458 University of California PtMications in Geology [Vol. 12 



It is interesting to note that in the vicinity of Arena the elastic 

 fling of the ground on the two sides of the fault was practically the 

 same in amount, and that the total differential displacement, 4.66 

 meters, as determined geodetically, checks closely with the displace- 

 ment of 15.5 feet at the fault measured on the ground by offset 

 fences, etc.^'^ 



In the foregoing discussion of the facts brought out by the geodetic 

 survey of the region north of the Golden Gate I have applied the 

 theory of elastic rebound as rigorously as the facts will permit. The 

 theory calls for a strained condition of the earth's crust as a necessary 

 preliminary to faulting. To account for this strain w^e must abandon 

 the notion that the displacement, found to have occurred between 

 surveys I and II, was a sudden movement which took place at the 

 time of the earthquake in 1868. This notion was tentatively adopted 

 by Hayford and Baldwin and affects many of the conclusions which 

 they drew from the discussion of the geodetic data. The displacement 

 which occurred between surveys I and II is here assumed to be the 

 expression of a northerly strain creep, due to the drag of the crust 

 riding on a suberustal tiow. The amounts of displacement for several 

 controlling stations and the dates of the surveys of these being known, 

 we have the rate of creep due to accumulating strain, and this may 

 be used to find the total slow displacement up to April 18, 1906. The 

 position before the earthquake is thus known of points which, after 

 the earthquake, were located geodetically, and the net displacement 

 at the time of the shock is thereby ascertained. But it is known 

 positively and independently of the geodetic survey that a portion 

 of this net displacement was due to slip on the San Andreas fault and 

 was therefore parallel to it. Another element of the net displacement 

 was apparently due to a transverse shift which carried the fault plane 

 with it, unless the Moeho-Diablo base had moved a corresponding 

 amount. 



EEGION SOUTH OF THE GOLDEN GATE 



The region south of the Golden Gate appears to have suffered a 

 more complicated series of displacements than that to the north. Here 

 we are confronted with evidence of greater variation of movement both 

 as to amount and direction, with a dominance of the southerly com- 

 ponent in the net result. Here, also, we have to reckon wnth the dis- 

 placements which caused the earthquake of 1868 and possibly that of 



i-t Earthquake Report, vol. 1, p. 60. 



