2IO 



MOVEMENTS AND DEFORMATIONS 



travel miles from their origin through continuous substance with 

 little result, and yet may then hurl a loose or unstable body to de- 

 struction. Earthquake waves striking the sea-border may thrust 

 the waters off shore, and the return wave may overwhelm the coast 

 (Fig. 209). Sea- waves doubtless arise also from sudden seismic 

 vibrations on the sea-bottom. 



Rate of propagation. The progress of a seismic wave varies 



EE The two epicentral tracts are indicated by arbitrary 

 == iaoseismal curves; the heavy line being the index. 



. SCALE OF MILES 



Fig. 210. Epicentral tracts of the Charleston earthquake, with isoseismal 

 lines (lines of equal disturbance). (Button, U. S. Geol. Surv.) 



