Seismicity 



81 



Figure 69. Positions of epicenters of earthquakes that occurred between January 1, 1934, and January 1, 1958. Only 

 epicenters of magnitude 3 or greater and of A and B quahty (position accurate to within 5 km) are shown. 



Only five shocks of magnitude 5 and two 

 of magnitude 6 occurred in the area of Chart 

 I. Compared to these shocks all the others 

 are of lower magnitude represent only a 

 minor quantity of energy (Gutenberg and 

 Richter, 1942; Wood, 1947). The energy in 

 the earthquakes during the 22 years is roughly 

 equal to that in about seven atomic bombs 

 of the 1946 Bikini variety, these being rated 

 as equivalent to earthquakes of magnitude 

 5.5 (Gutenberg and Richter, 1946). 



Movement on a strike-slip fault is followed 

 by earthquake waves having a first impulse 

 that theoretically is one of compression in 

 two quadrants and of dilatation in the other 

 two quadrants. Thus, Gutenberg (1941Z)) 



believed that an examination of the nature 

 of the first impulse of seismograms should 

 indicate the direction of movement on strike- 

 slip faults of the region. His study showed 

 that in the area south of the Transverse 

 Ranges, west of the San Andreas Fault, and 

 east of longitude 119° the records of almost 

 all earthquakes indicated right lateral move- 

 ments along the faults, according well with 

 the general direction of movement known 

 from geological data. 



Surface earthquake waves also provide in- 

 formation about the crust. In southern Cal- 

 ifornia Press (1956) has shown that the 

 phase velocity of Rayleigh waves indicates 

 that the crust above the Mohorovicic dis- 



