EARTHQUAKES AND SEAQUAKES 



69 



motions, which, if of large amplitude, may induce nausea, prevent 

 animals from keeping upon their feet, and wreck all structures 

 not specially adapted to withstand them. Heavy bodies are some- 

 times thrown up from the ground (Fig. 51), and at other times 



FIG. 51. Bowlders thrown into the air and overturned during the Assam 

 earthquake of 1897 (after R. D. Oldham). 



similar heavy masses are, apparently because of their inertia, more 



deeply imbedded in the earth. Thus gravestones and heavy stone 



posts are often sunk more deeply in the ground and are surrounded 



by a hollow and perhaps by small 



open cracks in the surface (Fig. 52). 



When bodies are thrown upward, it 



would imply that a quick upward 



movement of the ground had been FIG. 52. Heavy post sunk deeper 



suddenly arrested, while the burial into the ground during the 



of heavy bodies in the earth is prob- %3Z *?* 



ably due to a movement which 



begins suddenly and is less abruptly terminated. 



Seaquakes and seismic sea waves. Upon the ocean the quakes 

 which emanate from the sea floor are felt on shipboard as sudden 

 joltings which produce the impression that the ship has struck upon 

 a shoal, though in most instances there is no visible commotion in 



