EARTHQUAKES ORIGINATING BENEATH THE OCEAN. 125 



of land. But the surface- fissures so frequently described, small in size, 

 very numerous, and running in all directions, have an entirely different 

 origin. They are evidently produced by the shattering of the softer, 

 more incoherent, and inelastic surface-soil, and by the passage of the 

 earth-wave. Even the more elastic underlying rock is broken by the 

 same cause, but to a much less extent. 



Earthquakes originating beneath the Ocean. 



We have thus far spoken of earthquakes originating beneath the 

 land-surface. But three fourths of the earth-surface is covered by the 

 sea ; and we have already seen that other forms of igneous agency are 

 most abundant in and about the sea. As we might expect, therefore, 

 the greater number of earthquake-shocks occur beneath the sea-bed. 

 It is worthy of remark that this is especially true of the sea-bed imme- 

 diately bordering the continents. In such, the phenomena already 

 described are often complicated by the addition of the " Great Sea- 

 Wave." 



Suppose, then, an earthquake-shock to occur beneath the sea-bed ; 

 the following waves will be formed : 1. As before, a series of elastic 

 spherical waves will spread from the focus, until they emerge on the 

 sea-bed. 2. As before, a series of circular surf ace- waves, the outcrop of 

 the spherical waves, will spread on the sea-bottom until they reach the 

 nearest shore, and perhaps produce destructive effects there. 3. On 

 the back of this submarine earth-wave is carried a corresponding sea- 

 wave. This is called the "forced sea-wave" since it is not a free wave, 

 but a forced accompaniment of the ground-wave beneath. It reaches 

 the shore at the same time as the earth-wave. It is of little impor- 

 tance. 4. In addition to all these is formed the great sea-ivave ; or 

 tided wave, as it is sometimes, but wrongly, called. 



Great Sea- Wave. — This common and often very destructive accom- 

 paniment of earthquakes is formed as follows : The sudden upheaval of 

 the sea-bed lifts the whole mass of superincumbent water to an equal 

 extent, forming a huge mound. This movement of the sea-bed is not 

 due to the mere emergence of the earth-wave, for this is far too small to 

 produce such effects; but is due to bodily movement of the earth-crust 

 by displacement of a fissure which, as we have seen, is the usual cause 

 of earthquakes. The falling again of this water as far below as it was 

 before above its natural level generates a circular wave of gravity, 

 which spreads like other water-waves, maintaining its original wave- 

 length, but gradually diminishing its wave-height until it becomes 

 insensible. Usually, a series of such waves is formed. These waves are 

 often 100 to 200 miles across their base (wave-length) and fifty to 

 sixty feet high at their origin. Their destructive effects may be 

 inferred from the enormous quantity of water they contain. In the 



