26 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
ent distances below the surface which were straight and at right angles to the fault when 
the rock was unstrained became distorted in different degrees, the distortion from the 
surface downwards being somewhat as shown in fig. 14, where the three lines illustrate, 
in an exaggerated way, how the distortion of straight 
3 lines varies from the surface (1) to the bottom (8). 
Both the shearing strain and the strength of the rock 
, increase with the depth, but the rate of neither is 
known; the depth at which the rupture first occurs is 
the depth at which the shearing strain becomes too 
great for the rock to withstand. It is pretty certain that this would not be very near 
the surface, and also that it would not be at the lowest part of the subsequent fault, but 
somewhere between those two points; for, wherever the rupture began, the strain must 
have been increased on all sides, the fracture must have been extended downwards as 
well as in other directions, until the strain was generally relieved. The determination, 
by time observations, of the origins of the earliest disturbance and of the beginning of 
the heavy shock place them between the surface and a depth of 40 km. (25 miles). 

Fig. 14. 
THE DISTRIBUTION OF THE SLOW DISPLACEMENTS. 
We have no information regarding the absolute displacements of the land at a distance 
from the fault-line; we merely know that relative displacements occurred between the 
surveys of 1851-1865 and 1874-1892; and also between 1874-1892 and 1906-1907. We 
have for the sake of simplicity assumed that the regions at a distance from the fault and 



Fra. 15. 
on opposite sides experienced nearly equal and opposite absolute displacements; but this 
is entirely unnecessary. It is possible, indeed probable, that the region on one side of the 
fault and at a short distance from it remained stationary, and that the slow displacements 
were all in one direction. The fact that the eastern side was above, and the western side 
below the sea-level, does not in the least indicate which side remained stationary ; but the 
constancy in length and direction of the line from Mount Diablo to Mocho suggests that 
the eastern side was not displaced ; for it seems improbable that, if this side had moved, 



Fic. 16. 
the displacements would have been so nearly alike at the points mentioned that no change 
could be detected in the line joining them. Under this assumption our curve of displace- 
ments takes the form of the full line in fig. 15 instead of that in fig. 12. The curvature 
of this line between A and C is the same as in the former case; to the east of C the line is 
straight, and at some point to the west of A it again reaches its unstrained position. The 
total shearing force (represented by the broken line in fig. 15) has practically the same 
values as in the former case, except that it dies out near C’; and the applied forces per 
unit area (full line in fig. 16) do not differ materially from the former case except that 
they do not extend farther east than C. 
