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Distribution of Apparent Intensity in San FranciscoBy H. O. Wood. IntroductionIn presenting the results of this study, the subject-matter has been taken up as follows: First, brief mention is made of the physiographic features of the city. Map No. 4, of the atlas accompanying the report of the Commission, shows the location of the city and its physiographic environment, also a segment of the Rift and of the fault on which the earthquake of 1906 was generated, and the position of a similar fault where the shock of 1868 originated. The city lies between these two zones of faulting. Then follows a note on the general geology of the region, illustrated by a geological map, No. 17 of atlas, prepared by Professor Andrew C. Lawson, on which is shown the areal distribution of the more important rock formations and of the districts of "made" land. Then comes the description and classification of typical destructive effects examined in the field. An intensity scale is discust, and its relationships to the Rossi-Forel and Omori scales are determined as well as possible. By critical comparison with Omori's scale, approximate values are fixed for the grades in terms of acceleration. Illustrating this discussion, map No. 19 of the atlas, showing the areal distribution of intensity in terms of an especially devised scale, presents graphically the results of the investigations in the city. The methods employed in the preparation of the map are set forth; also the manner in which the intensity scale was utilized. In map No. 18 are shown several geological cross-sections with corresponding intensity profiles. As vertical coördinates of the latter, values of the grades determined approximately in terms of acceleration were utilized. Following the general discussion of the intensity is a detailed description of the evidence which characterized various localities and determined the intensity grades ascribed to them. Next are discust details of evidence in the localities where very high intensity prevailed, which are of general interest owing to the suggestions they offer, the problems they raise, or the warnings they proclaim. PhysiographyThe San Francisco peninsula rises with bold relief from the level of the sea to hill summits varying in altitude between 100 and 1,800 feet, with the broad Pacific to the west of it, the waters of San Francisco Bay to the east, and the Golden Gate on the north. Southward, trending slightly east, the peninsula runs for several miles, merging finally with the hills of the Santa Cruz Range which mark the eastern limits of the Santa Clara Valley. On the western shore, promontories such as Point Lobos, Mussel Rock, San Pedro Point, and Montara Point, where rock-cliffs rise out of the surf, alternate with stretches of smooth beach line. At the north, the hills come down to the shore, forming rocky points: Point Lobos, where the Cliff House stands; Fort Point, marking the narrowest part of the Golden Gate; and the minor promontories of Black Point and Telegraph Hill farther east. The eastern shore is marked by prominent rock ridges extending out into the Bay, while between these, reaching well back into the hills, are sharply limited valleys cut down to the level of the sea and filled with deep deposits of alluvium, thus forming a gently sloping floor from which the hills rise abruptly. Before the building of the city, tide marshes with their little tidal creeks occupied the floors of these valleys, near their mouths. ― 221 ―
The most important of these is the relatively large Mission Valley, opening into the Bay between Rincon Point and Potrero Point and extending back westward and then southward, with a minor fork to the northwest, fully a quarter of the way across the peninsula. Mission Creek, with its lagoon and contiguous marsh, before it was filled to provide street and building sites, extended from the Bay shore around the northern extremity of the hills of the Potrero. Another long narrow marsh occupied a part of the floor of Mission Valley, stretching eastward from the present site of the Post-office building for several blocks, and then turning southward to the old Bay shore. This marsh also has been filled to provide building sites. Another dominant valley is that of Islais Creek, stretching back to the southwest between the hills of the Potrero and those of Hunters Point. This valley is outside the city proper. The city and county of San Francisco occupy the northern end of the peninsula, bounded on the south by an arbitrary east-west line some 7 miles south of the Golden Gate. The city, properly speaking, occupies the northeastern third of this area, covering the summits and flanks of the sandstone hills known as Telegraph Hill, Nob Hill, and Russian Hill, on the north; and other unnamed summits on the west. It covers also the floor of Mission Valley and reaches well up on the flanks of the hills which culminate in the center of the area. On the outskirts of the city proper, except in the southwestern part, are small detached groups of dwellings in the hills or on the sands. Market Street is a broad thorofare running southwestward from the Ferry Building and the wharves, at the northeast corner of the city, thru Mission Valley to the flanks of the high hills in the center of the area. About the lower part of Market Street is the commercial center of the city. The City Hall, situated about 1 block north of this broad highway, and about 12 blocks southwest of the Ferry, was not far from the center of the city proper. The zone of faulting where the recent earthquake had its origin past under the sea from a point near the head of Bolinas Lagoon, 12 or 15 miles northwest of the Golden Gate, to a point half a mile north of the little headland of Mussel Rock, about 8 miles south of Point Lobos. The map, No 4, shows its location. The entire area of the city and county is east of the fault-zone. The southwest corner of the area is less than a mile distant from it. The vicinity of the Ferry Building, at the foot of Market Street, was the most remote of any point in the whole area, being between 9.25 and 9.75 miles away. The site of the City Hall is from 7.5 to 8 miles from the fault. The Cliff House, at Point Lobos, the most western point of the area, is about 3 miles east of it. Fort Point lies between 5.75 and 6 miles east of it. Potrero Point and Hunters Point, as well, are about 8.5 miles from the fault. Hunters Point is the most easterly point in the district. GeologyIt is desirable to insert here a brief abstract of the geology of the northern part of the San Francisco peninsula, for it will appear that the effects produced by the earthquake were largely influenced by the character of the underlying formations. Map No. 17 shows the distribution of the geological formations at the surface. It shows also the areas of "made" land. These areas were determined by plotting the shore line shown on the accurate chart published in 1853 by the U. S. Coast and Geodetic Survey, upon the latest accurate chart of the same bureau. In these districts the materials forming the surface have been transported to their present position by human agency. The depth or thickness of this "filled" stratum is variable and, for the most part, not definitely known. A little study, comparing the areas of rock with the topographic contours, shows that all the hills are of firm rock, mostly coated with a veneer of soil and vegetation, but frequently outcropping at the surface. In general, their lower flanks are more and more thickly covered with loose sand and alluvium the nearer approach is made to the floor of ― 222 ―
the valleys or the districts of sand-dunes. At the lower levels such loose materials cover the whole area very generally.
The thickness of these strata must be notably variable, considering the uneven configuration of the rock surface where it
emerges from this mantle, since it is probably no less irregular beneath the covering. Very little information is available
concerning the depths to which these uncemented materials extend. A well at the United States mint is about 176 feet deep
and is believed not to have reached bedrock. A boring that was sunk at the corner of 7th and Mission Streets past thru sand
and clay to a depth of 264 feet, but did not reach bedrock. In general the sands and clays fill deeply the major valleys,
Mission and Islais. The minor northwest fork of Mission Valley, called Hayes Valley along its lower part, is probably less
deeply filled. This is certainly true of its upper reaches, to which, in this report, the name Upper Hayes Valley is applied.
Minor valleys and gullies all over the area have thin coverings of sand and alluvium which quickly thin out where the slopes
of the hills begin to rise steeply.
From the ocean inland for a considerable distance extends an area covered with sanddunes. This district is limited irregularly at the east by the contour of the hills. The sands form a thick mantle near the ocean shore, which becomes thinner and thinner as it rises upon the lower flanks of the hills. As in the case of the materials filling the valleys, the rock floor upon which the sands rest is probably very irregular. Of the hills, the northern ridge is carved out of the firm sandstone of the Franciscan series. Along this ridge are the summits of Telegraph Hill, Nob Hill, and Russian Hill, with other unnamed hilltops to the west, separated from each other by little saddle-like depressions in the surface. The outlying summits of Black Point and Rincon Hill appear to belong genetically to this ridge. This body of sandstone abuts on the west against a mass of serpentine, which forms a narrow range of hills stretching southeastward across the peninsula from Fort Point to Potrero and Hunters Points. This serpentine is intrusive in the firm Franciscan rocks, chert, and sandstone. The southwestern boundary of the serpentine in the vicinity of Fort Point is determined by a fault which has a throw of about 1,000 feet. This fault may possibly extend quite across the peninsula along the southwestern limits of the serpentine, but the field evidence does not warrant any definite statement. The fault movement occurred so long ago that the present land surface gives no unequivocal indication of its position. Mission Valley cuts across the body of serpentine, separating the northern hills from the southern group. The northern group rises along the western boundary of Hayes Valley. The central and southern hills, and the ridge at the northwest of the city, are carved intricately from firm Franciscan rocks, sandstone, and chert, commingled with minor bodies of irruptive rock of basaltic character. The hills of the more remote southwest corner of the city and county are of softer rocks of more recent geological origin — sandstones and shales of the Merced formation. These are relatively little cemented. Readers interested in a more complete account of the geology should consult the detailed report on this peninsula. [1] Destructive Effects and Intensity ScalesTo some extent the earthquake caused damage to buildings and other structures in all parts of the city and county of San Francisco. The whole area was decidedly within the destructive zone. Still, over a large part of this area, far the larger part, the damage was slight both in amount and character. Almost everywhere chimneys were thrown down or badly broken, but in a few small localities most of the chimneys withstood the shock. Some probably were unhurt. Plaster on walls and ceilings was very generally damaged. So, probably, were frail partition walls and chandeliers, crockery and fragile household furnishings. Such effects were typical of large sections of the city. There ― 223 ―
were relatively small districts, however, in which brick and frame buildings of ordinary construction were badly wrecked or
quite destroyed. Pavements were fissured, buckled, and arched. Sewers and water-mains were broken. In places, portions of
streets were moved laterally several feet out of place. Well-ballasted street-car tracks, equipped with 8, 10, or 11 inch
rails, were arched and flexed or thrown into shallow wave forms. The whole land surface, sometimes for several blocks together,
was deformed into shallow waves of irregular extension, length, and amplitude. Effects of this degree of violence were pretty
closely confined, as has been stated already, to areas of "filled" or "made" land. Such characterize, therefore, only a small
portion of the city; but, as it happens, areas of commercial importance and of special interest for the scientific purposes
of this inquiry. In consequence they will require a relatively large share of attention.
