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Minor Geological Effects of the EarthquakeLandslidesThere are three types of landslides known in the Coast Ranges. For convenience in reference they may be designated as earth-avalanches, earth-slumps, and earthflows. The first and last of these are of somewhat exceptional occurrence, but the second is exceedingly common. These landslides are of geological importance as an agency concerned in the evolution of the geomorphy of the Coast Ranges of California to an extent equaled in few other regions; and it becomes a matter of interest to appreciate the rôle played by earthquakes in promoting the efficiency of this agency. The activity of all three kinds of landslides is related directly or indirectly to the earthquake of April 18, 1906. In order to appreciate certain phases of the relationship, it will be of advantage to state briefly, in a general way, some of the characteristics of these different types of landslides. In doing this, reference will first be made to the most commonly occurring type, the earth-slump. The other two may then be characterized by contrast with this type. Under normal conditions, earth-slumps appear chiefly as features of mature slopes which are in adjustment to the ordinary processes of rain erosion. They are also found, however, as notable features of immature slopes, at the base of which horizontal corrasion is active, as on sea-cliffs and stream-cliffs, supplanting under certain conditions the earth-avalanche which is chiefly found in such situations. On the mature slopes of the Coast Ranges of California, under present climatic conditions, the regolith or mantle of decomposed rock, on the more common rocks, appears to be accumulating at a somewhat faster rate than the rain-wash can remove it. This excessive accumulation of the regolith appears to be an important factor in producing conditions conducive to earth-slumps. The climate of the region is characterized by a pronounced alternation of dry and wet seasons. In the summer the soil and regolith on the hillsides are dried out to a considerable depth, in many cases down to the underlying firm rock; and as the desiccation proceeds the soil shrinks and cracks. The cracks thus formed permit the ready access of the early winter rains to the deeper portions of the soil and regolith. The concentration of the entire rainfall in one half of the year is also more conducive to the saturation of the ground than if it were distributed thruout the year. The climate is thus a contributory factor to the prevalence of earth-slumps. A factor of local importance is the character of the underlying geological formations. Where these consist of clays or shales, earth-slumps are much more liable to be inaugurated and to recur than where the rocks have little or no clay in them. The emergence of springs on hillsides is also a fruitful cause of earth-slumping where other conditions, particularly the last mentioned, are favorable. Another factor may be the recent subjection of the hill-slopes to grazing and tillage. In general, however, this interference with natural conditions appears to have been conducive to excessive corrasion and sapping, rather than to slumping. Grazing and tillage rob the surface of its natural protection of dead grass and other vegetation, which in the early winter season tend to restrain the rapid flow of the rain-water and its concentration in lines of corrasive activity. New lines of corrasion are thus inaugurated, and where the rocks are but slightly coherent ― 385 ―
new geomorphic forms, of the bad-land type, are evolved with startling rapidity. This corrasive process is sometimes complicated
by earth-slumping.
The activity of earth-slumping as a degradational process is, in general, a function of the amount of rainfall in any given season. Thus in the winter of 1889-1890, in which the rainfall was exceptionally heavy, earth-slumps thruout the Coast Ranges were much more active than in seasons of normal rainfall, and many new ones were started. In all such earth-slumps the saturation with water of the soil and regolith, and in some cases of the underlying formations, is an essential condition. This water is the main agent in loosening or disintegrating the material preparatory to the slip. It is also a motive power on account of the large addition which it makes to the weight of the unstable mass; and it is a transporting agent owing to the fluid or plastic nature which it imparts to it. The character of the movement in an earth-slump is noteworthy. The ground moved drops away from the slope in the form of a bite, leaving a lunate or horseshoe-shaped searp overlooking the sunken area. As the mass moves down, it generally encounters the resistance of more stable portions of the slope below, and is thus crowded upon itself. The plastic mass is in this way deformed, and the deformation amounts in many cases to an effective rotation of the moved portion upon a horizontal axis. The lower portion is thrust over the passive ground at its lower margin, and the slope of the surface of the moved part is greatly diminished and in many cases reversed. Between the reversed slope and the limiting scarp a depression is thus formed which may become a pool. The change in the slope thus occasioned gives rise to the landslide terrace. [1] This kind of movement may be slowly continuous for considerable periods, or it may be fitful, depending upon the supply of water. In a slumping tract the movement may be repeated at various levels, giving the slope an irregularly stept or terraced profile; and if the movement has been recent, numerous cracks and fissures traverse these terraces, particularly where they break away from the upper limiting scarp. The instability of the mass is an essential feature of the earth-slump. When not actually moving, its movement is imminent at all times, but with varying degrees of imminence, depending upon local conditions. This instability and imminence of movement is true of many slopes where no actual earth-slump has appeared, but where movement may be inaugurated at any time by an exceptionally heavy winter or by some other precipitating cause. Severe earthquakes constitute one of these precipitatory causes. Thruout the Coast Ranges of California the small residual stability of many earth-slumps was overcome by the vibration of the ground at the time of the earthquake of April 18, and they were caused to slump forward. In many other instances new earth-slumps were started, owing to the same general cause. Besides the earth-slump movements which were the immediate effect of the earthquake shock, there were doubtless others which were indirectly referable to the same cause. As will be shown in another part of this report, one effect of the earthquake was the derangement of the normal movement and amount of flow of underground waters, the general result being a temporary increase of flow. Inasmuch as many earth-slumps depend for their water upon springs, there can be little doubt that the increased flow had its effect upon these, and promoted their activity several days or possibly weeks after the shock itself. Another way in which the shock conduced to the activity of earth-slumps at a later date than the shock itself was by opening cracks and thus rendering the deeper portions of the unstable mass more accessible to the rains of the following winter. The movement of earth-slumps at the time of the earthquake was abnormally large and sudden, thus leading to the development of numerous open cracks, not only in the landslide proper, but also in the surrounding slopes above the limiting scarp. The effect of this ― 386 ―
would inevitably be the enlargement of the area of the slide in the wet season. Similarly on many slopes, particularly at
points not far distant from the Rift, numerous cracks were opened without actual slumping of the ground occurring in consequence
of the shock; but the conditions were thus provided for the slumping process the following winter. During the winter 1906-1907
many such slides were reported in a general way. Unfortunately detailed information as to their occurrence is as yet lacking.
It is to be noted that an exceptionally heavy rainfall conspired with the conditions established by the earthquake to produce
these landslides.
In the type of landslide thus far considered, the contained water, which is at once in part the cause and the means of the movement, accumulates relatively slowly, and it varies with the season, there being usually a more or less free drainage from the lower portion of such slides. There are, however, other landslides which are due to a relatively large and sudden accession of water to the unconsolidated materials of a slope. Such sudden accessions of water may be conceived to be produced in a variety of ways; such, for example, as a so-called "cloudburst" in a desert canyon, the slopes of which may be heavily mantled by earth and loose rock; or the breaking of a barrier which retains a bog or other body of water. For the present purpose, however, which is not that of an exhaustive systematic discussion of this class of phenomena, it will be sufficient to take note only of water which is expelled from the ground by the compressive action of the earthquake shock. Such landslides may be discriminated from earth-slumps by reason of their greater mobility, under the designation earth-flow. Earth-flows differ from earth-slumps not only in the much larger quantity of water involved in their mechanism as a moving mass, in the suddenness with which the water becomes efficient as a transporting agency, and in the rapidity of the movement; but also in the brevity of the entire process, its finality, and its non-recurrence. Besides these two types of landslides, there is still another, which is immediately associated with earthquakes as a cause of movement. This is the slide of dry earth and rock upon precipitious slopes or their fall from cliffs. Soil or other loose forms of earth may participate in such landslides, but the material is usually composed chiefly of rock which becomes increasingly shattered with the progress of the slide. Such landslides will here be referred to as earth-avalanches. They are distinguished from both earth-slumps and earth-flows by the character of the material and by the absence of water as an essential factor in producing movement. They also differ usually in the marked acclivity of the slopes on which they occur. They differ from earth-slumps, but resemble earth-flows, in the finality or completeness of the movement. They are not progressive movements, but sudden events; and there is no recurrence of movement of the material involved, altho the avalanche may recur at the same place. Besides these three types of landslide, another ought perhaps to be recognized. This is the form of superficial earth movement which occurred in consequence of the earthquake shock on the alluvial bottom-lands of many streams. It may appropriately be designated an earth-lurch. It varies from the opening of a mere crack, with a slight movement of the ground on one or both sides, to a violent and complicated deformation of the surface, usually accompanied by cracks and open fissures parallel to the trend of the neighboring stream trench. These cracks and fissures cut the ground up into strips or prisms which lurch toward the stream trench, or, it may be, toward an abandoned slough, the lurch usually being accompanied by a rotation of the prism. They are distinguished from all other forms of landslides by occurring on perfectly flat ground and by the fact that they are apparently referable directly and solely to the horizontal jerk of the earth movement during the earthquake shock. A brief account, which in some cases amounts only to a mention, will now be given of some of the various kinds of landslides set in motion by the earthquake. ― 387 ―
