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University of California PtMications in Geology [Vol. 12 



susceptible of satisfactory explanation on the hypothesis of a persistent 

 northerly strain creep throughout the entire region, with a southerly 

 rebound in 1868 of that part of it south of the Golden Gate. This 

 hypothesis involves the notion of an elastic prism released from com- 

 pressive drag so as to rebound in one direction only. The measure of 

 rebound will increase regularly in an arithmetical ratio from the fixed 

 or zero point toward the free end. Thus if Tamalpais were unmoved 

 in 1868, the rebound from compression of the block to the south would 

 increase the distance between that station and Black Mountain. For 

 points north of Black Mountain, but south of the locus of zero move- 

 ment in 1868, the increase of distance from Tamalpais would be less, 

 and for points south greater. There must be, of course, a limit to 



Fig. 19. Effect of relief of longitudinal strain on a lowly inclined fault. 



this expansive rebound, but at present we do not know where it is, 

 except that it is beyond Gavilan to the southward. The arithmetical 

 ratio of increase of the measure of rebound applies only to the sudden 

 movement of 1868 and not of course to the geodetically measured 

 distances, since the increase of distance is due to the rebound of that 

 year less strain creep between surveys I and III plus the rebound of 

 1906 for Black ]\Iountain and Loma Prieta, but without any increment 

 in 1906 for Gavilan. 



The notion that the entire region is subject to a persistent north- 

 erly flow stress, while only the part of it south of the Golden Gate 

 rebounded in 1868 on a presumed flat fault, implies a residual strain 

 in the region north of the Golden Gate. That this is mechanically 

 possible may be appreciated by reference to the diagram, figure 19. 

 Let AB represent the line of demarcation between the zone of flowage 

 and the zone of ruptxire. The arrows C and D represent the directions 

 of stress and strain respectively, and aa' is a flat fault upon which a 

 slip has just occurred. This fault intersects AB at E. Below E the 

 fault cannot persist as such into the viscous zone. The slip, therefore, 

 is only effective for the portion of the overriding crust to the right of 

 E and above aa'. In this segment of the crust the rebound takes 

 place. To the left of E above AB the stress has operated continuously 



