between Upper and Lower Crystal Springs Lakes. Beyond this it skirts the southwest side of the upper lake, partly in the water and partly on the projecting points, and finally leaves the lake about a 0.25 mile from its end, for the stage of the water of April, 1906, having here a course of S. 40° E.
The mean course of the fault, as thus closely followed from the vicinity of Mussel Rock to the end of Upper Crystal Springs Lake, a distance of about 15 miles, is S. 36° 30' E. But the trace is not a perfectly straight line. Between Mussel Rock and San Andreas
Characteristics of the fault-trace. — For this stretch of from 14 to 15 miles, Mr. Robert Anderson, who examined this territory under direction of Prof. J. C. Branner, describes the trace of the fault as marked by a belt of upturned earth resembling a gigantic mole-track. The rupture may be traced along every foot of the way when not below the waters of the lakes. It varies in width from 2 or 3 feet to 10 feet, but at times branches out into several furrows that include a space of 100 feet or more in width. Such branches sometimes join again after a short interval. Sometimes it forms a crack 2 or 3 feet wide and several feet deep, and in other places shows a vertical wall of soil on one side or the other, several feet high. The typical occurrence in turf-covered fields is a long, straight, raised line of blocks of sod broken loose and partly overturned. It is thus shown in plate 61A, B.
Associated with the fault fractures are many lateral cracks, extending away from the fault in a northward, or north slightly eastward, direction; that is, at an oblique angle to the northeast side. These cracks were especially abundant along the northeast side of the northern half of Crystal Springs Lake, and between there and San Andreas Lake. In places they run off every foot or few feet for a distance of 100 yards or more, and again they do not form for some distance. They vary in size from minute crevices in the earth to fractures a foot or more in width. Here and there they form lines of broken sod very like the main furrow in size, while they have a length of from a few feet to several hundred feet. At the great dam at the head of San Mateo Canyon, these cracks emerged from the lake and ran northward up on the hills for several hundred yards, breaking the fences where they crost. Plate 16A shows large lateral cracks of this description, already partly filled up, crossing a road that runs parallel to the fault at the upper end of Crystal Springs Lake. The main line of fracture is about 50 feet beyond the fence, and the cracks extend into the foreground at an angle of from 35° to 40° with the main faulttrace. The fence is pulled apart 40 inches in the two places which are shown in the photograph, and a total of 10 feet in ten different breaks in this locality, within a distance of 200 yards. Such lateral cracks as these were not noted on the southwest side of the fault.
The lateral cracks described above make an angle of 45° to the general line of the fault fracture. They appear to have been produced very much like the fracture lines in compression tests of building stones. There was evidently great pressure holding together the two faces along the fracture. A dam made of earth and rock divides Crystal Springs Lake into two parts. This dam crosses the fault-trace at right angles, and was offset but not badly cracked or injured by the movement. The fences that line the road were warped and their boards buckled thruout the distance across the dam. The earthquake rendered them too long for the distance from the hills on one side of the valley to those on the other. The inference is that a strong compression took place. The slicken-siding shown in plate 62A furnishes further evidence of compression. In the same way the heaving up of the sod into a long, raised mound, for most of the extent of the furrow, suggests lateral pressure. The formation of cracks a few inches to 2 or 3 feet wide in places along the furrow seems to contradict the theory of compression; but these are regarded as due to the irregular, crooked fracturing of the surface and the faulting of irregularities into juxtaposition with one another near the surface. The open cracks
Offsets on fences, pipes, dams, etc. — About a mile southeast from the point near Mussel Rock where the furrow was first noted as a clearly defined feature, the fault-trace passes thru the trough of a well-marked saddle. This feature is more accentuated than similar features at other points along this portion of the rift, tho many such are found. Southeast from this saddle there is recognizable in the topography a distinct line of former movement, lying east of the fault. No furrow follows the line continuously, but an occasional short fissure or crack runs along it for a little way. To the west of the place is a similar, but less well marked, topographic indication of a former movement. There is no evidence of any movement on this line at the time of the earthquake. At the point where the fault-trace crosses the road, less than half a mile farther on, the roadway and fence were broken, but the effects were so confused that the measure of the offset could not be determined. The apparent horizontal displacement was slight.
Still farther to the southeast, about 1.25 miles, the fault intersected a fence and not only caused it to be offset, but the intersection showed clearly the effect of the drag in the earth movement. The bearing of the fence is N. 68° E., so that it is approximately transverse to the line of the fault. On the west side of the latter, the fence suffered a displacement to the northwest of 13 feet from its former position, and this displacement was effected by a bending or curvature in the fence line extending westerly from the fault for a distance of over 200 feet. On the east side of the fault, the fence was bent away from its former position, in the same direction, about 7 or 7.25 feet, the bent portion extending easterly from the fault-trace about 45 feet. The two ends of the fence were thus offset on the line of the fault only 5.75 to 6 feet, altho the total displacement was 13 feet. The displacement is shown diagrammatically in fig. 29. At a point 330 yards beyond this, on the Rift, the fault-trace was found to be confined to a furrow about 6 feet wide, passing thru a little trough between an outcrop of Franciscan on the west and a fine conglomerate (Merced) on the east.
Nowhere along this portion of the fault-trace between the slide at Mussel Rock and San Andreas Lake was there observed any definite evidence of vertical displacement. There was a hint of slight upthrow on the western side, but it could not be tested by measurement. There were, in general, furrows on either side of the main fault, at various distances up to 200 feet. Some of these were persistent for considerable distances.
About 2 miles from the upper end of San Andreas Lake the fault encounters the 30-inch, laminated, wrought-iron pipe of the Spring Valley Water Company, which prior to the earthquake conveyed the water from Pilarcitos Lake to San Francisco. The metal of the pipe is about 0.1875 inch thick and coated with asphaltum. The pipe is buried in the soil at a depth of 3 to 4 feet. The point of intersection is near Small Frawley Canyon. Here the course of the pipe swings from a northwesterly to a more northerly course, and the fault consequently intersects it at an acute angle. At the point of intersection, the pipe was obliquely sheared apart and telescoped upon itself, effecting a shortening of about 6 feet. The amount of the transverse offset involved in the shear was about half the diameter of the pipe. The portion north of the break was moved east and telescoped southerly. For 0.875 mile southeast of this point, the path of the fault lay on the northeast side of the pipe and nearly parallel to it, but a short distance away. About 220 yards southeast of the intersection, where the pipe, buried a few feet below the surface, ascends a rising slope, the pipe had completely collapsed for a distance of several
At a point about a mile from the upper end of San Andreas Lake, the fault intersects a bend in the pipe at two places, and here again the pipe was telescoped. (See plate 60A.) The conditions at one of these intersections are thus described by Mr. Robert Anderson:
The pipe makes an angle of about 15° with the fault-trace, the end of the pipe on the north side of the fault running that much nearer the north. The ends of the pipe on opposite sides of the fracture were therefore thrust into each other. The furrow was at this place divided into several smaller ones, the disturbed zone covering an area of considerable width. The pipe was broken in three places within 100 feet. In one place it was telescoped 58 inches, as shown in plate 59B; in another 17 inches, and in a third, the one farthest north, 41 inches.
Near the head of the lake, the pipe was again intersected by the fault, with results described by Mr. Anderson as follows:
The pipe line runs almost parallel with the fracture, but slightly more to the west at this point, so that the acute angles made by the ends of the pipe with the furrow were in this case on opposite sides of the furrow to those in the two previous instances. In other words, the southeast end of the pipe was farther to the east than the southeast end of the
― 97 ―furrow. The movement was in the same direction as before, therefore a pulling apart of the pipe took place instead of a compression. There occurred two breaks in the pipe (see plate 59A), the main one at the crossing of the fault, and the other 150 yards away on the northeast side of the fault, but very near it, the pipe being almost parallel with it. At the main break, the pipe was pulled apart 59 inches, and at the other one 21.5 inches, making a total displacement of 6.666 feet. The pipe was not quite parallel with the fault and therefore there was a slight offset, at right angles to its direction, of 4 inches at the main break and 2 inches at the minor one, or a total of 6 inches. A fence which crost the fault at the main break is offset 6.5 feet. (Plate 60C.)
The index map, fig. 30 (p. 95), indicates the position of three dislocated fences which were surveyed by R. B. Symington, C.E. The fences are marked A, B, C. One of these fences, C, near the upper end of San Andreas Lake, is nearly normal to the trace of the fault, and its deformation extends over a zone 1,200 feet wide, the total displacement aggregating 16.9 feet. Here, as usual, the portion on the southwest side of the fault moved relatively to the northwest, but there was a distinct drag on the northeast side in the same direction. (See fig. 31.)
The offsets in three other fences southeast of San Andreas Lake are shown in figs. 32, 33, and 34 and plates 60D and 61B.
Thruout this 2-mile stretch within which the pipe line nearly parallels the fault-trace, the path of the latter is strongly marked by a prominent furrow in the sod, with the usual diagonal cracks and variable width. This furrow lies on the northeast side of the
At the San Andreas dam, the fault past thru a rocky knoll which serves as an abutment for the dam on both sides, the embankment being in 2 parts. The rocks were shattered and the road over the dam and the fence paralleling it were offset several feet in the usual direction. The ground here was traversed by several cracks, those on the southwest side of the fault branching southerly from it and those on the northeast side branching northerly. Below the dam a heavy wooden flume on a trestle within 50 feet of the fault-trace was demolished for about 60 feet of its length.
About 125 yards below the dam the fault past thru the lower end of a massively built brick and cement waste weir tunnel. The inside diameter of the tunnel was about 7 or 8 feet and the walls were 17 inches thick. At the intersection of the fault within this structure, the latter was stove in and smashed in pieces for a distance of about 28 feet. The tunnel was offset about 5 feet. In the shattering of the brick work, the cracks and ruptures in no case followed the cement between the bricks, but broke across the latter; the cement and its adhesion to the bricks being stronger than the bricks themselves, altho the bricks were evidently carefully selected and of good quality. Several cracks traversed the tunnel longitudinally and obliquely to the northeast of the part that was demolished. (See fig. 36.)
About 550 yards below the San Andreas dam, the fault-trace crost a boundary fence between the estate of D. O. Mills and the property of the Spring Valley Water Company, causing an offset of about 10 feet. Here the deformation of the fence was distributed over a zone 300 feet wide in the direction of the fence, or about 250 feet in a direction normal to the trace of the fault. A survey of the dislocated fence made by R. B. Symington, C.E., is shown in fig. 37. Half a mile below the dam, the fault again crost the Pilarcitos pipe. A note by Mr. Anderson as to the conditions at this intersection is as follows:
It is a 2-foot pipe made of iron 1 inch thick. The fault broke it at an upward bend. An elbow at the bend was crusht by the compression and thrown down, while the two remaining ends were brought about 22 inches nearer together. At the same time they were faulted past each other a distance of 20 inches. The pipe runs N. 25° E., making an angle of 65° with the fracture, which here runs N. 40° W. The telescoping at this angle, being 22 inches, represents 52 inches of faulting.
In this neighborhood the fault crost a wire fence nearly normally, the line of which had been carefully established by a series of stone monuments. The fence marks the boundary between the estates of D. O. Mills and A. M. Easton. The deformation of the fence as shown in the accompanying diagram, fig. 38, from a survey by R. B. Symington, C.E., extended over a zone at least 2,200 feet wide. On the southwest side of the fault-trace, the fence was displaced to the northwest a distance of 9.3 feet, and on the northeast side it was displaced to the southeast 3.4 feet, making a total displacement of 12.7 feet and showing a slight drag close to the line of the fault. There were two parallel cracks representing the fault about 90 feet apart, and the chief displacement took place on the west crack.
About 0.625 mile farther southeast, near the upper end of Crystal Springs Lake, the fault crost another fence showing a displacement of 9 feet. About 0.25 mile southeast of this place, the fault crost the Locks Creek 44-inch pipe line. Regarding this intersection Mr. Anderson writes:
Just above the northern end of Crystal Springs Lake, a 44-inch water main made of iron 0.125 inch thick runs up the hill from the lake valley in a direction about N. 28° E. This line is buried all the way under several feet of soil. The fault crosses it at the base of the hill, in its N. 37° W. course, thus making an acute angle of 65° with the pipe line. At the intersection of the fault and the pipe line, the heavy rivets of the pipe were torn out all the way around at a section joint and the two sections were jammed into one another a distance of 4 feet 4 inches. In addition to the telescoping of this pipe, a slight change in course was induced, so that the northeast end trended one or two degrees more toward the east than the other end. This was shown by the fact that the broken ends did not fit into each other squarely. There was no lateral displacement, the whole movement having been taken up by the telescoping, but there was a bending of the pipes at the point of the break, as mentioned. The main part of the pipe, at a distance from the fault, must have moved with the land. At the fault-trace there was a bend amounting to one or two degrees. Supposing the bowing to be simple, this amount indicated that the land must have carried the pipe the distance represented by the telescoping, or about 10 feet, within 300 to 500 feet of the fault on one side, and that beyond such a point the pipe must have preserved its normal course. As a matter of fact, this same pipe was broken on the northeast side of the fault about 400 feet further up the hill. The break occurred at the junction of 2 sections, the rivets having been sheared off and part of the rim torn away at the rivet holes. The ends were pulled apart 3.375 inches. Here the pipe resumed its former course, but owing to the slight amount of the pipe displayed by the excavation, it was impossible to see whether a return bend occurred or not. Beyond the break the direction was as before measured, approximately N. 28° E. No such break occurred on the southwest side of the fault. A crack was formed in the earth at right angles to the pipe for several yards on either side of the break.
The measurements of the engineers of the Spring Valley Water Company on the break and displacement of this pipe at the intersection above described by Mr. Anderson are given in the accompanying diagram, fig. 39.
About a mile southeast of the Locks Creek pipe line, the trace of the fault entered Crystal Springs Lake for the stage of water of April, 1906. At 2.5 miles farther
Where the fault intersects the causeway dam between Upper and Lower Crystal Springs Lakes, the dam was dislocated and offset about 8 feet. (Fig. 40.) This displacement was well marked in the roadway across the dam and in the fences which parallel it. The fences on both sides of the road were broken and the boards were buckled and shoved over each other; the telephone wires crossing the lake sagged considerably, showing that the movement brought the poles closer together. The facts indicate, as previously stated, that, in addition to the offset of the dam along the line of the fault, there was a notable compression in the direction normal to it. Beyond this dam the trace of the fault is partly beneath the lake and partly skirts its southwest shore (for the water level of April, 1906), and finally leaves the lake on that side about 0.25 mile from its southeast end.
Exposures of the fault-plane (R. Anderson). — In addition to the evidence given by fences and pipes, there is the displacement of land surfaces and actual exposures of the fault face at the surface. Examination of mounds, embankments, and shore lines crost by the rupture usually revealed a displacement of the surface, and an interruption of the old topographic outlines. In the case of mounds cut by the fracture, the displacement makes itself apparent in vertical scarps in consequence of the curved surfaces being faulted past each other. At the northwest face of a hillock, near where the furrow emerges from Crystal Springs Lake, the northeast side of the mound — the side away from the lake — has retreated relatively, leaving a portion of its lower slope in juxtaposition with the higher slope of the other side. A horizontal line across the exposed face would give the distance moved, provided no subsidence had taken place, which does not seem to have been the case. The distance could be only approximately measured, but it is at least 8 or 10 feet. A crack 2 to 3 feet wide and several feet deep separates the two walls locally. Looking at the other side of the same mound an irregular face several feet in height is exposed on the northeast side of the fault, the natural result of a longitudinal slipping. The faulting of raised surfaces after this fashion was discovered in various other instances. Large hills were crost only two or three times in this stretch of the fault. They were not so affected.
The banks of stream channels sometimes preserved evidence of the movement even more completely than did mounds. Almost every gully crost by the fracture suffered
The writer is indebted to Mr. C. E Durrell and Mr. F. D. Posey, of St. Matthews School, San Mateo, for the discovery of this interesting example of faulting.
A steep embankment of weathered serpentine and soil occurs at the southern end of San Andreas Lake, where it is crost by the fault. The zone of rupture is several feet in width and the broken material on the northeast side is shown projecting several feet beyond its previous position in continuity with the serpentine slope. A displacement of the shore line is observable at several places where the fault fissure enters the lake. Wherever cracks were opened, search was made for the disjointing of squirrel holes and other discrepancies due to shifting, but with rather unsatisfactory results. Roots, however, were found broken and displaced in accordance with the general movement as shown by other things. In general, the search for evidence in the separation of different zones of vegetation or of color in the earth, etc, failed to add anything of value to evidences of other kinds.
Vertical movement (R. Anderson). — No proof was found of a vertical movement along the fault line. Here and there occur small escarpments along the fissures, varying from a few inches to several feet in height. They were only local, however; they exhibited no constancy in the side of the fault upon which they appeared, and were invariably explainable either as fault exposures such as are discust in the previous paragraphs, or as
Crystal Springs Lake to Congress Springs
For our knowledge of the character and extent of the earth's movement on the fault for that portion of its course lying within the limits of the Santa Cruz quadrangle of the U. S. Geological Survey, or between Crystal Springs Lake and the vicinity of Congress Springs, we are indebted to observations made by Messrs. H. P. Gage, F. Lane, S. Taber, and B. Bryan, under the direction of Professor J. C. Branner. The notes of these gentlemen are preceded by a summary statement, and are arranged as far as possible in sequence from northwest to southeast in the following section:
Summary statement (J. C. Branner). — The fault-trace that follows the San Andreas Valley continues southeastward in a nearly straight line. Beyond Crystal Springs Lake, it passes thru the village of Woodside, the Portola Valley, crosses Black Mountain a mile southwest of the triangulation station, follows down the general course of Stevens Creek a distance of 5 miles, and thence, following the same general direction along the eastern slope of Castle Rock Ridge, passes off the eastern side of the Santa Cruz quadrangle near latitude 37° 10'. West of Stanford University it follows along the northeastern base of the mountains that lie between the Pacific Ocean and the Bay of San Francisco, but as it passes toward the southeast, it cuts into the range and leaves Black Mountain and Monte Bello Ridge on the northeast side, while south of Saratoga it keeps well within the mountains. A singular feature of the fault, as it appears at the surface, is that instead of following the bottoms of the valleys, it often skirts along the base of one of the enclosing ridges, as shown in the accompanying sections. (Fig. 41.) This is not an invariable rule, however.