Earth-AvalanchesEarth-avalanches were caused chiefly along the sea-cliffs of the coast on the morning of the earthquake, tho some also occurred on steep canyons within the zone of high intensity. On the coast the earth-avalanches were for the most part simply an exceptional incident in the normal process of cliff recession. Where the upland of the Coast Ranges approaches the shore, the horizontal corrasion of the waves maintains a steep sea-cliff; and the recession of the sea-cliff is effected by the repeated occurrence of earth-avalanches due to the undermining by the sea, combined with the disintegrating action of atmospheric agencies. There are thus always upon the face of the cliff masses of earth or rock, the fall of which is imminent and may easily be precipitated by a severe shock of earthquake. The most notable of the earth-avalanches occurred where the sea-cliffs are highest and steepest. This happens on the coast of Humboldt County, between Cape Mendocino and Point Delgada. Not only are the cliffs here particularly favorable for large earth-avalanches, but the coast here is close to the line of the fault which caused the earthquake, and so received an exceptionally severe shaking. For many miles of coast there was a general slipping of rock and earth into the sea, down very precipitious sea-cliffs ranging up to over 2,000 feet in height. Between Shelter Cove and Point Arena, the sea-cliffs are not so high nor so continuous, but there was nevertheless a very general, and locally large, shedding of material from their face; and the sea was muddy for many days after the earthquake in consequence of the dejection of the débris upon the shore, within range of the attack of the waves. From Point Arena southward to Fort Ross, the cliffs are low, being for the most part not in excess of 100 feet. Earth-avalanches were nevertheless of common occurrence along this stretch of coast. South of Fort Ross to Bodega Head the cliffs are again, as far as the mouth of the Russian River, several hundred feet high and very steep. Here again earth-avalanches were extensive. The rocks along this entire stretch of coast from Cape Mendocino to Bodega Head are prevailingly sandstones and shales. On the sea-cliffs on the north side of Bolinas Bay and west of the town of Bolinas, there was a very general crumbling and fall of the sea-cliff upon the beach. South of the Golden Gate, the most notable earth-avalanches were along the sea-cliffs between the city and Mussel Rock. This cliff has a length of about 6 miles and ranges in height from about 100 feet up to 700 feet, and is cut almost wholly in the strata of the Merced (Pliocene) series, which are inclined at angles varying from 15° to 75°. The rocks are for the most part rather soft and incoherent, tho there are numerous well-cemented and indurated beds in the series. This cliff converges on the fault at a small angle, and intersects it at its south end near Mussel Rock. The cliff was severely shaken and great quantities of earth and rock were caused to fall or slip down. The great earth-slump at Mussel Rock (Plate 129C, D) was also notably accelerated. A similar sudden movement of the ground occurred on the west side of Merced Lake, whereby a large section of the slope sank toward and into the lake, and a portion of the bottom of the lake was lifted above the surface by the deformational rotation of the collapsed ground. To the south of Mussel Rock there were several small earth-avalanches along the cliffs, and numerous cracks were formed near the brink of the cliffs which will in future doubtless lead to further falls from the cliff-face. Near San Pedro Point there was a large movement of the earth on the face of the high cliff. One earth-avalanche to the north of the Devil's Slide started about 800 feet above the shore and swept the face of the cliff, carrying away several hundred feet of roadbed. The slide occurred near the contact of sandstones reposing on granite, and both kinds of rock were involved. Smaller earth-avalanches occurred farther south on the sea-cliffs. ― 388 ―
Inland from the coast there were numerous earth-avalanches caused by the earthquake on the walls of steep canyons. One of the most noteworthy of these was on the north side of a short but deep canyon west of Chittenden and close to the line of the fault. (Plate 126A.) The rocks composing the side of the canyon are the bituminous shales of the Monterey series. The slope rises very steeply for about 500 feet and was quite dry before the earthquake, altho it was covered with spring vegetation. Areas of bare rock appeared thru this vegetation. At the time of the shock several earth-avalanches were started, and these slid suddenly down the slope, part of the material filling the bottom of the canyon and part remaining on the less steep lower portions of the slope. The larger masses were broken off up near the brink of the canyon. There was apparently little or no rotation of the sliding mass. The result was to gorge completely the lower part of the canyon with rock débris, to widen the upper part of the canyon, and to expose extensive surfaces of unweathered rock. On Deer Creek, in the Santa Cruz Mountains, an extensive earth-avalanche started near Grizzly Rock and moved westward down a steep, narrow canyon for about 0.25 mile. (Plates 124D and 125A.) It then changed its course thru an angle of about 60° as it entered a wider canyon of lower grade, and following this for another 0.25 mile, finally stopt at the Hoffmann Shingle Mill, which was wrecked. A fine growth of red-wood, some 200 feet in height, was mowed down, and covered to the extent of 10 acres or more with from 30 to 60 feet of débris. The trees were from 3 to 10 feet in diameter. The main canyon was filled with earth and rock for an average width of 80 yards and a length of 400 yards. The entire area of the slide was about 25 acres. The difference in altitude between the point where the slide started and the shingle mill, where it stopt, is 500 feet. According to Mr. G. A. Waring, the slide material has a depth of 300 feet and is composed of soil, clay, and shale. Mr. E. P. Carey, who examined and photographed this interesting earth-avalanche, states that it originated in rock that broke away in pieces from the steeply inclined slope at the head of the gulch, leaving a large theater-like space, the bare, light-colored rock walls of which were in sharp contrast with the surrounding green vegetation. The movement was faster in the center or deepest part of the gorge than on the margins. The rock was in general piled up higher along both sides than in the center, and many pieces became entangled in the standing or uprooted trees. A steep-walled tributary to the southeast of the main gulch supplied rock material to the main avalanche, and the 2 streams joined much as confluent glaciers do. The material involved in the avalanche showed every gradation from powder to angular pieces 30 feet or more in diameter. The surface was uneven thruout. Near the mill a man was killed by a tree that fell as the avalanche was advancing. Mr. Carey also reports another earth-avalanche located on the Petty ranch, about 4 miles southeast of the one just described. Here a huge rock mass, which embraces an area of about 12 acres at the headwaters of Cauley Gulch, broke away from a ledge and dropt, leaving a vertical scarp of 40 feet or more. The rock mass in this case was not shattered. It practically maintained its integrity. The narrow gulch below was unfavorable for free downward movement. As the block readjusted itself, its upper surface became nearly level, but was lower at the foot of the scarp than at its outer edge, thus indicating that it had suffered rotation. At a point about 1.25 miles west of the Mindego sink, on the ranch of Andrew Stengel, an earth-avalanche is reported by Mr. Albert C. Herre. It is on a small tributary of Alpine Creek, and about 4 miles southwest of the San Andreas fault at the point where the latter crosses Black Mountain into the head of Stevens Creek Canyon. The creek here is in a narrow, steep-walled canyon in the bituminous shale of the Monterey series. The soil on the canyon side was very shallow, and at the time of the earthquake it was shaken down into the bottom of the canyon, leaving the walls absolutely bare in places ― 389 ―
for a hundred yards at a stretch. The slide extends for 0.25 mile on both sides of the canyon. A similar earth-avalanche was
caused by the earthquake on the ranch of Judge Welch, not far from Long Bridge and within 2 miles of Saratoga. Mr. Herre reports
that here the soil on the northwest side of a small creek coming down from the Castle Rock Ridge, was shaken down for perhaps
0.5 mile, tho not continuously. In places the slid material filled up the creek-bed and totally changed the contour. It destroyed
the road to the ranches farther up the canyon, and wrecked some bridges. Along the upper part of the area affected, a vineyard
was destroyed; while farther down the canyon a heavy forest growth, consisting mostly of redwood, oak, alder, and laurel,
was obliterated. This slide lies in the path of the San Andreas fault.
Mr. Herre further reports a large slide on the Mindego Ranch, 20 miles southwest of Palo Alto. Here, on the north side of Alpine Creek, a tract of some 50 acres sank at the time of the earthquake, with little or no apparent forward movement. The tract sloped to the south and west, and formed part of a great, open hill pasture, with trees and underbrush about the lower or creek side. The creek-bed itself is filled with a growth of Douglas spruces and other trees. The land, which before the earthquake was steeply inclined, is now comparatively level, the eastern and northern part having sunk perhaps 100 feet, while that on the west has sunk but 10 or 15 feet. The surface of the sunken tract was greatly seamed and cracked, and part of it was flooded, owing to the springs uncovered; but otherwise it was unchanged in appearance. There was no piling up of earth, nor sliding of one portion over another. A fence crost the tract, and the posts on it sank so that but a few inches protruded above the surface; while some Douglas spruces also sank several feet into the earth. A number of cattle were on the land at the time of the earthquake, but were uninjured. It was a work of great difficulty to remove them, block and tackle being necessary. The creek-bed was apparently not affected, nor were the trees in it disturbed. There was no apparent movement of the earth into the canyon, but the whole mass seems simply to have been dropt from a steep slope to a nearly uniform level, surrounded by the high, blank, almost perpendicular walls of earth and rock from which it had been sundered. Many other earth-avalanches of minor importance were caused by the earthquake in various parts of the Santa Cruz Mountains. At Hidden Villa, 2 miles northwest of Black Mountain, large blocks of rock are reported to have rolled down the slopes. There were numerous slides along Stevens Creek, due chiefly to the caving of the creek banks. Along the ridge road southwest of Stevens Creek, sandstone blocks, some of them 6 feet in diameter, rolled down the hills toward the creek. Near Half Moon Bay considerable masses of granite were dislodged on a steep slope. (Plate 124c.) On the road along Pilarcitos Creek, an earth-avalanche brought down big blocks of sandstone upon the road. (Plate 126B.) At Boulder Creek a large portion of the soil was shaken loose from an abrupt hill 150 feet high, and fell to the level of the creek, carrying trees with it. At the north end of Ben Lomond Mountain, a slide carried trees and brush down to the creek. Near Olive Springs, 12 miles north of Santa Cruz, an earth-avalanche demolished Loma Prieta Mill and killed several men. At many places on the south side of Corte Madera Creek, huge masses of rock had been thrown down from the steep bluffs into the road, completely blocking it. About a mile from the summit of the ridge, where the Alpine road enters the Page Mill road, a slide carried away the entire roadbed for a distance of about 300 feet. On Purissima Creek a slide filled the road for a length of about 100 feet; another, between 0.25 mile and 0.5 mile long, dammed the creek to a depth of 25 or 30 feet. A large slide close to Wright Station partly dammed the stream. On the western slope of the ridge just west of Skyland, several earth-avalanches were caused by the shock; and great slides of a similar character occurred on both sides of Aptos Creek for 0.75 mile. Besides these, there were many smaller earth-avalanches ― 390 ―
in many parts of the Santa Cruz Mountains which can not be enumerated. There were also several such slides on the granite
slopes of Montara Mountain, farther north in the San Francisco Peninsula.
