EARTHQUAKES AND SEAQUAKES 



83 



which may be repeated and continued for some time. In the case 

 of the earth these later adjustments are the so-called aftershocks 

 which usually continue throughout a considerable period follow- 

 ing every great earthquake. Gradually they fall off in intensity 

 and frequency until they can no longer be felt, and are thereafter 

 continued for a time as rumblings only. 



Derangement of water flow by earth movement. The water 

 which supported the blocks in our experiment has represented 

 the more mobile portion of the earth's substance beneath its outer 

 zone of fracture. The surface water layers in the tank may, how- 

 ever, be considered in a different 

 way, since their behavior is remark- 

 ably like that of the water within 

 and upon the earth's surface during 

 an earth adjustment. At the instant 

 when adjustment takes place in the 

 tank, water frequently spurts upward 

 from the cracks between the sinking 

 end blocks; and if in place of one 

 of the higher center blocks we insert 

 one whose top is below the level of 

 the water in the tank, a " lake " will 



7 



be formed above it. When the ad- 

 justment occurs, this lake is im- 

 mediately drained by outflow of the water at its bottom along 

 one of the cracks between the blocks (Fig. 76). 



Such derangements of water flow as have been illustrated by 

 the experiment are among the commonest of the phenomena 

 which accompany earthquakes. Lakes and swamp lands have 

 during earthquakes been suddenly drained, fountains of water 

 have been seen to shoot up from the surface and have played for 

 some minutes or hours before their sudden disappearance in a suck- 

 ing down of the water with later readjustment. During the great 

 earthquake of the lower Mississippi valley in 1811, known as the 

 New Madrid earthquake, the earlier Lake Eulalie was completely 

 drained, and upon the now exposed bed there appeared parallel 

 fissures on which were ranged funnel-like openings down which 

 the water had been sucked. In other sections of the affected 

 region the water shot up in sheets along fissures to the tops of high 



[G. 76. Diagrams to illustrate 

 the draining of lakes during 

 earthquakes. 



