SEA-QUAKES 889 



Changes due to displacement are not to be classed in this category, 

 as they are due to the same cause which is responsible for the 

 earthquake, namely, the faulting. But topographic changes caused 

 by the shaking of the earth are properly classed here. Such are 

 the formation and sudden drainage of lakes ; the blocking of rivers 

 by land slips ; the shifting of river channels in level delta regions, 

 and other changes. After the Indian earthquake of 1897 no less 

 than 30 earthquake lakes were produced. The earthquakes of 

 1811-12 caused the appearance of many nevv^ lakes along the lower 

 Mississippi, owing, probably, to the local settling of the river sedi- 

 ment. One of these is Reelfoot Lake, in Obion county, Tennessee, 

 which has a length of more than 20 miles and a width of seven 

 miles ; the water in places covering the tops of submerged cypress 

 trees. Near Little Prairie a lake, many miles in length but only 

 from 3 to 4 feet in depth, came into existence. Upon its disap- 

 pearance it left behind a stratum of sand. On the other hand, Lake 

 Eulalie, 300 yards long and 100 yards wide, was suddenly drained 

 through parallel fissures which opened in its bottom. Many other 

 examples of this kind might be cited. 



This same earthquake was responsible for a local and temporary 

 reversal of the current of the Mississippi. Similar phenomena 

 have been observed in other rivers. 



Submarine Earthquakes and Sea-quakes. 



As might be expected, seismic disturbances are common along 

 the borders of the Pacific, where, as before noted, we have the 

 regions of down-warping, and, therefore, the region of stresses 

 and of readjustments to these stresses. From their proximity to the 

 sea the latter is, of course, strongly affected by the shocks, and sea- 

 quake waves, or tsunmnis, are the result. These, on account of their 

 height and velocity, are exceedingly destructive. Perhaps the most 

 memorable one is that which destroyed Lisbon in 1775. The sea- 

 quake originated on the ocean floor fifty or more miles off the 

 coast of Lisbon, and the vibrations were transmitted along the 

 surface of the water in a series of monstrous waves. The greatest 

 of these was sixty feet high, and was followed by others of decreas- 

 ing height. Spending their strength on the coast of Spain the 

 waves passed on with diminished amplitude, but were felt even 

 on the shores of the West Indies, across the whole expanse of 

 the Atlantic. 