These destructive effects vary in degree from place to place thru the whole range between the extremes cited. In some cases this variation is best shown by the character of these effects; again by the frequency of their occurrence. The change from strong effects to weak sometimes takes place rather abruptly within the distance of a block or two, or less. Commonly the localities where very violent effects were produced are themselves pretty sharply limited. In such cases, however, there is still a noticeable variation in the sort and amount of damage resulting at different points just outside their limits, along their peripheries. At other places the destructive effects change gradually thru a distance of several blocks. This areal variation in the degree of damage indicates clearly a like variation in the intensity of the shock. The effects produced are the direct results of the intensity manifested, since where nearly all kinds of structures are to be found in all districts, of whatever intensity, such factors as the individual strength of the injured structures must practically cancel in the aggregate result. Consequently the destructive effects furnish a measure of the intensity, not very precise, it is true, but the best available, since no seismographic instruments were maintained in the city. By a classification of these effects different grades of intensity can be recognized and defined. Several such classifications have been made by seismologists for this purpose. The best known of these is the Rossi-Forel intensity scale, which provides ten scale numbers. The first defines a shock just barely perceptible to a sensitive observer, or one recorded by a sensitive seismograph; the tenth, a great disaster. The four highest numbers of this scale, as republished by the present Commission in its Preliminary Report, are as follows: VII. Violent shock, overturning of loose objects; falling of plaster; striking of church bells; some chimneys fall. VIII. Fall of chimneys; cracks in the walls of buildings. IX. Partial or total destruction of some buildings. X. Great disasters; overturning of rocks; fissures in the surface of the earth; mountain slides. The range of intensity in the city did not exceed these limits. Probably it did not reach the higher numbers recognized by the scale number X. In only a few small localities were the minimum values of scale number VII prevalent. It is easy to see, however, that this scale distinguishes its three upper scale numbers in vague terms, particularly with regard to effects likely to be produced in a modern city. For this reason it was found unsatisfactory for the investigations in San Francisco. A scale of greater merit is that devised by Professor Omori, of Tokyo, given below: No. 1. Maximum acceleration is 300 mm. per sec. per sec. People run out of houses; brick walls of bad construction are slightly cracked; plaster of some old dozo (godowns) shaken down; wooden houses so much shaken that cracking noises are produced; trees visibly shaken; water in ponds rendered slightly turbid in consequence of the disturbance in the mud. ― 224 ―
No. 2. Maximum acceleration is 900 mm. per sec. per sec. Walls in Japanese houses cracked; old wooden houses thrown slightly out of the vertical; tombstones and stone lanterns of bad construction overturned; in a few cases changes are produced in hot springs and mineral waters; ordinary factory chimneys not damaged. No 3. Maximum acceleration is 1,200 mm. per sec. per sec. About one factory chimney in every four is damaged; brick houses of bad construction are partially or totally destroyed; a few old wooden dwellings and warehouses totally destroyed; wooden bridges slightly damaged; some tombstones and stone lanterns overtuned; shoji (Japanese paper-covered sliding doors) broken; roof tiles of wooden houses disturbed; some rock fragments thrown down from mountain sides. No. 4. Maximum acceleration is 2,000 mm. per sec. per sec. All factory chimneys are broken; most of the ordinary brick buildings partially or totally destroyed; some wooden houses totally destroyed; wooden sliding doors and shoji mostly thrown out of their grooves; cracks 2 or 3 inches in width, in soft or low ground; embankments slightly damaged here and there; wooden bridges partially destroyed; and ordinary stone lanterns overthrown. No. 5. Maximum acceleration is 2,500 mm. per sec. per sec. All ordinary brick houses very severely damaged; about 3 per cent of the wooden houses totally destroyed; a few tera, or Buddhist temples, are thrown down; embankments severely damaged; railway lines slightly curved or contorted; ordinary tombstones overturned; ishigaki, or masonry walls, damaged here and there; cracks 1 to 2 feet in width produced along river banks; water in rivers and ditches thrown over the banks; wells mostly affected with changes in their waters; landslips produced. No. 6. Maximum acceleration is 4,000 mm. per sec. per sec. Most of the tera, or Buddhist temples, are thrown down; 50 to 80 per cent of the wooden houses totally destroyed; embankments shattered almost to pieces; roads made thru paddy fields so much cracked and deprest as to stop the passage of wagons and horses; railway lines very much contorted; wooden bridges partially or totally destroyed; tombstones of stable construction overturned; cracks a few feet in width formed in the ground, accompanied sometimes by the ejection of water or sand; earthenware buried in the ground mostly broken; low grounds, such as paddy fields, very greatly convulsed both horizontally and vertically, sometimes causing trees and vegetables to die; numerous landslips produced. No. 7. Maximum acceleration is much above 4,000 mm. per sec. per sec. All buildings except a very few wooden houses are totally destroyed; some houses, gates, etc., projected 1 to 3 feet; remarkable landslips produced, accompanied by faults and shears of the ground. In the foregoing scale, in addition to these definitions by destructive effects, it will be noticed that a range of values for the acceleration is assigned to each scale number. These acceleration values have been tested experimentally by Professor Omori, and found accurate within narrow limits. Consequently it is called an absolute scale. It is the best intensity scale yet proposed. Since, however, it is defined in terms of damage produced upon Japanese structures, it would require constant critical interpretation in use in an American city. For this reason, it is believed to be not so well adapted to the purposes of this investigation as the scale proposed below. This is especially true since the values of the acceleration necessary to produce the destructive effects encountered here have not been determined by experiment. A use of the absolute scale would, therefore, pretend to an accuracy not attained with any certainty. The following scale will be referred to as the San Francisco scale: Grade A. Very violent. — Comprizes the rending and shearing of rock masses, earth, turf, and all structures along the line of faulting; the fall of rock from mountain sides; numerous landslips of great magnitude; consistent, deep, and extended fissuring in natural earth; some structures totally destroyed. ― 225 ―
Grade B. Violent. — Comprizes fairly general collapse of brick and frame buildings when not unusually strong; serious cracking of brick work and masonry in excellent structures; the formation of fissures, step faults, sharp compression anticlines, and broad, wave-like folds in paved and asphalt-coated streets, accompanied by the ragged fissuring of asphalt; the destruction of foundation walls and underpinning structures by the undulation of the ground; the breaking of sewers and water-mains; the lateral displacement of streets; and the compression, distension, and lateral waving or displacement of well-ballasted street-car tracks. Grade C. Very strong. — Comprizes brick work and masonry badly cracked, with occasional collapse; some brick and masonry gables thrown down; frame buildings lurched or listed on fair or weak underpinning structures, with occasional falling from underpinning or collapse; general destruction of chimneys and of masonry, brick or cement veneers; considerable cracking or crushing of foundation walls. Grade D. Strong. — Comprizes general but not universal fall of chimneys; cracks in masonry and brick work; cracks in foundation walls, retaining walls, and curbing; a few isolated cases of lurching or listing of frame buildings built upon weak underpinning structures. Grade E. Weak. — Comprizes occasional fall of chimneys and damage to plaster, partitions, plumbing, and the like. This scale obviously is simply a classification of the phenomena observed. It defines as many grades as the facts seemed to express in this field. It is more finely subdivided than the Rossi-Forel scale and, for conditions in a modern city, the definitions are better framed. It has less intrinsic merit than the Omori scale, for both scales cover a similar range of destructive effects, but the subdivision is finer and more evently spaced in the case of the Omori scale. Also the grades of the San Francisco scale can not be fixed by values of the acceleration, except approximately by comparison with the absolute scale. The fact, however, that it does not pretend to absolute values seems a point in its favor under the circumstances. And it is a practical scale for the phenomena dealt with. Altho rigorous values can not be obtained by such means, it is desirable to subject the grades to careful comparison with the numbers of the Omori scale in order to determine reasonably close acceleration values for them. A comparative study of the 3 scales is summarized diagrammatically in the accompanying table at the top of next page. [1] Some of the effects which serve to define Grade A are weaker than the maximum effects defining No. 6 of the absolute scale; and nowhere, not even in the vicinity of the fault, were most buildings totally destroyed. Grade B covers a wide range. Perhaps if the initial shock had been a little stronger; it could have been subdivided with some certainty. Grades C and D cover each a slightly lower range of values than the scale numbers 3 and 2, to which they correspond most closely. Grade E, as defined, is more narrowly limited than No. 1. These values, despite their lack of precision, constitute the best approximation to an absolute measure of energy developed, for each grade of intensity, which it appears practicable to attain. There were no instruments of precision to record the character and amount of the motion of the shock, hence estimates of other sort than this seem difficult to make. The fact must not be lost sight of, however, that it is only an estimate, based upon the interpretation of a series of destructive effects produced in very variable media under variable conditions and then compared with a similar series of destructive effects produced in structures of a different sort, for which pretty accurate acceleration values had been determined experimentally. ― 226 ―
It may be perhaps well to point out that Grade D lies between Nos. VII and VIII of the Rossi-Forel scale. This grade characterizes the greater part of the city, as the intensity map shows. Grade B, equivalent to Nos. IX and X of the same scale, is characteristic of very small areas only. Grade A is not exhibited in the city proper. Utilizing the San Francisco scale, intensity map No. 19 was prepared, which indicates the location and areal extent of the districts characterized by each grade of intensity. It presents graphically the results of the field work. In the field study, practically all of the city proper, including the large area devastated by fire, was thoroly traversed, excepting one or two isolated hilly localities where a brief examination showed no significant damage. Unbuilt districts were, of course, comparatively neglected, except where disturbances of natural objects were found or lookt for. Chimneys, buildings, streets, paving, curbing, sidewalks, car tracks, retaining walls, etc., were subjected to careful scrutiny, and such injuries as were observed were classified on the spot in terms of the San Francisco scale. The intensity indicated was recorded by a spot of color placed upon a field map of suitable scale (1,760 feet to the inch, or 1:21,120). Many photographs were made, some of which appear as illustrations in this report. Detailed field notes were made only when damage of unusual or striking character was encountered, or when it was perplexing. When effects were observed which seemed likely to be of value in analyzing the character of the earth motion, notes were made. Little indoor evidence was obtained or sought. It will thus be seen that the field study, while adequate for the purposes in view, did not constitute an expert engineering investigation, dealing with specific details and location of damage. Frequently there was doubt as to what grade of intensity should be assigned to a given city block, because of conflicting or inadequate evidence in the field. This is particularly true of districts swept by fire, especially where the intensity was low; for most ― 227 ―
of the effects which serve to define the lower grades were obliterated with the structures in which they were developed. Where
buildings were sparsely distributed, it was often hard to determine what grade of intensity was developed, for the evidence
was scattering and heterogeneous. Nevertheless, the map is a pretty faithful representation of the distribution of intensity,
and quite justifies the scientific and economic conclusions of a general nature that are drawn from it here.