In the Coast Ranges to the north of the Bay of San Francisco, earth-avalanches were not so common away from the coast as they were in the Santa Cruz Mountains. There were, however, two notable ones which deserve mention here. The first of these is the Maacama slide, 6 miles east of Healdsburg. (Plate 124A, B.) The slide is about 0.125 mile wide at the top, and 0.5 mile long. The rock is a stratified volcanic tuff, and the slip was down the dip of the beds. The avalanche cut its way thru a fir forest and dammed Maacama Creek. The other is the earth-avalanche which, on May 1, 1906, dammed Cache Creek to a depth of 90 feet at a point 4 miles below the confluence of the north and south branches of the creek. The rock which fell is red sandstone. The width of the slide is 300 feet and its height is 500 feet. The dam thus formed broke one week later. This earth-avalanche can not be so directly referred to the earthquake of April 18 as the others heretofore described, but it was probably indirectly caused by the shock. Earth-SlumpsBy far the most common manifestation of landslide phenomena was that here referred to as earth-slump. It would be wearisome to attempt to mention all the various earth-slumps stimulated by the earthquake, even if information were sufficiently detailed to make this possible. Only two of the more important slides which have come under the observation of geologists will be noted. Cape Fortunas earth-slump (F. E. Matthes).—This landslide, immediately south of Cape Fortunas, is by far the most extensive one on the northern coast. (See plate 127A, B.) In May, 1906, it projected into the ocean for about 0.25 mile, like a hummocky headland of irregular outline; indeed, it formed a new cape on the coast-line, but will doubtless rapidly be cut back by the action of the waves. Its length, in the direction of its movement toward the ocean, is estimated at little short of a mile; its width varies from 0.25 mile to 0.5 mile. Its total descent, from the summit of the higher scarps at its head to the level of the sea, is probably less than 500 feet. Its surface is exceedingly irregular, with many large humps and hollows. Over large areas the sod is more or less rhythmically broken by deep cracks extending at right angles to the direction of movement. These cracks are only a few feet apart, and the sod-blocks between them lie mostly in tilted attitudes, making the area exceedingly difficult to traverse. The general aspect is not unlike that of a much crevassed glacier. In some places, however, the mass seems to have been torn apart so completely that the sod is not merely broken but almost swallowed up or buried, the browns and yellows of the under soil being the prevailing tints. Around its head are a number of steep scarps, from 100 to 200 feet high. They are especially prominent on the north side, and again toward the southeast; but over considerable stretches between these two sets, the broken surface of the slide joins the unbroken hillsides to the east without significant offset. Owing to this, the slide is easily approached from the wagon road (from Centerville to Cape Town), which passes close by its head. The longitudinal profile of the landslide is one of gentle slopes for the most part; its declivity is not at all great, and in a few places even reversed slopes occur. Its noteworthy feature is not its vertical drop but its great forward movement. In a sense it has flowed like a partially plastic mass, expanding and advancing 0.25 mile beyond the coast-line, but descending only a few hundred feet. In its general aspect, as well as in the nature of its movement, the Cape Fortunas landslide is altogether different from those observed farther south, particularly along the mountainous coast both north and south of Point Delgada, which, in effect, did little ― 391 ―
else than revive a series of old landslide facets. This may not be apparent to an observer on the beach, but is quite striking
when the coast is viewed in its entirely from a vessel off-shore. These facets existed before this earthquake, and had been
recognized as such. They are conspicuously outlined against the dark timbered slopes behind them, rising from 1,000 to 2,000
feet above the shore, and affording an important series of landmarks for the mariner. In strong contrast with these bold mountain
forms is the region in which the Cape Fortunas landslide took place. The land here can scarcely be called mountainous; and
while it breaks off in cliffs at the coast and is traversed by many fairly deep draws, it is essentially a region of subdued
relief. Great declivities are notably absent, except in the sea-cliffs, and even these are only a few hundred feet high. At
the same time, evidences of former landslides may be seen on every hand. They are not extensive, as a rule, and are as likely
to occur on gentle slopes as on steep ones. In a few cases only is a marked downslip noticeable, resulting in the uncovering
of a steep scarp; in nearly every instance the dislocated mass appears not so much to have sheared off and dropt from its
former position, as to have expanded or slumped, with an accompanying subsidence of its surface. The billowy and irregularly
pitted appearance of these areas, together with the rank vegetation that covers them, afford the principal marks of identification.
Both from their characteristic form, suggestive of plastic flow, and from their mode of occurrence, it seems reasonable to
infer that ground-water plays an important rôle in their genesis. They are apparently masses which have changed their shape
in obedience to a lessening of cohesion in their interior, through saturation with water. Whether the movement be initiated
by an earth-tremor or not, it is in every case essentially an adjustment to a more stable position, rendered necessary by
a change in the physical constitution of the mass.
It is to this category of landslides that the one at Cape Fortunas belongs. While there are scarps in various places at its upper end, these are really insignificant features alongside of the extensive tract of the slide itself. What downslipping occurred on these scarps was merely an incident in the entire movement. Both in the large ratio between its horizontal advance and its vertical drop, and in its general appearance, the Cape Fortunas landslide is closely analogous to the numerous lesser slides referred to; and there is good reason for the belief that, like them, it consisted essentially of an adjustment of equilibrium in a partially water-saturated mass. It probably had long been imminent before the earthquake started it. San Pablo earth-slump.—At the time of the earthquake a landslide occurred on Mills' ranch, which is about 4 miles east of San Pablo. The slide is interesting from the fact that a previous geological mapping of the region indicated that the point where it occurred was on the line of a fault extending in a northerly and southerly direction through the Sobrante Hills. The slide was examined by Mr. E. S. Larsen, who describes it as follows:
The same slide was subsequently visited by Mr. F. E. Matthes, and the following descriptive note is by him. (See figs. 68 and 69.) The slip occurred east of a high ridge at the southern end of the Sobrante Hills. It covers the northeast half of an area whose terraced nature is indicative of a former landslide of much larger dimensions. The accompanying sketches show the general outlines, and a cross-section of the slide. It will be noticed that the slide does not extend all the way down the slope, its lower edges being fully 100 feet or more above the bottom of the gulch. The lower slopes were not materially changed, and but little débris fell into the stream-bed. A steep scarp has been produced east of the crest of the ridge. The downslip along this scarp does not exceed 50 feet, and decreases both to north and south. Along the north edge there has been a marked movement down and southward, the scarp there averaging 10 feet. Along the south side, on the other hand, the loosened mass had advanced over the old surface, presenting a bulging and cracked frontal scarp some 6 feet high. It appears from this that the movement took place, not along the line of greatest declivity, but in a direction somewhat more southward, as indicated by the arrow. The 2 hummocks probably existed before the slip occurred, but judging by their greatly cracked and rent surfaces, it seems likely that their height has been slightly increased. The main crack, which extends southward from the upper scarp, continues along the hillside in irregular zig-zags for some 300 feet south of the slide. (See plate 128A, B.) Other earth-slumps referred to under the section on the Distribution of Intensity are shown in plates 125B and 129A, B, C, D. Earth-FlowsMount Olivet Cemetery (A. C. Lawson). — Perhaps the best illustration of an earth-flow caused by a sudden accession of water to the incoherent materials of a slope, in consequence of the earthquake shock, is that which occurred in the upper part of Mount Olivet Cemetery, near Colma, 9 miles south of San Francisco. The locality is at the base of the San Bruno scarp, and about 2.75 miles northeast of the San Andreas ― 393 ―
fault at Mussel Rock. The steep slope of the scarp is underlain by hard sandstone of the Franciscan series, with but a thin
veneer of soil, or none at all. At the base of the scarp is the gentle slope of Merced Valley, underlain here by Pleistocene
and recent sands. The sands, partly eolian, lap up on the lower flanks of the scarp, and mantle the trace of the auxiliary
fault which follows its base. The sands thus vary in thickness from a feather edge to an unknown thickness, which it is believed
may be as much as a few hundred feet at no great distance from the base of the scarp. Traversing the gentle slope of the valley-floor
are several shallow arroyos, which head in incipient ravines in the face of the scarp. At the moment of the earthquake there
was a sudden outgush of sand and water at a point at the upper end of the cemetery, close to the base of the scarp and quite
near, if not immediately upon, the line of the buried fault-trace. This stream of sand and water, admixed with the loam of
the slope, flowed rapidly down the course of a shallow arroyo on a grade of about 1:25 with a depth of from 13 feet in its
upper part to about 3 feet in its lower. The front of the stream stopt abruptly at a point just beyond the roadway about half
a mile from the origin. The flow was so rapid that it carried away many small trees; a wind-mill was wrecked and the heavy
concrete blocks which served for its foundation were swept down, with other débris. One of the pumping stations of the cemetery
was demolished by it, and 2 horses were carried off their feet, and were extricated afterwards with difficulty. (See plates
130A, B and 131A.)