It will be seen from the map, No. 22, of the isoseismal lines on the Santa Cruz quadrangle that there are several other faults within the area of the quadrangle, but evidences of movement at the time of the earthquake have been found only on this San Andreas fault-line, with the possible exception of slight movements along part of the Black Mountain fault. Cracks in the ground occur here and there almost all over the area covered by the sheet, but the cracks away from the San Andreas fault are due to incipient landslides or to the settling of loose or wet ground, and are not otherwise related to the more profound faults.
The movement that took place along the fault in this portion of its course at the time of the earthquake was almost entirely a horizontal one. At several places evidences were seen of vertical displacement, but further examination showed in many instances that the appearances were deceptive or due to local causes. For example, where the fault crosses the top of Black Mountain there was apparently an upthrust on the northeast side of the fault. But it was found later that a great wedge-shaped piece nearly half a mile across had settled on the southwest side of the fault, producing this appearance.
The direction of the horizontal displacement is uniformly a relative southeastward movement of the land on the northeast side of the fault. The amount of displacement varies in this area from near zero to 8.5 feet. This variation appears to be due to the character and condition of the ground. Usually ground that was wet and incoherent at
Crystal Springs Lake to Portola. — Southeast of the southern end of Crystal Springs Lake are numerous cracks along the line of the fault. One less than 0.5 mile from the southern end of the lake past directly under a house, the chimney of which had fallen, and the building had burned to the ground. (28, map 22.) About 100 feet southeast of the road near this house (29, map 22) a crack 1.5 feet wide in places runs approximately parallel to the road. The cracked belt adjoining is about 4 feet in width, the downthrow being about 6 inches on the northeast side, the lateral thrust about 1 foot on the same side, the northeast side moving southeast relative to the opposite side.
About a mile southeast of the lake are large cracks, running approximately north and south, in places 1.5 feet wide. At a point 2 miles southeast of the lake, a crack about a foot wide is crost by a fence running N. 53° W. (27, map 22.) The top wire of this fence was broken by tension during the shock, and the post nearest the crack was snapt off at the ground, the adjoining post being uprooted, and bent over in the same direction as the broken one. The posts are of split wood about 5 inches in diameter, and the wires
Further southeast, down the road toward Portola, the main fault-trace crosses the road just north of the creek and within 12 feet of a giant redwood. The ground was raised and crumpled across the road, and the cracks extend both up and down the stream from this place. In a cluster of young redwoods southeast of this road a board fence is bent out of line, and huge cracks opened among the roots of the trees. A wire fence was pulled in two and one of the posts split. In this cluster of trees the fault past thru and split a big redwood stump.
Two fences crossing the crack at right angles near 12 (map 22) had been thrown out of line, their northeast portions being moved southeast relative to their southwest portions. They had been given an offset of 8.5 and 8 feet respectively. (Plate 63B.) A large oak tree standing on the crack was uprooted, while branches were snapt on a big white oak tree just south of the fault line.
Northwest of Searsville Lake, about a mile, there is a belt of cracked ground 7 or 8 feet wide, one crack being 1 foot in width. The apparent upthrust was in some places 2 feet on the northeast side. At other places there is no change of level. On the Portola road, just southwest of the Searsville Lake, parallel cracks with a trend N. 43° W., some of them 1.5 feet wide, were formed across the road and extended into the marsh to the northwest and into the woods on the southeast. Where they crost the road, the fence boards were broken and the earth shoved up in ridges; the northeast side of the crack moved southeastward.
The main fault fracture passes thru the Portola Valley and crosses the public road in front of a small 1-story house southeast of the village store. Where the fault crosses the road, the fences on both sides were torn in two, and in the prune orchard south of the road the rows of trees were displaced in some instances about 2 feet. The cracks in the road were about 6 inches wide, approximately parallel, and running nearly north-south, while the direction of the fault line itself was about northwest-southeast.
About a mile beyond Portola, a crack, measuring 2.5 feet in width in some places, crost a field, the cracked ground spreading out for 10 feet at intervals. Wooden fences crossing it were broken, water pipes bent and pushed up to the surface of the ground, and a dead tree near the line of the crack was thrown down. There was an apparent upthrust of about 2 feet on its northeast side.
Road from Judge Allen's southward. — Between 3 and 4 miles southeast of Portola, many cracks were visible extending in all directions. Several showed an uplift on the east or northeast side, which is also the downhill side. Some cracks were from 4 to 5 inches wide, and had a vertical throw of nearly a foot. In other places the downhill side had been thrust upward, and pieces of the crust shoved as much as 4 inches over the uphill side. Near the top of the ridge, just before reaching the point where the trail branches off, a 4-inch crack running S. 63° E. showed a 4-inch upthrow on the northeast (downhill) side. Southwest of the ridge and about 100 feet below the trail, an old landslide dating back to some time within the past year, covers about 2 acres. Around this slide the ground appeared to have been much cracked recently.
Along this trail the direction of the cracks varied considerably. One an inch wide in places, elsewhere branching into several smaller ones, was traced for about 150 yards, chiefly along the crest of the ridge. Its direction varied from due east to southeast, and the unpthrust on the west was sometimes as much as 3 feet. Going northwest down the crest of the ridge, numerous cracks crost in directions varying from southeast-northwest to northeast-southwest, several showing an upthrust varying from a few inches to a foot on the southwest side. At the foot of the trial, a large crack running down the center of the valley followed the road for about 100 yards, then cut across the fields. In places the crack was 2 feet wide, but in other places a ridge 3 feet high had been raised beside the road, and there were many parallel cracks within 50 feet of either side. There were upthrusts and downthrows, some as much as 1.5 feet, but the total change of level seemed to be nil.
Alpine road. — A fault branches from the main San Andreas fault in the Portola Valley and crosses the Alpine road just where the Portola road leaves the latter. At this fork several cracks were formed at the time of the earthquake. A water pipe 2 inches in diameter was buckled and lifted out of the ground here, and farther along the Portola road this same pipe was pulled apart. Following southward along the Alpine road, the next evidence of disturbance by the earthquake was where the main fault-trace crosses the road 0.75 mile south of where the Portola road forks. Here the road was so badly broken and cracked that it was not possible to ride or drive across the fracture until the place was repaired. (Plate 63A.) The fracture followed along the south side of the road for a distance of 300 feet, tearing up the bank with cracks, some of which were a foot or more across. Where the road bends toward the south, the fracture crost to the north side of the road, making cavities several feet deep. These cracks continued toward the northwest thru the underbrush, pulling apart a barbed wire fence and leaving many well-marked furrows thru the adjoining fields. About 30 feet north of the road, a white oak, somewhat weakened by decay and fire, was jerked off by the violence of the shock. To the southeast the fault-line is traceable by a well-marked furrow thrown up in the fields. Where the fracture crosses the Alpine road, there appears to have been an uplift of about 2 feet on the northeast side of the fault. This appearance may be due to the settling of a part of the ridge of incoherent materials to the south, or it may be due to the lateral thrust along a sloping surface.
Black Mountain. — The great mass of Black Mountain lies between the San Andreas fault and a branch fault (Black Mountain fault) which, starting in the Portola Valley, crosses the Page Mill road on the north side of the mountain about a mile south of Clarita vineyard. This area between the faults was badly shattered by the earthquake, tho it is not clear whether the abundant cracks found over the surface are to be attributed to the boldness of the topography or to the crushing of the wedge-shaped end of the fault block. Several days after the earthquake, 345 cracks, large and small, were counted along the county road (Page Mill) in a distance of less than 3 miles between these faults. These cracks ran in every direction, and some of them were clearly attributable to local topography, while others cut thru the mountains in apparent disregard of the topography.
The main fault-trace crosses the Page Mill road in a topographic saddle near three frame houses. The displacement occurred along two parallel and well-defined cracks some 30 feet apart. These cracks can be traced across the fields on both sides of the road. Toward the northwest they converge until they are only a few feet apart. Where they crost the road, the fracture was not a single clean-cut break, but made up of a series of small short cracks from 3 to 5 inches across, parallel with each other and "splintering" across the general direction of the fracture. The fences on both sides of the road were displaced about 3 feet, and there was an apparent drop of 18 inches on the southwest side of the fault. The horizontal displacement showed the northeast side to have moved
Page Mill road. — In following the Page Mill road up Corde Madera Creek from Mayfield, the first noticeable trace of the earthquake was a crack crossing the road due east and west, its width varying from 0.5 to 1 inch. Wagon-tracks showed a lateral displacement of 1 inch, the north side of the crack having moved west, relatively to its south side. This crack was traced a short distance into the fields beside the road, where it disappeared. Several smaller cross-cracks intersected it at intervals. There was no apparent vertical displacement. About 100 yards farther south were 3 smaller cracks varying from 0.25 to 0.75 inch in width. One ran N. 53° W., and another N. 23° W. The latter, being only 8 feet from a culvert crossing under the road, appears to have been deflected by this from a course running more nearly east. Here again was no evidence of vertical throw. Going on up toward the Alpine road from this point, more and more cracks were found, running approximately east and west, with the exception of several north and south ones where the road ran closely parallel to the stream. Less than a mile from the first crack, groups of cracks were accompanied by small slides of dirt from the hill to the west of the road, and farther on from the bluff to the east of it. The cracks ran nearly parallel with the axis of the branch valley lying northeast and southwest. Farther up the road, large cracks began to appear among smaller ones running parallel. The first of these was 2.5 inches across and ran S. 13° E., with a downthrow of 1 inch on the east side, and could be traced from 50 to 100 feet on either side of the road. For a mile farther up the road, the cracks became so numerous and complicated that it was impossible to map any individual ones. They intersected and ran in all directions, and were all of varying widths, the largest seen measuring 8 inches across. The size of this crack, however, was probably partly due to its position on the side of a hill. The larger cracks could be traced for several hundred feet. In some places crushing had taken place, and the layer of macadam on the road had been humped up and broken. In this same area are many small landslides, some large enough to cover the road; one has occurred since the earthquake.
Stevens Creek. — Following the road from the junction of the Castle Rock Ridge road with the road from Stevens Creek to Boulder Creek toward Stevens Creek, small cracks appeared crossing the road in a direction of N. 1° E. Further east nearer Stevens Creek, the road was badly broken up by the land sliding in two directions, N. 18° W. and N. 47° E. All along this region cracks varying from a fraction of an inch to 2 inches in width, and running from N. 43° W. to due north and south, appear every 10 feet or more, showing a badly broken-up area. Here and there such cracks resulted in landslides from the bank to the road. A crack about 2 inches wide ran N. 53° W. for some distance above the house, at the junction with the Stevens Creek road. On the Stevens Creek road, just after leaving the Saratoga road, there were cracks every 20 or 30 feet, running in the same direction, about N. 43° W. A mile and a half northwest of the place where Stevens Creek turns northeast, a strip of ground 2 feet in width and about 100 yards long had been broken up, with a downthrow of about 6 inches on the west side. The cracks ran N.
South of Congress Springs. — Near and northwest of the reservoir 2.5 miles southeast of Congress Springs, fissures from 4 to 6 feet wide ran nearly north and south, and past thru the earthen dam at the northwest end of the reservoir. (Plate 64A.) The intake pipes at the south end of the reservoir were disconnected, and the escaping water undermined a part of the southern dam of the reservoir. This reservoir is in a topographic saddle and has dams at both ends. The fault-trace passes thru this saddle. Where the bottom of the reservoir was exposed by the escape of the water, cracks of the fault-trace were exposed in the mud. Fences crossing the fissures showed but little displacement; the displacement moved the northeast side toward the southeast, relatively. The hills southeast of the reservoir have steep slopes of from 20 to 50 degrees. The cracks follow the east-facing slopes and the east side of these cracks had raised about 6 or 8 inches. Southeast of the reservoir the chimneys and water-tanks were down.
Congress Springs to San Juan
Mr. G. A. Waring, under the direction of Prof. J. C. Branner, studied the displacement along the fault from the vicinity of Congress Springs to its southern end near San Juan. The following is an account of the phenomena observed by him:
Cracks and displacements along the fault-trace (G. A. Waring). — Starting at the upper reservoir about 2 miles south of Congress Springs, the fault-trace was followed to its southern end near San Juan. From the upper reservoir, thru which the fault past, cracking the dams at each end, a fairly continuous series of cracks a few inches wide runs down the southwest side of Lyndon Creek about 2 miles to Mr. Edwards' place, "Glendora." Thruout this distance the individual cracks run S. 3° to 13° E., while the line as a whole trends S. 33° E. The relative movement of the northeast side of the fault is from 14 to 20 inches southeast. From Glendora the fractured zone becomes wider and not so distinct. The lower reservoir is slightly cracked and several fissures appear near it, but the main line of fracture seems to be nearly 0.5 mile west of it, showing in two or three cultivated fields. The whole ridge west of the reservoirs was severely shaken, however, for cracks 4 or 5 inches wide opened near Grizzly Rock and several large slides occurred in its neighborhood. One water-pipe running north and south on the Beatty place was broken, while one trending east and west was unhurt. No cracks were found crossing the ridge between Grizzly Rock and White Rock. The cracks were next found on the road about a mile east of B. M. 2135 of the U. S. Geological Survey, but they do not show in the vineyard to the southeast. On the ridge road, about 5 miles northwest of Wright Station, the fault again shows slightly in a few 2-inch cracks bearing S. 3° E., with a slight relative movement of the east side toward the north. Going down the slope from here to Wright, the cracks rapidly become larger.
At Patchin, 3 miles west of Wright Station, there are fissures over a foot wide trending mainly in the direct line of the fault (S. 33° E.). Several stretches of numerous small
The road at Corralitos is said to have been slightly cracked, and in the low hills between Valencia and Corralitos a few cracks were found; but the fault evidently runs fully 0.5 mile east of Corralitos. The mountain roads east and northeast of Corralitos were rendered impassable by landslides and by the bridges being injured. Crossing the road near Hazel Dell Creek is a band of small cracks 35 yards wide, trending S. 3° E. The fence on either side is not displaced, but the posts lean 30° to the southwest. About 0.25 mile farther northeast the stake fence on the northwestern side of the road is moved 10 inches out of line, and the ground just beyond has sunk a few inches. The fissures appear to die out in the marshy land west of Wm. McGrath's house, and they begin again a mile eastward, halfway up the slope. Thru this upland meadow region is a series of slides and sinks gradually rising in elevation. At a small ravine, fissures again appear and follow up it (S. 33° E.) for 0.25 mile, mainly as a great furrow from 2 to 6 feet wide. Three ponds near the divide lie directly in its path, but the cracks are only a few inches wide here. Thru the grain fields beyond they are not very evident until at the divide between the steep slope to the Pajaro River and the gentle westward drainage. Several cracks a foot or less in width show on the ridge, but the fault seems to set off about 100 yards to the northeast and to consist of east and west cracks, having loosened the whole slope for nearly a mile northward of Chittenden, causing great landslides. The fault-line crosses the Pajaro River at the railway bridge at Chittenden. The movement is shown by the disturbance of the concrete bridge piers. (See plates 17A, 65B, and fig. 43.) Thence straight across the low hills and fields on the opposite side of the river a line of cracks extends, passing 0.5 mile west of Mr. Canfield's house, "just where the earth cracked 16 years ago." This crack crosses the Sargent-San Jose road a mile north of San Juan,
The tunnel at Wright Station (E. P. Carey). — Mr. Everett P. Carey reports that he made an examination of the tunnel at Wright Station soon after the earthquake, and again on February 17, 1907. The result of his observations is incorporated in the following memorandum:
The length of the tunnel is 6,200 feet. Its direction is S. 48° 24'.5 W. A fissure crost the tunnel 400 feet from the northeast portal, along which there was a lateral displacement of 4.5 feet. The movement on the southwest side was northerly with reference to the northeast side. Nothing of this fissure can now be seen, as the tunnel along that part has been excavated, the walls timbered and entirely obscured from view. My description rests on my examination soon after the earthquake, before any work had been done. The strike of this fissure is N. 52° W., making an angle of 80° with the trend of the tunnel, and it dips at an angle of about 75° to the west. The walls of the fissure were well smoothed and slicken-sided, but I did not determine the direction of the striæ. Specimens from this fissure indicate that the fault occurred in sandstone, and that much movement had already taken place along the same fault in apparently a variety of directions. Specimens secured at the time have changed from a damp, sticky, clay-like mass to a relatively dry, hard, and crumbled condition. Streaks of light-colored sandstone occurred in this dark attrition material.
The damage to the tunnel itself consisted in the caving in of overhead rock; the crushing in toward the center of the tunnel of the lateral upright timbers, and the heaving upward of the rails, due to the upward displacement of the underlying ties. In some instances these ties were broken in the middle. In general the top of the tunnel was carried north or northeast with reference to the bottom. This seems to be the prevailing condition in the exposed part of the tunnel not yet repaired.
