432 University of California Publications in Geology [Vol. 12 



INTEODUCTION 



In his masterly discussion of the mechanics of the California earth- 

 quake of 1906 Reid^ makes it clear that the forces which moved the 

 earth 's crnst in the region affected were applied to its under side, that 

 these forces generated a strain in the elastic rocks of the crust, and 

 that, at the time of relief from strain by the fault slip, the sudden 

 movement of the ground was an elastic rebound. This conception of 

 elastic rebound on rupture planes in the crust on which strain is 

 suddenly relieved, was subsequently generalized by Reid,^ and is the 

 most satisfactory explanation of earthquakes that has been formulated. 



The notion that the stresses which generated the strain in the crust 

 were applied to the lower side of the mobile region is a logical neces- 

 sity which may be freely accepted, but the application of those stresses, 

 particularly the direction of their operation, is left by Reid undecided. 

 He points out that the phenomena observed may be explained on one 

 or the other of two hypotheses : 



(1) A shearing strain generated by two currents of the subcrustal 

 region flowing parallel to the fault in opposite directions and dragging 

 the overlying crust with them, so as to produce a slow differential 

 displacement of the sides of the zone of maximum shear. 



(2) A single current flowing northerly, parallel to the fault, 

 developing a zone of shear in the riding crust between the mobile region 

 to the west and the stationary region to the east of the fault. 



Reid's discussion of these two possibilities applies particularly to 

 the phenomena observed to the north of the Golden Gate. But his 

 treatment of the problem is general and apparently intended to be 

 comprehensive of the whole field of disturbance ; and he makes only 

 incidental reference to the results of the geodetic survey in the region 

 to the south of the Golden Gate, some of which appear to be anomalous. 



In the Report of the State Earthquake Investigation Commission 

 I made the following comment^ on this southern portion of the field : 



In the region about Monterey Bay the most interesting fact brought out by 

 the geodetic resurvey is that the combined effect of the earlier movement and 

 that of 1906 is a soutlierly migration of the earth's crust on both sides of the 



1 Eeport of the State Earthquake Investigation Commission, vol. II, Carnegie 

 Institution, 1910. 



2 The elastic rebound theory of earthquakes, Univ. Calif. Publ. Bull. Dept. 

 Geol., vol. 6, no. 19, 1911. 



3 Vol. I, p. 151. 