On the map, color in northwest-southeast bars (A, B, C, D, E) represents districts marked by unequivocal evidence. Continuous lines indicate the position of well-determined boundaries between areas affected by different grades of intensity. Color applied in northeast-southwest bars (a, b, c, d, e) represents districts in which the evidence was scanty or circumstantial; and dotted lines indicate the position of boundary lines which were determined but vaguely by the phenomena in the field. Detailed Description of the Evidence By Localities [1]No district designated upon the map as exhibiting intensity of Grade E, so far as the writer could find, exhibited any destructive effects of a more violent kind than the fall of chimneys. The really typical measure of intensity for these localities was the cracking and falling of plaster. Without exception, these are places where the firmly cemented bedrock of the Franciscan formations is either exposed directly or covered with a very thin mantle of soil. This lowest grade of intensity does not, by any means, characterize all places where the firm bedrock is exposed at the surface. It was rather developed on the summit portions of the rocky hills. The tops of Telegraph Hill and Russian Hill are districts in which a large part of the chimneys withstood the shock. This was also the case with the upper slopes of the chert hills about the head of Market Street, at the center of the area. Scarcely any injuries resulted on the hills of the Potrero; and one or two small serpentine hills just north of Market Street were likewise immune. Similarly, the Hunters Point serpentine ridge was subjected to a shock of low intensity; at least, a hasty survey pointed to this conclusion, tho the evidence was sparse and not thoroly examined. San Bruno Mountain, however, was about as near to the zone of faulting as Point Lobos, where most of the chimneys were thrown. Intensity of Grade D is believed, therefore, to have been developed upon the summit of San Bruno Mountain. The general fall of chimneys, slight cracking of brick work, and such damage, denoting intensity of Grade D, characterizes the northeastern half, or possibly two-thirds, of the city and county, except in localities where special conditions, chiefly lithological, modify it. Districts of exposed bedrock on the flanks of the hills, and of sand and alluvium wrapped as a thin mantle about their lower slopes, exhibited this degree of damage. Consequently a large area was affected by this grade of intensity which does not, in general, require detailed discussion; no violent nor specially significant effects being produced. Where, however, the loose earth covering is thicker, the magnitude and frequency of damage increases. Market Street, between Second and Fourth Streets; Mission Street, between First and Third Streets; and Howard Street, between Second and Third Streets, together with the blocks in the neighborhood of Market Street on Montgomery and Kearney Streets, Grant Avenue, Stockton and Powell Streets, form a district in which the effects denote an intensity only a little short of Grade C. A large proportion of the buildings were excellent structures which individually withstood the shock well. In consequence, it was difficult to draw a line in this region between districts marked by broken chimneys and cracked brick walls, and those where more serious damage was certainly developed. The resistant character of the excellent buildings and the thoro obliteration by the fire of evidence produced in poor structures, render the determination of the intensity as Grade D somewhat doubtful. ― 228 ―
In the blocks adjacent to Point Lobos Avenue and Clement Street, between First Avenue and Sixteenth Avenue, in the sand-dune district, damage — mostly of Grade D — was prevalent. This locality is the part of the city nearest to the seat of the disturbance, and the cover of sand which rests upon the uneven bedrock is unevenly thick; therefore irregular variations of intensity are to be expected. Nevertheless it is not easy to fix the boundaries between Grade C and Grade D in this part of the city. Along Oak and Fell Streets, and the Panhandle Parkway from Broderick Street west, the intensity closely approaches Grade C without seeming quite to reach it. Along Washington Street and its immediate vicinity, from Baker Street west to Spruce Street, on the crest of the sandstone ridge, the intensity is higher than for most other localities of exposed bedrock. Fallen chimneys and cracks in foundation walls were more prevalent than in most areas so situated. On bedrock at Point Lobos, also, the effects indicate an intensity pretty close to Grade C, but this locality is nearer the fault than any other Franciscan outcrop save the western slopes of San Bruno Mountain. We may say in general, therefore, that Grade D is the intensity developed on bare rock foundations, or on rock only moderately coated with soil, in the northeastern part of the city and county of San Francisco. In the low lands of the valleys, and along portions of the water-front, the sand and alluvial deposits are thicker and the destructive effects were increased in magnitude and in prevalence; also thruout a large part of the sand-dune tract at the west, wherever evidence was obtained, increased intensity was found to prevail. All over Mission Valley and Hayes Valley, including Upper Hayes Valley, brick walls were cracked and some gables and walls actually fell. Buildings placed on weak underpinning were frequently displaced slightly from the vertical. In a few cases, weak frame dwellings collapsed as a result of the giving way of weak foundation structures. Most chimney stacks were broken. In no part of this large district was evidence of this kind lacking, altho the majority of the structures were fairly substantial frame dwellings, and were of course not seriously damaged. There was much indoor damage, but no investigation of this was undertaken. At the outer margin of this area, marked by an intensity of Grade C, the destructive effects were weaker, indicating an intensity just above Grade D. Where the district adjoins localities which suffered a still severer shock, the damage was of greater magnitude and more prevalent. Besides this gradation there were, within the limits of the district, several little localities where the characteristic destructive effects were conspicuously numerous. In the neighborhood of O'Farrell Street, between, say, Mason and Taylor Streets, brick work was sadly cracked. Photographs made before the fire (plate 87A) show that some building fronts were thrown out on O'Farrell Street in this vicinity. Many of the buildings hereabouts were mediocre structures at best, but injuries were too generally distributed to be ascribed wholly to structural weakness. The damage was not of great magnitude and did not indicate intensity of Grade B, so far as could be made out from the ruins after the fire. Near the City Hall there was a small locality conspicuous for the damage produced. The City Hall itself made a picturesque ruin (plates 82 and 83A), as all the world knows, but the character of the construction was probably a large factor in its destruction. Nevertheless ugly cracks in other buildings near by indicated intensity somewhat higher than was common in the valley district as a whole. Just south of Jefferson Square some weak buildings quite collapsed, and foundation walls were generally cracked and crusht. Wooden underpinning showed a tendency to lurch and throw buildings slightly out of the vertical. Similar effects prevailed along Folsom and Treat Streets for two or three blocks south of Eighteenth Street. ― 229 ―
The blocks between the old tide-marsh area, extending east from near the Post-office, and the former course of Mission Creek, give evidence in the form of cracked foundation walls, broken concrete cellar floors, etc., of intensity values high in Grade C. The fire did much to destroy evidence here, as it was a district of wooden dwellings. From near the corner of Third and King, to the corner of Folsom and Steuart Streets, there is a narrow fringe of land constituting the water-front around Rincon Point. The land is partly natural and partly made. Few structures are found on it which are not built on piling, whether they be warehouses on the docks or good modern buildings. Some parts of the area are devoid of buildings. No evidence was disclosed in this tract indicating intensity higher than Grade C. Significant evidence was scarce. Cracked brick walls here and there served to fix the degree of intensity. Onward to the north and to the west from the corner of Steuart and Folsom Streets, extends a narrow sinuous area around Telegraph Hill to the vicinity of Black Point, which is designated upon the map as affected by an intensity of Grade C. Such effects as badly cracked brick walls, some of which fell, the fall of cornices and gables, etc., are said to have been developed here. Such evidence as could be made out amid the fire ruins tends to confirm this. This region divides the water-front area of made land, where high intensity was developed, from sandstone hills, where a lesser shock was experienced. It will be discust further in connection with phenomena of special significance. A low-lying crescent-shaped area of alluvium and sand, with a little made land near the shore, extends westward from Black Point to Fort Point. South, east, and west the hills rise steeply. Formerly a tide-marsh completely separated the alluvial flats from the sandbar at the shore. Part of this, near its mouth, has been filled. Buildings are scantily distributed all over the area. Evidence in the form of structural damage is not, therefore, met frequently. The filled land of the tide-marsh is devoid of structures. There are several little localities in this district marked by damage denoting intensity of Grade B. These will be mentioned later. For the most part, frame buildings occasionally tilted a little out of the vertical, and cracked and crusht foundation walls are typical of