440 



University of California Publications in Geology 



[Vol. 12 



4. The movement of 1868 can be explained consistently on the 

 hypothesis of a single northerly current; and the supposed distension 

 of the region which formed the basis of the discussions of Rothpletz 

 and Wood becomes unreal. 



POSSIBILITIES OF TEANSVERSB SHIFT 



There being by assumption no sudden movement in 1868, the dis- 

 placement between surveys I and II in the region north of the Golden 

 Gate must be the expression of strain creep ; and, the dates being 

 known, we have the rate of strain creep. Thus, Tamalpais between 

 1854 and 1882 was displaced 1.64 meters in the direction 168° by strain 



1.64 



creep the rate of which was therefore apparently — = .058 meters 



per year. Similarly Chaparral was displaced 1.83 meters in the 

 direction 173° between 1856 and 1891, which is at the rate of .052 

 meters per year. For Ross Mountain the rate of strain creep is in the 

 same way found to be .053 meters per year in the direction 182°. 

 Using this rate we may determine the position of geodetic stations just 

 prior to the earthquake of 1906, and so arrive at the true amount and 

 direction of displacement by slip on the San Andreas fault in that 

 year. This method, however, involves the further assumption that the 

 straight line connecting the positions of a given station, as determined 

 by surveys I and II, is the direction of strain creep. But this is not 

 necessarily the case, as will appear from the following considerations. 

 The stations we are concerned with are all in a region of accumulating 

 strain, and the strain was relieved by a slip on the San Andreas fault. 

 Now faults are things with which geologists are familiar ; and in many 

 of the exposures of faults which they have occasion to examine in 

 mines, etc., the phenomena known as "drag" are well displayed. The 

 term drag is, however, comprehensive of two distinct phenomena, 

 although the distinction is not always kept clearly in mind. These 

 are : (1) the bending of the rocks in the zone of shear prior to the slip 

 on the fault, and (2) the smearing out in the fault plane of the 

 products of attrition at the time of the slip on the fault. Both effects 

 are of importance in the study of the San Andreas fault and the 

 movements connected therewith. The bending in the shear zone of a 

 fault is best exemplified in stratified or laminated rocks, particularly 

 where the strata are thin and strong layers alternate with weak. It 

 is, as commonly observed, a plastic deformation ; and the structure 