According to Mr. M. Jensen, the superintendent of the cemetery, the entire flow had been accomplished within 3 minutes from the time of the shock, and he was at its source within 20 minutes after it occurred. The height of the flow within a few hundred feet of its source was attested by the mud upon the trunks of some eucalyptus trees near its margin. This mud extended up to 13 feet above the bottom of the arroyo. This, however, doubtless indicates the height of the front of the stream as it past this point. As the flow advanced, its surface near its source rapidly dropt; and by the time the front had reached the roadway the stream was probably no deeper at its source than at its terminus. Indeed, it seems to have been somewhat less, as there was a marked tendency for the sand to pile up at the front by reason of the negative acceleration at the front due to loss of water. After the moving mass had come to rest and partially dried out, it was found that it had left a streak of muddy sand on the bottom of the arroyo averaging 100 feet wide and about 3 feet thick. Taking the length of the flow as 900 yards, this gives the total volume of the compacted wet sand as 89,100 cubic yards. The cavity in the slope caused by the evacuation of this sand and loam was not measured, but was estimated to have a width of 150 yards, a length of 300 yards in the direction of the flow, and an average depth of 2 yards. On this estimate, its volume would be about 90,000 cubic yards, which agrees quite closely with the estimated volume of the material ejected. The sand, after it had ceased flowing and had been drained and compacted, undoubtedly held in the voids between the grains not less than 25 per cent of its volume of water. An additional 15 per cent would probably give it the necessary fluidity for flow down a slope of 1:25. But as the flow was swift, there was an excess of water, so that probably 25 per cent would have to be added to give it the properties manifested in the actual flow. The sand, however, in its original position before the time of the earthquake, probably did not contain more than 20 per cent of water, since the upper or soil layer had been somewhat dried out by the air. To the original sand of the slope, therefore, there must have been added 30 per cent of its volume of water to cause it to behave as it did. This amounts to 27,000 cubic yards. This water came from ground immediately below the source of the flow; and it came in a moment, at the time of the earthquake. It is only another way of stating the facts to say that it was squeezed out. There was ― 394 ―
no disturbance of the soil on either side of the cavity, even in its immediate vicinity. On the shoulder to the southeast,
where the trace of the auxiliary fault passes over practically bare rock, no evidence of movement was detected on critical
examination. The expulsion of the water was a purely local phenemenon. In attempting to explain the cause of it, or to ascertain
the local subterranean conditions which conspired with the earthquake shock to bring about the event, it should be noted first
that on the line of the fault-trace there are longitudinal depressions, which appear to be in part structural and in part
due to erosion following the fault. If one of these depressions should locally have the character of a sink, without free
drainage, then the sand which filled it would be saturated with water in consequence of the rains of the previous winter.
It is believed that the compressive action of the earth-wave passing through such a pocket of saturated sand, and reflected
perhaps more than once from the containing rock walls, would be adequate to expel 27,000 cubic yards of water from the deeper
portion and add it suddenly to the more superficial portion of the formation, thus bringing about the earth-flow. It may be
stated in this connection, although it has no conclusive bearing upon the question involved, that the sands of the valley
generally are an abundant source of well water, and that there is a surface well within a few hundred feet of the source of
the earth-flow, lower on the slope. There was very little water in the arroyo before the earthquake and a very insignificant
stream afterwards, the latter being probably referable to the drainage from the ejected sand.
Vicinity of Half Moon Bay (Robert Anderson). — The earthquake shock caused the appearance of an unusual amount of water at the surface in many places. This was noticeable in the vicinity of San Bruno, where several short streams running into the bay were flooded by an unusual volume of water during the first days following the earthquake, in spite of the fact that it was perfectly clear weather. Instances have been cited in the present writer's notes on the results of the earthquake in the San Francisco Peninsula, of water with a salty taste or milky color issuing from springs after the shock, and of streams being muddy and flowing with increased volume. These facts, and the fact that water continued to issue after the earthquake at the points where earth-flows occurred, and where it had not been in evidence before, and that earth-flows occurred sometimes on convex slopes where the concentration of water under normal conditions would be unlikely, seem to be explainable only by the theory that underground conduits were disturbed and made more open, that new channels of escape for the water were formed, and that water was actually squeezed out of the hills in some cases by compressive movements. This flowage of water to the surface, in increased amounts and sometimes at new places, caused the formation of the earth-flows. The places where these debacles occurred may or may not have been previously points of concentration of seepage water, and the soil already in part saturated. But it is supposed that the content of water was increased by the shock, possibly in extreme cases by the gushing up of a large volume; and that this increment of water, with its disintegrating, weighting, lubricating, and direct forcing power, aiding the attack of the shock on the soil, was the main cause of the earth-flows. There is little evidence as to when the flows were first set in motion; whether at once during the shock, or later after the lapse of some brief period of time that may have been necessary for the accumulation of the water in extra large quantities. Earth-flows originated in valleys, in gullies, or on hillsides. Where the weight of the earth, combined with the weight of the added water, was sufficient and the substratum of the soil was rendered plastic, gravity caused it to creep like a lava-stream, leaving a hollow in the place from which it came and a fan or tongue of débris down the slope below. Movement was especially apt to ensue where the ground had been previously wet, the intensity of the earthquake shock being particularly great at such points and ― 395 ―
the tendency of the vibrations being to set the mass in motion. Earth-flows occurred in many places in the Coast Ranges, and
probably thruout the region in which the shock was heavily felt. The writer found many of them, large and small, on the San
Francisco Peninsula and in the Santa Cruz Mountains, also in the Mount Diablo and Mount Hamilton Ranges.
Following are descriptions of 5 earth-flows that occurred on the morning of the earthquake in the neighborhood of Half Moon Bay, which is on the coast 25 miles south of San Francisco: One of them was formed in the hills bordering the terrace at Half Moon Bay, immediately south of Frenchman Creek, 1.5 miles north of the town, and a mile from the sea, at an elevation of 100 feet. It is pictured in plate 132A. At this place the earth caved away in a crescent-shaped area on a slope of only 18°, and flowed out in two long arms so as to leave a hole 4 feet deep, surrounded by vertical walls of unaffected soil. The flow occurred at a fairly high point on a gently undulating incline. The discharged earth was divided by a mound, at a point 150 feet below the summit of the are, and followed two courses which were determined by gullies on both sides. Much of the débris overflowed the central mound at the same time, and inundated the barley fields to a depth of 2 to 4 feet, for 100 feet farther. On both sides of the central mound the caving away continued to the same depth. In the left-hand fork it stopt within a few feet, and the flow did not extend very far beyond. In the right-hand fork a cut 100 feet long and 50 feet wide was made, the earth flowing down from it 250 feet farther over the grain field, as shown in plate 132A. Thus the whole length of the slide was 500 feet. The width of the main hole was on the average about 100 feet, and the length, as already mentioned, 150 feet not including the arms. In this hollow in the hillsides many dry blocks of sod carrying growing grain — usually in an upright position — were left stranded 4 feet below the surface of the hill by the removal of the subsoil. The fence that crost this area was broken and carried away and partly buried. Where the caving ceased in the right fork, a ridge of débris was piled up across the mouth of the hole, much higher than the stream of loose material that flowed farther. Similar ridges were heapt up across the path of the flow, where the breaking away of the hill stopt in the other arm and at the upper end of the central mound. The south or right arm of the flow extended down the hill at an angle gradually decreasing from 18° to less than 5°. Large parts of the fence were carried on its surface for 300 feet. Plate 132A gives a detailed view of the lower extremity of the right arm. The stream came to an abrupt stop, like a quickly cooled lava flow, and preserved a face 1 to 2 feet in height above the grain field. The surface of the flow consisted largely of blocks of sod, usually almost upright, which were carried down from the hole without much moistening, or transformation into material capable of flowing. The bulk of the flow was a moist aggregate of earth fragments possessing something of their previous form and grading into mud, which assumed a semi-fluid consistency underneath. The bottom of the hole, and the flow itself, remained too muddy to walk on for weeks after the earthquake, and the field below the lower end of the large arm was left marshy, tho it had not been so before. It is to be noted that several fairly heavy rains followed the earthquake after an interval of several days, and before these earth-flows were visited; but these were not sufficient to account for the amount of moisture observed. The chief effect of the water was in the ground at a depth of 3 or 4 feet below the surface. It rendered the soil sufficiently fluid to enable it to flow down the gentle slope, probably partly oozing from under the surface crust and partly transporting the sod with it. Most of the surface was carried down with the main flow, the stranded surface ― 396 ―
blocks that remained in the cavity being accountable for as fragments from the broken edges subsequently giving way and being
carried only a short distance as the upper end of the flow came to rest. In this way, probably, the walls were trimmed, for
the cut in general was left remarkably clean.