I examined with particular care the walls of the tunnel at several points where the damage to the timbers appeared to be greatest, more especially between 1,400 feet and 2,200 feet in from the opening at Wright. At each place I found several fissure lines running somewhat irregularly, but in general parallel to the fissure already described 400 feet in from the entrance at Wright. These fissures all contained more or less attrition material. Three of them I had an opportunity to examine better than the others. In each case two distinct sets of striæ were found, one set vertical and the other set horizontal. The horizontal set was clearly more recent than the vertical set, and to all appearances might have been formed the day before. The three slicken-sided faults mentioned were the only ones that lookt as if recent movement had occurred. The rocks in the tunnel look like sandstones and jaspers of Franciscan age. According to the evidence, so far as it went, the whole of the top of the mountain is fissured thruout in such a way that a large movement could be distributed among several fissures and thus account for a relatively slight motion along any one fissure. The measuring of any minor movements in the tunnel would be difficult because of the caving in of the rocks at such points. It would seem, too, that such movement could occur without materially altering the line of the tunnel at that point, so far as the timbering is concerned.
As far as learned no recognized fissures or faults have been crost by the workmen thus far, except the one 400 feet from the northeast portal. Nothing corresponding to the fissure passing Morrell's house has yet been found in the tunnel.
Engineers' measurements of displacement. — The reconstruction of the tunnel at Wright Station necessitated an instrumental survey of the displacement in so far as it immediately affected the structure. The results of this survey have been placed at the disposal of the Commission by Mr. J. D. Matthews, assistant resident engineer in charge of the work. The plot of the survey is given in fig. 42. The plot shows that, while the tunnel is traversed by only one fault fracture, at a distance of about 400 feet from the northeast portal the deformation has been distributed over a distance of nearly a mile. This deformation of the tunnel, or its departure from a straight line, is measured from a line drawn from the northeast portal to a point on the same side of the tunnel 675 feet in from the southwest portal. It indicates a bending of the ground to the northwest in the direction of the relative displacement on the southwest side of the fault. That is to say, the bending is in the opposite direction to that which would be characteristic of the drag of a fault. A possible explanation of this phenomenon is that the ground pierced by the tunnel was in a state of excessive elastic stress, even at the time the tunnel was constructed; and that the relief effected by the rupture rendered resilience operative and so caused the ground to be flexed in the sense opposite to that of a drag. The nature of the deformation of the ground on the northeast side of the fault is not yet known. It may be here mentioned, in regard to the effect of the fault upon the steel bridge at Chittenden, that, in addition to the cracking and displacement of the supporting piers, as noted by Mr. Waring, the distance between the abutments was lengthened about 3.5 feet, according to measurements supplied to the Commission by Mr. J. H. Wallace, Assistant Chief Engineer, Southern Pacific Company, and illustrated in the accompanying diagram, fig. 43.
Geodetic Measurements of Earth Movements
Published by permission of the Superintendent of the Coast and Geodetic Survey.
By John F. Hayford and A. L. Baldwin
The Coast and Geodetic Survey has done much triangulation in California to serve as a control or framework for its surveys along the coast and other surveys. The results of all the triangulation, south of the latitude of Monterey Bay, together with the primary triangulation to the northward, have already been published.
See Appendix 9 of the Report of the Coast and Geodetic Survey for 1904, Triangulation in California, Part I, by A. L. Baldwin, Computer.In 1906 the results of the triangulation in California, from the vicinity of Monterey Bay northward, were being prepared for publication. The reports from various sources in regard to the effects of the earthquake of April 18, 1906, indicated that there has been relative displacements of the earth's surface of from 2 meters (7 feet) to 6 meters (20 feet) at various points near the great fault accompanying the earthquake. These were relative displacements of points on opposite sides of the fault and had been reported along all parts of the fault for 185 miles, from the vicinity of Point Arena, in Mendocino County, to the vicinity of San Juan, in San Benito County.
Preliminary Report, State Earthquake Investigation Commission, Berkeley, May 31, 1906.The average relative displacement was said to be about 3 meters (10 feet). Displacements of that size would so change the relative positions of points which had been determined by triangulation and so change the lengths and directions of the lines joining them that the triangulation would no longer be of value as a means of control for accurate surveys. The value of the triangulation could be restored only by repeating a sufficient amount of it to determine definitely the extent and character of the absolute displacements. It was, therefore, decided to repair the old triangulation, damaged by the earthquake, by doing new triangulation.
If the displacements of a permanent character had occurred in a narrow belt only, close to the fault, but a few triangulation points would have been affected. The available evidence, however, indicated that the movements probably extended back from the fault for many miles on each side, and that the new triangulation necessary for repair purposes must, therefore, cover a wide belt.
The new triangulation to repair the damage was completed in July, 1907. In addition to serving this practical purpose, it has shown the character of the earth movements of 1906, which were found to extend back many miles on each side of the fault. These are very interesting results from a purely scientific point of view. Moreover, there came to light, during the study of the movements of 1906, entirely unexpected evidence of earlier earth movements, probably in 1868, which also affected a large area.
The purpose of this paper is to set forth fully the amount and nature of these two great displacements of large portions (at least 4,000 square miles) of the earth's crust and to indicate the degree of certainty in regard to these displacements warranted by the evidence.
Extent of New Triangulation
The new triangulation done during the interval July 12, 1906, to July 2, 1907, extends continuously northwestward from Mount Toro, in Monterey County, and Santa Ana Mountain, in San Benito County, to Ross Mountain, and the vicinity of Fort Ross, in Sonoma County. This new continuous triangulation, as indicated on map No. 24, extends over an area 270 kilometers (170 miles) long and 80 kilometers (50 miles) wide, at its widest part. It includes the station known as Mocho, about 11½ miles northeast from Mount Hamilton
This continuous scheme consists of a chain of primary triangulation comprizing the eleven occupied stations, Mount Toro, Gavilan, Santa Cruz Azimuth Station, Loma Prieta, Sierra Morena, Mocho, Mount Tamalpais, Point Reyes Hill, Tomales Bay, Sonoma Mountain, and Ross Mountain; triangulation of the secondary grade of accuracy extending from the stations, Mount Tamalpais, Mount Diablo, Rocky Mound, and Red Hill, to the Pulgas Base near the southern end of San Francisco Bay, and triangulation of a tertiary grade of accuracy in three different localities; namely, in the vicinity of Colma, west of San Francisco Bay, along Tomales Bay, and in the vicinity of Fort Ross, Sonoma County.
The primary and secondary triangulation are shown on map No. 24, and the tertiary triangulation on map No. 25. On these two maps the straight blue lines indicate lines over which observations were taken in the new triangulation. The small red circles indicate stations marked upon the ground, of which the relative positions were fixt by the triangulation. Observations were taken in both directions over each blue line which is unbroken, thruout its length. Observations were taken in one direction, only, from the solid end toward the broken end, over each blue line which is broken at one end. A station from which no blue line is drawn unbroken was not occupied. The position of such a station was determined by intersections from the occupied stations.
In addition to this continuous triangulation, a detached piece of new triangulation of the secondary grade of accuracy, connecting old triangulation stations, was done in the vicinity of Point Arena. (See map No. 25.) This makes the total number of old triangulation stations which were recovered and redetermined 61.
In connection with the new triangulation, astronomic determinations of azimuth or true direction were made by observations on Polaris at the stations Mount Tamalpais, Mocho, and Mount Toro.
Four different observers, each with his own complete outfit and party, were engaged in the new work for an aggregate period of 35 months. The observers were all field officers of the Coast and Geodetic Survey, with previous experience in triangulation.
Mr. J. F. Pratt, Assistant, was in the field from August 4, 1906, to July 2, 1907, and made the observations at the five primary stations, Ross Mountain, Sonoma Mountain, Tomales Bay, Point Reyes Hill, and Mount Tamalpais.
Mr. W. B. Fairfield, Assistant, was in the field from August 11, 1906, to May 29, 1907. He did nearly all of the Tomales Bay triangulation, made the observations at the primary stations, Mocho and Sierra Morena, and did a part of the secondary triangulation in the vicinity of Pulgas Base.
Mr. C. H. Sinclair, Assistant, was in the field from July 14, 1906, to April 10, 1907. He made the observations at the primary stations, Santa Cruz Azimuth Station, Loma Prieta, Gavilan, and Mount Toro, and also did a part of the secondary triangulation in the vicinity of Pulgas Base.
Mr. Edwin Smith, Assistant, was in the field from July 12 to July 24, 1906, engaged in making the reconnaissance and other preparations for triangulation along Tomales Bay. He was then called away on other duty and Assistant Fairfield completed the Tomales Bay triangulation. Between September 26, 1906, and February 26, 1907, Mr. Smith did
These observers remained at their work continuously in spite of many delays and discomforts due to fog, rain, snow, gales, and roads which were at times nearly or quite impassable. To them must be given the credit for overcoming the difficulties and securing the observations of the necessary high grade of accuracy.
The Old Triangulation
The old triangulation fixing the positions of the points before the earthquake of April 18, 1906, was done in many years, extending from 1851 to 1899, as a part of the regular work of the Coast and Geodetic Survey and without reference to the possible future use of this triangulation as a means of determining the movements of permanent character due to earthquakes. During the earlier years certain parts of this old triangulation had existed as detached triangulation not connected with other parts. Before 1906, however, all parts of the old triangulation had been connected with each other by triangulation to form one continuous scheme. It was also connected with other triangulation extending to many parts of the United States, including many of the interior states, as well as the Atlantic and Gulf Coasts.
In connection with studies of the evidence as to the earth movements set forth in this paper, it is important to note briefly the dates of the old triangulation which serves, in connection with the new triangulation of 1906-1907, to determine changes in positions of marked points on the earth's surface.
During the years 1854-1860 primary triangulation was carried from the stations, Rocky Mound, Red Hill, and Mount Tamalpais, northward to Ross Mountain, thru a primary scheme practically identical with that shown on map No. 24, except that the station Bodega was occupied in this earlier triangulation, tho not in 1906-1907.
Tertiary triangulation, following substantially the scheme shown on map No. 25, was also done in 1856 to 1860, along Tomales Bay, starting with the line Tomales Bay-Bodega, of the primary triangulation referred to in the preceding paragraph. In connection with this work, the station Chaparral of the Fort Ross triangulation, shown on map No. 25, was also determined.
Primary triangulation was done during the years 1851 to 1854, connecting the group of stations, Mount Diablo, Rocky Mound, Red Hill, with the Pulgas Base, the scheme being somewhat different from that shown on map No. 24, but equally direct and strong.
During the years 1854-1855, 1864, 1866, primary triangulation was done connecting the stations in the vicinity of Rocky Mound, referred to in the preceding paragraph, with stations Gavilan, Santa Cruz, and Point Pinos Light-house around Monterey Bay. This triangulation, for the greater part of its length, consists of a single chain of triangles, affording, therefore, comparatively few checks upon the results.
This practically completes the statement of triangulation done before 1868 which is concerned in the present investigation. The extent of the triangulation done between 1868 and 1906 is stated separately in the following paragraphs.
Northward of the line Mount Diablo-Mount Tamalpais, but one station of the primary scheme, shown on map No. 24, was determined by primary triangulation in the interval 1868-1906; namely, Ross Mountain. It was determined directly from the stations Mount Tamalpais, Mount Diablo, and Mount Helena of the transcontinental triangulation.
See The Transcontinental Triangulation, Special Publication No. 4, pp. 597-608.
During the years 1876-1887, primary triangulation was extended southward (by substantially the same scheme as that shown on map No. 24, except that station Gavilan was omitted) from the line Mount Diablo-Mount Tamalpais to the line Mount Toro-Santa
Secondary triangulation near Point Arena, forming the western extremity of the transcontinental triangulation, was done in the interval 1870-1892, the scheme being substantially the same as that shown on map No. 25, except that all stations were occupied. The triangulation fixing the initial stations, Fisher and Cold Spring, has been published.
See The Transcontinental Triangulation, Special Publication No. 4, pp. 597-610.
Tertiary triangulation in the vicinity of Fort Ross was done in 1875-1876, following a scheme similar to that shown on map No. 25, and starting from the line Bodega Head-Ross Mountain, as determined before 1868.
Tertiary triangulation was done during various years from 1851-1899, extending from the vicinity of the Pulgas Base northward, spanning San Francisco Bay, to the Golden Gate, and thence southward to the vicinity of Colma, including stations shown on sketch No. 4 on map No. 25. The greater portion of this triangulation was done before 1868, but it is impracticable to separate the computations into two parts dealing with triangulation before and triangulation after 1868, respectively.
Permanent Displacements Produced By the Earthquakes of 1868 and 1906
The following tables, Nos. 1, 2, and 3, show the permanent displacements of various points as caused by the earthquakes of 1868 and 1906. These permanent displacements were determined by comparisons of the positions of identical points upon the earth's surface as determined by triangulation done before and after the earthquakes in question.
While for the sake of brevity in statement these movements are referred to the earthquakes of 1868 and 1906, the evidence furnished by the triangulation simply indicates the fact that the displacements in question took place sometime during the two blank intervals within which there was no triangulation done fixing the points in question; namely, the interval 1866-1874, including the 1868 earthquake, and the interval 1892 to July, 1906, including the 1906 earthquake. Neither does the triangulation furnish any evidence indicating whether the displacements took place gradually, extending over many months and possibly years, or whether they took place suddenly. The evidence connecting the displacements of 1906 with the particular earthquake and indicating that they were sudden comes from other sources and will be commented upon later in this report.
