424 



University of California Publications. 



[Geology 



with the distance from the fracture, but never became less than 

 the displacement. A' A", of the left block after the line A'C was 

 drawn. 



The last experiment illustrates, as well as a simple experiment 

 can, what occurred at the time of the California earthquake; 

 the sudden fling of the rock when the rock fractured along the 

 San Andreas fault was due to the elastic forces set up in it by 

 an earlier relative displacement of the regions on opposite sides 

 of the fault, just as the fling of the jelly was due to the elastic 

 forces set up by the relative shifting of the wooden blocks. As 

 already mentioned, observations in the field showed that at the 

 time of the earthquake there was a relative movement of the 

 two sides, at the fault-surface, amounting to about 21 feet. The 

 surveys show that the actual displacements which took place 

 between surveys II and III diminished as the distance from the 

 fault became greater ; on the east side the displacement prac- 

 tically died out at a distance of six or ten miles from the fault, 

 and on the west side the displacement apparently became equal 

 to that of the Farallon Light House at about the same distance. 

 All the phenomena are in close accord with the last experiment 

 described above. The main difference consists in the fact that 

 a straight line across the fault on the earth's surface did not 

 break up into two straight lines, as in the experiment, but into 

 two curved lines. We ascribe this curvature to the fact that 

 the forces which produced the displacement of the ground were 

 applied below the crust of the earth, whereas in the experiment 

 they were applied to the outer boundary of the jelly. 



The elastic rebound near the fault-surface, of course, took 

 place suddenly at the time of the earthquake ; between surveys 

 I and II, and between II and III, there were relative shifts of 

 very extensive regions, the fault-line being the line of separation 

 between them for the second interval ; but the surveys do not 

 determine whether these shifts took place suddenly at the times 

 of the great earthquakes of 1868 and 1906, or whether they were 

 the effect of a slow, gradual movement continuing through the 

 years. The experiments we have described might have been 

 varied, and instead of a slow displacement of the block, grad- 

 ually setting up an elastic shear, we might have set up the shear 



