GENERALIZATIONS CONCERNING EARTHQUAKES 29 



DIFFERENCES BETWEEN THE SEISMIC CONDI- 

 TIONS OF NORTHERN AND SOUTHERN 

 CALIFORNIA 



The geologic and geographic differences between 

 Northern and Southern California, which are reflec- 

 tions of the relative earth movements, are very 

 great. These are pointed out in a separate and later 

 chapter. Some of these have an important bearing on 

 the question of their relative seismicity. Among them 

 may be mentioned : 



(1) The positions of the great centers of popula- 

 tion relative to the master and active fault lines and 

 the abyssal escarpments. Students of diastrophism 

 will understand the significance of such conditions 

 without the necessity of my dwelling upon them. 



(2) The positions of the centers of populations near 

 or distant from the immediate strand line. The im- 

 portance of this is due to the fact that the weight 

 and moisture of the advancing and receding tidal 

 waters may sometimes be the force that pulls the seis- 

 mic trigger. 



(3) The testimony of the changes of level which 

 have taken place in the respective localities within 

 recent time. 



Both coasts show evidences of elevations and sub- 

 sidences in later geologic time, as well as differences in 

 geologic materials. The southern coast and islands 

 show evidence of late Pleistocene and Recent eleva- 

 tion, over 1100 feet in all. The comparable part of 

 the northern coast in the vicinity of San Francisco ap- 

 parently shows greater subsidence during the similar 

 time. 