the destructive effects found here. Leading down into this area from near the corner of Polk Street and Pacific Avenue is a minor valley, once deeply trenched, but now modified by alluvial and artificial filling. Along its course most chimneys were thrown and foundation walls were cracked and crusht generally. Two little places, where intensity of Grade B was developed, are situated in the trough. They are discust below. A group of small areas, 4 in number, together with a small spur of Mission Valley, situated along a line extending northwest from about the junction of Sixteenth and Dolores Streets, is designated upon the map as characterized by intensity of Grade C. It chances that the northwest extremity of this line coincides with the location of an old fault-zone, mentioned above as partly determining the southwestern limits of the serpentine. In one of these localities, practically bounded by Maple, Spruce, Washington, and California Streets, brick walls and foundations were cracked conspicuously. A building at the corner of Maple and Washington Streets had a balcony supported by pillars above its front entrance. This was thrown down, and the walls were cracked rather badly. It was probably a structure ill adapted to resist earthquake shock. Still it stands directly upon a bare ledge of serpentine, and upon similar rock in the Potrero, an equal distance from the fault, intensity of only Grade E was developed. At the corner of Maple and California Streets, the Hahnemann Hospital, a new brick building, sustained severe damage, particularly the east wing. If neighboring structures showed any destructive effects comparable with this, intensity of Grade B would be indicated. But they do not. Some cause peculiar to the building itself is responsible for the exaggeration of the intensity. Probably the newness of the masonry was a contributory factor. The surface material here is sand, but it can not be very thick. ― 230 ―
Along the same line, near the corner of Waller and Portola Streets, is a little locality of sharp intensity, quite within the lower range of Grade B. It occupies about a block. In the adjoining blocks chimneys fell generally, houses were disturbed slightly on their foundations, and foundation walls were cracked. Here a thin layer of sand occupies the bottom and lower slopes of a sharp little valley. There are low serpentine hills just to the east, with higher chert hills to the west. In the vicinity of the corner of Van Ness Avenue and Clay Street, there is a low place, or saddle, in the crest of the sandstone ridge where, without apparent lithological cause, there were manifestations of some violence. Some apparently good buildings displayed conspicuous cracks. It is believed that this damage may be in part ascribed to explosions of dynamite used in checking the fire, but in many cases the cracks do not appear to be due to this cause. There is doubt as to the meaning of the evidence here. In the western part of the city proper, the Richmond district, the Sunset district, and Golden Gate Park, there are several places where chimneys were quite generally destroyed and houses were shifted slightly on their foundations. Loose sand covers the rock to an unknown depth, but this mantle is probably not very thick. Lake Street, in the vicinity of Fifth, Sixth, and Seventh Avenues, is one of these localities, where, for instance, the Maria Kip Orphanage exhibited conspicuous cracks in its brick walls, as well as fallen gables. In the Home for the Aged, not far away, cracks in the brick walls were numerous. Dwellings of wooden frame construction were less seriously damaged; but even these were much more noticeably affected than others at a little distance. The buildings of these charitable institutions were probably not very well constructed. A smaller area, on Eleventh Avenue between California and Clement Streets, shows one frame dwelling quite ruined by collapse. (See plate 88A.) This was due to the giving way of a high-posted wooden underpinning. Houses near by are comparatively little affected. It is suggested that this locality is a place filled by grading. Along First Avenue, between Point Lobos Avenue and A Street, a considerable length of the west wall of the Odd Fellows Cemetery was thrown over to the east. This was a concrete wall 5 or 6 feet high, with a thickness at the base of from 1 to 1.5 feet. It was reënforced near the top by a 2-inch gas pipe running the length of the wall. Houses on the west side of the street were slightly shifted on their basements. On Third Avenue, between Point Lobos Avenue and Clement Street, the underpinning of houses was disturbed. The French Hospital buildings, which occupy the entire block bounded by Point Lobos Avenue, Fifth Avenue, A Street, and Sixth Avenue, showed ugly, X-shaped cracks in the brick walls, especially in the central towers. Some brick work fell from the gables, and the chimney stack was broken. In this part of the city buildings are isolated or in small clusters, with unbuilt districts of blown sand intervening. Consequently evidence was scarce and unsatisfactory. The Park Emergency Hospital, near the southeast corner of Golden Gate Park, had its walls badly cracked and its gable thrown out. It is a small, 1-story, sandstone building, with a wooden frame. Its site was loose sand of unknown depth, probably extensively graded. Evidently it was not an excellently built structure. The restaurant at the children's playground in the Park was wrecked. (Plate 86.) The Museum in the Park, not far from the corner of Eleventh Avenue and Fulton Street, was a wooden framed building, with brick and plaster walls. These were cracked very badly, and considerable portions fell. Near by considerable brick and stone fell from the cornice of the music stand. Ugly cracks traversed the hemispherical arch, constructed of sandstone blocks, which served as a sound reflector. The building was made of sandstone blocks, backed with brick. In some of the columns, several of the blocks are moved ― 231 ―
out of place. Two or three smaller buildings in the immediate neighborhood were also notably damaged. Intensity equivalent
to high values of Grade C was certainly developed hereabouts. In some cases it undoubtedly reached low values of Grade B.
Yet the glass walls and roof of the conservatory, and of the aviary close by, were not appreciably damaged. This discrepancy
shows clearly that some purely local factor determined the amount of damage.
Buildings on the beach sands near the Cliff House, close to the sandstone cliffs of Point Lobos, were strongly shaken. A small 1-story brick pumping station had its walls badly cracked and portions thrown down; its chimney stack also was broken. Weak underpinning in some neighboring frame buildings yielded perceptibly. Here also an abrupt transition is noticed from intensity of Grade C on the sands to Grade D on the sandstone cliffs. Near Lakeview, fairly well built frame buildings on dune sand of unknown thickness were caused to lurch and shift their positions. Ocean Avenue, between Ingleside and the sea, tho almost devoid of structures, shows by the unearthing, bending, and even breaking of drainage and water pipes, and by fissures in the road and asphalt paving, a change of intensity from Grade C to Grade B. Localities of Lesser Importance Affected by Intensity of Grade BIn the neighborhood of the crossing of Steiner and Sutter Streets, there is an irregularly bounded district a little larger than a city block in which several buildings not conspicuously weak were totally destroyed. St. Dominic's Church, at the corner of Steiner and Bush Streets, was a complete ruin, as the illustration (plate 92A) shows. Its steeple towers were ruined, its roof fell in, and all its walls were so badly cracked that it became a menace to the neighborhood. If the shock had occurred during the hours of religious service, few would have escaped from the building alive. Probably it was not a building of the most excellent construction; but, on the other hand, it did not appear to be built flimsily. It certainly suffered a most violent shaking. Near by small frame dwellings were pitched from their underpinning. On Geary Street, just above Fillmore Street, two wooden-framed brick buildings standing side by side — the Albert Pike Memorial Temple (Masonic) and a Jewish Synagogue—were utterly wrecked, as the illustration shows. (Plate 87B.) The Girls' High School, near by on O'Farrell Street, at Scott Street, poorly and flimsily built, was badly damaged. Its walls were much cracked and portions of the gable walls were thrown down. This district of Grade B intensity is on the floor of Upper Hayes Valley and is surrounded by a relatively broad area in which Grade C effects prevail. It lies near the base of the hills which hem in the valley on the east. The surface strata are sand and alluvium extending to no great depth, unless the slopes of the bedrock hills change suddenly where they pass under the mantle of loose materials. No explanation can be offered for the occurrence of this limited area of high intensity (Grade B) unless it be that the district has been converted into "made" ground by extensive grading in the preparation of the surface for building sites and streets. At the corner of Vallejo Street and Van Ness Avenue, fissures were formed in the asphalt paving, sidewalk pavements were thrust over the curbing, and water-mains and sewers were broken. Buildings were thrown out of the vertical, and foundations and lower story walls were shifted and crusht. The walls about the foundation of one brick building were actually deformed into undulations with much consequent cracking. This building was so badly damaged that it had to be taken down. Surrounding this corner is a small ovoid district, about 2 blocks in extent, in which the intensity was clearly of Grade B. This was once a sharp ravine and had been filled to a depth of 40 feet in order to provide a ― 232 ―
suitable grade for streets and buildings. The filling was shaken together and moved slightly downhill.
