1921] Latvson: The Mobility of the Coast Ranges of California 465 



Gate. If this be so, then there must be a locus in the vicinity of the 

 Golden Gate, transverse to the direction of that movement, on which 

 the movement was zero. If as I have already suggested, the earthquake 

 of 1868 was caused by rebound on a flat fault, then the measure of 

 that rebound, being the relief from elastic compression, would be 

 progressively and regularly greater from north to south. If this be 

 so then we may locate approximately the zero point of the slip on the 

 flat fault. For we can take the measures of the rebound at Loma 

 Prieta and Red Hill and therefrom construct a gradient, the rebound 

 being proportional to the southerly departure of the stations from the 

 zero point of slip. Thus in figure 14 let AB represent the distance 



B' 













4. 







x-—^ 









Zero . — 



- — TT54ni| 

 1 



1 .68m 





2.67m 





.57m 



D A C B 



50 miles 



Fig. 14. Gradient of displacement by elastic rebound in 1868 between San 

 Bruno Mountain and Loma Prieta. 



between Red Hill and Loma Prieta, and make AA' and BB' propor- 

 tional to the sudden shifts of these stations in 1868. Then the line 

 A'B' affords the measure of the same shift for any other station to the 

 north of Loma Prieta. The zero point of shift is thus found to be at 

 San Bruno Mountain, fifty miles from Loma Prieta. The shift of 

 1868 at Black Mountain, CC", was 2.67 meters, and at Pulgas West 

 Base, BD' , was 1.54 meters. The shift at Sierra Morena was the same 

 as that at Red Hill. 



Black Mountain. — Using now this inferential information we may 

 arrive at the movements of those stations which were located by 

 surveys I and III but not by II. In the case of Black Mountain let 

 A in figure 15 be the position of the station in 1854 and B the position 

 in 1906p. Between 1854 and 1868 the movement would have been by 

 strain creep .812 meters in the direction 163° to C ; thence by rebound 

 2.67 meters in the direction 320° at the time of the earthquake of 1868 

 to D; thence between 1868 and 1906a, by renewal of the strain, 2.2 

 meters in the direction 163° to E ; thence by rebound in 1906 south- 

 easterly along hh' parallel to the San Andreas fault an unknown dis- 

 tance compounded with an apparent transverse shift which brought it 

 to B. The component of the transverse shift normal to the fault is 

 .78 meters. 



