IGNEOUS AGENCIES. 



159 



Nature of Earthquake-Waves. In any case, it is 

 evident that an earthquake is produced by concussion of 

 some kind somewhere in the interior of the earth, usually 

 at a depth of from six to ten miles. The concussion 

 gives rise to a series of elastic earth-waves, spreading in 

 all directions spherically, like sound-waves, until they 

 reach the surface, and then spread in all directions on the 

 surface as a circular wave, as in Fig. 84. The interior 



40m'~ 



FIG. 84. Section and perspective of a portion of the earth's crust shaken by an 

 earthquake, showing origin, x; section of the spherical waves, a', &', c', etc., and 

 perspective of the outcropping surface waves, a, &, c, etc. 



point of origin (x) is called the focus, or centrum ; the 

 point of first emergence (a), the epicentrum. It is the 

 passage of a series of these circular waves beneath the 

 feet of the observer at any point (d) that gives rise to 

 the actual observed phenomena ; so that the scientific 

 discussion of earthquake phenomena is little else than 

 the discussion of such earth-waves emerging and spreading 

 on the surface. 



Earthquakes occurring- beneath the Sea. We have 

 thus far spoken of earthquakes occurring beneath the 

 land ; but three fourths of the earth-crust is covered with 

 water, and therefore it is probable that the larger number 

 of earthquakes have their origin beneath the sea-bed. 

 Besides, as we shall see hereafter in treating of mountain- 

 chains, marginal sea-bottoms are particularly liable to 

 movements. When an earthquake occurs beneath the 

 sea-bed, there are some additional phenomena, which 

 must now be discussed. 