Another flow of similar character took place 3 miles north-northwest of the town of Half Moon Bay, on the creek next west of Frenchman Creek. It is shown in plates 132B and 133A. On the morning of the earthquake an acre of the gently sloping alluvial floor of a broad, short valley tributary to the main creek on the east caved and flowed out, leaving an excavation 10 feet deep, where before it had been almost level and where there had been no stream channel. In this case, the water already gathered in this basin-like valley, which here had had no means of prompt escape, was an important aid in the formation of the flow, aside from the sudden accession of water that probably caused the earthquake. The presence of a large amount of water and the forcible movement during the earthquake shock resulted in the loosening and undermining of the ground and its transportation as a fluent mass. The angle of slope was about 5°. The flow carried out thousands of tons of earth in this manner and spread it over about 2 acres of meadow land, to an average depth of 1.15 to 3 feet. Plate 132B gives a view of this earth-flow, showing the pit from which it was derived. Covering much of the surface of the flow and the floor of the hole are to be seen blocks of sod which have been carried right side up as if the material had moved en masse. The amount of water in evidence shows clearly how the earth was softened and enabled to move. The picture was taken two weeks after the earthquake. At that time water was still seeping up from underground, and out of the lower portions of the broken walls, while the ground near the surface of the valley was quite dry. The water had formed two definite rivulets thru the débris, at an elevation above the surrounding meadow, and was running in continuous streams, fast cutting a channel for itself and removing the soft material. Considerable water was dammed back in the hole by a 4-foot ridge of débris piled across the mouth of the hole, as in the case of the previously described earth-flow. This mound of earth, along the line where the stream left the caved-in area and flowed over the preexisting slope, was probably piled up at the last by the remnants of the flow gliding down and heaping themselves up as a barrier at the mouth of the hole. The cavity, about an acre in extent, has 10-foot walls which gradually decrease in height lower down the valley, the bottom of the hole being more nearly level than the valley-floor. Plate 133A shows part of this flow in detail. Some of the great blocks of sod around the edges have not been removed, altho the material from underneath has gone. Concentric cracks not visible in the pictures extend around the edge of the hole and for 50 feet above its upper end, showing that the area affected is broader than appears at first sight, and that the work is not yet all accomplished. The material of the valley-bottom is a coarse, arkose earth, derived from decomposing granite, and containing many rock fragments. A flood of earth covers about 2 acres of the meadow. Water was present in this earth-flow in greater amount than in any other that was examined. The nature of the material may be judged of by the abrupt face of the stream where it stopt. The edge makes a steep angle with the meadow and rises to an average height of 2 feet above it. Yet the fact that this mass of earth was able to move more than 300 feet after it left the lower end of the hole, and spread into an even and thin layer over a wide extent of nearly level meadow, shows that it was fairly soft. It was moved on a basal layer of semi-fluid mud and sand, with the aid of the weight of the overlying and partly disintegrated earth. ― 397 ―
The largest of the earth-flows seen occurred in the canyon south of the house of Mr. Nunez, 2.5 miles east-northeast of the town of Half Moon Bay, at an elevation of about 500 feet. It originated in a manner similar to the others, but in a canyon along which there is a distinct but ordinarily dry stream channel. A long, irregular hole from 4 to 7 feet deep was excavated near the head of the valley, and a great volume of earth flowed down its curving course for 0.25 mile, as far as the Nunez house, and there stopt, being in part diverted into the main creek to which the valley is there tributary. According to the testimony of witnesses, the flow reached the end of the 0.25 mile in 0.5 hour after the earthquake shock. It was seen gliding slowly down and engulfing the orchard just back of the house. According to observers on the Nunez ranch, the earth-flow was not accompanied by any water; but two weeks later, when examined by the writer, it preserved every evidence of having been muddy. Especially was this true at the bottom, where great masses of mud still had the consistency of jelly. It is probable that there was no flowing water on the surface of this or other earth-flows at the time of their formation, and that the presence of water in the flow was not evident to the casual observer because of the comparative dryness of the material on its upper surface. The slope of the canyon down which the moving body of land crawled is about 25° near the head and decreases to 15° farther down. The flow filled this to a width of 100 feet on the average, and to a depth varying from 10 to 20 feet. The inertia of the mass is illustrated by the fact that in the early stage of the flow the earth was piled 20 feet higher on the hill, on the inside of the big curve made by the canyon, not far below the pit, than it was when the flow came to rest. The marks at this elevation were probably made very soon after the main mass was discharged from the cavity, before it had spread very widely. The central portion of this earth-flow is pictured in plate 131B, where it appears as a ridge many feet high rising above the tall grass on the hillside, on the right of the picture. The pressure of the material at the head of the flow, as it started, was so great that the earth bulged up over the sides in places, in such a way as to force upward great blocks of sod and turn them on edge or completely over, away from the rim of the hole. The flow assumed the form of two lateral ridges and a central depression, or channel. The ridge on the west or inner side of the curve was considerably the higher. The form was due partly to the concavity of the valley; but chiefly, it is thought, to the tendency of the more fluid material to follow the deepest possible path along the gully under the center of the flow. Thus the drier material was retarded at the sides. Subsequent to the first starting of the flow, a stream of semi-fluid mud and sand continued to run down the central channel, covering its sides with a coating of mud and leaving flowage striations on it. This channel and its markings are exhibited in plate 131B. Two weeks after the earthquake, when the photograph was taken, water was running in this channel and had cut down into it several feet deeper. Its bottom, however, was still from 5 to 10 feet higher than the bottom of the underlying preëxistent water course, where water had not flowed before at this time of the year. The man in the picture is standing at the bottom of the gully. To the left of him, the hammer and note-book mark the top of one of the parts of the lateral ridge which is here divided into several hummocks. To the right is the other and higher lateral ridge. The foreground was formerly covered by a dense thicket of willow trees. These willows have been completely buried, except at the sides where some dead branches protrude. A fence that crost the canyon was torn away for 100 feet, and not a trace of it could be found. The fence shown in the picture is one newly built in its place. Two other smaller earth-flows occurred just over the hill westward from the last one described. They are shown in plate 133B, the canyon on the left being the one occupied ― 398 ―
by the Nunez flow. One of these 2 earth-flows, that at the right of the picture, started near the top of the ridge in a depression
in the slope, formed a hole 75 feet long and 40 feet wide, and coursed down a narrow runnel having a gradient of 25° to the
bottom of the hill, a distance of 600 feet. Enough earth issued to fill up the rather deep ditch in the gully clear to the
bottom of the hill and to bury the grain field on both sides to a depth of 1 to 2 feet. In this case, as in the preceding
one, there were formed lateral ridges higher than the center, so as to leave a groove between. Down this channel there flowed
softer material, which lined the sides of the lateral ridges with a smooth coat of mud and left conspicuous flowage marks.
The flow thus raised a ditch for itself above the level of the slope. The earth-flow probably assumed this form by leaving
behind, at the sides, the material least capable of flowing, and by concentrating its most liquid parts along the deep central
line.
The other earth-flow was near by, on the convex face of the knoll in the center of the picture. A similar cavity was produced, from which the contents were spread out broadly. It is a good example of the starting of a gully, as there was no depression before. One branch of this earth-flow came straight down the hill and slightly toward the canyon on the left; the other branch came down toward the gully in which the first-mentioned of these 2 earth-flows occurred. Thus drainage lines were started which ultimately may separate the central hill from the ridge on the right, of which it is now a continuation. The left arm of the flow on the hill may develop a channel, as explained below, which will cause the drainage from this hill, which is now toward the foreground, to pass into the canyon on the left. Similar landslides, tho usually of smaller size, occurred thruout the region neighboring the fault visited by the writer, and even in districts at a considerable distance from the fault. Frequently they were not definitely referable to the earth-flow type, but resembled more closely earth-slumps formed without the aid of a suddenly increased water supply. It was often difficult, especially in cases where the movement was slight, or the slide was in the embryonic stage, to determine whether the earthquake at those points had caused a flow of water or not. In the instances so far described, it was pretty certain that it had; but in many others the phenomena were explainable as being the result of moisture that was already collected before the earthquake. Many slips were formed on hillsides and along the embankments of mountain roads, and along the cracks formed by the shock in moist and loosened soil. Often these slips were arranged one above another, the perpendicular faces due to slipping having the appearance of step faults. In such cases the weight of the moved mass and the amount of water was not sufficient to cause the material to flow. There were examples of such slips along the coast hills north of San Pedro Point, near the road halfway between San Bruno and San Andreas Lake, near the road from Belmont to Crystal Springs Lake, 0.5 mile southeast of the San Mateo Alms House, and in many other places on the San Francisco Peninsula. In some places bare ridges had their lines of symmetry broken into little knolls and irregularities by these slips, a common occurrence in the hills of soft sand formations in the northern part of the San Francisco Peninsula. All the slips just referred to illustrated the gradation between earth-slumps and earth-flows. Doubtless in many of them a small amount of water did gather as a result of the earthquake. Relation of earth-flows to rainfall (Robert Anderson). — The rainfall previous to the earthquake, tho possibly of little importance in connection with the more extreme types of earth-flows, in which practically all the work was done by a head of water brought from underground by the shock, bears a close relation to the less extreme types, and to the geologically very important doubtful types intermediate between the earth-flows and earth-slumps. In a dry year the number and size of all of these would probably have been much less. Had covering of slopes been unsaturated, areas might not ― 399 ―
have been so ready to break forth at a sudden accession of water from below; and the rainfall not having been great, there
might not have existed such a plentiful source of underground water to be drawn from. The following review of the rainfall
conditions may be of value in indicating a relation between the preparedness of the ground and the number and importance of
flows and slumps.
