1921] Lawson: The Mobility of the Coast Ranges of California 441 



imposed upon the rocks in the development of the shear zone remains 

 after the general elastic strain has been relieved by slip on the fault. 

 The development of this bending shear involves a shortening of dis- 

 tances measured transverse to the shear zone, as may be appreciated 

 by an inspection of the diagrams in figure 2. If a and 1) be the 

 boundaries of a shear zone in three stages of its development, I, II, 

 and III, it is apparent that a and h are closer together in the later or 

 more acute stages of the shear than in the earlier. If the shear zone 

 were 10 meters wide at the initial stage it might easily be reduced to 



a b a b a b 



Fig. 2 Fig. 3 



Fig. 2. Three stages of bending shear to illustrate transverse shortening. 

 Fig. 3. Path of a point under combined strain creep and transverse shortening. 



8- meters or less at a later stage, just prior to rupture. Such shorten- 

 ing would proceed at an increasingly rapid rate as the shear developed, 

 and would involve a creep of the region towards the fault on each 

 side of it. If, for example, we consider the case of the San Andreas 

 fault and assume that it traverses such a shear zone as I have described, 

 the primary strain creep to. which the shear is due would be modified 

 in direction by combining with the secondary creep due to shortening 

 transverse to the shear zone during its development. Thus in the 

 diagram, figure 3, if AB be the direction of primary strain creep and 

 CD the azimuth of the fault, then, as a point to the east of the fault 

 moved from A toward B it would be deflected to the left by the 

 secondary creep due to shortening; and the point in arriving at E 

 from A would have followed the curved path AE. 



But while I have assumed, for the purpose of elucidating the 

 principle, that the slip on the San Andreas fault was preceded by the 



