EARTHQUAKES. 



extending along a considerable line of operation, passing away from the observer, the sound- 

 waves will be rumbling noises, and may be more or less conf<>und<-d from the different velocities 

 uith \\liidi tln-y air t raiisinit led tlm.ii^li tin- several media. When n< fracture* or explosion* 

 occur, sound-waves may be wholly wanting. If the amount and peculiar character of the 

 disturbance at the centre of impulse be such as by partial disturbances to set in motion wave* 

 of sound through tin- earth or sea, before any sufficient impulse has been given to propagate a 

 M tisil.lr sh'.ck, the rumbling noises may precede the actual shock considerably. So also when 

 the centre of impulse is beneath highly elastic crystalline rock, overlaid by soft rock and allu- 

 vion, the sound-wave through the earth, passing first horizontally and then vertically through 

 the small distance to the surface occupied by the less elastic materials, will reach the ear quicker 

 than if it passed first vertically and then horizontally. But as the earth-wave must pursue the 

 latter course, the sound-wave and a slight vertical shock will be first sensible, and the principal 

 waves some time after. This is precisely what occurs, not only during extraordinary earth- 

 quakes, but also at most of the tremors observable in Chile. Sixth. Some time after the shock, 

 the great sea-wave rolls in to land. The impulse given to the bed of the sea originated this and 

 each of the other waves simultaneously ; but the great earth-wave moves with a velocity de- 

 pendent upon a function of the elasticity of the crust of the earth, whilst that of the sea-ware 

 depends upon a function of the depth of the sea; and thus the former immediately leaves the 

 latter pursuing its slow career. Should the depth of the sea be uniform, the wave is propa- 

 gated with uniform velocity in all directions ; but where the depth decreases, not only will the 

 initial annulus* change form, but the velocity of propagation will decrease, and as the wave 

 advances toward the beach, its front slope becomes short and steep, the rear first long and 

 gentle, then more and more depressed about its centre, until finally it separates into two or 

 more smaller waves. These lose their equilibrium on arriving where the depth of water below 

 the mean surface is less than their altitude, and, toppling over, fall in breakers on the shore. 

 Consequently, where deep water continues close to the land, the great sea-wave comes in as a 

 long, wide and unbroken swell, one such waVe arriving for every earth-wave generated. But 

 before it reaches the coast, the water retires to some distance from the ordinary shore-line, and 

 then, after an instant's pause, the great wave rolls with fury upon the devoted land. Within 

 bays like Talcahuano, Coquimbo, and Caldera, where the water continues deep only to the 

 mouths, the momentum of such waves is overwhelming. Valparaiso, with deep water less than 

 half a mile oif the land, would only have a swell proportionate to the height of the impulse. 

 The sea will then again retreat and a second and a third or more waves of oscillation succeed 

 the number dependent on the form and slope of the beach. As with the earth- wave, it by no 

 means follows that the sea-wave will reach an observer with the same direction of motion as it 

 set out. Such result can only occur when there is uniform depth of water between the origin 

 and nearest point of the coast ; at every change of depth, or deviation of the coast line from a 

 tangent to the advancing curve, the direction will be distorted. 



All the coast of Chile, Valparaiso excepted, has suffered excessively under the effects of these 

 waves at every great earthquake recorded by historians until that of April, 1851. We should, 

 therefore, have a right to believe them submarine, even without the corroborative testimony of 

 approximate direction ; and the absence of oceanic disturbance, on the last occasion referred to, 

 is the best proof of its inland origin. That earthquakes of oceanic origin should be excessive, 

 is readily comprehensible. Only a violent effort of the internal forces can overcome the enor- 

 mous pressure in deep water. The heterogeneous formations of the coast, uneven in surface 

 and broken by fissures, presents many weak districts which readily yield to slight accumulation 

 of the disturbing agent. Apparently two such seats of disturbance exist near the coast of 

 Chile one near the latitude of Concepcion, the other near that of Copiapo, 600 miles to the 



* The height of the crest at formation of the wave will be nearly in proportion to the reciprocal of the square root through 

 which it has diverged. British Ass. Rep., 1847. 



