MOVEMENTS AND DEFORMATIONS OF THE EARTH'S BODY. 531 



earthquake shocks has. therefore, its key in the suddenness and strength 

 of rather minute vibrations of the earth-matter^ but it is also dependent 

 on the freedom of motion of the bodies affected. The rocks of the deeper 

 zones, where the matter is sensibly continuous, transmit the seismic 

 ^ibrations T\ithout appreciable disruptive effect, so far as kno^\TL, though 

 the origin of crevices has been assigned to this cause; but bodies at the 

 surface are fractured, overturned, and hurled from their places. The 

 reason is doubtless this: AVithin a great mass firmly held in place by 

 cohesion and pressure on all sides, the forward motion of a particle 

 develops an equal elastic resistance, and it is quickly thro^^m back again 

 and the wave passes on. At the surface, where bodies are freer to move, 

 the stroke of the vibration projects the body, and so, instead of vibratory 

 resilience, the chief energy is converted into mass-motion. The tap 

 of a hammer sends an almost imperceptible vibration along the floor, but 

 this vibration may throw a glass ball, beneath which it runs, into the 

 air. So the minute vibrations of earth-matter may travel miles from 

 their origin through continuous substance with little result, and then so 

 suddenly thrust a loose body on the surface, or the base of a colunm, 

 or the foundation of a house, as to rack it with differential strains, or 

 even to hurl it to destruction. So, too, earth-waves striking the sea- 

 border may thrust the waters off shore by their sudden impact, and the 

 reaction may develop a wave AA'hich overwhelms the coast. Such waves 

 may doubtless arise from a sudden stroke of seismic vibrations on the 

 sea-bottom. The great gaping fissures that sometimes open during 

 earthquakes occur oft en est where the surface on one side is less well 

 supported than on the other, as on a slope, or near a bluff -face or a river- 

 channel. lA'hen in such situations the earth is once suddenl}' forced in 

 the direction of least resistance, it is not always met by sufficient elastic 

 resistance to throAv it back. Sometimes, however, there is an elastic 

 return, and the fissure closes forcibly an instant after it is opened. 



Direction of throw. — Immediately above the point of origin, techni- 

 cally the epiceJitrum or epi-focd point, bodies are projected upwards. 

 When crushing takes place in such a case, it is due to the upthrust or to 

 the return do^Mifall. At one side of the epicentrum the thrust is oblique 

 in various degrees, and is usually more destructive, if not too far from 

 the epicentrum. The destructiveness commonly increases for a cer- 

 tain distance from the epi-focal point, and then diminishes. Under 

 ideal conditions, the greatest effects are found where the vibration 