During the first three months of 1906 the rainfall was exceptionally heavy thruout California, being on an average thruout the whole State more than 9 inches in excess of the normal for that period. Up to the beginning of 1906, the amount of rain for the season was 4.5 inches below the average; but owing to the great excess during the late winter and early spring months the total for the year up to the first of April, the month in which the earthquake occurred, was nearly 5 inches above the normal. During January, February, and March the rain was heavy and continuous. Nearly all the rain of the season was during these months immediately preceding the earthquake month. Practically no rain fell between April 1 and April 18. All of the rainfall data available in the monthly reports of the Weather Bureau for California, compiled by Professor McAdie, has been used for calculating the amount of rain in 8 counties south of San Francisco. These are San Francisco, Alameda, San Mateo, Santa Clara, Santa Cruz, Monterey, San Luis Obispo, and Santa Barbara. The average rainfall at 46 different places distributed thru these counties was 22.59 inches from September, 1905, to April 1, 1906, between 2 and 3 inches above the normal for this region. The excess would have been greater but for the lightness of the rainfall during the autumn term, which was 3.55 inches, or several inches less than the average for former years. During the spring season up to April 1, the precipitation was excessive. During the three months that preceded the earthquake, 19.04 inches of rain fell, or 84.30 per cent of the whole precipitation up to that time. During the first half of April, there was practically no rain at all. Thruout this region, as well as thru California as a whole, March was a very rainy month; especially heavy downpours coming everywhere in the State during the last days of the month. It was the rainiest of the months except in parts of Santa Clara and Santa Cruz Counties, where more fell in the month of January. The majority of the earth-flows and earth-slumps that occurred were near the coast, although the amount of rain that fell was not as large there as it was farther back in the mountains. The coast region, however, is subject to heavy fogs, which precipitate some moisture and help to prevent evaporation of the moisture already present. These fogs were probably a factor in causing the earth-flows and earth-slumps near the sea. The principal cases described were near Half Moon Bay. The records from Point Montara, only a few miles away, showed that the rainfall in this vicinity was heavier than at any other point along the coast south of San Francisco. During the spring season up to April 1, it amounted to 23 inches, and during the autumn season it amounted to 12 inches. The table shows that the heaviest rains were in the Santa Cruz Mountains. At Boulder Creek, in Santa Cruz County, 55.70 inches of rain fell during January, February, and March alone, and 16 inches fell during the four months preceding. During the spring of 1906, a large part of the precipitated moisture remained in the ground, which was previously dry, and the amount of evaporation was minimized by the continuous succession of cloudy and rainy days. The year afforded an example of the concentration of an excessive annual rainfall into a short period, with all the conditions favorable for the absorption and retention of the moisture in the ground. For this reason, conditions favored the production of debacles of various kinds in the loose material covering slopes. The earth-flows that have been discust are more or less similar to the flows occasioned by the bursting of peat-bogs. The causes of their origin and their nature appear to be ― 400 ―
much the same. [1] Sir William Conway has given an account of a mud-avalanche, [2] a swift torrent of mud, water, and great rocks, in the Himalayas, somewhat similar in nature to these earth-flows. Streams
and torrents of mud somewhat analogous but usually of glacial or lacustrine origin have been known to flow in the Alps. Mention
of these has been made by T. G. Bonney. [3]
Earth-flows are important as giving rise to new drainage lines and modifying old ones. They are also powerful transporting agents. The initiation of a new drainage line is a matter of importance. Once started, it is a point of vantage for the attacks of agents of erosion, which thereupon are able to increase their work at an accelerating rate of speed. Only next in importance is the definition and fixing of embryonic depressions and gullies. Both these processes are carried out vigorously by these earth-flows, besides other processes such as the enlargement of valleys and channels already formed, the transportation of material, the destruction of the regularity of contours, and the transformation of surface rock material into a form easily removable otherwise, thus in every case supplying better leverage for further destructive action. Earth-flows usually originate in minor depressions or in already well-formed gullies or valleys, these being the places most subject to the concentration of water; but in some instances they occur on the convex face of a slope, where the removal of soil develops a depression for the first time, and a new drainage line is made possible. The soft débris that is removed, although piled higher than the surrounding slope, lends itself easily to the formation of rivulets by the water that rises and collects in the excavation that is left. These small water-courses, once formed, control the line of flowage, and result in a sort of superimposed drainage when they have worn through the débris to the original slope below. Earth-flows of the above varieties, large and small, with the closely related types of earth-slumps, are thus among the important initial steps in the development of drainage lines in the California hills. Earth-LurchesOf the three kinds of landslides thus far referred to, the first two, earth-avalanches and earth-slumps, occur quite commonly independent of earthquakes. Of the third kind, or earth-flows, the only examples that have been presented are immediately connected in genesis with the earthquake of April 18, although it is conceded that sudden accessions of water to loose earth might arise in other ways and occasion earth-flows. As regards the fourth type, the earth-lurch, it is difficult to conceive for it any other origin than an earthquake, since it is caused directly by the horizontal jerk of the ground and can not be produced in any other way. In the detailed account of the distribution of apparent intensity, a brief account of these superficial movements of the ground has been given and need not here be repeated. They are best exemplified on the flood plain of the Eel River, west and north of Ferndale; the flood plain of the Russian River; the flood plain of Alameda Creek, near Alvarado; the flood plain of Coyote River near Milpitas; the flood plain of Pajaro River; and the flood plain of the Salinas River. (Plates 136A, B and 137A, B.) In all these localities cracks were formed in the alluvium, generally parallel to the stream trench, and the ground between the cracks was caused to lurch horizontally toward the stream, usually with a rotation of the moved mass, which gave to it the profile of a Basin Range fault-block in miniature, the portion of the moved strip farther from the stream collapsing into the vacuity caused by the lurching. ― 401 ―
Along the beach or sand-spit which separates the Salinas River from the Bay of Monterey at Moss Landing, there was a marked lurching of the spit toward the trench of the river as illustrated in plates 134A, B and 135A, B. Lurching of soft ground was also exemplified on the tidal mud flats of Tomales Bay, and on the "made land" of San Francisco; but there being no trench in these cases, the movement caused a ridging of the surface with compensating depressions. In the case of the made land in San Francisco, and perhaps generally, the deformation of the surface due to lurching was complicated by the settling together of the loose material. Cracks and FissuresThe cracks in the ground which appeared at the time of the earthquake fall into different categories. Of these there are two distinct classes: 1. The crack or fissure of the main fault, which is a superficial expression of the deep rupture of the earth's crust that caused the earthquake. Associated with this as a subclass are the auxiliary cracks and fissures which are superficial expressions of branch ruptures or subparallel ruptures, generally close to the main rupture in the Rift zone. In this class would also belong any cracks due to supplementary faulting in the general zone of disturbance, if such supplementary faulting exists, which is doubtful except in special instances. 2. The second general class includes those cracks and fissures which were caused by the earthquake, as a result of the commotion of the ground, and have, therefore, been designated as secondary. The main crack, or fault-trace, and the auxiliary cracks satellitic to it, have been described in the section of the report dealing with the earth movement along the fault. The secondary cracks, inasmuch as they are an indication of the intensity of the shock at any locality, have been described or referred to in the section dealing with the distribution of intensity. A brief review of the phenomena of cracks in the ground, apart from the main fault-trace and the auxiliary cracks in the Rift zone, will, however, be given, even at the risk of some slight repetition. Since some of the cracks to be referred to can not with certainty be placed in one or the other of the two fundamental classes above indicated, it will be found convenient not to force that classification in all cases. Along the zone of the Rift there were many secondary cracks, as well as those classed as auxiliary; but it was not in every case possible to discriminate between them. These secondary cracks occurred both on hill slopes and in alluvial bottoms. On the hill slopes they were very commonly associated with landslides, or marked the inception of landslides; and these have already been discust. On the bottom lands of streams or embayments in the Rift zone, cracks in the ground were exceedingly common for the entire length of that portion of the Rift along which the fault extended. In very many cases these cracks were associated with the lurching of soft incoherent materials, just as the cracks on the hillsides were associated with more common phases of landsliding. There were also, however, many cracks quite dissociated from the deformation of the surface due to lurching, although there was doubtless in these cases an ineffective tendency to lurching. Beyond the zone of the Rift, cracks were observed at many localities. These were most common on the bottom-lands of the streams, notably the Eel River (plate 138A, B), the Russian River (plate 139A, B), Coyote Creek (plate 140A, B), and other streams at the south end of the Bay of San Francisco, Pajaro River (plate 141B), San Lorenzo River, and the Salinas River. Many other smaller streams might also be mentioned. In these cases the cracks were usually associated with the phenomena of lurching of the alluvial deposits, though many cracks also occurred where no such association was apparent. They were in nearly all cases found to be parallel or sub-parallel to the nearest ― 402 ―
portion of the stream trench. They very commonly extended for several hundred feet, in some instances for several hundred
yards, and were characteristically arranged in linear series. The cracks in the series in some cases overlapt en échelon, and in others they were in groups of parallel cracks in belts a few hundred feet wide. In no case was there any suggestion
that they were more than purely superficial phenomena. A unique manifestation of surface cracks is that described by Matthes
and Crandall in the vicinity of Livermore. (See plate 141A.)
On the hillsides and ridge crests, at points not within the Rift zone, cracks were of common occurrence. Most of these were connected with landslides, as has been indicated in the section dealing with that subject. Roadways and artificial embankments were particularly susceptible to damage from such cracks. But some of the cracks had no apparent connection with landslides, actual or incipient, and these are of especial interest. The most northerly are those described by Mr. E. S. Larsen in the region northwest of Covelo, Mendocino County, as set forth in the record of intensity. Some of the cracks described by Mr. Larsen crost the crests of rocky ridges; and altho it was not possible to follow them for great distances, they evidently extend down into the rock. It is remarkable that in the district where these cracks occur, there was no evidence of a local rise in intensity and, therefore, nothing to suggest that they were the seat of a supplementary local earthquake. The probable interpretation of the occurrence is that they are secondary cracks of a rather exceptional kind, in ground that required no very severe shaking to rupture it superficially. Cracks of a similar character were noted by Mr. C. E. Weaver in the Clear Lake district and on the flanks of Mount St. Helena. On the San Francisco Peninsula, similar cracks were observed by Mr. R. Crandall on Cahill Ridge and Sawyer's Ridge, and are described by him in his account of the distribution of intensity in that region. In the Santa Cruz Mountains, such cracks were common and are described more or less in detail in the section on the distribution of intensity. In general they appear to be the result of the earthquake rather than a contributory cause, although in some cases it is quite possible that they may have been local ruptures of the nature of auxiliary cracks and so gave rise to subordinate vibration. Effect of the Earthquake Upon Underground WatersSignificance of the PhenomenaPerhaps the most interesting and significant fact which the study of the earthquake has brought to light, apart from the great fault along the Rift, was the general disturbance of underground waters. In earthquakes generally, the phenomena which appear at the surface of the earth have become well known and, indeed, almost commonplace in recent years; but what transpires in the earth's crust below the surface, as the earth-waves generated at the seat of disturbance pass through it, is as yet a matter of uncertainty and inquiry. The effect of the shock upon the movement of underground water, as manifested by the behavior of springs and wells, throws light on this question. A few pages are, therefore, devoted to recording information of this kind. It appears from the reports that have come in that springs and wells were very generally and variably affected throughout the disturbed area, indicating a sudden derangement in the normal movements of such water. This derangement could only have been effected by the changes in spaces in the rocks in which the waters in the subsurface region are contained, whether flowing or stagnant. These spaces are of 4 general kinds: (1) interstitial spaces, or so-called voids, between the constituent fragments of imperfectly compacted rocks, such as sands, gravels, sandstones, conglomerates, tuffs, ― 403 ―
etc.; (2) the cracks and fissures which traverse the more firmly compacted forms of the same rocks, or others, such as granite,
lava, etc., which occur only in a solid or coherent condition; (3) the vesicular spaces and tunnels of lavas, and (4) the
spaces of dissolution which occur frequently in relatively soluble rocks, notably limestone. The occurrence of water which
does not permeate the rocks nor flow thru them, but is contained in small discrete cavities in rocks, such as the liquid inclusions
in igneous rocks and in the constituent minerals of sedimentary rocks, is here ignored. Thruout the Coast Ranges of California,
limestones are not abundant and spaces of dissolution are believed to have played no part in the changes which were manifested
in the behavior of springs and wells. The same remark holds with reference to vesicular and tunneled lavas. These changes
were thus confined to the voids of porous and usually little coherent rocks and to cracks and fissures which traversed the
coherent rocks, whether porous or not.