On Lombard, between Gough and Octavia Streets, is a little area, less than a block in extent, in which the destructive effects were of Grade B. No particularly notable effects were produced. It is a district of made land, formerly the site of a little lagoon in the sands, known as Washerwoman's Lagoon. A portion of Union Street, between Pierce and Steiner Streets, not more than a quarter of a block in length, where a filling had been made to equalize the street grade, was shaken down into the adjacent building lot on the north. The north sidewalk was shifted about 10 feet to the north, and deprest about 10 feet below its original level. The south sidewalk was deprest a few inches and shifted to the north from 2 to 3 feet. The paving and the cable conduit suffered more severe damage than at any other point in the city. The photograph (plate 88B) conveys a graphic conception of the very great violence which occurred here. The phenomena have no general significance, however, despite their striking character, being merely a sliding of unconsolidated material not supported on the sides. But that such places are dangerous building sites, especially in regions subject to seismic disturbances, is unequivocally demonstrated. Along the north shore water-front, between Fillmore and Steiner Streets, from Bay Street to the water's edge, was a plot of made ground occupied by a gas-producing plant. Here brick walls were cracked and partly thrown down; part of the wooden framework was wrenched out of position, and the chimney stack was broken. One of the large gascontainers was badly wrecked, but whether its destruction was caused directly or in some secondary way, as by rapid leakage, is not known. The intensity was clearly Grade B. Along Lyon, Baker, and Broderick Streets, north of North Point Street, is a small locality 2 blocks wide and 4 blocks long, where the Baker Street sewer was broken and frail frame buildings were thrown out of the vertical. This district was partly made land, but the greater part was on the point of a sand-pit. Unquestionably extensive grading had been done to prepare the ground for building. In Golden Gate Park, near the Museum, the granite railing of a stone-arch bridge was shattered by the shock. This was a low balustrade, with many turned granite posts set closely together, supporting a flat, massive granite top-rail. Such damage as it sustained appears to indicate an intensity of Grade B. The bridge was built on loose sand of no great thickness. On Fulton Street, between Twelfth and Thirteenth Avenues, there was much slumping of the street-filling down into the Park adjacent; and exactly the same sort of damage occurred on H Street, between Ninth and Fourteenth Avenues. Altho, under the definitions, the damage produced in these localities denotes intensity of Grade B, it is believed that the energy of the shock was not greater than elsewhere in their immediate neighborhoods. They were especially susceptible to damage from earthquake shock, being practically loose earth embankments. Strawberry Hill, in Golden Gate Park, is a chert knob rising abruptly in the sand wastes. Its summit had been leveled, but it is not known whether this was done by cutting off the top and filling out the upper slopes, or by filling alone. The altitude given for the present hill is the same as that given in the earliest accurate surveys. Much artificial stone work, and a circular concrete observatory building 2 stories high, had been erected upon the leveled hilltop. This building was of weak design, having a row of columns, with windows between, which rested upon a foundation wall 3 feet high and supported a heavy secondstory balcony. The construction itself was probably good, but the observatory was utterly ruined by the shock. (Plates 84 and 85.) The entire lower story was sheared out of position, and part of the balcony fell. The cement floor showed numerous cracks arranged in a roughly concentric way. ― 233 ―
The whole periphery of the hilltop was broken into a series of concentric blocks or steps, and the outer ones moved down the hill from 2 to 3 feet. The artificial stone work was badly cracked and dislodged. These phenomena indicated that the material used in grading the upper slopes had settled somewhat, with consequent rupture of the surface and wrecking of the building. No other explanation can be urged for such striking damage on this hill, in view of the small damage produced on other rock summits in the city. All the driveways in the western part of Golden Gate Park showed scattered narrow fissures. There were but few structures here, and they did not show significant damage. These were low, strong, frame buildings. It is a district which was extensively graded in the work of landscape gardening, and is underlain by a deep sand deposit. It appears to have suffered a shock of intensity of the middle range of Grade B. Phenomena of Especial InterestAbout the Ferry Building, at the foot of Market Street, is a district of "made" land, shown on map No. 17, in which high intensity was manifested. Here buildings of all sorts were crowded close together. Wooden buildings, I story to 3 stories high, with brick or stone work fronts, were interspersed among ordinary brick buildings from 2 to 6 or 8 stories in height. Mingled with these was a considerable number of modern, class A, office buildings. Here the fire burned fiercely and caused great havoc, heaping the streets and the cellars of buildings with fallen brick and stone and twisted beams and girders. For weeks after the conflagration many of the streets were completely hidden under the débris. So much of the damage due directly to the shock was thereby concealed or obliterated, that no adequate knowledge of the direct effects of the earthquake could be obtained in this part of the city; tho eye-witnesses tell of cornices and gables which fell, and of walls and roofs which collapsed at the time of the shock. After the fire had past, standing walls revealed ugly, sinuous cracks, in rudely parallel systems, which were not due to fire nor to dynamite. Masonry blocks in the walls of excellent modern buildings were broken as by a blow. Rivets were sheared off in parts of the framework of steel structures, and tension rods in such frames were badly stretched. Tubular cast-iron columns, supporting floor girders, were broken off near their bases in cellars where they rested upon piling. The concrete casing of piles was frequently broken. Wherever the intensity was high, the tendency to crack or crush near the base, as tho a sharp blow had been struck there, was notably conspicuous. In spots the streets sank bodily, certainly as much as 2 feet, probably more. Accompanying this depression, concrete basement floors were broken and arched, as if to compensate for it. The surface of the ground was deformed into waves and small open fissures were formed, especially close to the wharves. Buildings on the water side, along East Street, generally slumped seaward, in some cases as much as 2 feet. The damage was greatest close to the water's edge, growing less as the solid land was approached, gradually at first, then more rapidly. These phenomena seem to suggest that the materials used in filling were shaken together so as to occupy less space with the accompanying development of waves, fissures, and structural damage The more recent the filling, the more it would be compacted; hence the greater prevalence and magnitude of destructive effects near the water's edge. As well as could be made out from the inadequate evidence left by the fire, the district which suffered intensity of Grade B is limited on the landward side by a line drawn from Filbert Street to Market Street, between Battery and Front Streets; thence between First and Fremont Streets to a little south of Folsom Street, where the line turns and runs eastward to the wharves. Flanking this district on the landward side is a narrow, sinuous area limited by a line drawn from Filbert Street to Green Street, just east of Sansome Street; thence between Sansome and Montgomery Streets to Market Street; thence to the corner of Mission and First Streets; thence between First and Fremont Streets ― 234 ―
― 235 ―
to a point south of Folsom Street; thence easterly nearly to the wharves. Between Washington and Sacramento Streets, this
boundary is barely east of Montgomery Street. Immediately west of these districts, low intensity prevailed.
