The California earthquake of April 18, 1906 : report of the State Earthquake Investigation Commission, in two volumes and atlas.

Review of Salient Features

The differential displacement of the earth's crust effected by the movement on the San Andreas fault on April 18, 1906, may for convenience be resolved into two components, the horizontal and the vertical. Of these the horizontal movement was the more important and was susceptible of measurement, giving minimum values for the amount of displacement in this direction practically all along the trace of the fault, except at the extreme north and extreme south. The vertical movement was small compared with the horizontal, and was established satisfactorily only in the region to the north of the Golden Gate.

Two kinds of evidence of vertical displacement were available. The first of these was the formation of scarps along the fault-trace, and the second was the change on portions of the coast of the level of the land relatively to sea-level. The scarps that appeared as features of the fault-trace were in part fresh facets where none had existed before the earthquake and in part accentuations or additions to old scarps due to former movements. In both cases exact measurements were rendered difficult by the drag of the soil along the rupture, and by the complication due to the larger horizontal movement. But making all allowances for the masking effect of drag of the soil, it is certain that the height of these scarps, or of the additions to old ones, was quite variable, even in the same general locality, within a range of a few inches up to about 3 feet. It is suggested that this variation is referable in considerable measure to the drag and adjustment of materials in the zone beneath the soil; so that the true displacement of the firm rocks lies between the extremes observed.

The evidence of vertical displacement, based on the recognition of scarps, indicates a slight upward movement of the crustal block on the southwest side of the fault in the northern territory. South of the Golden Gate there is no very satisfactory or consistent

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evidence of differential vertical movement. For many segments of the fault-trace in this region, there is no suggestion of displacement of this kind. In other portions, notably in the vicinity of Black Mountain and southward, the movement appears to have been distributed over a considerable zone, with the formation of many auxiliary cracks. Upon the latter scarps were formed, but these in some cases faced the northeast and in others the southwest, and the resultant effect is not known. Judging from the localities where the movement was not so distributed, but was confined to a narrow zone, the differential vertical displacement was nil.

Similarly, the evidence of vertical displacement, based on a comparison of the relative position of land and sea-levels before and after the earthquake, is limited to the region north of the Golden Gate. The Point Reyes Peninsula appears, from this class of evidence, to have been probably upraised slightly by the fault movement; but the evidence is not entirely conclusive.

Observations conducted by the Coast and Geodetic Survey thruout the year succeeding the earthquake, at the tide-gage station near Fort Point in the Golden Gate, show that the relative level of land and sea at that point is the same as it was before the earthquake. Since this station lies on the northeast side of the fault, the observation would appear to indicate that any upward movement of the crustal block on the southwest side was an absolute one.

The horizontal displacement on the fault, as measured on fences, roads, and various structures which crost the fault-trace, is also apparently quite variable, ranging from a foot or less up to 20 or 21 feet. This variation is probably due to a number of causes. The principal one of these is the fact that the displacement was not always confined to the sharp line upon which an offset was observed at any locality. Auxiliary cracks, distributed over a zone not uncommonly a few hundred feet wide, took up portions of the displacement; and these auxiliary cracks doubtless escaped observation in many cases. Indeed, owing to the yielding character of the superficial mantle of soil and regolith, it is probable that many of these auxiliary cracks did not appear as ruptures at the surface. Besides this distribution of the displacement on auxiliary cracks satellitic to the main rupture, the deformation of the ground along the latter, both superficially and in its deeper portions, was probably variable. The extent of this drag is shown in a few instances that have been susceptible of measurement; notably the fence at Fort Ross, surveyed by Mr. E. S. Larsen, on which a displacement of 12 feet was distributed over a distance of 415 feet on the southwest side of the fault-trace; the roadway near Point Reyes Station, where a displacement of 20 or 21 feet was distributed over 60 feet; the fence south of Mussel Rock, surveyed by Mr. H. O. Wood, in which a displacement of 13 feet was distributed over 250 feet on the southwest side of the fault-trace and 40 feet on the northeast side; the 3 fences surveyed by Mr. R. B. Symington near San Andreas Lake, one showing a displacement of 16.9 feet, distributed over more than 1,100 feet, the second a displacement of 10.4 feet distributed over more than 300 feet, and the third a displacement of 12.7 feet distributed over more than 2,200 feet; and the tunnel at Wright, surveyed by the engineers of the Southern Pacific Company, showing a displacement of 5 feet distributed over nearly a mile on the southwest side of the fault-trace. These instances are doubtless indicative of the general character of the deformation of the ground in the immediate vicinity of the fault, and aid in understanding the variable expression of the amount of offset at the main fault-trace. The recognition of the distribution of the movement on auxiliary cracks, some of which may not have appeared at the surface, and the deformation of the ground along the zone of rupture, justifies the conclusion that, except under peculiar conditions — such, for example, as in the marsh at the head of Tomales Bay — the maximum figures obtained for the displacement by the measurement of offsets at the surface must be a minimum expression for the true extent of the

