I909-] REID— SEISMOLOGICAL NOTES. 305 



;i 

 veys it had shifted 5.8 feet more in nearly the same direction, making 



a total shift in about 50 years of IO-.4 feet. 



Observations in the field on the offsets of fences and roads 

 showed that at the time of the earthquake there was a relative move- 

 ment of the two sides at the fault-surface, amounting to something 

 like 20 feet, and it is only reasonable to suppose that this movement 

 was equally divided between the opposite sides of the fault. The 

 surveys show that the actual displacement which took place between 

 II. and III. diminished as the distance from the fault became 

 greater; on the east side the displacement practically died out at a 

 distance of four or five miles from the fault, and on the west side the 

 displacement became equal to that of Farallon Light House at about 

 the same distance from the fault. All the phenomena were in close 

 accord with the experiments described above. The main difference 

 consists in the fact that a straight line on the earth's surface across 

 the fault and at right angles to it 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 dis- 

 placement of the ground were applied below the crust of the earth, 

 whereas in the experiment they were applied at the outer boundary 

 of the jelly. 



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

 suddenly at the time of the earthquake ; and the surveys show that 

 between I. and II., and between II. and III. there was a relative 

 shift of very extensive regions on opposite sides of the fault, but 

 the surveys do not determine whether these shifts took place sud- 

 denly at the times of the great earthquakes of 1868 and 1906, or 

 whether they were the effect of a slow, gradual movement con- 

 tinuing through the years. We must turn to other considerations 

 to decide this point. In the experiments we have described the 

 elastic rebound was greatest at the ruptured surface, became 

 progressively less at greater distances from this surface, and the 

 jelly in contact with the wooden blocks did not partake of the 

 movement at all. The experiments might have been varied and 

 instead of a slow shift of the block gradually setting up an elastic 

 shear, we might have set up the shear suddenly; but this was not 



PROC. AMER. PHIL. SOC. XI.VIII. I92 U, PRINTED SEPTEMBER 7, I909. ' 



