EARTHQUAKES 211 



appreciably. The violent vibrations on the surface near the 

 rpirentrum (point above the focus) are the most irregular, and 

 strong vibrations generally have greater speed than weak ones. 

 Vibrations propagated to great distances through and around the 

 earth are loss irregular in rate. Those which follow the surface 



BOIUAV INI. CO., N.Y, 



Fig. 211. Coseismal lines (lines connecting places feeling the shock at the 

 same time) for each minute; Herzogenrath (Germany) earthquake of October 22, 

 1873. (Lasaulx.) 



travel about 1.85 miles (3 km.) per second. Those which go through 

 the earth travel more rapidly, at rates ranging from about 3.9 miles 

 to about 5.7 miles per second. 



Distribution. Over large parts of the globe, severe earthquakes 

 are rare, but in certain regions they are, unfortunately, frequent. 

 Earthquakes are likely to be rather frequent where geologic changes 

 are in rapid progress, as along belts of young mountains where 

 stresses are not yet adjusted, or at the mouths of great streams where 

 deltas are accumulating, or about volcanoes where temperatures 

 and strains are changing, or on the great slopes, particularly the 

 submarine slopes, where adjustments to inequalities of stress are 

 in progress. Not a few, however, occur where the special occasion 

 is not obvious. 



Geologic Effects of Earthquakes 



Geologically, earthquakes are of less importance than many 

 gentler movements. Disastrous as they are to human affairs, they 

 leave few distinct marks which are more than temporary. 



