Popular Science Monthly/Volume 69/July 1906/The Geological Prelude to the San Francisco Earthquake
|THE GEOLOGICAL PRELUDE TO THE SAN FRANCISCO EARTHQUAKE|
BY GEO. H. ASHLEY
WASHINGTON, D. C.
SINCE the San Francisco earthquake, the reading and scientific public has become acquainted with the fact, if not already known, that the recent disaster was the result, not of volcanic activity, but of the activity of the ordinary mountain-making forces. In a large measure they have become acquainted with the further facts that mountain-making forces have long been, and still are, active in the immediate region about San Francisco; that as a result of these activities the rocks of the regions are folded and faulted; that the faulting is of major importance; that the recent disturbance is ascribed by the geologists to movements of adjustment along one or more of these fault planes; that investigation after the earthquake along some of these lines gave abundant evidence of differential movement visibly affecting the surface.
These facts, now widely known, start questions along several lines of inquiry. One of these lines, having an important bearing on the probabilities of future trouble, involves the geologic evidence as to the recency of the observed earth movements; the relative value of this last displacement as compared with past displacements, both remote and near; the character and amount of geologically recent movements; in short, as given in this paper, a résumé of the recent geologic history of the San Francisco peninsula and the observed evidence upon which the statement of that history is based.
As our interest increases with the recency of the events the earlier history will be passed over rapidly and increasing attention given to the later events.
California in Mesozoic time was the theater of profound geologic activity—the movements of subsidence, the vast volume of sedimentation, the intrusion of great sheets of igneous rocks, and the final folding, crushing and faulting were possibly not exceeded anywhere in the world during that period. In Tertiary time the same notable activity continued. The last expression of that activity in the immediate neighborhood of San Francisco consisted of a subsidence beginning apparently just at or before the end of Miocene time and continuing probably a little over into the Quaternary. Coincident with this subsidence was sedimentation that locally resulted in the laying down of over 4,700 feet of sediments. A remnant of these deposits, known as the Merced series, stretches from the city limits of San Francisco southward along the coast for-i miles, beautifully exposed in bare cliffs 700 feet high. A study of the geology shows that this fragment lies between two fault planes, beyond which no trace of it is found for many miles. Calculation shows that on either side of this depressed block there has been elevation during Quaternary time of about 11⁄2 miles with pari passu removal of the Merced series and deep erosion of the underlying basement. The time interval is best measured by the erosion of the mile of Merced sediments. As exposed to-day upturned along Seven-mile Beach, these consist mainly
Fig. 2. Section of Seven-mile Beach and Beyond; from mouth of Lake Merced, showing position and exposure of Merced series of rocks. This remnant of this series lies between the San Bruno fault plane at the north and the San Andreas fault at the south. Its entire absence from the San Bruno Mountains to the north and Mount Montana to the south is part of the basis for the theory that since its deposition there has been uplift along the two faults which lifted the outside of these faults nearly or quite a mile and one half above sea-level, and that erosion, not only planed down the folded rocks of this block, but entirely removed the Merced rocks either side of this block.
Fig. 3. Figure showing Position of Recent Marine Deposits above Seven-mile Beach and the relative movement along the two fault planes.
Since then two events are clearly shown in the records; these are subsidence and differential uplifts. The subsidence carried most, if not all, of the San Francisco peninsula below its present elevation, flooding the valleys and leaving the hills of the city largely an archipelago. Marine deposits were laid down on top of the clearly recognizable sand dunes and wash deposits, containing in many places the trees mentioned above. Following that came local uplift, raising these marine deposits to elevations of over 700 feet above sea-level just
Fig. 4. Mussel Rock from the South, showing marine Pleistocene (a) overlying sand dunes (c) and igneous rocks (b).
about San Francisco, and double that farther down the coast. These last movements were differential, and it is quite possible that in some cases the two took place together. In the same way, it is quite possible that while in parts of the area the evidence suggests elevation continuing at present, elsewhere subsidence is in progress, though not so evident. The point of special interest in this connection is the fact that this differential movement about San Francisco is clearly a movement of the fault blocks and reveals slipping along these planes of faulting of hundreds of feet in times so recent as to suggest that much of it may have taken place since the human occupation.
Considering first the evidence of movement, a good example is furnished by the San Andreas fault, where it reaches the ocean at Mussel Rock. At the foot of the parallel San Bruno fault a few miles to the northeast the recent marine deposits are about at sea-level. Coming south along the sea-cliff or southwest, these deposits are clearly seen rising, making distinct benches in the little stream valleys where they overlie the wind and wash deposits of the preceding stage, these wash deposits in places being full of half exposed spruce logs, which the neighboring inhabitants use for firewood. Before the San Andreas fault is reached, these deposits have risen to over 700 feet above sea-level, lying on the truncated upturned edges of the Merced strata. As soon as the trace of the San Andreas fault has been crossed, the top of these sediments is found at only 220 feet above sea-level, whence it descends until within a short distance it reaches the level of the beach. Here is therefore clear evidence of movement along the fault plane of several hundred feet since this last submergence. The question of most interest then becomes: How recent was this last uplift?