The permanent displacements indicated in tables 1, 2, and 3, must be carefully distinguished from the vibrations of a more or less elastic character which take place during earthquakes. These vibrations die down in a few seconds, minutes, or hours. While they are in progress, a given point on the earth's surface is in continuous motion along a more or less complicated path which turns upon itself and leaves the point, at the end of the vibration, near the initial position. The displacements indicated in tables 1, 2, and 3, on the other hand, remain for years, possibly for centuries. They are of a permanent character. The displaced point remains in the new position until another displacement occurs in some later earthquake, or possibly by slow relief of strain accompanied by a creeping motion which causes a new permanent displacement. In tables 1, 2, and 3, the first column gives the name of the station by which it may also be identified on map 24 or on map 25, or both. The second column gives its latitude at the time indicated in the heading. The third column gives the seconds, only, of the new latitude at the later time indicated in the heading. The fourth and fifth columns have the same significance with reference to the longitude that the second and third have with reference to the latitude of each point. The sixth column gives the north and south component of the displacement
|Station||Latitude Before 1868||Latitude After 1868||Longitude Before 1868||Longitude After 1868||Southward Component of Displacement||Eastward Component of Displacement||Direction of Displacement||Amount of Displacement||Degree of Certainty|
|Rocky Mound||37°||52'||57.237''||57.253''||122°||14'||30.510''||30.507''||- 0.49||+ 0.07||188°||0.50||1.6||Doubtful.|
|Red Hill||37||33||04.717||04.730||122||05||41.003||40.982||- 0.40||+ 0.52||232||0.65||2.1||Doubtful.|
|Mount Tamalphais||37||55||27.455||27.507||122||35||45.228||45.242||- 1.60||- 0.34||168||1.64||5.4||Certain.|
|Farallon Light-house||37||41||58.210||58.250||123||00||03.579||03.605||- 1.23||- 0.64||153||1.39||4.6||Certain.|
|Point Reyes Hill||38||04||48.325||.....||122||52||00.801||.....||(- 1.51)||(- 0.31)||(168)||(1.54)||(5.0)||Inferred, certain.|
|Tomales Bay||38||10||55.456||.....||122||56||46.733||.....||(- 1.56)||(- 0.22)||(172)||(1.57)||(5.2)||Inferred, certain.|
|Bodega||38||18||23.680||.....||123||00||03.726||.....||(- 1.62)||(- 0.11)||(176)||(1.62)||(5.3)||Inferred, reasonably certain.|
|Ross Mountain||38||30||20.528||20.583||123||07||09.223||09.221||- 1.70||+ 0.05||182||1.70||5.6||Reasonably certain.|
|Bodega Head||38||18||29.249||.....||123||03||45.417||.....||(- 1.62)||(- 0.11)||(176)||(1.62)||(5.3)||Inferred, certain.|
|Tomales Point||38||12||45.732||.....||122||58||14.449||.....||(- 1.57)||(- 0.19)||(173)||(1.58)||(5.2)||Inferred, certain.|
|Foster||38||08||13.410||.....||122||54||23.271||.....||(- 1.54)||(- 0.26)||(170)||(1.56)||(5.1)||Inferred, certain.|
|Smith||38||14||51.518||.....||122||56||08.865||.....||(- 1.58)||(- 0.17)||(174)||(1.59)||(5.2)||Inferred, certain.|
|Mershon||38||10||55.295||.....||122||54||06.016||.....||(- 1.56)||(- 0.22)||(172)||(1.57)||(5.2)||Inferred, certain.|
|Hans||38||07||58.492||.....||122||52||02.072||.....||(- 1.54)||(- 0.28)||(170)||(1.57)||(5.2)||Inferred, doubtful.|
|Hammond||38||04||45.046||.....||122||48||34.993||.....||(- 1.51)||(- 0.31)||(168)||(1.54)||(5.1)||Inferred, doubtful.|
|Chaparral||38||29||33.905||33.964||123||10||56.207||56.216||- 1.82||- 0.22||173||1.83||6.0||Reasonably certain.|
|Loma Prieta||37||06||40.971||40.912||121||50||36.521||36.423||+ 1.82||+ 2.42||307||3.03||9.9||Certain.|
|Station||Latitude Before 1868||Latitude 1906-07||Longitude Before 1868||Longitude 1906-07||Southward Component of Displacement||Eastward Component of Displacement||Direction of Displacement||Amount of Displacement||Relation to Fault||Degree of Certainty|
|Rocky Mound||37°||52'||57.237''||57.262''||122°||14'||30.510''||30.515''||- 0.77||- 0.12||171°||0.78||2.6||32||20||E||Doubtful.|
|Red Hill||37||33||04.717||04.738||122||05||41.003||40.975||- 0.65||+ 0.69||227||0.94||3.1||19||12||E||Certain.|
|Mount Tamalpais||37||55||27.455||27.492||122||35||45.228||45.228||- 1.14||0.00||180||1.14||3.7||6.4||4.0||E||Certain.|
|Farallon Light-house||37||41||58.210||58.277||123||00||03.579||03.669||- 2.07||- 2.20||133||3.02||9.9||37||23||W||Certain.|
|Point Reyes Hill||38||04||48.325||48.470||122||52||00.801||00.906||- 4.47||- 2.56||150||5.15||16.9||2.7||1.7||W||Certain.|
|Tomales Bay||38||10||55.456||55.606||122||56||46.733||46.841||- 4.62||- 2.63||150||5.32||17.5||2.1||1.3||W||Certain.|
|Bodega||38||18||23.680||23.695||123||00||03.726||03.694||- 0.46||+ 0.78||239||0.90||3.0||2.0||1.2||E||Reasonably certain.|
|Ross Mountain||38||30||20.528||20.572||123||07||09.223||09.204||- 1.36||+ 0.46||199||1.43||4.7||7.0||4.3||E||Reasonably certain.|
|Sonoma Mountain||38||19||24.539||24.579||122||34||27.894||27.891||- 1.23||+ 0.07||183||1.24||4.0||34||21||E||Certain.|
|Pulgas E. Base||37||28||36.265||36.258||122||08||08.143||08.129||+ 0.22||+ 0.34||302||0.41||1.3||12||7||E||Doubtful.|
|Guano Island||37||34||23.655||23.649||122||15||43.475||43.479||+ 0.18||- 0.10||28||0.21||0.7||10||6||E||Doubtful.|
|Pulgas W. Base||37||28||48.787||48.764||122||15||15.681||15.673||+ 0.71||+ 0.20||344||0.74||2.4||3.5||2.2||E||Reasonably certain.|
|Bodega Head||38||18||29.249||29.417||123||03||45.417||45.443||- 5.18||- 0.63||173||5.22||17.1||2.2||1.4||W||Certain.|
|Tomales Point||38||12||45.732||45.874||122||58||14.449||14.549||- 4.38||- 2.43||151||5.01||16.4||2.0||1.2||W||Certain.|
|Foster||38||08||13.410||13.577||122||54||23.271||23.398||- 5.15||- 3.09||149||6.01||19.7||1.9||1.2||W||Certain.|
|Smith||38||14||51.518||51.522||122||56||08.865||08.839||- 0.12||+ 0.63||259||0.64||2.1||2.6||1.6||E||Certain.|
|Mershon||38||10||55.295||55.284||122||54||06.016||06.008||+ 0.34||+ 0.20||330||0.39||1.3||1.1||0.7||E||Certain.|
|Hans||38||07||58.492||58.479||122 52 02.072||02.093||+ 0.40||- 0.51||52||0.65||2.1||0.5||0.3||E||Doubtful.|
|Hammond||38||04||45.046||45.037||122||48||34.993||35.064||+ 0.28||- 1.73||81||1.75||5.7||1.2||0.7||E||Doubtful.|
|Chaparral||38||29||33.905||33.927||123||10||56.207||56.187||- 0.68||+ 0.48||216||0.83||2.7||1.6||1.0||E||Doubtful.|
|Black Mountain||37||19||09.810||09.761||122||08||48.462||49.402||+ 1.51||+ 1.48||316||2.11||6.9||1.4||0.9||E||Certain.|
|Loma Prieta||37||06||40.971||40.895||121||50||36.521||36.390||+ 2.34||+ 3.23||306||3.99||13.1||4.8||3.0||E||Certain.|
|Santa Cruz Az. Sta.||36||58||42.106||42.027||122||03||18.728||18.702||+ 2.44||+ 0.64||345||2.52||8.3||19||12||W||Certain.|
|Gavilan||36||45||21.068||20.961||121||31||11.504||11.341||+ 3.30||+ 4.04||309||5.22||17.1||6.4||4.0||W||Certain.|
|Pt. Pinos Light-house||36||38||01.551||01.399||121||55||58.939||58.795||+ 4.68||+ 3.58||323||5.89||19.3||39||24||W||Certain.|
|Point Pinos Lat. Sta.||36||37||59.413||59.279||121||55||31.685||31.578||+ 4.13||+ 2.66||327||4.91||16.1||39||24||W||Certain.|
By combining the values given in columns 6 and 7, the direction and amount of the displacement were obtained as shown in columns 8, 9, and 10. In column 8 the direction of displacement is given, reckoned as geodetic azimuths are usually reckoned, clockwise around the whole circumference from south as zero. In this reckoning, west is 90°, north, 180°, and east, 270°. Column 9 gives the amount of displacement in meters and column 10 gives it in feet. Column 11 shows the approximate distance of the point from the fault of 1906, measured approximately at right angles to the fault. In this column E indicates that the point is to the east of the fault and W that it is to the west.
For example: The fifth line of table 1 indicates that during the earthquake of 1906 the Farallon Light-house moved 0.83 meter north and 1.57 meters west, or, in other words, moved 1.78 meters (5.8 feet) in azimuth 118°, or 62° west of north, and that it is 37 kilometers (23 miles) from the fault of 1906 and to the west of it.
In the heading, the expression "Before 1868" refers to years within the interval 1851-1866. The expression "After 1868" refers to years within the interval 1874-1891, and "1906-1907" refers to dates within the interval July, 1906-July, 1907.
The latitudes and longitudes given in tables are all computed upon the U.S. Standard Datum and differ somewhat from those now in use on the charts and maps of this region. They are, however, the latitudes and longitudes to which all charts and maps should ultimately conform.
Table 1 shows the displacements which occurred on April 18, 1906; table 2 shows the displacements which occurred in 1868, and table 3 shows the total, or combined displacements in both 1868 and 1906.
For some cases, as, for example, Point Reyes Hill, the separate displacements were not directly determined by the triangulation but only the combined displacements. In such cases, if probable values could be derived for the separate displacements, indirectly, by inference from surrounding points, they were so derived and placed in the table. In each case, such inferred displacements are clearly distinguished in the table from others which were determined directly by measurement, by leaving the third and fifth columns blank and by having the values in the sixth to tenth columns enclosed in parentheses.
All of the displacements given in tables 1-3 are computed upon the assumption that the two stations, Mount Diablo and Mocho, remained unmoved during the earthquake of April 18, 1906. The reasons why this assumption is believed to be true will be set forth fully in a later part of this report.
In the tables the points are separated into seven groups for convenience of discussion. Each group of points is fixt by a portion of the triangulation which may conveniently be considered as a unit in discussing the magnitude of the possible errors of the triangulation. The discussion of the observed displacements and the degree of certainty in regard to them is given after the tables and deals with each group in succession.
The apparent displacements, as shown in the above tables, are of course in part due to the unavoidable errors in the triangulation and in part are doubtless actual displacements of the points. The triangulation furnishes within itself the means of estimating its accuracy. If the observations were absolutely exact, the sum of the observed angles of each triangle would be exactly 180° plus the spherical excess of that triangle, and moreover the computation of the length of the triangle sides would show no discrepancies, starting from a given line and ending on a selected line, but proceeding thru the various alternative sets of triangles which it is possible to select connecting said lines. In any
The triangulation, both old and new, was adjusted by the method of least squares. This method of computation, as applied to triangulation, takes into account simultaneously all the observed facts in connection with a group of triangulation stations and also all the known theoretical conditions connecting the observed facts; such, for example, as those mentioned in the preceding paragraph, in regard to closures of triangles and discrepancies in length. It is the most perfect method of computation known. The results of the computation are a set of lengths and azimuths (true directions) of lines joining the triangulation stations and of latitudes and longitudes defining the relative positions of the stations which are perfectly consistent; that is, contain no contradictions one with another and are the most probable values which can be derived from the observations. In such a computation, the measures of the accuracy of the computed results appear in the form of corrections to observed directions from station to station, which it is necessary to apply in order to obtain the most probable results given by the computation. The greater the accuracy of the observations the smaller are the corrections to directions.
In the problem in hand, in which, at least for some points, the observed apparent displacement is of about the same magnitude as the possible error in the apparent displacement due to accumulated errors of observation, it is necessary to make a careful estimate of the errors of observation and of the uncertainties of the computed displacements. This has been done and the estimates are given in general terms in the following text and are indicated in the last column of the tables. These estimates will help the reader to avoid drawing conclusions in detail not warranted by the facts.
Group 1. Northern part of primary triangulation.—In this group, as shown by tables 1-3 (see also map 24), there are 11 points of which the positions were redetermined after the earthquake of April 18, 1906. Of these, 9 had been determined before 1868 and 7 between 1868 and 1906.
There is about 1 chance in 3 that each of the two apparent displacements of Rocky Mound, 0.50 meter (1.6 feet), in 1868 (table 2), and 0.34 meter (1.1 feet), in 1906 (table 1), is simply the result of errors of observation. Similarly there is about 1 chance in 3 that the apparent displacement of Red Hill in 1868, 0.65 meter (2.1 feet), is the result of errors of observation. The chances are about even for and against the apparent displacement of Red Hill in 1906, 0.30 meter (1.0 foot), being simply the result of errors of observation. The effect of errors of observation upon the apparent displacements are larger at these two points than they otherwise would be on account of the difficulty in this vicinity of separating the triangulation into two complete schemes, one before 1868 and one after that date, each strong and complete.
According to the evidence furnished by the triangulation, the apparent displacement of Ross Mountain in 1906, 0.53 meter (1.8 feet) in azimuth 309° (51° E. of S.), is probably the result of errors of observation. This apparent displacement as computed depends on the accumulated errors of the two triangulations from Mount Diablo to Ross Mountain, a distance of 130 kilometers (81 miles). The apparent displacement of 0.53 meter almost directly toward Mount Diablo corresponds to a shortening on the line Ross Mountain-Mount Diablo by 1 part in 250,000, too small a change to be detected with certainty by the triangulation.
On the other hand, there is about 1 chance in 15 that the apparent displacement of Ross Mountain in 1868, 1.70 meters (5.6 feet), is due to errors of observation. It is reasonably certain that this is a real displacement.
The chances are about even for and against the apparent displacement of Point Reyes Light-house in 1906, 1.09 meters (3.6 feet), being due simply to errors of observation.
There is about 1 chance in 7 that the apparent displacement of Bodega, shown in table 3, is due to errors of observation. It is reasonably certain that this is a real displacement.
For the remaining six points in group 1, Sierra Morena, Mount Tamalpais, Farallon Light-house, Point Reyes Hill, Tomales Bay, and Sonoma Mountain, each of the apparent displacements given in the tables as observed is real, being in each case clearly beyond the maximum which could be accounted for as errors of observation.
Prof. George Davidson has believed for many years that Mount Tamalpais moved during the earthquake of 1868 and that the triangulations made before and after that date showed such a displacement. Accordingly in 1905, at his request, a reëxamination was made at the Coast and Geodetic Survey office of the evidence furnished by the triangulations, and the conclusion was reached that a real displacement of Mount Tamalpais occurred in 1868. At that time, however, convincing evidence was not discovered that any other triangulation station moved in 1868. In the more extensive studies made in connection with the present investigation, and with the additional skill acquired in recognizing the effects of earthquakes upon triangulation, it became evident, as shown in table 2, not only that Mount Tamalpais moved in 1868, but also that the Farallon Light-house and Ross Mountain moved at that time, the three apparent displacements being clearly beyond the range of possible errors of triangulation. The displacements for these three stations are similar. The amount of the displacement is least at Farallon Light-house, 1.39 meters (4.6 feet), and greatest at Ross Mountain, 1.70 meters (5.6 feet). The azimuth of the displacement is least at the Farallon Light-house, 153° (27° W. of N.), and is greatest at Ross Mountain, 182° (2° E. of N.). (See map 24.) The apparent differences in direction and amount of the three displacements may or may not be real. It is certain therefore that in 1868 the large part of the earth's surface included between these three stations, at least 700 square miles, moved about 1.5 meters (4.9 feet), in about azimuth 168° (12° W. of N.).
Within the triangle defined by the three stations, Mount Tamalpais, Farallon Light-house, and Ross Mountain, which certainly were displaced in 1868, are the three stations, Point Reyes Hill, Tomales Bay, and Bodega, of group 1. It is therefore believed to be reasonably certain that these stations were displaced at that time. The probable displacements were interpolated from the three displacements observed at the first three stations, taking into account the relative positions of the stations. The resulting interpolated displacements are shown in table 2. Other evidence, tending to show that these interpolated values of the displacements are real, will be brought forward later.
For the three stations, Point Reyes Hill, Tomales Bay, and Bodega, the positions were determined before 1868 and after the earthquake of 1906, but not during the interval 1868-1906; hence the computation of the positions determined by triangulation for these stations furnishes simply the combined displacements of 1868 and 1906 as shown in table 3. As noted in the preceding paragraph, the displacement of 1868 has, for these three stations, been interpolated from surrounding stations and entered in table 2. The differences
The differences were taken separately for the meridian components and the prime vertical components and then combined to secure the direction and amount of the resultant.between these inferred displacements in table 2 and the observed combined displacements in table 3 were then taken and are shown in table 1, as inferred displacements in 1906. As indicated in the marked column of table 1, these inferred displacements are believed to be certain for two of these points and somewhat doubtful for the third, Bodega.
The doubtful apparent displacements at Rocky Mound and Red Hill in 1868 (see table 2) agree with other displacements which are certain, in having a decided northward component.
In table 1, showing the displacements of 1906, there are three stations, Sierra Morena, Mount Tamalpais, and Farallon Light-house, at which observed displacement is certain,
From these five stations, one may deduce four laws governing the distribution of the earth movement which occurred on April 18, 1906. First, points on opposite sides of the fault moved in opposite directions, those to the eastward of the fault in a southerly direction and those to the westward in a northerly direction. Second, the displacements of all points were approximately parallel to the fault. Third, the displacements on each side of the fault were less, the greater the distance of the displaced points from the fault. Fourth, for points on opposite sides of the fault and the same distance from it, those on the western side were displaced on an average about twice as much as those on the eastern side.
If the proof of these four deduced laws rested upon the evidence of these five stations only, it would be insufficient to convince one. Much other evidence in proof of these four deduced laws will be shown in this report. The laws are here stated in order that they may be kept in mind and tested by the evidence as presented.
The apparent displacements of the remaining five points of group 1 may now be compared with the stated laws.
The displacement of Point Reyes Light-house, believed to be determined with reasonable certainty, is apparently about 1.6 meters (5 feet) greater than and differs about 32° in direction from the displacement which might be inferred from the above laws and comparison with the surrounding stations.
The displacement of Bodega, of which the determination is somewhat doubtful, is just what would be inferred from the deduced laws, as its amount is greater than for Mount Tamalpais, corresponding to the fact that it is closer to the fault, and its azimuth agrees within 2° with that of the fault.
The displacement of Ross Mountain, of which the determination is doubtful, agrees very closely in amount with that at Mount Tamalpais and differs only 15° in direction. Ross Mountain is on the same side of the fault as Mount Tamalpais and at practically the same distance from it.
The apparent displacements of Rocky Mound and Red Hill, 32 and 19 kilometers (20 and 12 miles) from the fault and to the eastward of it, of which the determinations are doubtful, agree with the laws in being small but are contradictory as to direction.
For Sonoma Mountain the triangulation serves to determine the combined displacements of 1868 and 1906 as shown in table 3, but not the separate displacements, as this station was not involved in triangulation done between 1868 and 1906. The combined displacements at Sonoma Mountain are of about the same amount and are in approximately the same azimuth as displacements of 1868 at Mount Tamalpais, Point Reyes Hill, Tomales
Group 2. Southern end of San Francisco Bay. — In this group there are three new points not yet considered and Red Hill which has already been considered in group 1. The three new stations, Guano Island, Pulgas East Base, and Pulgas West Base (see map 24), were determined in 1851-1854 and again after the earthquake of 1906. No determination was made between 1868 and 1906, hence these points are entered in table 3, the combined displacements of 1868 and 1906 being determined, but not the separate displacements.
A study of the errors of the triangulation shows that the apparent displacement of Guano Island, 0.21 meter (0.7 foot), is probably due to errors of observation, and that there is one chance in three that the apparent displacement of Pulgas East Base, 0.41 meter (1.3 feet), is also due to errors of observation.
The determination of the displacement of Pulgas West Base, 0.74 meter (2.4 feet), is reasonably certain, there being about one chance in twelve that it is due to errors of observation.
Tho the determinations of the separate apparent displacements of Red Hill in 1868, 0.65 meter (2.1 feet), and in 1906, 0.30 meter (1.0 foot), are each doubtful, the combined displacement as observed, shown in table 3, 0.94 meter (3.1 feet), is certain.
It is therefore reasonably certain that there was a relative displacement of Pulgas West Base and Red Hill as indicated in table 3, Red Hill moving 0.94 meter (3.1 feet), in azimuth 227° (47° E. of N.), and Pulgas West Base 0.74 meter (2.4 feet), in azimuth 344° (16° E. of S.). This lengthened the line Pulgas West Base to Red Hill, 16 kilometers (10 miles) long, 0.50 meter (1.6 feet), or one part in 32,000. It also changed the azimuth of this line by 11'', from 240° 44' 35'' to 240° 44' 24'', rotating it in a counterclockwise direction.
The red arrows on map 24, showing apparent displacements, indicate that the apparent displacements of Guano Island and Pulgas East Base, which are considered doubtful, are not inconsistent with the displacements of Red Hill and Pulgas West Base. Apparently the area included between these four stations was distorted by stretching and rotated in a counterclockwise direction.