It is of interest to inquire whether all or only a portion of this district in which high intensity was developed is "made" land. In the map (fig. 51) is reproduced a portion of the U. S. C. & G. S. chart, "City of San Francisco and its Vicinity," published in 1853 from surveys made in 1851-1852. On it the dot-and-bar line represents the course of the "original high-water line according to plot of Wm. M. Eddy's survey dated 1852." The "zero contour" which determines the configuration of the shore, except where wharves put out, is shown by a continuous line; it is not expressly defined, but it is believed to represent mean low-water, as the soundings are measured from this level. It is needless to point out that this contour is drawn farther seaward than the original high-water line. The portion thus delimited has an area of not less than 20 city blocks, partly or wholly occupied by buildings. Quite outside the "zero contour," as shown on this map, are 8 complete blocks and portions of others — an area of not less than 10 city blocks, partly or wholly built upon. If, then, confidence may be placed in the location of the original high-water line of the Eddy survey of 1852, there were already in San Francisco 30 blocks of "made" land, occupied wholly or in part by buildings before the end of 1853, less than 4 years after the sudden rush to California which followed the discovery of gold in 1849. The revised chart of 1857 shows that very little additional land was made in this district in the succeeding four years. Without conflicting evidence from other surveys, and no such evidence has been found, the high-water line established by the Eddy survey can not be discredited. Still it is proper to state that these facts raise some doubts as to the accuracy of its delineation, and that the evidence developed by the earthquake does not tend to dispel these doubts. The gradation in the effects produced by the shock, from great magnitude at the water-front to small at the former land margin, would suggest that at least the marginal district where only Grade C intensity was developed, tho outside the location of the original high-water line, might not be made land, altho it has undoubtedly been somewhat elevated by grading. Very little stress can be laid on this suggestion, however, for these districts suffered very severely in the earthquake shock of 1868; but the materials used in filling were then, of course, shaken together, and in addition, the slow settling together from year to year has undoubtedly compacted the earlier made land much more than that recently "made." Besides, the exhibition of damage depends upon the character of the structures in a given locality, as well as upon the ground, and it is to be noted that the buildings along Kearney, Montgomery, and Sansome Streets comprized a larger percentage of excellent structures than the streets nearer the wharves. The problem is thus complex, and very likely unsolvable; but there remains the haunting suggestion that the "original high-water line" does not constitute the landward boundary of the "made" land, properly speaking. At any rate, it is very clear that that which was known to be "made" land suffered much more severely than that which was known to be natural alluvium. It is important to recognize that, despite the great intensity manifested near the water-front, first-class modern buildings, such as the Ferry Building, built upon deep piling or grillage foundations, were not imperiled by injuries to their walls or framework. Some rivets were sheared off; some tension rods were stretched; an occasional girder was dislodged, and cracks were formed here and there in the brick and stone walls. Large financial loss was unquestionably occasioned, but buildings of this type were not in serious danger of collapse nor of being toppled over, either during or after the shock. Nevertheless conservative engineers recognize that even these structures were weakened. They recognize, too, that future shocks may exert greater energy, and they are trying to devise buildings better able to resist the peculiar stresses of earthquake shocks. The ― 236 ―
general public should share their interest, and uphold and enforce the provisions they deem it wise to make against future
disasters.
A good indication of the value of deep piling as a foundation structure was furnished by the conduits of the cable-car system on lower Market Street. On account of the constant tendency of the whole district to subside from year to year, as the filling material became more closely compacted, these conduits were constructed upon piling to secure permanence of grade. On both sides of them the street sank in places as much as 2 feet, and the pavement was broken, fissured, and thrown into waves. These tracks did not escape entirely, but for several days, before street repairs were made, they constituted a narrow raised path along the center of the street. Altho in this part of the city the fire did much to conceal the earthquake damage, a few little spots, especially along the water-front, where water was available, escaped its devastation. A building on Spear Street near Folsom, occupied by the National Bolt Works, illustrates what must have occurred in the case of many small brick structures. Its side wall was thrown down and the entire structure lurched out of plumb. To be sure, this building was heavily loaded on its second floor; still it was not so badly damaged as many partly standing walls in near-by districts swept by fire. The earthquake cracks, being sinuous, and recurring with a rude parallelism, were easy to distinguish from cracks opened by heat, or by the stresses induced by the wrenching away of falling walls or by dynamite. Buildings erected upon good foundations withstood the ordeal well, even when the streets around them were deprest and fissured. The Appraisers' Building furnishes a good illustration of this; it is substantially built of brick upon a piling foundation, at the corner of Washington and Sansome Streets, and still stands without significant damage. The levels of its foundation walls were not disturbed. (See fig. 53.) High intensity was developed thruout a small elongate district having a width of about two blocks, which extends from near the corner of Eighth and Mission Streets to the vicinity of Fourth and Brannan Streets; from this point the boundaries are irregular and very sinuous, leading to the water-front at about the crossings of Third Street with Berry and Channel Streets. A glance at the geological map, No. 17, shows that the regularly bounded portion of this district corresponds very closely with the area of a former tide-marsh, drained and flooded by one or two small tidal streams. The former shore line of Mission Bay was just north of Brannan Street, between Fourth and Fifth Streets, so that the irregular seaward portion of the district lies outside the old shore. This is one of two localities in the city, the other being a "made" land tract along the former course of Mission Creek, in which destructive effects of great magnitude were conspicuously developed. Only in very close proximity to the fault was greater violence manifested. For blocks the land surface, paved streets, and building plots alike, were thrown into wave forms, trending east and west about parallel to the length of the area. The amplitude and wave-length of these earth billows, and the distances to which they extend, are indefinite and irregular. The fissuring and slumping, and the buckling of block and asphalt pavements into little anticlines and synclines (arches and hollows), accompanied by small open cracks in the earth, characterize the land surface. This slumping movement or flow took place in the direction of the length of the area, and its amount was greatest near the center, or channel, where the street lines were shifted east-ward out of their former straight courses, by amounts varying from 3 to 6 feet. A satisfactory photograph of this phenomenon was not obtainable, owing to the quick convergence of parallel lines in perspective, but to the observer in the field it was a very striking result of the shock. The greater part of the district was occupied by wooden dwellings and shops, with a small percentage of mediocre brick buildings and a few of substantial construction. The fire swept the area clear. Not even heaps of débris remained to cover the ground, most ― 237 ―
of the destructive effects being obliterated, along with the structures in which they were developed. Enough remained, however.
Foundation walls and sidewalk pavements were broken and flexed; sharp little anticlines were produced in the street by the
arching of block paving, as on Russ Street between Folsom and Howard Streets (plate 88C); granite curbing was broken and thrust
up into an inverted V, as on Moss Street, between Folsom and Howard Streets (plate 88D); there were fissuring and slumping
in the block pavement, as along Columbia Street between Folsom and Harrison Streets (plate 89A), and sharp flexures of the
paved streets and car tracks, as on Sixth Street just south of Howard Street. These effects point simply and clearly to the
great magnitude of the intensity thruout the greater part of this old swampy district.
Attention has already been directed to the slumping or flow movement to the east along the long axis of the area. The heavily ballasted car-tracks on Bryant Street, at the crossing with Fourth Street, were sharply flexed laterally, tho bounded by block paving. (Plate 89B.) This was at the eastern end of the district where the marsh formerly bent to the south around the flanks of Rincon Hill, a mass of firm sandstone rising from the floor of Mission Valley. No similar sharp flexures were encountered along east-west streets in the western or central portion of the district, tho lateral displacement and flat, sinuous curvings of the street lines were common enough; notably on Harrison Street between Fifth and Sixth Streets, and on Folsom Street between Fourth and Seventh Streets. Both these streets cut across the direction of the flow movement at a small angle. These phenomena are easy to understand if, as seems certain, Rincon Hill served as a solid buttress against which the flow to the east was arrested, causing sharp crumpling of the surface near the buttress, with less disturbance farther away. This was combined with a slight tendency to flow southward in the southeastern part of the district. The shaking caused the materials used in filling to settle together and occupy less space, so that the surface over the whole district was lowered by amounts varying from a few inches to 3 feet or more. This is clearly seen in the change of street levels along the margin of the solid ground, where the car rails are bent downward in little monoclines. Occasionally a structure with a relatively good foundation remains at its former level, with the whole neighborhood deprest about it. Such a case is exemplified on Sixth Street, a little south of Howard Street, near the margin of the area. (Plate 89C). The flow movement is thought to be due simply to the action of gravity, the loose, water-soaked material being compacted into less volume by the shaking. Besides this sinking of the district, and its flow movement, mention has been made of the deformation of its surface into irregular waves, trending approximately east and west parallel with the length of the district. Along the streets running approximately north and south, at right angles to the elongation of the area, car rails were bent abruptly to the side, or raised in arches, and sharp anticlines were formed in the block pavements. Large square concrete slabs, used for sidewalk paving, were thrust one over the other; and in one or two cases a slab entirely covered an adjoining one. These phenomena indicate shortening by compression in the north-south direction. On the other hand, however, a stretching of the surface is shown by fissures in the paving; by places where wedge-like blocks were deprest below the general level; and by the rails of car tracks which were pulled apart in amounts varying from 8 to 12 inches. Owing to the relatively great and very variable structural strength of paved streets and heavily ballasted car tracks, these phenomena are not developed regularly nor frequently enough to afford a satisfactory test of the hypothesis that they are directly associated with the wave forms into which the surface of this district was thrown. Besides, owing perhaps to the varying rigidity of the materials which make up the surface of the streets and building plots, the wave forms themselves, tho generally prevalent, are not persistent in their extension. The compression and distension ― 238 ―
effects, however, are believed to be due to the same cause as that which generated the wave forms; for there is no evidence
of any true shortening, or lengthening, of the north-south dimension of this district, nor is there any probability of this
having occurred.