In the discussion of certain earth-flows in the preceding section of this report, the initiation of which is ascribed to a sudden accession of water from the underlying formations, attention has been already directed to an extreme phase of the disturbance of the normal conditions of the ground-water. In those cases the ground-water was suddenly expelled or squeezed out of saturated, incoherent formations at the time of the shock. They are extreme manifestations of a tendency which affected the ground water generally thruout the disturbed region. In this connection, it may be well to direct attention more particularly than has hitherto been done to the behavior of water contained in the alluvium of the river-bottoms. One of the most common phenomena in such situations was the expulsion of water in jets from apertures which suddenly appeared in the flat-lying ground. The water was usually thrown into the air for several feet; in some cases it was reported to be as much as 20 feet, and the ejection continued for several minutes after the earthquake. The continuance of the ejection after the shock indicates that an elastic stress had been generated in the saturated ground, which thus found relief in the expulsion of the contained water or that there was a gravitational settling together of the material, which diminished the spaces occupied by water. The vents thus established were very numerous, and were in many instances closely spaced; more frequently a few to the acre, and occasionally isolated. These vents were easily recognizable for weeks and even months after the earthquake, in the form of craterlets. The water in its passage to the surface brought up considerable quantities of fine sand, which, from its prevailingly light bluish-gray color, was evidently derived from considerable depth. On the flood plain of the Salinas River, the sand was recognized by the people of the neighborhood to be the same as that of a stratum of sand pierced by wells at a depth of 80 feet. The craters were usually distinctly funnel-shaped and were rimmed by a circular flat ridge of sand which, by reason of its light color, was in marked contrast to the surrounding surface. They varied in diameter from 1 to perhaps 10 feet. In some instances the funnels were several feet deep; in others the feeble action in the closing stages of the eruption had caused them to fill up with sand. They were quite analogous to the craterlets described and pictured in Dutton's account of the Charleston earthquake. [1] (See plates 142A, B and 143A, B.) These craterlets occurred on practically all the saturated alluvial bottoms of the streams within the zone of destructive effects, and also on the tidal mud flats of Tomales Bay. They are significant of the compression to which such water-laden, incoherent formations were subjected by the passage of the earth-waves at the time of the earthquake or by the consequent settling of the ground. They thus afford us, in part at least, a key to the behavior of many springs and wells. Most of the springs of the Coast Ranges are in solid rock, though they may emerge on a hillside mantled with regolith ― 404 ―
and soil. Such springs, as a general rule, had their flow increased at the time of the earthquake. The tendency to compression
in firm rocks would not be so effective as in the case of noncoherent sediments, but it would make itself manifest in the
generation of an elastic stress which would die out and merge with the normal gravitative stress very gradually. There would
also be an effective tendency to bring together the walls of cracks and fissures whose planes lay transverse to the path of
propagation of the compressive wave. Both of these tendencies would make for an expulsion of the water. The expulsion could
not, in most cases, be effected suddenly, however, owing to the great frictional resistance; and simply resulted in an increased
flow of the springs at the surface, which would continue during the life of the abnormal elastic stress. The duration of this
stress appears in some cases to have lasted but a few days; in other cases it continued for 2 months, as inferred from the
abnormally large flow of the springs. This variation would depend on local conditions, such as the superficial or deep source
of the water, the character of the rocks, the degree to which it was seamed with cracks, etc.
This same general explanation would apply to artesian wells, in which the water acquired and maintained an increased head for some time. In some such wells, where the water stood normally at some little distance below the surface, it overflowed and flooded the ground in some instances. In other cases, where the supply was not artesian, but shallow wells reached the ground-water, the level of the latter rose. This general tendency was complicated in some instances by other effects of the earthquake. Several surface wells had their level lowered, and others went dry. This sudden drop in the level of the ground-water can be explained only by a sudden draining off of the underground waters to lower levels, and this might be effected by the opening up of the ground superficially, in consequence of the shock. A similar explanation would apply to the few springs which had their flow diminished or cut off altogether. This draining off of the waters of higher levels would also augment the flow of springs and wells at lower levels and may in some cases have been the principal cause of observed increases of flow. The noteworthy case of the spring near Ukiah, described below, which ceased flowing and remained dry thruout the following summer and fall, but resumed its flow with the advent of the winter rains, suggests that the fissure in the rock from which the spring welled served as the limb of a siphon and that the water in the siphon was drained off in consequence of the agitation and opening of the ground at the time of the shock. The winter rains refilled the siphon limb and so brought about a resumption of the flow. One of the most common reports regarding the shallower wells was the roiling of the water by the admixture of earthy matter, doubtless due to the agitation of the ground and the loosening up of the incoherent material at the bottom of the wells. Record of Springs and Wells AffectedA brief and partial record of springs and wells affected by the earthquake follows: Montague, Siskiyou County (C. H. Chambers). — A sulfur spring was formed at a point 2 miles south of the town of Montague. Hot water ran from it for 2 days, after which it cooled off. A soda spring 9 miles east of the town doubled its flow. The water of many springs was muddy for several days after the quake. Denny, Trinity County (P. L. Young). — At a small quartz mine near Denny the shock doubled the amount of water flowing from the tunnel. Peanut, Trinity County (Mrs. E. Diller). — There was an increase in the water in the ditch which comes from a small gulch. The increased flow had not diminished up to May 6, 1906. ― 405 ―
Briceland, Humboldt County (J. w. Bowden). — The pressure on the flow of natural gas was doubled in the vicinity. Covelo, Mendocino County (E. S. Larsen). — Some springs and wells in the vicinity went dry, while others flowed more freely. Laytonville, Mendocino County (A. S. Eakle). — A sulfur spring had its volume of water increased at least threefold by the shock, according to report. Mendocino, Mendocino County (O. H. Ritter). — Wells in the lower part of town became full to overflowing and a heavy seepage of water was observed in the yard of the Alhambra Hotel. (W. Mullen.) — The flow of a number of springs was increased. Ukiah, Mendocino County (S. D. Townely). — The water in the well at the Observatory was very noticeably roiled for several days after the shock. The Ukiah press for April 27 reports some very marked changes in the flow of springs near Ukiah. A spring near the E. Clemens Horst Company's ranch, which supplied water for domestic and ranch purposes, stopt flowing after the earthquake. The ranch is about 2 miles north of Ukiah and a little west of the center of the valley, and the spring is in the foot-hills on the edge of the valley, nearly a mile to the west of the ranch. Pipes connected the spring with 2 tanks on the ranch, the spring having supplied the ranch with water for a great many years. The foreman, John Eldred, states that the day after the earthquake it was noticed that no water was flowing into the tanks from the spring. Investigation showed that the spring, which comes out of rock, was absolutely dry. Mr. Eldred and his men worked for two or three weeks, digging, drilling, and blasting, in the effort to regain a supply of water; but these efforts were futile and were finally abandoned. A well 75 feet deep was dug on the ranch and a wind-mill erected. Eldred went to the site of the spring several times during the summer and early fall, but there was no water. Upon going to the place in the early part of the winter, after the rains had begun, it was found that the spring was again flowing with a largely increased volume of water. He estimated that the flow was about doubled. The spring was still flowing with the increased volume on March 15, 1907. Hemlock, Mendocino County (C. D. C. Bowen). — Some springs flowed more abundantly after the shock. Lake County (C. E. Weaver). — At Highland Spring, in Lake County, none of the springs dried up, but one new soda spring was formed in the Franciscan rocks. The mineral springs in all parts of the county are reported to have increased in flow and number. The artesian wells in Scott's Valley, west of Lakeport, have diminished in flow, and several have stopt flowing. Many wells have dried up, but this was not confined to any particular locality or part of the county. The shock apparently had no effect upon the waters of the northern part of Clear Lake, nor upon the springs in that part of the district. Lakeport, Lake County (J. Overholser). — The flow of many springs increased on account of the earthquake, while the flow of artesian wells decreased. Annapolis, Sonoma County (G. W. Fiscus). — Wells and springs have gone dry in places, and water has come to the surface where there was none before. Sebastopol, Sonoma County (R. M. Hathaway). — The wells in this vicinity were all stirred up, the water becoming filled with sediment, as tho a heavy rain had washt in surface water. A small brook a little to the left of a fissure in the soil on the Blundon place had its flow of water so increased that the owner of the place had his attention called to it by the roaring of the water. Santa Rosa to Sonoma, Sonoma County (E. S. Larsen). — At the city pumping station, 1.5 miles east of Santa Rosa, there are 4 wells, dug 50 feet and connected with a tunnel 450 feet long. Within each well there is a bored well 8 inches in diameter and 108 feet ― 406 ―
deeper than the dug well. The water began to rise immediately after the shock, and is 15 feet higher than before, altho the
pumps have been run to their full capacity.