There is no evident method of ascertaining whether the displacement of Pulgas West Base took place in 1868 or 1906 or in part at each time. The displacement is nearly in the direction corresponding to the laws governing the displacements of 1906, as already stated in connection with group 1. Pulgas West Base is to the eastward of the fault of 1906 and slightly nearer to it than Mount Tamalpais and Ross Mountain and hence, according to the laws referred to, should be displaced in the same direction as these two points (see table 1), and by a similar amount. This is the fact.
Group 3. Vicinity of Colma. — There are nine points in group 3 all determined by triangulation in 1899 or earlier, and redetermined after the earthquake of 1906 (see table 1). The earlier determination was made by secondary and tertiary triangulation, extending from the vicinity of Pulgas Base northwest, spanning San Francisco Bay to the Golden Gate, and thence southward to Colma. The earlier positions of these nine points are subject to the effect of accumulated errors in this chain of triangulation about 60
In the triangulation of 1906-1907, the position of San Bruno Mountain, which is in the midst of this group, was determined by secondary triangulation in connection with group 2 as indicated on maps 24 and 25, a direct and strong determination. The new azimuth was also carried into the triangulation of group 3 with a high degree of accuracy in this same manner. No new determination was made of the starting length in group 3. It was assumed that the length San Bruno Mountain to Black Ridge 2 had remained unchanged during the earthquake of 1906 and the old value of that length was used in the computation of the triangulation of 1906-1907. As a check upon the assumption that this length remained unchanged, it is to be noted that the azimuths of this line before and after the earthquake of 1906 were found to differ only by 9.3'', which is within the possible range of errors of observation in the earlier triangulation.
For the reasons stated above, the apparent absolute displacements shown in table 1 for group 3, as referred to Mocho and Mount Diablo as fixt points, are probably due to errors of observation.
On account, however, of the fact that seven of the nine points in this group are within a rather small area, their relative displacements are determined with considerable accuracy, the errors of length and azimuth having less effect in producing errors in relative positions, the smaller the area covered by a triangulation. Montara Moutain Peak and Bonita Point Light-house are each determined with a low grade of accuracy. They are each far from the stations occupied in the triangulation and the lines which determine them intersect at a small angle; hence even their relative displacements are uncertain. The relative displacements observed for the remaining seven points after omitting these two are certain, being beyond the possible range of errors of observation.
The apparent absolute displacements for this group of points (see table 1 and map 25) indicate that all points on the eastern side of the fault moved in a southerly direction, and those on the western side in a northerly direction; that the displacements tend to be parallel to the fault, the more doubtful displacements showing the greater angles with the fault; and that the amounts of the displacement are in the inverse order of the distances of the stations from the fault, with two exceptions. These exceptions are San Pedro Rock, of which the relative displacement is determined with sufficient accuracy to establish this as a real exception; and Bonita Point Light-house, for which the apparent displacement as observed is so uncertain that this apparent exception has but little significance. Of the four points, all on the western side of the fault, of which the relative displacements are believed to be certain, as indicated in table 1, the azimuths of the displacements vary from 151° to 169°, with a mean of 157° (23° W. of N.). The azimuth of the fault in this vicinity is 144° (36° W. of N.).
The relative displacements on opposite sides of the fault and near to it are less in this group (2 to 3 meters) than for points at a similar distance from the fault in group 1; namely, Point Reyes Hill, Tomales Bay, and Bodega (5 to 6 meters).
Group 4. Tomales Bay. — There are seven points in this group (see tables 1 to 3 and maps 24 and 25). These were fixt in 1856-1860 by tertiary triangulation extending southeastward along Tomales Bay from stations Tomales Bay and Bodega of group 1. They were fixt again in practically the same manner in 1906 after the earthquake.
With these seven points may advantageously be considered the three points, Point Reyes Hill, Tomales Bay, and Bodega, which were fixt in group 1.
No one of these ten points was determined between 1868 and 1906, hence the observations served to determine the combined displacements of 1868 and 1906, as shown in table 3, but not the separate displacements. The separate displacements have been determined by interpolation from surrounding stations for the three points, Point Reyes Hill, Tomales Bay, and Bodega, as indicated in the discussion of group 1. The same process has also been applied to the seven points of group 4.
Starting with the interpolated displacements of 1868 for the three points, Point Reyes Hill, Tomales Bay, and Bodega, as shown in table 2, and with map 25 before one, it was a simple matter to interpolate separately the meridian components and the prime vertical components of the displacements of 1868 for the seven stations of group 4. This amounts practically to interpolating the displacements for these points from the three observed displacements of 1868 at Mount Tamalpais, Farallon Light-house, and Ross Mountain. The resulting interpolated displacements of 1868 are shown in table 2. Each of these being subtracted, component by component, from the corresponding combined displacement of 1868 and 1906, as shown in table 3, leaves the displacement of 1906 as shown in table 1.
A study of the possible accumulated errors in the triangulations shows that all of the seven displacements of 1906 in group 2 are certain except for Hans and Hammond. There is about one chance in five that the apparent displacements of 1906 for these two points are simply due to errors of observation.
The ten displacements of 1906 in this group show clearly the four laws already suggested in regard to such displacements. All points to the eastward of the fault moved southerly and those of the western side, northerly. Four of the five points to the westward of the fault moved in azimuths between 141° and 143° with a mean of 142° (38° W. of N.). The azimuth of this part of the fault is about 145° (35° W. of N.). The azimuth of the fifth displacement on the west side, at Bodega Head, is 172° (8° W. of N.). The azimuths of the three reasonably certain displacements of points to the eastward of the fault vary from 323° to 348° with a mean of 334° (26° E. of S.), which is within 9° of being parallel to the fault. Of the five points to the westward of the fault, the one nearest to the fault, Foster, has the greatest displacement. The other four, all between 2.0 and 2.7 kilometers from the fault, have nearly equal displacements. The five displacements for points to the eastward of the fault show a slight tendency to stand in inverse order from the distances from the fault. But one only of these displacements differs by more than 0.42 meter (1.4 feet) from the mean of the five, and the estimated distances from the fault vary only from 0.5 to 2.6 kilometers. When the uncertainty of the position of the fault beneath Tomales Bay is considered, as well as the small variation in distance of these ten points from the fault, difficulties are to be expected in detecting the relation between displacement and distance from the fault in this group. The mean displacement of the points to the eastward of the fault is 1.86 meters (6.1 feet) and of the five points to the westward 2.1 times as much, namely, 3.88 meters (12.7 feet).
Group 5. Vicinity of Fort Ross. — There are twelve points in this group, all determined by secondary triangulation in 1875-1876 and again in 1906, the scheme of triangulation being in each case substantially the same as that shown on map 25. The base from which these positions are determined is not independent of observations made before 1868, but is gotten by making the observations preceding that date conform to those made between 1868 and 1906. From the small size of the necessary corrections to the observed angles, and from the fact that the position of Ross Mountain, which predominates the group, is determined by observations made entirely after 1868, the error of assuming that these twelve points belong to the period between 1868 and 1906 is deemed negligible.
For one point, Chaparral, observations made in 1860 furnish a determination of the position before 1868, and hence the displacement of this point in 1868 (see table 2) is determined as well as its displacement in 1906. The displacement of 1868 agrees closely, within less than 0.13 meter (0.4 feet) in amount and 9° in direction, with the displacement at that time at Ross Mountain, 5.7 kilometers (3.5 miles) to the eastward.
A study of the possible accumulated errors in the triangulation shows that five of the observed displacements in this group, as referred to Mocho and Mount Diablo, are clearly beyond the range of possible errors of observation; namely, those at Fort Ross, Funcke, Timber Cove, Stockhoff, and Pinnacle Rock. For the remaining seven displacements, there are from one to two chances out of ten that they are due entirely to errors of observation, and these displacements are therefore reasonably certain. The relative displacements of pairs of points on opposite sides of the fault and near to each other in this group are certain, being in every case clearly beyond the range of possible errors of observation.
The apparent displacements in 1906 of the twelve points in this group conform closely to the four deduced laws governing such displacements. The seven points to the westward of the fault moved in a northerly direction, in azimuth varying from 137° to 158°, with a mean of 144° (36° W. of N.). The azimuth of the fault in this region is about 141° (39° W. of N.). All five points to the eastward of the fault moved southerly, in azimuth varying from 301° to 328° with a mean of 318° (42° E. of S.). All of the points in this group are within 3.2 kilometers (2.0 miles) of the fault and therefore give little opportunity to ascertain whether the amounts of the displacements show any relation to distances from the fault. Such a relation is not clearly discernible among the observed displacements. The evidence of the apparent displacement at Ross Mountain (see table 1), 6.2 kilometers (4.2 miles) to the eastward of the fault, indicates a decrease of displacement with increase of distance from the fault in that direction. The average displacement of the five points to the eastward of the fault is 1.44 meters (4.7 feet) and that of the seven points to the westward is 1.5 times as great, namely, 2.11 meters (6.9 feet).
Group 6. Point Arena. — In this group there are ten points determined by secondary triangulation in 1870 to 1892 that were redetermined by secondary triangulation in 1906, starting from the stations Fisher and Cold Spring, 11.2 and 13.5 kilometers eastward from the fault respectively. (See map 25.) A study of the possible errors in the triangulation shows that all of the observed displacements in this group are certain, each being clearly greater than the maximum possible errors of observation. There is a possibility that the assumption that the two stations, Fisher and Cold Spring, remained unmoved in 1906 is in error. The movement, if any, of these stations was probably about the same for both stations and in a southerly direction and parallel to the fault. If such a movement of these stations occurred, the computed displacements in 1906, shown in table 1 and on map 25, are all too small for stations to the eastward of the fault, and too great for stations to the westward of it.
The agreement of the observed displacements of the ten points in this group with the four deduced laws is close. The six points to the westward of the fault moved in azimuths varying thru a range of 5° only, from 159° to 164°, with a mean of 162° (18° W. of N.). The fault in this vicinity is said to change in azimuth, near the point where it crosses the coast-line, from about 144° to about 164° (16° W. of N.), curving to the eastward. The four points to the eastward of the fault moved in azimuths varying from 324° to 340° with a mean of 330° (30° E. of S.). The station Shoemake, comparatively near to the fault, 1.5 kilometers (0.9 mile), on the west side, showed a displacement much larger than any of the other five points on that side, all of which are from 5.7 to 7.6 kilometers from the fault. The two points to the eastward of the fault which are within less than 1 kilometer of it were displaced nearly twice as much as the other two which are nearly 4 kilometers from the fault. The average displacement for the four points to the eastward
Group 7. Southern part of primary triangulation. — In this group, extending southward from the line Mocho-Sierra Morena, there are nine points (see map 24) of which the positions were redetermined after the earthquake of 1906. Of these, one, Loma Prieta, had been formerly determined both before and after the earthquake of 1868; five others had been determined before 1868 but not after, and three had been determined after but not before 1868. (See tables 1 to 3.) In this group, therefore, but one point is available to show the displacement of 1868.
The triangulation of 1854-1855, starting from the line Ridge to Rocky Mound near the Pulgas Base, consisted of a single chain of triangles with all angles measured, down to the line Loma Prieta-Gavilan. The Point Pinos Light-house and the Point Pinos Latitude Station were connected with this chain, and with checks, by observations in 1854, 1864, and 1866.
The main triangulation of 1876-1887, from the line Mount Diablo-Mocho to the line Mount Toro-Santa Ana, consisted of a strong chain of figures with many checks, being substantially as shown on map 24 if Gavilan be omitted and all stations occupied. In this triangulation, however, no complete independent determinations with checks were made of Black Mountain, Santa Cruz Azimuth Station, Gavilan, Point Pinos Light-house and Point Pinos Latitude Station.
The triangulation of 1906-1907 was made as shown on map 24. Two separate least square adjustments were made of the main scheme connecting the points Mount Diablo, Mocho, Sierra Morena, Loma Prieta, Mount Toro, Gavilan, and Santa Ana.
In the first adjustment, it was assumed, as for the computations of other groups, that Mount Diablo and Mocho only remained unmoved during the earthquake of 1906. This first adjustment showed an apparent displacement of Santa Ana in 1906 of 3.26 meters (10.7 feet), in azimuth 288° (72° E. of S.), but an examination in detail of the possible accumulated errors in the triangulation showed that this apparent displacement was probably due to errors of observation. The new primary triangulation is much weaker in the figure defined by the five points, Mocho, Loma Prieta, Mount Toro, Gavilan, and Santa Ana, than elsewhere for two reasons. First, the length must be carried without a check thru the triangle Loma Prieta, Mocho, Mount Toro, of which only two angles were measured and this triangle is very unfavorable in shape for an accurate determination of length. Second, it so happened that the least accurate observations made in the primary triangulation were in this triangle or in its immediate vicinity.
In the second and adopted adjustment it was assumed that Santa Ana, as well as Mount Diablo and Mocho, remained unmoved during the earthquake of 1906. The astronomic azimuth had been observed at Mount Toro in 1885 and again after the earthquake of 1906. These two observations measured the absolute change in azimuth of the line between Mount Toro and Santa Ana and indicated it to be 2.5'', the later azimuth being the greater. This was utilized to strengthen the adjustment.
In view of the evidence of stations farther north, the assumption that Santa Ana remained unmoved is reasonably safe. Santa Ana is about 27 kilometers (17 miles) to the eastward from the point at which the fault disappeared near the village of San Juan. There is no station anywhere in the triangulation more than 6.4 kilometers to the eastward of the fault for which any displacement in 1906 was determined with certainty.
If Santa Ana was displaced in 1906, the erroneous assumption introduces an error into the computed displacements at the stations Gavilan, Mount Toro, Point Pinos Light-house, and Point Pinos Latitude Station, of about the same amount as the actual displacement at Santa Ana. The error produced in the computed displacement at Santa Cruz Light-house and Santa Cruz Azimuth Station must be much smaller, and no error
The displacements of Loma Prieta in 1906 and 1868 (see tables 1 and 2) are both certain.
The displacements of Black Mountain, Santa Cruz Azimuth Station, Gavilan, Point Pinos Light-house, and Point Pinos Latitude Station, as shown in table 3, are also certain. These are all combined displacements of 1868 and 1906. These stations were not determined between 1868 and 1906, hence it is not possible to determine directly from the observations the separate displacements. If it be assumed that the displacements in 1868 of the last four of these points were the same as that observed for Loma Prieta (see table 2), then the inferred displacements for each of these points in 1906 is as shown at the end of table 1. These inferred displacements for these points are, however, very doubtful as they depend upon a determination of the displacement of 1868 at a single point, Loma Prieta, which is 24 kilometers (15 miles) from Santa Cruz Azimuth Station and more than 48 kilometers (30 miles) from each of the other stations. It should be noted also that the displacement of Loma Prieta in 1868, which is certain, is very different from that of the other four points, Mount Tamalpais, Farallon Light-house, Chaparral, and Ross Mountain, for which the displacements of 1868 have been determined directly by observations. It is a displacement to the southward instead of to the northwestward and is much larger than for the other three points.
The determination of the displacement of Mount Toro as shown in table 1 is somewhat uncertain. There is still more uncertainty in regard to the apparent displacement at Santa Cruz Light-house.
The very small apparent displacement, 0.12 meter (0.4 foot), of the Lick Observatory small dome in 1906 is probably due to errors of observation.
The two points in this group to the eastward of the fault show apparent displacements in 1906 in accordance with the laws deduced from other groups: Lick Observatory, far from the fault, 36 kilometers (22 miles), having an apparent displacement so small as to be uncertain; and Loma Prieta, within 4.8 kilometers (3.0 miles) of the fault, having an apparent displacement of 0.97 meter (3.2 feet) in a southerly direction and within 9° of being parallel to the fault which here has an azimuth of about 312° (48° E. of S.).
Mount Toro is the only station to the westward of the fault in this group for which a determination of the displacement of 1906 is not very doubtful. The displacement in 1906 of 0.95 meter (3.1 feet) at Mount Toro is in a northerly direction with a slight inclination to the westward in fair agreement with the deduced laws. Mount Toro is beyond the end of the portion of the great fault of 1906 which has been traced on the surface.
The apparent displacement of Santa Cruz Light-house in 1906, of which the determination is doubtful, is closely parallel to the fault and in a northerly direction, corresponding to other points to the westward of the fault.
The inferred displacements of 1906 for four points shown at the end of table 1 are all very doubtful, and little significance should be attached to them or to the fact that they are somewhat contradictory to each other and all have a southerly tendency, whereas all other points to the westward of the fault of 1906 moved in a northerly direction. As a check on this conclusion, it should be noted that the inferred displacement for 1906 for Santa Cruz Azimuth Station differs by 72° in direction and 1.26 meters (4.1 feet) in amount from the observed displacement of 1906 for Santa Cruz Light-house, a point only 3.9 kilometers (2.4 miles) away. The observed displacement for Santa Cruz Light-house is much less uncertain than the inferred displacement for Santa Cruz Azimuth Station and hence the contradiction throws additional doubt on the latter and the other three points for which the inference is made in like manner.
Tho the inferred displacements of these four points for 1906 are all very doubtful, the observed combined displacements of 1868 and 1906 for these four points, as shown in table 3, are all certain, being clearly beyond the possible range of errors of observation. So also are the combined displacements of 1868 and 1906 for Loma Prieta and Black Mountain. It appears then that the combined effects of the earthquakes of 1868 and 1906 were to move the whole region from Black Mountain to Point Pinos to the southeastward by from 2.11 to 5.89 meters (6.9 to 19.3 feet). The mean azimuth of these six displacements is 321° (39° E. of S.). The most startling evidence of the combined effects of the two earthquakes is the increase of 3 meters (10 feet) in the width of Monterey Bay from Santa Cruz Azimuth Station to Point Pinos Light-house, both of these points having moved in a southerly direction, but the latter much more than the former. The length of the line Santa Cruz Azimuth Station to Point Pinos Light-house is only 39.8 kilometers (24.7 miles). The increase is therefore one part in 13,000.