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movement in the firm rocks below. For the middle half of the extent of the fault-trace from Point Arena to Crystal Springs Lake, these maximal measurements are very commonly from 15 to 16 feet, and these figures may thus be taken as a minimum expression for the amount of the displacement on the fault for this segment. In the southern quarter of the extent of the fault-trace, the maximum offset is about 8 feet, and this may similarly be taken as a general minimum expression for the displacement on this segment, except for the extreme south end, where it dies out. The amount of displacement at the northern end of the fault has not been ascertained.

The geodetic measurements of the earth movement, as presented in the paper by Messrs. Hayford and Baldwin, are of extreme interest and form one of the most important contributions to the study of the earthquake. The evidence of displacement observed along the fault-trace affords measurements of the total relative movement only, while the geodetic work gives us an approximate measure of the absolute movement on either side of the fault, and the distribution of the movement away from the fault. The results of this geodetic work are not only set forth in detail by the paper of Messrs. Hayford and Baldwin, but they are also admirably summarized, so that all that seems necessary in this place is to discuss very briefly these results from a geological point of view.

A notable feature of the paper is the discovery of a movement of the earth's crust which antedates the earthquake of April 18, 1906, and which is referred to the earthquake of 1868; altho it is recognized that the date and duration of the movement cannot, on the data available, be positively determined. Inasmuch as the time of this movement is left an open question, and is referred to the year 1868 largely as a matter of convenience in discussion, it may be of advantage to inquire briefly whether or not it may have some other significance than that of a sudden movement occurring in that year.

Altho, as shown in another part of this report, the earthquake of 1868 was related to a rupture or series of ruptures of the ground at the base of the hills on the northeast side of San Francisco Bay, there was no evidence of a large relative displacement such as occurred in 1906. It seems reasonable to suppose that if the earlier movement in question had occurred suddenly in the same way as that of April 18, 1906, we should have had a similar manifestation of faulting within the region affected. Since there was no such manifestation the reference of the earlier movement to the earthquake of 1868 may be fairly questioned, and another hypothesis entertained to explain it, particularly if this hypothesis harmonizes in some considerable measure with the results of the geodetic survey.

This hypothesis is that the earlier movement is not immediately or exclusively associated with the earthquake of 1868, but is the expression of the strain in the earth's crust which led to the rupture or slip of 1906 and the consequent earthquake. That rupture presupposes a condition of strain, and it is difficult if not impossible to conceive of such a sudden disruption except as a relief from strain. Such strain involves the idea of slow displacement; and if a series of points had been established in the territory affected at different dates, with reference to some base beyond it, a measure of this slow displacement or creep of the earth's crust might have been obtained.

The strain culminated in a slip on an old rupture plane and may fairly be supposed to have been more or less symmetrically distributed with reference to that plane, so that when relief was effected by slip, the movement involved would be equal in amount on the two sides of the fault.

This hypothesis and its implications appear to fit fairly well with the results of the geodetic resurvey, particularly for that portion of the territory where the earlier movement can be most satisfactorily discriminated from the displacement of 1906. For example in the Tomales Bay region there are ten points, viz.: Bodega Head, Tomales Point, Tomales Bay, Foster, and Point Reyes Hill on the west side of the fault of 1906, and Bodega, Smith, Mershon, Hans, and Hammond on the east, at which the two movements

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are separated. These stations are found to have moved in a nearly north direction an average amount of 1.56 meters in the interval between "before 1868" (1856-1860) and "after 1868" (1874-1891). Since the values upon which this average is based were arrived at in part by methods of interpolation, there is no great variation from the average at any of the ten stations. The interval within which this northerly movement took place is rather indeterminate, but may be placed doubtfully at 32 years.

