1921] Laivson: The Mobility of the Coast Ranges of California 471 



and the strain is still maintained. To the right of E between AB and 

 aa' the stress has also operated continuously and the strain is main- 

 tained. In the portion of the crust that has been relieved of strain 

 an expansion has occurred, the measure of which is proportional to 

 the distance from E, the zero point of the rebound in a given section. 

 In the region south of the Golden Gate the locus of E passes through 

 San Bruno Mountain. 



SUMMAEY 



I have now reviewed, in rather summary fashion, the results of the 

 geodetic surveys and have interpreted them in the light of the elastic 

 rebound theory of faulting. The working hypothesis of the paper is 

 the validity of the theory, which states that faults and consequent 

 earthquakes are due to sudden relief from compressive strain by 

 rupture, or by movement on old rupture planes. In the seismic region 

 of the middle Coast Ranges the causal stress of this strain is probably 

 a northerly slow subcrustal flow which is indicated by the displacement 

 of several geodetic stations in the interval between surveys I and II. 

 The more important of these stations are to the north of the Golden 

 Gate and to the east of the San Andreas fault. In each case, the 

 amount of displacement and time interval being known, the rate of 

 movement is ascertained. The movement itself is interpreted as the 

 expression of strain creep due to the northerly stress. The northerly 

 migration of the Ukiah station of the International Latitude Service, 

 and of Lick Observatory, sustains the hypothesis of a northerly 

 strain creep which affects the whole region. The strain generated by 

 the northerly stress has a twofold distribution: (1) A longitudinal 

 strain, opposed in direction to the general stress, which is relieved 

 by lowly inclined, deep faults having a strike normal to the direction 

 of stress; and (2) a transverse strain, due to the unequal distribution 

 of the stress in the horizontal sense, which is relieved by vertical faults 

 having a strike oblique to the direction of stress. The longitudinal 

 strain for the portion of the region south of the Golden Gate was 

 relieved at the time of the earthquake of 1868, while the transverse 

 strain for the whole of the region considered was relieved by a slip on 

 the San Andreas fault in 1906. The absence of any evidence of 

 rebound in 1868 in the territory north of the Golden Gate is the 

 warrant for a^ssuming that none occurred. The hypothesis that the 

 rebound of 1868 in the territory south of the Golden Gate was south- 

 erly, and that the measure of this rebound increased regularly to the 