Not much significance should be attached to the fact that Point Pinos Latitude Station has apparently moved one meter less than Point Pinos Light-house. This one meter is the difference of the combined displacements of two earthquakes. It is subject to the errors of observation in two determinations of each point by triangulation in somewhat different ways. Moreover, the determination of the position of the Latitude Station after the earthquake of 1906 was made without a check. It is for this reason that the displacement at Point Pinos Light-house is considered to be the more reliable determination of the two.
Distribution of Earth Movement; Summary
In reaching the conclusions stated below, the evidence has been studied much more in detail than it has been given in the preceding pages. The conclusions are based on both the positive and negative evidence. The positive evidence is given by the displacements marked "certain" or "reasonably certain" in tables 1, 2, and 3. The negative evidence is given by displacements marked "doubtful," of which Rocky Mound is an example. At this point the observed apparent displacement of 1906 was only 0.34 meter (1.1 feet). The accuracy of the triangulation is such that it is practically certain that any displacement of this station as great as one meter would be detected. Hence the evidence given by this station is that the displacement, if any, was less than one meter and probably was less than 0.3 meter.
Maps 24 and 25 should be consulted while reading the following conclusions.
During an earthquake in 1868 or about that time, about 1,000 square miles of the earth's crust, comprized between the four stations Mount Tamalpais, Farallon Light-house, Ross Mountain, and Chaparral, were permanently displaced to the northward about 1.6 meters (5.2 feet), in azimuth 169° (11° W. of N.). The indications are that this whole area moved as a block without distortion or rotation; at least the triangulation furnishes no evidence competent to prove either distortion or rotation of the block (about a vertical axis), or to locate accurately any boundary of the block. It is probable that the block included Sonoma Mountain. It is reasonably certain that Rocky Mound and the group of points near the southern end of San Francisco Bay, Red Hill, Pulgas Base stations, and Guano Island, were not on this block, tho they were probably displaced somewhat irregularly during the earthquake of 1868.
During the earthquake of 1868, or about that time, Loma Prieta was permanently displaced about 3.03 meters (9.9 feet), in azimuth 307° (53° E. of S.). This displacement is in a direction at an angle of 138° with that of displacements of same date, referred to in the preceding paragraph. Loma Prieta moved to the southeastward, whereas Mount Diablo, Farallon Light-house, Ross Mountain, and Chaparral moved to the northward.
It is reasonably certain that Santa Cruz Azimuth Station, Point Pinos Light-house, Point Pinos Latitude Station, and Gavilan were similarly displaced. It is probable that the last
The combined effects of the earthquakes of 1868 and 1906 have increased the distance between Mount Tamalpais and Black Mountain, see map 24 and table 3, by 3 meters (10 feet). The distance is 79 kilometers (49 miles) and the increase is therefore one part in 26,000. The Golden Gate lies between these two stations. It is interesting to note that the length of part of the Pacific Coast including the Golden Gate has been increased just as the distance across Monterey Bay has been increased.
During the earthquake of April 18, 1906, displaced points on opposite sides of the great fault accompanying the earthquake moved in opposite directions, those to the eastward of the fault in a southerly and those to the westward in a northerly direction. Among all the points there are but two apparent exceptions to this rule, namely, Rocky Mound and Red Hill. For both these stations the apparent exceptional movement is so small as to be probably due simply to errors of observation and therefore it is not signifieant.
During the earthquake of 1906, the permanent displacements of all disturbed points were approximately parallel to the fault. When the difficulties encountered in determining the direction of these displacements are considered, it is remarkable that the observed displacements follow this law so accurately as they do. The nearest fixt points to which each displaced point is referred are from 30 to 140 kilometers distant (20 to 90 miles). The total displacements are from 0.5 to 4.6 meters (2 to 15 feet). Among all the points examined, there are but five for which the apparent changes in distance from the fault are not so small as to be probably due to errors of observation. The Farallon Light-house apparently moved at an angle of about 27° with the fault and its increase in distance from the fault of 0.8 meter is reasonably certain. As Mount Tamalpais, nearly opposite to Farallon Light-house across the fault, moved practically parallel to the fault, there was either an opening of the fault beneath the sea in this region or an increase in length of the earth's crust, in a direction at right angles to the fault, of one part in 50,000 (0.8 meter on 44 kilometers, or 3 feet on 27 miles). Point Reyes Light-house also apparently receded from the fault, moving in about the same direction (within 5°) as the Farallon Light-house, but the determination of the displacement of the Point Reyes Light-house is so weak that this apparent displacement has little significance. It is reasonably certain that Bodega Head approached the fault from the western side, while Bodega, on the eastern side of the fault, about opposite, moved parallel to the fault. The apparent closing up of the fault or shortening of the crust at right angles to the fault is 1.6 meters (5.2 feet) between these two points only 5.4 kilometers (3.4 miles) apart. This is one part in 3,400. It is possible that as much as one-half of this apparent closing up is due to errors of observation, but it is reasonably certain that not all of it is due to that cause. Similarly it is reasonably certain that Peaked Hill in the Fort Ross group receded from the fault on the east side and Pinnacle Rock approached it on the west side, the apparent amounts being 0.4 meter (1.3 feet) and 0.7 meter (2.3 feet) respectively. It is reasonably certain that San Pedro Rock in the Colma group approached the fault from the west side, the apparent amount being 1.1 meters (3.6 feet).
During the earthquake of 1906, the displacements on each side of the fault were less the greater the distance of the displaced points from the fault. On the eastern side of the fault, ten points at an average distance of 1.5 kilometers (0.9 mile) from the fault have an average displacement of 1.54 meters (5.1 feet); three points at an average distance of 4.2 kilometers (2.6 miles) have an average displacement of 0.86 meter (2.8 feet), and one point, Mount Tamalpais, at 6.4 kilometers (4.0 miles) from the fault, has a displacement of 0.58 meter (1.9 feet). These fourteen points are the only ones on the eastern side of the fault for which the observed displacements were determined with reasonable certainty. For no point to the eastward of the fault at a greater distance than 6.4 kilometers (4.0
In receding from the fault, either to the eastward or to the westward, the displacement decreases more rapidly near the fault than it does farther from the fault. According to the averages given in the preceding paragraph, the decrease in displacement on the eastern side near the fault is at the rate of 0.25 meter per kilometer (that is, 0.68 meter on 2.7 kilometers) and farther away the rate is 0.13 meter per kilometer (that is, 0.28 meter on 2.2 kilometers). Imagine a straight line before the earthquake of April 18, 1906, starting at the fault and extending eastward at right angles to it. According to this investigation, after the earthquake this line became a curved line concave to the southward, the point at the fault being displaced southward and distant points on the line remaining fixt. Also according to the above figures, the part of the line which is from 1.5 to 4.2 kilometers from the fault was deflected from its former direction about 52 seconds and that part from 4.2 to 6.4 kilometers from the fault was deflected about 26 seconds, and the deflection probably decreased gradually to zero at distant points. To the westward of the fault the rate of decrease of displacement, according to the averages in the preceding paragraph, near the fault is 0.15 meter per kilometer (that is, 0.57 meter on 3.8 kilometers), and farther away only 0.02 meter per kilometer (that is, 0.60 meter on 31 kilometers). Accordingly the imaginary straight line at right angles to the fault and extending westward from it has become concave to the northward, the point at the fault being displaced to the northward and very distant points remaining fixt. The deflection from its original direction is about 31 seconds for the part from 2 to 6 kilometers from the fault and about 4 seconds on an average for the part from 6 to 37 kilometers from the fault.
For points on opposite sides of the fault of 1906, and at the same distance from it, those on the westward side are displaced on an average twice as much as those on the eastern side. This statement applies especially to points within 10 kilometers (6 miles) of the fault. For points farther away, the ratio becomes more than two to one. It is important to note that this statement applies to displacements, not distortions. The distortion, exprest in angular measure, discust in the preceding paragraph, is nearly the same on the two sides of the fault, being somewhat less close to the fault on the western side than on the eastern side.
The amount of relative displacement of the two sides of the fault by sliding along the fault, as detected by the triangulation, shows no variations for different parts of the fault along its whole length from Point Arena to San Juan, with one exception, which are sufficiently large to be clearly not due to erros of observation. This exception is the region near Colma where, as already noted, relative displacements seem to be unusually small.
The permanent displacements and distortions which took place at the time of the earthquake of April 18, 1906, may be pictured by imagining a series of perfect squares drawn on the surface of the ground before the earthquake, with their sides parallel and perpendicular to the fault. At the time of the earthquake every square to the eastward of the fault moved bodily in a southerly direction parallel to the fault, the squares more distant from the fault moving less than those near to it. All sides of squares parallel to the fault remained straight lines, unchanged in length and direction. For the squares to the eastward, the sides perpendicular to the fault became curved lines concave to the southward and changed direction as a whole by rotation in a counterclockwise direction, the change being 52 seconds or more for squares near the fault, and less for more remote squares. The angles of the squares all took new values differing from 90° by quantities
It is not probable that the actual displacements and distortions were perfectly regular as indicated in the word picture of the preceding paragraph, but the apparent departures from this perfectly regular ideal, of the displacements and distortions detected by the triangulation are nearly all so small as to be possibly due to errors of observation. Attention has been called to the few exceptions, of which one can be certain, which have been detected. The earth-movements of April 18, 1906, were remarkable for their regularity of distribution.
The triangulation of 1906-1907 has extended eastward clearly beyond the region of appreciable permanent displacements by the earthquake of 1906. The disturbed region evidently extended to the westward out under the Pacific beyond the possible reach of the triangulation. To the northward of Point Arena there is little probability of much success if an attempt were made to determine additional displacements by triangulation, for the known fault of 1906 touches the coast for but a short distance anywhere north of Point Arena, and triangulation to the northward of Point Arena before the earthquake consisted simply of a narrow and weak belt of tertiary triangulation. It had been intended to extend the triangulation of 1906-1907 far enough to the southward to reach outside of the disturbed region. It was supposed until after the observing party left the southern end of the triangulation that this had been accomplished, but when the additional evidence given by the office computations became available, it was evident that the most southern points determined are still within the disturbed region. The fact that the visible evidence of the fault of 1906 does not extend farther southward than San Juan indicates that there are probably few points to the southward of Mount Toro and Point Pinos for which the displacements were large enough to be detected by triangulation.
Discussion of Assumptions
Certain things have apparently been assumed in this investigation; for example, that appreciable permanent displacements occurred during the earthquake of 1868 as well as during the earthquake of 1906; that the permanent displacements in 1906 occurred suddenly, and that the stations Mocho and Mount Diablo remained unmoved in both earthquakes.
These are called apparent assumptions because in a real sense they are not assumptions but are instead facts detected gradually in studying for fifteen months upon a steadily increasing mass of evidence. However, treating them as assumptions, their validity has been reëxamined in the light of all the evidence, and to make this report complete, it is now necessary to state why they are believed to be true.
It has been tacitly assumed that the permanent displacements of 1906, detected by the triangulation, took place suddenly. It is certain from evidence entirely distinct from the triangulation that on April 18, 1906, relative displacements by sliding along the great fault of that date took place suddenly, that is within an interval of a few seconds, without much crushing or separation of the sides of the fault, and that these relative displacements
For the displacements credited to the year 1868 in this report, the case is different. It had been known from previous examination of the evidence given by triangulation that Mount Tamalpais had moved between 1859 and 1876. In the course of the detailed studies of the triangulation in connection with the present investigation, it was found that other triangulation stations had moved at or about 1868. It was discovered that wherever triangulation in this part of California before 1868 had been connected with triangulation done after 1868, it was necessary, in order to obtain consistent results, to apply abnormally large corrections to the observed angles. By trial it was found that wherever the observations of angles were separated into two groups and separate computations made connecting identical points marked upon the ground, one group comprizing observations before 1868 and the other observations after that year, that the corrections necessary to obtain consistent results from each set of angles were much smaller than before, and about the normal size to be expected from the instruments and methods of observation used. The evidence proves that permanent displacements took place at or about 1868 of a magnitude which the triangulation could detect with certainty. The particular year in which the displacements took place is not fixt, however, by the triangulation, but simply the fact that they occurred within the interval of several years which elapsed in each part of the triangulation between the last observation before 1868 and the first observation after that year. For this reason considerable care has been taken in stating the dates of the triangulation for each locality. In 1906, it was known that sudden permanent displacements took place on a certain day, hour, and minute along a great faultline and these displacements were similar to those detected later by triangulation. So far as the writers know, no evidence has been found that such large sudden relative displacements took place in 1868 or about that year, but it is known that a very severe earthquake in this region occurred in 1868. Hence the observed displacements, referred in this report to 1868 for the sake of brevity, may have occurred in some other year near 1868 and may have occurred by a gradually creeping motion extending over several years.
No other abnormal discrepancies in the triangulation within this region are known to exist. If there are such discrepancies, produced by displacements of the triangulation stations by earthquakes, they are so small as to be effectually masked by the unavoidable errors of observation. In other words, any other permanent horizontal displacements by earthquakes within this region between 1850 and 1907 must have been much smaller than the displacements of 1906 and 1868.
It has been assumed that there was no permanent displacement of stations Mocho and Mount Diablo during the earthquake of 1906. What is the evidence that this assumption is true?
The true direction or azimuth from Mocho to Mount Diablo was determined by observations upon the stars in 1887 and found to be 144° 57' 35.71''. In 1907 it was redetermined by observations upon the stars and found to be 144° 57' 35.66'', differing by only 0.05''
The probable error of observed azimuth in 1887 was ± 0.21'' and in 1907 ± 0.20''. The expression "probable error" is here used in the technical sense in which it is used in connection with the least square method of computation.Hence these observations show positively that the true direction from Mocho to Mount Diablo had not changed between these dates by as much as 1'' and probably had not changed by as much as 0.3''.
The true direction or azimuth of the line Mount Tamalpais to Mount Diablo was determined by observations upon the stars in 1882 and again in the same manner in 1907. In 1882 it was found to be 274° 15' 15.04'' and in 1907, 274° 15' 14.49'', 0.55'' less than before. The azimuth of the line Mount Tamalpais to Mount Diablo was computed separately from the triangulation between 1868 and 1906, and from the triangulation after the earthquake of 1906 and the two values found to be 274° 15' 19.46'' and 274° 15' 17.89'' respectively, the second being 1.57'' less than the first. This apparent decrease of azimuth as determined by the triangulation agrees within 1.02'' with the decrease of 0.55'' determined independently by astronomic observations.
The discrepancy of about 4'' on each date between the azimuth determined by astronomic observations and the azimuth determined by triangulation is what is known as "station error" in azimuth and is due to the deflection of the vertical at the observation station. It does not enter into the present discussion, which is based on differences of azimuths of the same kind, either astronomic or geodetic, on different dates at the same station.This agreement is within the range of possible errors of observation. In the two computations of the triangulation, the line Mocho-Mount Diablo was assumed to have the same azimuth before and after April 18, 1906; hence the close agreement noted indicates that the azimuth Mocho-Mount Diablo remained unchanged.
In the investigation which has been made, it was found that the absolute displacement decreased with increased distance from the fault and that no displacement sufficiently large to be detected with certainty was found farther to the eastward of the fault than Mount Tamalpais, 6.4 kilometers (4.0 miles) from it. Mocho and Mount Diablo are 53 kilometers (33 miles) from the fault; hence it seems certain that the displacements, if any, at Mocho and Mount Diablo must have been extremely small. It may be objected that this is reasoning in a circle, inasmuch as the computed displacements depend upon the assumption that Mocho and Mount Diablo stood still. Cleared of this objection, the argument reduces to the following. The triangulation shows no relative displacements in 1906, large enough to be determined with certainty, of Mocho, Mount Diablo, Rocky Mound, Red Hill, and Lick Observatory, a group of points far to the eastward of the fault, whereas many points nearer to the fault showed large relative displacements as referred to each other, with a marked tendency to be greater the nearer to the fault are the groups of points compared. Hence the reasoning is valid that Mocho and Mount Diablo remained unmoved, these being two points in a group showing no displacements relative to each other, the whole group being far from the fault and these two particular stations being the two points most distant from the fault.
If either Mocho or Mount Diablo had moved in April, 1906, in such a direction as to decrease (or increase) the azimuth of the line joining them, the effect of the erroneous assumption, used in the computation of the triangulation done after the earthquake that the azimuth had remained unchanged, would have been to produce a set of computed apparent displacements which would be represented by red arrows on map 24, all indicating a rotation in a clockwise (or counterclockwise) direction around Mount Diablo, the lengths of the arrows being proportional to their distances from Mocho and Mount Diablo. The fact that the computed apparent displacements of 1906, as shown by the red arrows on maps 24 and 25, do not show any such systematic relation to each other, indicates that the line Mocho-Mount Diablo remained unchanged in azimuth on April 18, 1906.
Similarly, if either Mocho or Mount Diablo had moved on April 18, 1906, in such a direction as to increase (or decrease) the distance between them, the effect upon the computations of apparent displacements would have been to produce a set of red arrows on maps 24 and 25, all pointing toward (or from) Mocho and Mount Diablo, the lengths of the arrows being proportional to their distances from Mocho and Mount Diablo. No such systematic relation appears among the arrows.
Another item of evidence is still available which indicates that the absolute displacement of points far to the eastward of the fault was zero on April 18, 1906. From 1899 to date a series of observations of latitude by observations upon the stars has been in progress continuously for the International Geodetic Association at Ukiah, California. The purpose of these observations is to detect variations in latitude due to any cause. The observations are of an extremely high grade of accuracy and they are made on every clear night. Dr. S. D. Townley, in charge of these observations, made a special study of the 233 observations made during the interval April 4-May 4 inclusive, 1906, to determine whether any sudden change of latitude took place on April 18.