The warm spring at Peters' ranch was little affected, except that for a day or so after the shock the water in the spring was lower. At Conrad ranch, northwest of Melita, there are numerous warm springs, about 100°, all along the base of a hill, which have had their flow increased very much. Mr. Striddle thinks that there is ten times as much water as before, and that it is a little warmer. He also tells me that the flow is gradually decreasing again. The springs at Melita, along the north side of the hill, have behaved much like those at Conrad's. I am told the creek about 2 miles to the north has risen considerably since the shock. A mile north of Kenwood there is a well which was dried up about a year ago by an earthquake, and had to be dug deeper. This shock did not seem to affect it. Glen Ellen Springs continue to be changed, usually increasing their flow, tho a few springs went dry. At McEwan's Ranch, 3 miles west, both cold and hot springs are flowing much more water. At the State Home at Eldridge, a warm spring started about 0.75 mile east of the town. Hot springs at Agua Caliente have nearly trebled their flow, and the temperature has risen from 112° to 114°. A spring which required pumping before now flows. Boyes Hot Spring has increased a little and now flows without pumping. The temperature has also increased a little. Several years ago an earthquake stopt the flow, so that pumping has been required until this last shock. At Sonoma the wells and springs supplying the city are flowing more than before. Veterans' Home, Napa County (A. Brown). — The earthquake caused the springs to flow more fully for about 2 months, after which they returned to normal. Napa, Napa County (T. Hull). — In many cases springs increased their flow. Redding, Shasta County (L. F. Bassett). — Some springs have been reported to have increased their flow and to have a lower temperature. McCloud River, Shasta County (Chico Enterprise). — Springs in the limestone belt above Baird, which were formerly cold and clear, became warm and milky. Allegheny, Sierra County (W. A. Clayton). — The earthquake changed the flow of water in mines and springs. Suisun, Solano County (E. Dinkelspiel). — Mr. Miller's gas well, 3 miles northwest of Suisun, gave threefold greater volume of gas for 2 weeks before the earthquake than it did afterward. Martinez, Contra Costa County (R. Wulzen). — Alhambra Creek is said to have risen 2 feet after the earthquake. A small stream to the east of the town, which is usually dry by May 1, now has considerable water. The same is reported of another stream south of town. A well in the vicinity, in which the water has always been several feet below the surface, is reported to be filled almost to the surface. Stockton, San Joaquin County (R. Crandall). — An old disused gas well at the City and County Jail had a flow of water started in it by the earthquake. This flow continued for about two weeks, after which time it began to diminish. In a gas well, at the City and County Hospital, both the gas and water flow had been doubled and had continued so up to the time of my visit. Ripon, San Joaquin County (T. H. Wren). — I have 18 acres of alfalfa land, which watered with an inch less water over the head-gate in 1905, in 17 to 20 hours. This year it took 25 hours to water 13 acres, all conditions being the same as last year except that the land was more packed and should have watered quicker. Others have made the same observation. Sunol, Alameda County (R. Crandall). — The level of the ground-water around Sunol was affected considerably. In most of the wells the water rose, some overflowing for ― 407 ―
a short time. The postmaster gave 4.5 feet as the measured rise of the water in his well. The spring which furnishes the town
supply is said to have been diminished by one-fourth of its flow. Two other changes in water supply were reported: one being
the starting of a new spring near one of the western Pacific camps in Niles Canyon; the other the rejuvenation of an old sulfur
spring near Sunol, which had not flowed for many years.
Calaveras Valley, Santa Clara County (G. F. Zoffman). — The springs near Mr. Robert Ingleson's house, in section 22 on the ridge east of Calaveras Valley, became muddy after the shock and remained so for two or three days. The volume of water discharged by the springs increased to about four times the usual amount. Alvarado (E. W. Burr). — At the Alvarado Sugar Mill, in several wells, formerly flowing artesian wells, the water-table is now a few feet below the surface, the water-level having risen at the time of the earthquake. In the accompanying table are given the heights of water in a number of wells about the mill, referred to an assumed level 30 feet above an assumed base. These wells were observed daily before and after the earthquake. In most of them the water suddenly rose. The readings show that in a few cases the water rose from 1 to 2 feet. A well which used to be considered nearly dry began showing daily fluctuations, overflowing nearly every morning for some weeks after the earthquake. The figures here given are for measurements made on April 9 and 14, preceding the earthquake of April 18, 1906, and the measurements made on April 21 and 28 of the same month, and May 5 subsequent thereto. [1]
San Francisco Peninsula (R. Anderson). — Thruout the central portion of the San Francisco Peninsula, the chief geological effects, aside from the actual displacement along the fault and the slumping and settling of alluvial ground, were the increased circulation of water and its discharge at the surface. The normal flow of water from springs was much disturbed. The water was usually muddy or milky. It is reported to have flowed salty from one spring for 2 days after the earthquake; after this it returned to its usual purity. Streams were considerably swollen temporarily, and water frequently came to the surface where it had not made its appearance before. (R. Crandall.)—At Mr. Ebright's place, at the lower end of the lake in Pilarcitos Canyon, the spring water used for house supply is said to have been milky white the day of the earthquake. At Byrne's store, on the Half Moon Bay road, 0.5 mile west of Crystal Springs Lake, it was reported by the keeper that the water from their spring on the day of the shock was muddy and was not tasted; on the second day it had a very salty taste, and on the third day was again normal. Santa Clara Valley (J. C. Branner). — At Menlo Park, a mile nearer Fairoaks Station, an artesian well flowed faster after the shock. At the Seale place, on the Embarcadero ― 408 ―
road, from the railway crossing at Palo Alto toward the Bay of San Francisco, a well was reopened. Other wells showed an increased
flow and brought up sand. At Guth Landing, and southward along the road into Mountain View, the flow from bored wells had
increased. A wind-mill which had for years pumped water from a well was no longer necessary, but the artesian water was muddy.
At the Ynigo Ranch, 3 miles northeast of Mountain View Station, there was an artesian well which had, before the shock, flowed
slightly or not at all, and a wind-mill was used to raise the water. After the shock it was found that the casing had been
shoved up 2 feet, damaging the pump. The flow of water was increased, and black sand was brought up. Another well at this
ranch was unaffected. Along the Jagel Landing road, 2 artesian wells had increased pressure after the shock. An old artesian
well filled with stones had begun to flow for the first time in several years.
(H. H. Taylor.) — The water in an artesian well 215 feet in depth, near Millbrae, was roiled by the earthquake and remained so for several days. San Jose, Santa Clara County (G. F. Zoffman). — Water and mud are reported to have spurted from many artesian wells. (W. S. Prosser.) — A well near San Jose was reported as having increased in flow the day before the earthquake. Gilory, Santa Clara County (M. Connell). — It is reported on good authority that at Gilroy Hot Springs the temperature of the water rose nearly 10° and the flow increased to 5 times the usual volume. Bellvale, San Mateo County (Miss L. E. Bell). — Some springs dried up and others broke out with a great gush of water, where no water had flowed before. An oil well from which tepid salt water, oil, and gas had been flowing since 1898 became suddenly dry and a similar flow began in another well 2,000 feet deep, at a distance of 600 feet to the east of the first well, where before nothing had been found. Wright, Santa Cruz County (Miss F. Beecher). — Most of the springs are running with a greater flow since the earthquake; but the water in our well on top of the ridge sank rapidly to the level it usually holds in August. The water in all wells was very roily for some days. Summit Hotel, near Wright, Santa Cruz County (H. R. Johnson). — The well at the summit, from which the Summit Hotel obtains its water, has its bottom on solid rock. After the shock the level of the water in the well rose 12 feet. Boulder, Santa Cruz County (J. C. Branner). — At a sawmill near Boulder Creek, water stopt running from a hitherto permanent spring, but another in the neighborhood was flowing more freely than before. Felton, Santa Cruz County (Miss F. Locke). — All the springs on the property of Miss S. Anderson, a mile east of Felton, greatly increased in flow. Soquel, Santa Cruz County (W. E. Wheaton). — I have a drilled or bored well, yielding a magnificent flow of clear water. From three to four weeks previous to the earthquake this 75-foot well began to show signs of agitation below the surface. Every few days water heavily mixed with sand and ground chalk rock was pumped up. I knew that something was going wrong down under the earth, owing to the action of this well. When the quake came, it drove both fine and coarse sand into the casing, which put the well out of commission entirely. Chittenden, Santa Cruz County (G. A. Waring). — Near Chittenden a marked increase was noted in the flow of oil and water, and more gas and sulfur appeared. In the neighborhood of Santa Ana Peak, the flow of springs was increased. Prunedale, Monterey County (H. H. McIntyre). — Water started in many places where there had been little or none before the earthquake. ― 409 ―
Salinas, Monterey County (G. A. Daugherty). — In many places water came up thru open fissures; in one place about 8 miles from Salinas, the water covered about 80 acres of land. (B. M. Abbott.) — Water spouted from holes in the ground to a considerable height, and flooded the fields. San Ardo, Monterey County (G. A. Waring). — At San Ardo, quicksand was thrown up in a well, seeming to lessen the flow considerably. Paraiso, Monterey County (A. S. Eakle). — At Paraiso Springs, the quake affected the underground waters. According to the owner, Mrs. Romie, the supply of water from the springs had been diminishing for some time, and the temperature had been decreasing. Immediately after the shock it became necessary to put in a large pipe to carry off the water, and the temperature has resumed its normal state. Lonoak, Monterey County (J. Rist). — The earthquake caused springs to flow more; and the water rose in some wells. San Benito Valley to San Joaquin Valley (G. F. Zoffman). — In some places about 5 miles northwest of Bell's Station, on the Pacheco Pass road, springs were reported to be flowing 2 or 3 times as much water as they had previous to April 18. At a ranch-house 7 miles from the pass, on the east side of Pacheco Pass, the increase in the flow of water from springs in the neighborhood was said to have been noticeable. Springs were reported to have opened up considerably thruout the region around Emmet P. O. Stone Canyon, Monterey County (G. F. Zoffman). — In the neighborhood of Stone Canyon Coal Mine, the people claimed that there was a sudden rise of the water of the wells immediately after the earthquake. Dudley, King's County (O. D. Barton). — The gas spring on sec. 22, township 25 S., Range 18 E. was started into great activity by the earthquake. Formerly there were 7 places where gas could be seen occasionally blowing off through a shallow pool of water. Now there are more than 50 places where gas blows off continuously. The quantity of water was greatly increased. Beneath these gas springs the ground is dry and hot. Bakersfield, Kern County (A. G. Grant). — Artesian wells 30 miles north of Bakersfield were rendered muddy by the earthquake. Gold, Madera County (T. J. Rhodes). — Several springs increased about one-third to one-half in volume. Steamboat Sprïngs, Nevada (J. A. Reid). — At these springs the water is constantly boiling. For about 3 days after the earthquake, the volume was considerably increased, and the water became noticeably turbid with mud. On the north end of the highest sinter terrace, where heretofore the waters had been invariably clear, considerable quantities of mud were discharged. This material is now lying dry on the white surface of the sinter and is gradually being blown away. At the extreme north end of the active springs, where several mud springs have always existed, the change was noticed in the increased activity. One in particular formed a low cone of dark-colored mud, which is now dried and cracked. |