In addition, then, to the flow movement and the settling together of the loose materials causing depression, there was some sort of rhythmic movement in this loose earth which produced wave forms in the surface, with places of compression and places of stretching. It probably was this movement which was most effective in producing structural damage. It is not believed that these surface waves were traveling waves "frozen" as the shock subsided. If they had been of that character, the ground surface should be more broken than it appeared to be; for in relatively rigid materials such waves must develop open fissures along the crests, which would close with crushing in the troughs. It must be noted, without any attempt at explanation, that the destructive effects of great magnitude which have been described above, are practically confined to the "made" land which occupies the old marsh site. Southeast of Brannan Street, where formerly lay Mission Bay, such effects are of less magnitude, in general; are less regular in their occurrence and are, on the whole, less prevalent. The complete devastation caused by the fire in this neighborhood leaves little to indicate the actual damage to the buildings wrought by the earthquake. Certain hotels or apartment houses are known to have collapsed, and many fatalities must have occurred. Probably a few dwellings were thrown down. A fairly large percentage of the buildings, one must believe, were rendered dangerous for occupation, even tho not completely thrown down. The new United States Post-office building (plate 94B), at the corner of Seventh and Mission Streets, was just on the margin of the district. It is a steel and granite structure, resting upon a foundation of piling driven to a considerable depth, but not as far as some had considered advisable. At its southwest corner, the streets are deformed into great waves, some with an amplitude of at least 3 feet, causing fissures and sharp compressional arches in the pavement and sidewalks. Some of the granite flanking structures, which did not rest upon the pile foundation of the building, shared this undulatory movement. In consequence, the building appears badly damaged to the casual observer. It is quite true that the structure was terribly shaken and greatly damaged — such injuries as the destruction of mosaics in the arches of the corridor helped to increase the loss — but the structure was not in peril of collapse, tho one of the low walls had to be supported by timbers. For the most part, the building survived the ordeal, and is in a safe condition for use. As stated briefly above, a similar district of high intensity occurs in an area of made land along the lower portion of the former course of Mission Creek. This district varies in width from 1 to 2 blocks, extending from near the corner of Ninth and Brannan Streets westward for about 3 blocks, then southwestward for about 2 blocks more; and finally, westward some 4 blocks more to a point on Nineteenth Street just east of Dolores Street. Mission Creek was formerly a sinuous tidal stream, with narrow fringes of salt marsh about its banks. Near its mouth the stream wound around a rocky point where the serpentine hills of the Potrero rose abruptly from its southern bank. Here, along its margin, is found the most sudden transition from high to low intensity that is anywhere encountered in the city. Along Dore Street, a narrow alley running from Bryant Street to Brannan Street, between Ninth and Tenth Streets, the street pavement was broken into a series of waves. The photographs, plate 89D, looking along Dore Street from Bryant toward Brannan Street; plate 90A, looking from Brannan Street in the reverse direction; and plate 90B, showing in detail the trough of one of these waves, with the fissuring of the pavement near the farther crest, indicate more clearly than words the great intensity manifested here. Less than 2 blocks south on the hill slopes, more than ― 239 ―
50 per cent of the chimneys were left standing, and no serious structural damage was noted. No comment seems needed to establish
clearly the fact that the change in the character of the ground, this being the only variable factor, is in some way the cause
of the change in the degree of intensity.
On Ninth Street, east of Dore Street, between Bryant and Brannan Streets, the block pavement was badly damaged by fissuring, slumping, and the formation of surface waves. Frame dwellings were thrown from their underpinning, and a few collapsed. Plate 91A shows a wave trough near Bryant Street, with the resulting disturbance of the pavement. The dwellings immediately in the trough have dropt from their foundation posts. In plate 91B, looking along Ninth Street from near Brannan Street, is shown the depression and fissuring of the street and its slumping or flow westward toward the former channel of a short branch of Mission Creek, which occupied the present location of Dore Street. Streets, curbing, car tracks, etc., are deflected from 6 to 8 feet from their former positions. The frame dwellings were not destroyed, but a careful examination of the picture will show that most of them are badly injured. Many were left in a dangerous condition by the shock. On Tenth Street, between Bryant and Brannan Streets, less violence was noted and the slumping of flow eastward (toward the channel of the little branch of Mission Creek) is scarcely noticeable. Again, along the creek bed from Folsom Street, between Seventeenth and Eighteenth Streets, to the vicinity of Valencia Street at Eighteenth, great destruction was conspicuously prevalent. Less than a third of the frame dwellings in this tract retained their vertical positions, and a few collapsed completely. Others remained standing only by leaning against each other. The south side of Howard Street, between Seventeenth and Eighteenth Streets, which escaped the fire, furnishes a good illustration of the damage produced here. (See plate 93A.) As in other places, the streets were deprest, fissured, and thrown into waves. (Plate 90C.) Car rails were arched and bent laterally in a violent fashion. (Plate 92B.) Sewers and water-mains were broken. At Eighteenth and Valencia Streets there was a serious break in the water-pipe. Here, on both sides of the street, the ground sank about 6 feet, causing the roadway to arch in a very noticeable way. (Plate 93B.) Ten-inch car rails were bowed up into arches from 24 to 30 inches in height. The Valencia Street Hotel collapsed so that occupants of the fourth story could step out into the street. Casualties in this district can never be known accurately, owing to the immediate onset of the fire, and the complete devastation it produced. On land made by filling in, "The Willows," a marshy tract formerly extending up the Eighteenth Street Valley from Mission Lagoon, near the corner of Nineteenth and Guerrero Streets, there was observed a considerable slumping or flow movement of the surface. The photograph (plate 94A) shows the Youth's Directory, a charitable institution for boys, where the street and building were moved northward and slightly eastward, toward the former channel and downstream, fully 6 feet. Enough evidence has been cited to demonstrate that high intensity prevailed thruout this district. Here, as in the other tract of made land which occupies the site of the old tidal marsh, the materials used for filling were shaken together, and caused a general depression of the surface over the whole district, accompanied by slumping or flow movements. The surface was deformed into waves, with accompanying fissures and sharp compressional arches. Here too, as in the tract previously described, the materials used for filling constitute a relatively thin rigid layer deposited upon the marshy fringes or in the shallow waters of the creek. The creek (see map No. 17) formerly extended for about 2 blocks eastward from Ninth and Brannan Streets before it reached the old shore line of Mission Bay. This portion ― 240 ―
of its course is now occupied in large part by the railway tracks and structures of the Southern Pacific Company; and the
exceptionally strong foundation necessarily provided for the railway line probably explains why less damage was found here
than one would at first have expected. At any rate, the greatest damage noted was the cracking of brick walls and the falling
of cornices.
The space formerly occupied by Mission Bay has been partly filled to provide building sites, and of course the materials used in filling were deposited in water. The district is occupied in part by structures of great strength, such as railway tracks; in part it is devoid of buildings. Thruout the district, evidence was insufficient and inconclusive. Except near the former outlet of Mission Creek, and in the area further north formerly occupied by the tidal marsh, the destruction produced does not denote intensity higher than Grade C. Apparently, therefore, land made by filling up spaces of open water is less dangerous, on the whole, than land made by depositing a thin rigid layer of filling upon a tract of marsh land. This, at least, is the lesson in San Francisco. The reasons for it are not very clear. Space forbids a discussion of theories which can not be adequately tested. It may be noted, however, that much of the material used in filling in areas of water has been broken rock derived from the grading down of neighboring rocky hills. Near the corner of Waller and Portola Streets, not far north of the head of Market Street, is a locality, less than a block in extent, where houses were shifted slightly on their foundations; their upper stories were moved farther eastward (downhill) than the foundations, as a result of shearing in the framework of the basement or of the first story of the buildings. (Plate 90D.) There also occurred minor bucklings and breaking of the thin asphalt pavement. The intensity, which belongs low in the range of Grade B, diminishes rapidly in all directions, and the district is surrounded by a band where the intensity is Grade C. Here a thin layer of sand reposes upon the slopes of a little upland valley between the low serpentine hills to the east and the high chert hills to the west. The effects are such as would be produced by a shaking downhill of this thin sand layer, with the structures which rest upon it. This seems the best explanation of high intensity in this district. Attention, nevertheless, must be directed to the fact that this, and three other districts shown on the map, No. 19, lie roughly along a straight line which nearly coincides with the western boundary of the serpentine body. At its northwest end, this boundary is known to be determined by a fault of considerable throw, constituting consequently a weak place in the crust of the earth here. It is not known how far southeast the fault extends, and it is not unlikely that it cuts entirely across the peninsula. The recurrence of these little districts of comparatively high intensity suggests that it continues as far south as Market Street, at least, and that such a zone of weakness was especially suited to the production of high intensity by the shock. This hypothesis can not be conclusively tested, but it is interesting and important enough to merit presentation and to receive attention in the event of future earthquakes. In support of the statement made in the foregoing pages that the intensity increases markedly as one approaches the fault, independently of the character of the ground and other factors, the following evidence is presented: Forty-eighth Avenue, between K and N Streets, is a district underlain by deep sand where extensive grading operations were undoubtedly necessary to convert an area of sand-dunes into streets and building lots. Here small, substantial frame dwellings were shifted bodily from 1 to 2 feet out of position, and the streets were slightly dislocated. Telegraph poles were thrown down or caused to lean over so much that only the tension of the wires kept them from falling completely, and lamp posts were overthrown. The dwellings suffered little structural damage, owing to their small, substantial character, and to their being built close to the ground; so that when shifted from their underpinning, ― 241 ―
they had but a few inches to fall. Still, it is the opinion of the writer that the intensity developed here was little, if
any, short of the maximum on the made land in the city, tho the conditions were not such as to permit so great damage.
