278 



PHYSICAL GEOLOGY 



instead of vertical 

 (Figs. 276, 277) and 

 to measure from 8 

 to 20 feet. Some 

 vertical movement 

 also occurred, but it 

 was inconsiderable. 



Distribution of 

 Earthquakes. — A 

 study of the distribu- 

 tion of earthquakes 

 gives a clue to their 

 cause. They occur 

 (1) in volcanic re- 

 gions, where the 

 earth's crust is sub- 

 jected to high tem- 

 perature and to 

 strains produced by 

 explosions ; (2) along 

 belts of young and 

 growing mountains, 

 where strains are being relieved from time to time; (3) along coasts, 

 where the sea bottom descends steeply from the shores, especially 

 where they are bordered by high mountains. The conditions last 

 mentioned are well illustrated in Japan, where the earthquake 

 records since the beginning of the seventeenth century show that 



Fig. 276. — Fence parted by an earthquake fault, 

 1906. The fault fracture is inconspicuous, although the 

 horizontal displacement is eight and a half feet. Near 

 San Francisco. (U. S. Geol. Surv.) 



Fig. 



277. — Diagrams illustrating the nature of the fault which produced the 

 San Francisco earthquake. (After Gilbert.) 



a severe shock has occurred on an average of once in two and 

 a half years. By far the greater number of these have been 

 accompanied by a movement along the scarp of the great Tuscarora 

 Deep, which lies a short distance off the coast. (4) Earthquakes 