Under the hypothesis here presented this movement continued at a probably uniform rate for the next 16 years up to the time of the earthquake of 1906. This would give us a total northerly movement for the interval from 1856-1860 to 1906 of 2.34 meters. Now the northerly component of the combined earlier and 1906 movements, shown in table 3 of Hayford and Baldwin's paper, is on an average 4.95 meters for the five stations west of the fault-line. This includes the sudden movement of 1906 plus the slow creep of 2.34 meters above deduced. The value of the northerly component of the sudden movement of those points in 1906 is thus 4.95 - 2.34, or 2.61 meters. Similarly the southerly component of the combined movements for the five stations to the east of the fault is found to be on the average 0.09 meters. The southerly component of the sudden movement of 1906 was therefore 0.09 + 2.34, or 2.43 meters. The absolute movement on the two sides of the fault on April 18, 1906, was thus nearly the same in amount.

The reference of the earlier movement to a slow creep thus appears to harmonize with and therefore tends to confirm the a priori assumption that the absolute movement of 1906 should have been the same on both sides of the fault. Were data available as to the time at which other groups of stations were determined in position, it is probable that a similar result would be reached. We may consider, therefore, that the earlier movement is better explained on the hypothesis of slow creep, continuing up to April 18, 1906, than on the assumption that it occurred at or about the time of the earthquake of 1868. This conclusion applies to the region north of San Francisco Bay. To the south of the Bay the data available are inadequate for a satisfactory separation of the two movements, except in the case of Loma Prieta, and here the earlier movement appears to have been southerly.

Another result of the geodetic resurvey which points to a slow creep of the region under strain precedent to April 18, 1906, is the distribution of the displacement on that date. The measurements of the absolute displacement on the two sides of the fault show that it was notably greater near the fault than at points remote from it. Thus if we imagine a series of points in a straight line transverse to the fault before the earthquake that line was so deformed that the segment to the west of the fault curved northerly and the segment to the east curved southerly in approaching the fault-trace. This deformation can be most readily explained by supposing that the series of points upon the assumed straight line were determined as to position in the first instance upon the surface of a portion of the earth under elastic strain, so that when relief was effected by rupture, the points tended to assume positions relative to one another which they would have had if they had been determined before the advent of the strain.

It may be further pointed out that the conclusion reached by Hayford and Baldwin to the effect that the absolute movement on the west side of the fault was on the average twice as great as the movement on the east side is founded on the assumption of the stability of the base-line Diablo-Mocho. In view of the unknown extent of the earth movement of April 18, 1906, it would seem preferable to make the assumption that the relief from strain was approximately distributed equally on the two sides of the fault and from this infer the amount of the southeasterly displacement of Diablo and Mocho. The assumption that Diablo and Mocho were not affected by the disturbance of April 18, 1906, is based on the following considerations:

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1. There was no change in the azimuth of the Diablo-Mocho line.

2. There was no change in the length of that line.

3. There was no appreciable change in the relations of these two stations to certain others nearer the fault.

4. The latitude of Ukiah remains the same as before the earthquake.

The first three of these conditions would be fulfilled if the region including all the stations occupied had moved in unison southeasterly with but little or no rotation, a possibility which it is difficult to deny. The fourth consideration does not preclude this possibility since the amount of movement involved is probably less than the errors of the method used for the determination of the latitude of Ukiah.

In the region about Monterey Bay the most interesting fact brought out by the geodetic resurvey is that the combined effect of the earlier movement and that of 1906 is a southerly migration of the earth's crust on both sides of the San Andreas rift. It is probable from direct observations of relative displacement along the fault-trace in 1906 that the southwesterly block moved northwest as far as the rupture extended. If this be accepted, then the southerly net movement on the west side of the south end of the fault is due to the predominance of an earlier southerly movement. This agrees with the positive and certain earlier displacement of Loma Prieta. Accepting the southerly character of this earlier movement as certain, there is forced upon us the remarkable fact that the direction of displacement in the region about Monterey Bay is the reverse of that of the earlier movement for the region north of San Francisco Bay. This means that the earlier movement was distensive in character, displacing the territory to the north of San Francisco Bay northerly, and that to the south southerly while the vicinity of the Bay itself was relatively neutral. It appears, moreover, that the southerly displacement was differentially diffused, since the amount of displacement of the south side of Monterey Bay was notably greater than that of the north side, resulting in a widening of the Bay by about 10 feet.

Similarly the distance between Tamalpais and Black Mountain, both on the same side of the San Andreas rift, has been increased by a like amount. The significance of this general distension involved in the reversal of the direction of displacement to the north and south of San Francisco Bay, and of the differential character of this distension, without known rupture, at Monterey Bay and San Francisco Bay, can not at present be stated. The problem requires prolonged study and repeated measurements to secure the necessary data for a proper discussion. It is evident, however, that we are here confronted with some of the most interesting phenomena in the mechanics of the earth's crust, phenomena which call for deliberate investigation extending through years and decades and conducted on a wisely planned program.