This investigation is published in the Publications of the Astronomic Society of the Pacific, Vol. XVIII, No. 109, Aug. 10, 1906, under the title The Latitude of the Ukiah Observatory before and after April 18, 1906.He found no such change. The observations are competent to determine with reasonable certainty any change as great as 0.03'', corresponding to 1 meter (3 feet). It is therefore reasonably certain that the southward component of the motion, if any, of the pier on which Dr. Townley's latitude instrument was mounted at Ukiah, was less than one meter on April 18, 1906. Ukiah is about 42 kilometers (26 miles) from the fault and to the eastward of it. Mocho and Mount Diablo are much farther from the fault (53 kilometers). It is important to note that latitude observations determined the absolute displacement rather than the relative displacement and that they are independent of observations at any other station.
For the reasons set forth above, it is believed to be certain that the permanent displacement, if any, of either Mocho or Mount Diablo on April 18, 1906, must have been extremely small.
During verbal discussions of the earthquake of April 18, 1906, it has been suggested more than once that one of its possible effects may have been to change the position of the earth with relation to its axis of rotation and so produce a change of latitudes. If an appreciable effect of this kind were possible, the validity of the above reasoning in regard to the latitude observations at Ukiah would be questionable. Accordingly, a computation of this possible effect has been made.
The formula and method of computation is shown in Traité de Mécanique Céleste, par F. Tisserand, Paris, 1891, Gauthier-Villars, Tome II, pp. 485-487.It was found that if it be assumed that the mass displaced in a northerly direction to the westward of the fault comprized 40,000 square kilometers (15,600 square miles) of the earth's crust, having a mean latitude of 38° and thickness or depth of 110 kilometers (68 miles), that this material had an average density of 4.0 and that the northerly component of the displacement was 3 meters (10 feet), the position of the pole of maximum moment of inertia would be displaced by 0.0007'', corresponding to 0.002 meter (0.006 foot). This is a limiting value certainly much larger than the actual value, for all assumptions entering the computation as to the area, depth, density, amount of displacement, and mean latitude have been made such as to make the computed value certainly too great. Moreover, the similar displacements of contrary direction to the eastward of the fault would partially cancel those on the westward side which have been considered. When the pole of maximum moment of inertia is displaced, the pole of rotation is not immediately changed with reference to the earth. The pole of rotation tends always to seek the pole of maximum moment of inertia and travels around it in an irregular path. It is the instantaneous position of the pole of rotation with reference to the earth which fixes the latitude at any instant. Hence
Similarly, the possible effect of the displacements on the deflections of the vertical, that is, upon the direction of gravity at any point, is too small to be considered.
The displacements near Point Arena were computed upon the assumption that the triangulation stations Fisher and Cold Spring remained unmoved during the earthquake of 1906. Is this assumption true? The station farthest to the eastward from the fault at which a displacement in 1906 has been detected with certainty is Mount Tamalpais, distant 6.4 kilometers and displaced 0.53 meter. Also the rate of decrease of displacements at this distance has been found to be 0.13 meter per kilometer of increase of distance from the fault. At this rate, the displacement would become zero at about 11 kilometers from the fault. Fisher is 11.2 and Cold Spring 13.5 kilometers from the fault; hence it is reasonably certain that if the displacement was not zero, at these two stations, it was so nearly zero that it could not have been detected with certainty.
A high degree of accuracy has been claimed for the triangulation. There is abundant evidence available from which to determine the actual accuracy, as has been indicated in an earlier part of this report. A large amount of time has been spent in studying this evidence in order to insure that the estimates of the accuracy of the determination of the various apparent displacements might be reliable. The methods necessarily followed in estimating the accuracy are too technical and too complicated to be included in this report. Two illustrations of the degree of accuracy attained in the observations may prove interesting, however.
The position of the Lick Observatory small dome was determined after the earthquake of 1906 by intersections upon it from four stations, Loma Prieta, Sierra Morena, Red Hill, and Mocho. There were discrepancies among these observations which were adjusted by the method of least squares and a resulting most probable position adopted and used in computing the apparent displacement given in table 1. The mean observation from Loma Prieta his 0.38 meter (1.2 feet) to the left of the position adopted for the dome. The mean observation from Sierra Morena hit 0.22 meter (0.7 foot) to the right, that from Red Hill 0.01 meter (0.03 foot) to the left, and that from Mocho 0.11 meter (0.4 foot) to the left of the adopted position. The words "right" and "left" refer in each case to the Lick Observatory dome as seen from the station named. The distance of the four observation points from the Lick Observatory were, Loma Prieta 31 kilometers (19 miles), Sierra Morena 59 kilometers (37 miles), Red Hill 46 kilometers (29 miles), and Mocho 17 kilometers (11 miles).
Similarly the determination of the position of the Lick Observatory before the earthquake depended upon observations taken from seven stations, Santa Ana, Mount Toro, Loma Prieta, Sierra Morena, Mount Tamalpais, Mount Diablo, and Mocho. The line from Mount Tamalpais, 106 kilometers (66 miles) long, mist the adopted position by 0.36 meter (1.2 feet). The other six all came nearer than this to the adopted position.
The Farallon Light-house was determined between 1868 and 1906 by intersections upon it from three stations, Mount Helena, Mount Tamalpais, and Sierra Morena. The mean observation from Mount Helena, distant 112 kilometers (70 miles), mist the adopted position by 0.30 meter (1.0 foot) and the other two lines came closer. In 1906-1907 the Farallon Light-house was determined by intersections upon it from the six stations Ross Mountain, Tomales Bay, Point Reyes Hill, Sonoma, Mount Tamalpais, and Sierra Morena.
One other assumption remains to be examined. The displacements of 1868 were computed on the assumption that the line Mount Tamalpais to Mount Diablo had a certain length and azimuth before 1868 and a certain different length and azimuth after 1868; Mount Tamalpais being supposed to be in a new position, but Mount Diablo unmoved. The two positions for Mount Tamalpais were derived from certain computations based in turn on assumptions that certain other stations remained unmoved in 1868, or practically so.
The azimuth of the line Mount Tamalpais to Mount Diablo was determined by observations upon stars in 1859, and again in 1882; the later observations made the azimuth. 7.84'' greater than earlier observations. The two adopted azimuths from the computations of triangulation referred to above also differ by 5.38'', the later adopted value being the greater.
The fact that the two independent determinations of change of azimuth, one astronomical and one geodetic, agree within 2.46'' is a strong proof that the adopted geodetic azimuths are correct, 2.46'' being within the possible range of the various observations.
Following the same reasoning as for Mocho and Mount Diablo, the computed displacements of 1868, as shown by red arrows on maps 24 and 25, indicate that the two azimuths and two lengths used for the line Mount Tamalpais to Mount Diablo, before and after 1868, must be very close to the truth.
Changes in Elevation
The preceding portions of this Report have dealt with permanent horizontal displacements caused by the earthquake of 1906. It is important to know whether permanent displacements in the vertical sense also occurred. Upon this point the observations of the Coast and Geodetic Survey furnish evidence for a small area, involving parts of San Francisco, both sides of the Golden Gate, and Sausalito, 1.25 miles north of the Golden Gate.
At the time of the earthquake an automatic tide-gage was in operation at the Presidio Wharf, in San Francisco, on the southern side and about 1.25 miles to the east of the narrowest part of the channel thru the Golden Gate. The gage had been in operation at that point continuously since July 17, 1897, and is still in operation.
The record made by this gage on April 18, 1906, showed an oscillation, with a range of about six inches, in the water surface evidently produced by the earthquake, but it showed no evidence of a change in the relation of the gage zero to mean sea-level. In other words, the record for that day does not indicate that the tide-staff had been changed in elevation by the earthquake.
To detect any possible small change in elevation it is, of course, necessary to examine much more record than that for a single day. The examination has now been extended by computation to include a whole year of observations since the earthquake for comparison with nine years of observations before it.
The following table shows the reading of mean sea-level on the fixt tide-staff for each of ten years, as determined by taking the mean of the hourly ordinates of the tidal curve. The annual means are taken rather than means for any other period in order to eliminate annual inequalities, presumably due to meteorological causes, which affect the means for separate months. May 1 is taken as the beginning of the complete year available after the earthquake. Since it is not convenient, in the computation, to separate any month's observation into two parts, the year is commenced on May 1, rather than on April 18, the date of the earthquake. The first year, 1897-1898, is incomplete because the observations were not commenced until July 17, 1897.
|Period||Readings of Mean Sealevel on Tide-Staff||Means|
|July 17, 1897 to Apr. 30, 1898||8.339||8.318|
|May 1, 1898 to Apr. 30, 1899||8.298|
|May 1, 1899 to Apr. 30, 1900||8.528||8.520|
|May 1, 1900 to Apr. 30, 1901||8.550|
|May 1, 1901 to Apr. 30, 1902||8.430|
|May 1, 1902 to Apr. 30, 1903||8.584|
|May 1, 1903 to Apr. 30, 1904||8.509|
|May 1, 1904 to Apr. 30, 1905||8.667||8.652|
|May 1, 1905 to Apr. 30, 1906||8.659|
|May 1, 1906 to Apr. 30, 1907||8.631|
The ten annual means show an unmistakable tendency to fall into three groups, as indicated by the means shown in the last column of the table. Within each group there is no apparent tendency to increase or decrease. Between the first and second groups the reading of mean sea-level increased 0.202 foot and between the second and third groups, it again increased 0.132 foot. Such an increase corresponds to a subsidence of the zero of the tide-staff with reference to mean sea-level. An examination of the monthly means indicates that probably the subsidence occurred suddenly in each case, the movements taking place about June, 1899, and April, 1904. The record must not be considered as proving positively that these two subsidences took place. The changes are not clearly beyond the range of possible error in the determination of mean sea-level on account of irregular changes in the water surface due to causes not clearly understood, tho they are beyond the possible range of instrumental errors.
The annual mean for the one year after the earthquake, 1906-1907, agrees with the two preceding annual means within less than 0.04 foot. In no other case in the table do three successive annual means agree so closely with each other as these three. Apparently, therefore, no change in the elevation of the zero of the tide-staff occurred at the time of the earthquake.
As further evidence that no appreciable change in the elevation of the tide-staff took place on April 18, 1906, the following table is submitted. Corresponding months of two years, one before and one after the earthquake, are compared to avoid the effects of annual inequalities. The comparison indicates that no change took place in April, 1906.
|Mean =||+ .028|
The zero of the tide-staff was connected by leveling with the group of bench-marks near the gage at various times during the interval 1898-1907, including a determination after the earthquake. The leveling showed no appreciable change in the relation in elevation of the bench-marks and the tide-staff. Hence, the preceding statements in regard to a possible subsidence of the tide-staff on two occasions and in regard to its constancy of elevation on April 18, 1906, also apply to this group of bench-marks.
Before the earthquake the Coast and Geodetic Survey had done leveling which connected the gage at the Presidio Wharf with various bench-marks in San Francisco from Fort Point to the Union Iron Works, and with bench-marks at Sausalito. This leveling was not of the grade of accuracy known as precise leveling nor was it done continuously. There are also available for use in the present investigation certain relative elevations of bench-marks before the earthquake furnished to the Coast and Geodetic Survey by the city engineer of San Francisco. These include a bench-mark near the gage at the Presidio Wharf.
After the earthquake Mr. B. A. Baird, Assistant, Coast and Geodetic Survey, ran a line of precise levels from the Presidio gage to Fort Point and Sausalito, and to the eastward thru San Francisco, to the Union Iron Works, connecting with various old bench-marks.
There were 26 bench-marks connected by the leveling before the earthquake which were recovered with certainty by Mr. Baird and the elevations redetermined by him. The following table shows the elevations of these bench-marks before and after the earthquake and their apparent changes in elevations. All of the elevations in the table are referred to the same datum, which is the reading 8.514 feet (2.5951 meters) on the fixt tide-staff at the Presidio Wharf, that being approximately mean sea-level. All the elevations are computed on the supposition that the zero of the tide-staff at the Presidio Wharf remained unchanged at the time of the earthquake.
The table shows no appreciable change of elevation of the bench-marks at the Presidio Wharf. The maximum apparent change in elevation is 7.0 mm. (0.3 inch), a quantity within the possible range of error of the leveling. Mr. G. K. Gilbert, Geologist of the U.S. Geological Survey, at the close of an examination made soon after the earthquake and before the leveling had been done, exprest the opinion that if this group of bench-marks had not changed their relative elevations, they probably had not changed in relation to the tide-staff. It is probable, therefore, that these two bench-marks and the tide-staff maintained their absolute elevations unchanged.
At Fort Point, the three bench-marks near the shore show an apparent rise of 74 mm. (2.9 inches) on an average, and bench-mark 9, high up on Fort Point, shows a slightly smaller apparent rise, 59 millimeters (2.3 inches). All these are on ground supposed to be stable. The rise indicated by the city leveling, in the last column, is considerably smaller.
The two bench-marks at Sausalito show an apparent rise of 37 millimeters (1.5 inches). It is not certain that this represents a real change in elevation as referred to the zero of the Presidio tide-staff. The errors of the old and new leveling, including the crossing of the Golden Gate (about 1.25 miles) in each case, may account for the apparent change. In the leveling before the earthquake the elevation was transferred from Presidio to Sausalito by water-levels and also by wye leveling with a difference of 13 millimeters (0.5 inch). In the precise leveling after the earthquake, the two independent crossings of the Golden Gate, each depending on many hours of observation, differed by 30 millimeters (1.2 inches).
The three bench-marks at and near Fort Point showed small apparent changes in elevation.
From an examination made soon after the earthquake Mr. G. K. Gilbert, Geologist, exprest the opinion that the bench-marks at Lafayette Park were probably more stable
|Locality||Character of Benchmark||B.M.||After Earthquake Coast and Geodetic Survey 1906-1907||Before Earthquake||New-Old (Coast and Geodetic Survey)||New-Old (City)|
|Coast and Geodetic Survey 1877-1905||City Levels 1901-1906|
|Presidio Wharf||Zero of tide-gage||11||- 2.5951||- 2.5951||.....||0.0||.....|
|Hinge socket of door of brick warehouse.||12||3.8932||3.9002||3.9002||- 7.0||- 7.0|
|Copper bolt in granite post.||15||2.7426||2.7371||.....||+ 5.5||.....|
|Fort Point||Copper bolt in natural rock.||4||6.7585||6.6797||.....||+ 78.8||.....|
|Copper bolt in granite post.||5||14.7958||14.7237||.....||+ 72.1||.....|
|Copper bolt in granite sea-wall.||6||3.9275||3.8554||3.8895||+ 72.1||+ 38.0|
|Brass plate on concrete emplacement.||9||60.7745||60.7151||60.7232||+ 59.4||+ 51.3|
|Sausalito||Copper bolt in rock||2||1.3909||1.3564||.....||+ 34.5||.....|
|Granite post||3||11.6073||11.5672||.....||+ 40.1||.....|
|Van Ness and Lombard Aves.||Star on iron plate in street.||27B||29.4047||.....||29.3967||.....||+ 8.0|
|Fort Mason||Granite post||28||32.5727||32.5606||32.5493||+ 12.1||+ 23.4|
|Fort Mason||Granite post||29||31.0876||31.0854||31.0649||+ 2.2||+ 22.7|
|Lafayette Park||Granite post||24A||101.7846||.....||101.7412||.....||+ 43.4|
|Pendulum pier||25||113.9662||114.0414||.....||- 75.2||.....|
|Transit pier||27||115.3477||115.4222||.....||- 74.5||.....|
|Union Iron Works||Brass spike in brick bldg.||50||3.7860||3.8384||.....||- 52.4||.....|
|Window shutter socket||47||4.4482||4.4004||4.4299||+ 47.8||+ 18.3|
|Bolt in wall of bldg||48||6.2121||6.1695||.....||+ 42.6||.....|
|19th and Bryant Sts||Copper bolt in brick bldg.||58||13.6176||13.5889||13.5883||+ 28.7||+ 29.3|
|Magdalen Asylum, Potrero Ave.||Copper bolt in brick bldg.||61||23.3281||23.3063||23.2977||+ 21.8||+ 30.4|
|Appraisers' Bldg.||Iron Rod||40B||3.3068||.....||3.3241||.....||- 17.3|
|Potrero Ave. and Division St.||Fire hydrant||44I||5.9000||.....||5.9695||.....||- 69.5|
|17th and Carolina Sts.||Nail in doorstep||City||6.0238||.....||5.9978||.....||+ 26.0|
|Mariposa St. between Penn. and Iowa Sts.||Bolt in concrete on bridge over S. P. tracks.||S. P.||10.4666||.....||10.4110||.....||+ 55.6|
|Cal. and Montgomery Sts.||Water table of Parrott Bldg.||41||5.1488||5.0173||.....||+ 131.5||.....|
|East and Mission Sts.||Iron pillar of brick bldg.||43||2.4828||2.8523||.....||- 369.5||.....|
|Folsom between Main and Beale Sts.||Granite post set in brick wall.||44||5.4835||5.5516||.....||- 68.1||.....|
For the three bench-marks at the Union Iron Works, the table shows a contradiction, two of them having, apparently, increased in elevation and one having decreased. The greatest change is, however, only 52 mm. (2.0 inches). The Union Iron Works is said to be partly on filled ground.
The two bench-marks near the Magdalen Asylum apparently increased in elevation as shown by both the Coast and Geodetic Survey and city leveling.
