Mr. Maturt on the Dynamics of Earthquakes. 89 
which have not partaken of the elevation. This wave will be negative above the 
neutral plane, and positive below it. 
Again, after the plate or crust has been so elevated, whether quickly or 
slowly, if the constraining forces be suddenly relaxed, so that the plate, like a 
bent bow, is permitted to become straight again, that is to say, drops down from its 
state of flexure to its former level, or partially towards it, the resilience—the sudden 
return of the extended particles above the neutral plane, and of the compressed 
particles below it to their condition of repose—will produce and propagate in all 
directions a similar wave of elastic compression, which will be positive above the 
neutral plane, and negative below it. Thus, sudden elevation, or sudden depres- 
sion of a tract of country, must always be attended with the production and transit 
through the surrounding crust, whose level has not been disturbed, of an elastic 
wave, or true earthquake shock, even although not preceded in either case by 
dislocation or fracture of the rocky crust ; the amount of extension and compres- 
sion of the particles above and below the neutral plane having been, in these cases, 
within the elastic limits of the particular rocks constituting the elevated crust. 
But again, the elevating forces may act rapidly, as in the case of Monte 
Nuovo, elevated 440 feet in forty-eight hours, in 1538, and continue to act 
until, at length, they do produce fracture and dislocation of the crust above ; 
or they may act very slowly, yet continuously, and at last produce fracture and 
dislocation. In the former case, one wave of elastic compression will be produced 
at the moment that the rocky crust is suddenly bent; and another, and much 
greater one, at the moment that fracture takes place by its giving way, and the 
constraint of the extended and compressed particles is thus released. In the 
latter case, namely, of very slow elevation ending in fracture, there will be no 
earth wave, and no shock until the fracture actually occurs. 
When the earth wave impulse arises from sudden elevation, or sudden 
depression, of a local district, within its elastic limits, and therefore not accompa- 
nied by fracture, the earthquake shock will be felt at a distance; but there may 
be no noise heard, no previous or subsequent rolling sounds in the earth. 
When fracture occurs, on the contrary, the awful noise (the “bramidos 7 
truenos” of the Spanish Americans) will be heard, and, as already explained, may 
precede, continue during, and be heard after, the shock reaches a distant point. 
In the former case also, viz., impulse by sudden elevation or depression, with- 
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