342 GEOLOGY 



the sea-border may thrust the waters off shore, and the return 

 wave may overwhelm the coast (Fig. 276). Sea-waves doubtless 

 arise also from a sudden stroke of seismic vibrations on the sea- 

 bottom. 



Direction of throw. Immediately above the point, line, or plane 

 of origin (epicentrum or epi-focal point, line, or plane), bodies are 

 projected upwards. Elsewhere at the surface plane, the thrust 

 is oblique. The destructiveness commonly increases for a certain 



Fig. 276. Great sea-wave on the coast of Ceylon. (Sieberg.) 



distance from the epi-focal point, line, or plane, and then diminishes. 

 Lines drawn through points of equal effect (isoseismals, Fig. 277) 

 are not usually circles or regular ellipses, and their departures from 

 these forms represent differences of elasticity, etc. As most earth- 

 quakes originate from lines, planes, or masses, rather than points, 

 there are differences in the intensity of vibration at different points 

 on the lines or planes of origin, and these differences introduce 

 inequalities in propagation and in surface effects. 



Rate of propagation. The progress of a seismic wave varies 

 appreciably. The violent vibrations on the surface near the place 