Of these bench-marks, the thirteen in the five groups at Fort Point, Sausalito, Fort Mason, Union Iron Works, and Magdalen Asylum, showed an average apparent rise at the time of the earthquake of 35 millimeters (1.4 inches) as determined by the Coast and Geodetic Survey leveling. As the leveling simply gives relative elevations the question arises, Does this quantity represent an average rise of the thirteen benchmarks or does it represent a settlement of the zero of the tide-gage and the adjacent bench-marks at the Presidio Wharf? The tidal observations are not competent to determine this question with certainty. The general experience with determinations of mean sea-level, from long series of tidal observations, warrants the statement that the error in determination from a single year is as apt to be greater as less than 0.75 inch (19 millimeters) and that it may sometimes be as great as 2.5 inches (64 millimeters). It is possible, therefore, that the two bench-marks at the Presidio Wharf and the zero of the tide-gage have settled 35 millimeters or that it is, in part, a subsidence at the Presidio and in part a rise at the other places.
The elevations of the group of four bench-marks in the table commencing with 40B at the Appraisers' Building, were determined before the earthquake by the city engineer, but not by Coast and Geodetic Survey leveling. These four, in various parts of the city, show no apparent change in elevation greater than 69 millimeters (2.7 inches). Two of them apparently rose and two subsided.
The apparent changes in elevation of the three bench-marks in the table, commencing with 41 at California and Montgomery Streets, are not supposed to have much significance in connection with the question of whether a general change of elevation took place. These three bench-marks were each subject to local disturbances during the earthquake or were near or on filled ground.
In ten cases the old leveling determined elevations of hydrants and the new leveling determined elevations on hydrants in the same locations but known, from the descriptions, to be different from the old hydrants. Similarly, in seven other cases, the old leveling established the elevations of points on curbstones, steps, or doors, and in each of these cases in the new leveling it was found to be impossible to recover the old point accurately. In all of these 17 cases there is, therefore, only an approximate connection between the old and the new leveling. The evidence from these bench-marks has all been examined carefully and does not lead to any different conclusion from that which may be drawn from the table above.
The general conclusion from both the leveling and the tidal observations is that, within the region examined, there occurred no general change of elevation of sufficient magnitude to be detected with certainty.
It is an opportune time, at present, on account of local changes in elevation at various bench-marks, to adopt the best possible determination of mean sea-level which is available up to date and to refer all new elevations determined since the earthquake to that datum. Accordingly, the reading 8.652 feet (2.7371 meters) on the tide-staff at the Presidio,
It is uncertain, as already indicated in this report, whether this correction of - 0.0420 meter is due to improvement in the determination of the relation of mean sea-level to the tide-staff or to a subsidence of the tide-staff and adjacent bench-marks in 1904 or earlier, or to both.
Note on the Comparison of the Faults in the Three Earthquakes of Mino-Owari, Formosa, and California
By F. Omori
The three great earthquakes of Mino-Owari (Central Japan) on October 28, 1891, of Kagi (Formosa) on March 17, 1906, and of California on April 18, 1906, were each accompanied by the formation of remarkable geological faults, whose total lengths were about 100, 50, and 430 kilometers respectively. The dislocation in the California earthquake was formed partly along, and partly off, the coast of California, belonging to the category of longitudinal faults.
The dislocation in the Mino-Owari and Kagi earthquakes were, on the other hand, formed nearly at right angles to the course of the Main Island (Nippon) and the axis of Formosa Island respectively, both belonging to the category of transverse faults.
Notwithstanding these differences, there are certain similarities among the three cases. Thus, in each of the three earthquakes, the direction of motion at different places in the immediate neighborhood of the fault was not perpendicular, but more nearly parallel, to the strike of the latter. This seems to indicate that the formation of the faults was mainly due, in each case, not to such actions as a simple falling down or sudden creation of a cavity underground, but to the existence of shearing stresses in the plane of fracture possibly of two opposing forces acting either from the center toward both ends of the fault-line, or toward the center from both ends.
The accompanying figure is a diagrammatic illustration of the three faults, the line ab indicating, in each case, a straight line (say, road) which suffered a shearing movement in such a way that the part b on the depressed side was displaced to the new position b', and generally transformed into a curve.
From the figure it will be seen that there existed in each fault what may be called the central point, where the disturbance of the ground is greatest and about which the shear and depression along the line of dislocation is more or less symmetrical. In the case of the Mino-Owari earthquake the central point was in the vicinity of the village of Midori in the Néo-Valley, where a very remarkable depression of the ground took place. The corresponding point on the Formosa fault was between the villages of Bishō and Kaigenkō. In the California earthquake the northern half of the fault was in part under the ocean, but the central point was probably in the vicinity of the Tomales Bay, the greatest amount of disturbance having occurred there.
The greatest vertical dislocation of 18 feet occurred in the Mino-Owari earthquake, while the greatest horizontal shear occurred in the California earthquake. In the latter the vertical displacement was only 1 or 2 feet, while in the former there was also a large horizontal shear of about 18 feet. In the Formosa earthquake, whose magnitude was much smaller than the other two, the vertical and horizontal displacements of the ground were each of a moderate scale, the maximum amounts being 6 and 8 feet respectively. The maximum (vibratory) motion in the Mino-Owari earthquake showed a tendency of being directed from the central point toward each end; while, in each of the two other earthquakes, the same motion was, as far as can be ascertained, directed from one end toward the center. Again, the direction of the maximum (vibratory) motion was, in the Formosa earthquake, the same as that of the shear of the depressed ground. In the two other earthquakes, however, the reverse was the case. These differences are probably due to the variation in the manner of the action of the force along the fault-plane which finally produced the dislocations.
Review of Salient Features
The differential displacement of the earth's crust effected by the movement on the San Andreas fault on April 18, 1906, may for convenience be resolved into two components, the horizontal and the vertical. Of these the horizontal movement was the more important and was susceptible of measurement, giving minimum values for the amount of displacement in this direction practically all along the trace of the fault, except at the extreme north and extreme south. The vertical movement was small compared with the horizontal, and was established satisfactorily only in the region to the north of the Golden Gate.
Two kinds of evidence of vertical displacement were available. The first of these was the formation of scarps along the fault-trace, and the second was the change on portions of the coast of the level of the land relatively to sea-level. The scarps that appeared as features of the fault-trace were in part fresh facets where none had existed before the earthquake and in part accentuations or additions to old scarps due to former movements. In both cases exact measurements were rendered difficult by the drag of the soil along the rupture, and by the complication due to the larger horizontal movement. But making all allowances for the masking effect of drag of the soil, it is certain that the height of these scarps, or of the additions to old ones, was quite variable, even in the same general locality, within a range of a few inches up to about 3 feet. It is suggested that this variation is referable in considerable measure to the drag and adjustment of materials in the zone beneath the soil; so that the true displacement of the firm rocks lies between the extremes observed.
The evidence of vertical displacement, based on the recognition of scarps, indicates a slight upward movement of the crustal block on the southwest side of the fault in the northern territory. South of the Golden Gate there is no very satisfactory or consistent
Similarly, the evidence of vertical displacement, based on a comparison of the relative position of land and sea-levels before and after the earthquake, is limited to the region north of the Golden Gate. The Point Reyes Peninsula appears, from this class of evidence, to have been probably upraised slightly by the fault movement; but the evidence is not entirely conclusive.
Observations conducted by the Coast and Geodetic Survey thruout the year succeeding the earthquake, at the tide-gage station near Fort Point in the Golden Gate, show that the relative level of land and sea at that point is the same as it was before the earthquake. Since this station lies on the northeast side of the fault, the observation would appear to indicate that any upward movement of the crustal block on the southwest side was an absolute one.
The horizontal displacement on the fault, as measured on fences, roads, and various structures which crost the fault-trace, is also apparently quite variable, ranging from a foot or less up to 20 or 21 feet. This variation is probably due to a number of causes. The principal one of these is the fact that the displacement was not always confined to the sharp line upon which an offset was observed at any locality. Auxiliary cracks, distributed over a zone not uncommonly a few hundred feet wide, took up portions of the displacement; and these auxiliary cracks doubtless escaped observation in many cases. Indeed, owing to the yielding character of the superficial mantle of soil and regolith, it is probable that many of these auxiliary cracks did not appear as ruptures at the surface. Besides this distribution of the displacement on auxiliary cracks satellitic to the main rupture, the deformation of the ground along the latter, both superficially and in its deeper portions, was probably variable. The extent of this drag is shown in a few instances that have been susceptible of measurement; notably the fence at Fort Ross, surveyed by Mr. E. S. Larsen, on which a displacement of 12 feet was distributed over a distance of 415 feet on the southwest side of the fault-trace; the roadway near Point Reyes Station, where a displacement of 20 or 21 feet was distributed over 60 feet; the fence south of Mussel Rock, surveyed by Mr. H. O. Wood, in which a displacement of 13 feet was distributed over 250 feet on the southwest side of the fault-trace and 40 feet on the northeast side; the 3 fences surveyed by Mr. R. B. Symington near San Andreas Lake, one showing a displacement of 16.9 feet, distributed over more than 1,100 feet, the second a displacement of 10.4 feet distributed over more than 300 feet, and the third a displacement of 12.7 feet distributed over more than 2,200 feet; and the tunnel at Wright, surveyed by the engineers of the Southern Pacific Company, showing a displacement of 5 feet distributed over nearly a mile on the southwest side of the fault-trace. These instances are doubtless indicative of the general character of the deformation of the ground in the immediate vicinity of the fault, and aid in understanding the variable expression of the amount of offset at the main fault-trace. The recognition of the distribution of the movement on auxiliary cracks, some of which may not have appeared at the surface, and the deformation of the ground along the zone of rupture, justifies the conclusion that, except under peculiar conditions — such, for example, as in the marsh at the head of Tomales Bay — the maximum figures obtained for the displacement by the measurement of offsets at the surface must be a minimum expression for the true extent of the
The geodetic measurements of the earth movement, as presented in the paper by Messrs. Hayford and Baldwin, are of extreme interest and form one of the most important contributions to the study of the earthquake. The evidence of displacement observed along the fault-trace affords measurements of the total relative movement only, while the geodetic work gives us an approximate measure of the absolute movement on either side of the fault, and the distribution of the movement away from the fault. The results of this geodetic work are not only set forth in detail by the paper of Messrs. Hayford and Baldwin, but they are also admirably summarized, so that all that seems necessary in this place is to discuss very briefly these results from a geological point of view.
A notable feature of the paper is the discovery of a movement of the earth's crust which antedates the earthquake of April 18, 1906, and which is referred to the earthquake of 1868; altho it is recognized that the date and duration of the movement cannot, on the data available, be positively determined. Inasmuch as the time of this movement is left an open question, and is referred to the year 1868 largely as a matter of convenience in discussion, it may be of advantage to inquire briefly whether or not it may have some other significance than that of a sudden movement occurring in that year.
Altho, as shown in another part of this report, the earthquake of 1868 was related to a rupture or series of ruptures of the ground at the base of the hills on the northeast side of San Francisco Bay, there was no evidence of a large relative displacement such as occurred in 1906. It seems reasonable to suppose that if the earlier movement in question had occurred suddenly in the same way as that of April 18, 1906, we should have had a similar manifestation of faulting within the region affected. Since there was no such manifestation the reference of the earlier movement to the earthquake of 1868 may be fairly questioned, and another hypothesis entertained to explain it, particularly if this hypothesis harmonizes in some considerable measure with the results of the geodetic survey.
This hypothesis is that the earlier movement is not immediately or exclusively associated with the earthquake of 1868, but is the expression of the strain in the earth's crust which led to the rupture or slip of 1906 and the consequent earthquake. That rupture presupposes a condition of strain, and it is difficult if not impossible to conceive of such a sudden disruption except as a relief from strain. Such strain involves the idea of slow displacement; and if a series of points had been established in the territory affected at different dates, with reference to some base beyond it, a measure of this slow displacement or creep of the earth's crust might have been obtained.
The strain culminated in a slip on an old rupture plane and may fairly be supposed to have been more or less symmetrically distributed with reference to that plane, so that when relief was effected by slip, the movement involved would be equal in amount on the two sides of the fault.
This hypothesis and its implications appear to fit fairly well with the results of the geodetic resurvey, particularly for that portion of the territory where the earlier movement can be most satisfactorily discriminated from the displacement of 1906. For example in the Tomales Bay region there are ten points, viz.: Bodega Head, Tomales Point, Tomales Bay, Foster, and Point Reyes Hill on the west side of the fault of 1906, and Bodega, Smith, Mershon, Hans, and Hammond on the east, at which the two movements
Under the hypothesis here presented this movement continued at a probably uniform rate for the next 16 years up to the time of the earthquake of 1906. This would give us a total northerly movement for the interval from 1856-1860 to 1906 of 2.34 meters. Now the northerly component of the combined earlier and 1906 movements, shown in table 3 of Hayford and Baldwin's paper, is on an average 4.95 meters for the five stations west of the fault-line. This includes the sudden movement of 1906 plus the slow creep of 2.34 meters above deduced. The value of the northerly component of the sudden movement of those points in 1906 is thus 4.95 - 2.34, or 2.61 meters. Similarly the southerly component of the combined movements for the five stations to the east of the fault is found to be on the average 0.09 meters. The southerly component of the sudden movement of 1906 was therefore 0.09 + 2.34, or 2.43 meters. The absolute movement on the two sides of the fault on April 18, 1906, was thus nearly the same in amount.
The reference of the earlier movement to a slow creep thus appears to harmonize with and therefore tends to confirm the a priori assumption that the absolute movement of 1906 should have been the same on both sides of the fault. Were data available as to the time at which other groups of stations were determined in position, it is probable that a similar result would be reached. We may consider, therefore, that the earlier movement is better explained on the hypothesis of slow creep, continuing up to April 18, 1906, than on the assumption that it occurred at or about the time of the earthquake of 1868. This conclusion applies to the region north of San Francisco Bay. To the south of the Bay the data available are inadequate for a satisfactory separation of the two movements, except in the case of Loma Prieta, and here the earlier movement appears to have been southerly.
Another result of the geodetic resurvey which points to a slow creep of the region under strain precedent to April 18, 1906, is the distribution of the displacement on that date. The measurements of the absolute displacement on the two sides of the fault show that it was notably greater near the fault than at points remote from it. Thus if we imagine a series of points in a straight line transverse to the fault before the earthquake that line was so deformed that the segment to the west of the fault curved northerly and the segment to the east curved southerly in approaching the fault-trace. This deformation can be most readily explained by supposing that the series of points upon the assumed straight line were determined as to position in the first instance upon the surface of a portion of the earth under elastic strain, so that when relief was effected by rupture, the points tended to assume positions relative to one another which they would have had if they had been determined before the advent of the strain.
It may be further pointed out that the conclusion reached by Hayford and Baldwin to the effect that the absolute movement on the west side of the fault was on the average twice as great as the movement on the east side is founded on the assumption of the stability of the base-line Diablo-Mocho. In view of the unknown extent of the earth movement of April 18, 1906, it would seem preferable to make the assumption that the relief from strain was approximately distributed equally on the two sides of the fault and from this infer the amount of the southeasterly displacement of Diablo and Mocho. The assumption that Diablo and Mocho were not affected by the disturbance of April 18, 1906, is based on the following considerations:
1. There was no change in the azimuth of the Diablo-Mocho line.
2. There was no change in the length of that line.
3. There was no appreciable change in the relations of these two stations to certain others nearer the fault.
4. The latitude of Ukiah remains the same as before the earthquake.
The first three of these conditions would be fulfilled if the region including all the stations occupied had moved in unison southeasterly with but little or no rotation, a possibility which it is difficult to deny. The fourth consideration does not preclude this possibility since the amount of movement involved is probably less than the errors of the method used for the determination of the latitude of Ukiah.
In the region about Monterey Bay the most interesting fact brought out by the geodetic resurvey is that the combined effect of the earlier movement and that of 1906 is a southerly migration of the earth's crust on both sides of the San Andreas rift. It is probable from direct observations of relative displacement along the fault-trace in 1906 that the southwesterly block moved northwest as far as the rupture extended. If this be accepted, then the southerly net movement on the west side of the south end of the fault is due to the predominance of an earlier southerly movement. This agrees with the positive and certain earlier displacement of Loma Prieta. Accepting the southerly character of this earlier movement as certain, there is forced upon us the remarkable fact that the direction of displacement in the region about Monterey Bay is the reverse of that of the earlier movement for the region north of San Francisco Bay. This means that the earlier movement was distensive in character, displacing the territory to the north of San Francisco Bay northerly, and that to the south southerly while the vicinity of the Bay itself was relatively neutral. It appears, moreover, that the southerly displacement was differentially diffused, since the amount of displacement of the south side of Monterey Bay was notably greater than that of the north side, resulting in a widening of the Bay by about 10 feet.
Similarly the distance between Tamalpais and Black Mountain, both on the same side of the San Andreas rift, has been increased by a like amount. The significance of this general distension involved in the reversal of the direction of displacement to the north and south of San Francisco Bay, and of the differential character of this distension, without known rupture, at Monterey Bay and San Francisco Bay, can not at present be stated. The problem requires prolonged study and repeated measurements to secure the necessary data for a proper discussion. It is evident, however, that we are here confronted with some of the most interesting phenomena in the mechanics of the earth's crust, phenomena which call for deliberate investigation extending through years and decades and conducted on a wisely planned program.
Provision for Measurement of Future Movements on San Andreas Fault
The extent of the movement on the San Andreas fault on April 18, 1906, was measured imperfectly and inexactly by offsets of fences, lines of trees, roads, pipes, dams, creeks, shore lines, etc. The distribution of the displacement in the immediate vicinity of the fault, the drag and compression of the soil, the uncertainty as to the former orientation of the lines offset, and other adverse conditions rendered the determinations unsatisfactory to a certain degree. With one exception, the measurements obtained in this way are suspected of being less than the true amount of relative displacement of the firm rocks below the surface materials.
With the object of obtaining a more exact measurement of any future movements that may take place on the same fault, the Commission