On Ocean Avenue and X Street, near where the former outlet of Lake Merced flowed, fissures were developed in the street and in the sands on either side, and water was squeezed out so as partly to flood the roadway. Drain pipes were unearthed and bent or twisted. From the former outlet of Lake Merced, where W Street meets the Grand Ocean Boulevard, or Great Highway, southward along the ocean, low cliffs of soft rock — the Merced sandstones and shales — rise abruptly from the beach. These mount gradually as we go southward, until at Mussel Rock they attain a height of 500 feet. All along this line of cliffs, and for a short undetermined distance inland, the rock masses were cracked, broken, and traversed by narrow fissures. These effects grow more and more numerous and of greater and greater magnitude until, a short distance north of Mussel Rock, the fault is reached. A short distance south of X Street, a small, substantial frame dwelling, built upon a good foundation under the cliffs by the beach, was almost overturned. South of this there were no structures along the beach except the seaward end of the Lake Merced Tunnel, an hydraulic arch which was slightly broken, tho embedded in the rocks of the Merced formation. All along the faces of these cliffs, much material fell or slid down to the beach. ConclusionsThis investigation has clearly demonstrated that the amount of damage produced by the earthquake of April 18 in different parts of the city and county of San Francisco depended chiefly upon the geological character of the ground. Where the surface was of solid rock, the shock produced little damage; whereas upon made land great violence was manifested. Other things being equal, there was a decrease of intensity from the southwest toward the northeast, as the distance from the zone of faulting increased. Other conditions, however, exerted a controlling influence. There was, for instance, much greater contrast, in the destructive effects produced, between the summit of Telegraph Hill and the vicinity of the Ferry Building, about a quarter of a mile apart and at practically the same distance from the fault, than there was between the damage produced near the Ferry Building and along the trace of the fault itself. (Consult the intensity map and profiles.) In this part of the zone of destruction, change in distance from the fault clearly did not influence the gradation in intensity, so much as did change in the character of the ground. AddendaSubsidence of made land.—The unstable character of the made land on the waterfront of San Francisco has long been known. This instability made itself manifest in a progressive subsidence which, in the course of years, rendered it quite difficult to maintain the grade of the streets. An effort was made by Mr. C. E. Grunsky, when he was city engineer, to determine the rate of this subsidence, and the following extract from his report as city engineer for the year 1902-3 is not without interest in connection with the violent disturbance of the ground caused by the earthquake in the areas of made land:
― 242 ―
Possible premonitory movements (Miss H. C. Lillis). — Mr. McConnell, a jeweler, located on Post Street between Montgomery and Kearney Streets, states that 4 days before the earthquake he found one of his windows broken in nearly 50 pieces tho none of the pieces had fallen out; and supposing that some one had tried to enter his shop, he sent for a detective. Captain Calunden came, and on looking over the premises declared that it was not the work of a burglar but was due to the settling of the building. He found the building out of plumb. This would indicate a settling of the ground before the shock. One of his workmen who lived in the Mission found his cellar door closed so that difficulty was experienced in opening it. This occurred the same day as that on which the glass was broken. Effect of the shock near the beach (W. D. Valentine). — We were residing on Forty-eighth Avenue, between K and L Streets, within a few hundred feet of the ocean, about 0.5 mile south of the park. In our section the shock was violent. It awakened me instantly, and for a few seconds I was unable to rise, as I was thrown back in the effort. Meanwhile I was carefully watching the movements of an extremely tall and heavy oaken wardrobe which stood almost in the middle of the floor. The top first swung to the west, then to the north, then to the east, and fell directly to the south with such force that it went to pieces. Our heavy upright piano and various heavy articles of furniture were thrown completely over. The sand in our basement raised from 1 foot to 18 inches. A wide and long 3-foot depression was raised level. Our lot, which was 120 feet deep, was shortened at least a foot, which was shown by the folding of the fence. Electric-light poles in the street in front of us, which were in the sand, were thrown down north, east, south, and west. There was a fissure for about a block, between Forty-seventh and Forty-eighth Avenues, about 3 feet wide and 6 or 8 inches deep, ― 243 ―
which was of course in the sand. There were also other blow-holes in the sand, which emitted water and sulfurous odors.
Effect of the shock on the gas plant and pipes (E. C. Jones). — The earthquake movement was apparently from north to south, inferred from the fact that bookcases and china closets placed east and west were almost invariably tipt over, or their contents thrown out; while those placed north and south were in most cases undisturbed. Gas-mains in streets running east and west were broken and drawn apart, while those in streets running north and south were crusht together and telescoped, or else raised out of the ground in inverted V's. This rule applied generally, with but few exceptions. On Jackson Street, between Drumm and Davis Streets, which is made land, the street main was laid on a line of piles which went to hard pan. The piles were not purposely driven to sustain the pipe, but happened to be in the line of the main when it was laid. This pipe broke over the center of each pile, 9 in number, and was not broken in the made ground where it was unsupported. During the latter part of the first shock, there was a rotating motion which had the effect of twisting gas-holders out of their guide frames. The foreman of the North Beach Station was looking at the 2,000,000-foot storage holder, and described it as follows:
These two holders were heavily framed with latticed girders, and did not leave their guides by the rotating movement of the earthquake. The storage holder at the Pacific Gas Improvement Company's Works was twisted around 2 feet from the guide rails, while at Martin Station the 1,500,000-foot storage holder was twisted 5 feet on the lower section, 8 feet on the middle section, and 12 feet on the upper section. At this plant the 4,000,000-foot generator was moved bodily 2.5 inches to the south. All connections were of steel, and no joints were broken. A barn at the North Beach Station, corner of Laguna and Bay Streets, was resting upon wooden uprights about 16 inches high. These uprights were tipt over, and the barn moved the length of the uprights toward the south; that is, after the earthquake it stood 16 inches on the sidewalk. The buildings at the different plants did not suffer according to their relative strength. Some brick buildings of comparatively poor construction were unharmed. Other buildings of great strength, with heavy footings on good foundations, were shaken to the ground, particularly those running east and west; while buildings of the same or less strength, with foundations not so good, but running in a direction north and south, were but little injured. Effect on certain street railways (T. Mallally). — There does not seem to have been any actual shortening of the length of the street railways of the United Railroads of San Francisco; but the rails in one location traveled about 3 feet in a northerly direction. This location was in the valley and was marsh land, beginning at a point about 100 yards north of Holy Cross Cemetery, where the rails parted, and ending about 1,000 yards north of Holy Cross, where the rails buckled up in the air. We had to cut out about 3 feet at this point, and add 3 feet where it parted at the other end. Of course there was a decided movement of the rails all along, in a lateral direction, which left the tracks out of alinement, but was not enough to prevent operation of cars. This condition would indicate that the fill in the marsh land moved in a northerly direction about 3 feet, but that the actual distance along our line has not been appreciably changed. ― 244 ―
Deformation of the U. S. Government buildings. — For the purpose of determining the extent of the deformation of the three U. S. Government buildings — the new Post-office, the Appraisers' Building, and the Mint — the Coast and Geodetic Survey, at the request of the Commission, determined on July 12, 1906, the relative levels of the four corners of each of these structures, as indicated in the accompanying notes and figures. The leveling was done by Mr. C. H. Sinclair. The memorandum of Mr. Sinclair's results, which was placed at the disposal of the Commission by the Superintendent of the Coast and Geodetic Survey, is as follows: New Post-office. — Fig. 52 shows by numbers the positions of the stations occupied; and the points at the corners, the relative levels of which were determined, are indicated by their orientation. The southwest corner is the lowest and is the only one that settled materially, being about 0.393 foot = 4.72 inches lower. The outer walls have cracks in many places. This is a fairly good showing for a bad foundation. Sights nearly equal except at II, where backsight is about 100 feet and foresight is about 300 feet. Street so low that readings could not be made any other way. Cars and drays passing all the time. Wind bad. Rod held on circular molding (tower, ½ circle section) which was the lowest projection built into the wall that was common to the four extreme corners.
Appraisers' Building. — Fig. 53 shows by numbers the position of the stations occupied; and the points at the corners are indicated, as before, by their orientation. The northwest corner is 0.909 foot = 10.908 inches above the southwest corner. The northeast corner is 0.909 foot + 0.054 = 0.963 foot = 11.556 inches above the southwest corner. The southeast corner is 0.080 foot = 0.96 inch above the southwest corner. The rod was held on top of water-table at each of the four corners, and the sights were nearly equal in length. The south side of the building is about 11.23 inches lower than the north side. ― 245 ―
Mint. — Fig. 54 shows by number, as in the former cases, the positions of the stations occupied and the points at the corners are indicated by their orientation. The southwest corner is the lowest, being 0.498 foot = 5.976 inches (mean) below the northwest corner. The walls on the north side are badly scaled by the heat. No serious cracks were noticed in the outside. The rod was held on top of the water-table at each extreme corner of the building. Street cars constantly passing on both Mission and Fifth Streets, also heavy drays. The wind was very troublesome. Sights were nearly equal. The deformation indicated by the above measurements can not be wholly referred to the earthquake, since it is quite probable that the structures had settled somewhat before that event. It appears, however, to be desirable to put the measurements on record for future reference.
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