460 University of California Publications in Geology [Vol. 12 



could see the great Avave rapidly approaching. He rushed to the side 

 of the road and had caught hold of the fence by the time the shock 

 broke. Mr. J. A. Graves was in the field a mile or so south of Colma 

 with his father. Looking north they saw first San Bruno Mountain 

 bobbing up and down ; then they saw the effect of the shock on a 

 freight train between them and the mountain, and finally they felt 

 the shock themselves and were thrown by it to the ground.^* 



These two bits of testimony, recording observation on both sides of 

 San Francisco Bay, seem to me to prove that the earth wave was 

 generated to the north of these observers and moved southward. 



In view of all the evidence, including personal testimony, open 

 cracks and geodetic measurements, the most satisfactory hypothesis 

 that can be formulated with regard to the direction of the sudden 

 displacement of the ground in 1868 is that it was southerly. It was 

 doubtless a rebound from elastic strain, and the relief from strain 

 was doubtless effected by slip on a fault. There is no evidence of the 

 outcrop of this fault, and as the region is geologically well known it 

 is fairly certain that it does not emerge at the surface, unless possibly 

 soutli of ]\Ionterey Bay. I am constrained, therefore, to believe that 

 the slip which caused the sudden southerly movement in 1868 took 

 place on a lowly inclined fault deep in the earth's crust. 



We thus have as our working hypothesis for the event of 1868 : a 

 slow sul)crustal flow in a northerly direction, generating a strain in 

 the overriding crust, relief from this strain by rupture and slip on a 

 flat fault, and the southerly rebound of the block above the fault, 

 causing the earthquake. Thus explained the sudden movement of 

 1868 appears to differ from that of 1906 in the fact that it was an 

 expression of the relief of the longitudinal strain, while the later 

 movement was due to sudden relief from transverse strain. 



Summary statement of displacements. — In the region south of the 

 Golden Gate, the displacements which were measiired geodetically 

 between 185-1 and 1906 are, by this hypothesis, the resultant of the 

 following separate and distinct movements : 



(1) A northerly strain creep persistent throughout the entire 

 period covered by the surveys. 



(2) A southerly elastic rebound from the longitudinal strain which 

 had developed up to 1868. 



(3) A southerly elastic rebound from the transverse strain by 

 reason of slip on the San Andreas fault in 1906. 



IT Op. cit., p. 443. 

 18 Op. cit., p. 445. 