Farther south on the ocean side of the San Francisco peninsula the coast is fringed with a raised beach, such as occur so abundantly along much of the California coast. The top of this beach is 75 to 100 feet above sea-level. At Purisima, the creek of that name runs out across this raised beach through a slightly cut channel to the very edge of the plane where it drops as a water-fall directly into the surf. As the rocks through which it would have to cut to reach sea-level are the only partly consolidated Merced series, there is striking evidence of the recency of the raising of the beach. Again just south of San Francisco are many places in which the recently raised deposits and in slighter degree the underlying Merced rocks have been trenched to depths of as much as 75 feet since the production of the Coast and Geodetic Survey map of this region in 1869. The fact that so large a part of these recent deposits still remains in view of this rapid erosion impresses one in the field most profoundly.
There is another line of evidence to which the writer refers with some hesitation. Indian shell mounds abound along the California coast. In many cases these now occur spread out in thin sheets, apparently forming the surface layer of the raised beaches over such large areas that in his first study of them he was deceived and considered them in many cases as marine deposits. As he remembers them now, he can not help thinking that in many cases they have been reworked by water before the final uplift. In one case 'the writer found an Indian skeleton, evidently formally buried, half exposed in the side of a stream channel so narrow as to force one to the conclusion that the channel has been entirely cut since the burial of the body. This is only a fraction of the evidence that in the field leads one to consider this last uplift as a thing of yesterday, and in all probability of to-day also, or, in other words, that these differential uplifts are still in progress.
A final question of maximum interest is: Is there physiographic or
Fig. 6. A Portion of the U. S. Geological Survey Map from Mussel Rock southeastward nearly to San Andreas Lake, showing topographic environment of a chain of ponds and undrained basins believed to be due to recent earthquake movements.
other evidence that there has been sudden slipping along these fault lines in very recent years, comparable in importance with movements of two months ago, especially as expressed in fault scarps? According to the preliminary report of the earthquake committee, the rupture of April 18 shows a horizontal displacement averaging 10 feet, and a vertical displacement not to exceed four feet. How long such a surface disturbance can be subsequently recognized is a question. The San Andreas fault belt is well situated for the noting of any such displacement for several miles southeast of Mussel rock, in many cases the actual fault planes emerging in sublevel pasture land at the surface. Neither the writer's notes nor his memory now yield any evidence of such a scarp. On the other hand, the fact that where such evidence might be seen has long been subject to the tramping of cattle renders its absence of less value. In this connection he does not give any value to a small scarp noted just back of Mussel Rock. At the time, it was considered to be a land slip. A photograph taken of it suggests the possibility of its having a deeper meaning.
But if such scarps are lacking, there is abundant evidence of another kind bearing on this subject. I have spoken of the San Andreas fault belt. Such it appears to be rather than a single clearly-defined break. Along this belt between San Andreas lake and where the belt meets the ocean at Mussel Rock is a string of drainless depressions occupied with water part or all of the year. In one or two cases these can be clearly seen to lie directly in one of the lines of faulting. That they are the result of fault movements seems highly probable. When were they made? That they were made within the last few centuries can not be asserted, yet the fact that so many of these shallow basins still exist, neither filled nor drained, notwithstanding that in many cases it is but a stone's throw to the head of a drain with a high gradient, suggests such a possibility. The possible cause of these basins is suggested in what appears to have formerly been one, now trenched from two directions at the head of Wood's Gulch, a small ravine cutting the cliffs of Seven Mile beach, a mile north of Mussel Rock. The ravine follows a fault with downthrow of 800 feet. At the head a cirque-like cut exposed an overhanging fault scarp of 100 feet or more. Against this face there appears to have gradually filled in wash from the adjacent hills, wind-blown sands and detached fragments from the fault face, until the whole thing was buried and later covered with the marine deposits of the last submergence. Judged from what is left of this filling, it must at one time have strongly resembled the undrained basins just described. The evidence suggests that this fault scarp was produced by a single movement. An elephas tusk found about
75 feet from the top of the filled in deposit agrees with the other evidence in placing the time of this movement back to the land period preceding the recent submergence.
Summing up, the evidence seems to warrant the following statements:
1. Since the beginning of Quaternary time there have been differential movements of uplift along fault planes on the San Francisco peninsula amounting in several cases to nearly or quite a mile and one half.
2. That these movements have been followed by subsidence and subsequent uplifts, involving sliding of several hundred feet along the older fault planes.
3. That these last movements are geologically of extremely recent date, that possibly a considerable part of these movements have come since the occupation by the Indians, and probably have continued to the present.
4. That some of the movements along the fault planes, geologically, in fairly recent times, seem to have produced more striking physiographic results than any produced the past spring.
5. That while the recent movement may have relieved the stress which the rocks were under to such an extent that it will be many years, or possibly centuries, before another such a disturbance will take place, on the other hand, a comparison of the few feet of motion in April with the hundreds of feet of movement that have taken place in very recent time suggests that fault adjustments of equal or greater violence are liable to occur at any time in the future. And since similar conditions are known to occur all over the Pacific Coast region, no place in that whole district can claim immunity.