304 



REID— SEISMOLOGICAL NOTES. 



[April 24, 



time of the slip, the same movement took place in the neighborhood 

 of this line, as near AC, and A'C was broken into two parts, A'E' 

 and D'C ; the total slip, D'E', being equal to DE. A third experi- 

 ment was tried ; the left piece of wood was shifted i cm. and a 

 straight line was drawn across it; it was then shifted a half centi- 

 meter more and the straight line took the position A"C" in Fig. 3. 

 When the jelly slipped along the surface, ft', the line broke into the 



t 

 Fig. I. 



Fig. 2. 



Fig. 3. 



two parts, A"E" and D"C" ; the slip, D"E'\ being equal to the total 

 displacement of the left side. Two characteristics of the movement 

 are to be noted ; the total slip on the ruptured surface equalled the 

 total relative displacement of the blocks of wood ; and, at the time 

 of the slip the blocks remain stationary, and the whole movement 

 at that ti'ne was an elastic rebound of the jelly to a condition of no 

 strain. 



These experiments illustrate as well as simple experiments could 

 what occurred at the time of the California earthquake of April 18, 

 1906. Fortunately, early surveys had been made of this region 

 which Dr. Hay ford, in the report of the California Earthquake Com- 

 mission has, for the sake of discussion, divided into two groups ; I., 

 the surveys made from 1851-65; II., those from 1874-92. A third 

 survey (III.) was made after the earthquake in 1906-7. These 

 surveys extended from Mt. Diablo, about 33 miles east of the fau't, 

 to Farallon Light House, about 22 miles west of it. They showed 

 that between the I. and II. surveys Farallon Light House had shifted 

 relatively to Mt. Diablo, 4.6 feet north-northwest, practically in a 

 direction parallel with the fault-line; and between II. and III. sur- 



