Geology, Vol. 1 [of 3]

by Thomas C. Chamberlin; Rollin D. Salisbury

Earthquakes.

When the tremors spring from sources within the earth itself and are of appreciable violence, they are recognized as earthquakes. The sources of earthquake tremors are various. The most prevalent is probably the fracture of rocks and the slipping of strata on each other in the process of faulting. The interpretation of movements of this class has now been so far perfected that the length and depth of the fault, the amount of the slip, and the direction of the hade are capable of approximate estimation.[224] To the same class belong the movements due to slumping. They are illustrated by the sliding and arrest of great masses of sediment along the steep fronts of deltas, and of the accumulations of deep-sea oozes on steep submarine slopes. Such slumping is, in reality, superficial faulting. Seismic tremors often attend volcanic eruptions, and are then probably attributable to the sudden fracture and displacement of rock by the penetration of lava, or by rapid and unequal heating. They are perhaps also due sometimes to the sudden generation or cooling of steam in underground conduits, crevices, and caverns, the action possibly being in some cases of the “water-hammer” type. In rare instances, probably, the bursting of beds overlying pent-up non-volcanic gases may give origin to earthquakes. A more superficial source of earthquake vibrations is the collapse of the roofs of subterranean caverns.

Seismic vibrations seem to be in part compressional, in part distortional, in part (on the surface) undulatory, and in part irregular. The distortional are especially significant, as they seem to imply a solid medium of transmission.

Points of origin, foci.—It is probable that nearly or quite all earthquake movements start within the upper ten miles of the crust, and most of them within the upper five. Some of the earlier estimates indeed placed the points of origin as deep as 20 or 30 miles, but in these cases the necessary corrections, discussed below, were neglected. Most of the recent and more accurate estimates fall within the limits given.

The method of estimating the depth of the centers of disturbance consists in observing the directions of throw or thrust of bodies at the surface, and in regarding these as representing the lines of emergence of the earthquake-waves. By plotting these lines of emergence, and projecting them backwards to their underground crossings, a first approximation to the location of the focus is reached (the lines EF′, [Fig. 446]). From the nature of the case, the observations of the angles of emergence cannot be very accurate, but an effort is made to limit the error by making the number of observations great.

Two systematic corrections are to be applied to all such estimates, the one for varying elasticity and density, and the other for varying continuity. Both reduce the estimated depth. In making the correction for varying elasticity, it must be noted that the velocity of vibrations varies directly as the square root of the elasticity, and inversely as the square root of the density. The velocity is also accelerated by increase of temperature. The elasticity, temperature, and density all increase with depth. Theoretically, the increase of velocity due to the increasing elasticity and temperature of increasing depths, overbalances the retardation due to increasing density, and recent observations on the transmission of seismic waves through deep chords of the earth have confirmed this conclusion. The path of the vibration will, therefore, be curved toward the surface, as pointed out by Schmidt and illustrated in [Fig. 446], taken from his discussion.[225] From this it is clear that the focus is not so deep as implied by the simple backward projection of the lines of emergence.

Fig. 446.—Diagram illustrating by closed curves the different rates of propagation of seismic tremors from a focus F, and, by lines normal to these, the changing directions of propagation of the wave-front. It will be seen that the paths of propagation curve upwards in approaching the surface. If the lines of emergence, as at E and E, be projected backwards, as to F′, the points of crossing will be below the true focus.

A second correction must be made for the differences of continuity of the upper rock in the vertical and horizontal directions. In the outer part of the earth, the continuity in horizontal directions is interrupted by vertical fissures. Were these not usually filled with water, they would soon kill the horizontal component of the seismic wave, and the residual portion would be directed almost vertically to the surface, for the width of the fissures is almost always greater than the amplitude of the seismic vibrations. The water restores the continuity, in a measure, but not perfectly, for the elasticity of water is much less than that of rock. It is clear that in horizontal movement there must be a constant transfer from rock to water and from water to rock, and this must retard, as well as partially destroy, the vibrations. In a vertical direction, however, the rocks rest firmly upon one another, and this gives measurable continuity, the only change being from one layer or kind of rock to another. It seems certain, therefore, that the vertical component of the seismic wave will be less damped and less retarded in transmission than the horizontal. It will, therefore, reach the surface sooner and will have the greater effect on bodies at the surface, not only for the reasons given, but also because it emerges more nearly in the line of least resistance and of freest projection. On this account, a second correction must be added to the correction for elasticity, and this must further reduce appreciably the first estimate of the depth of the focus.

Observation shows that in some way a seismic wave becomes separated in transmission into portions of different natures and speeds, but their interpretation is yet uncertain. These separated portions probably consist of the compressional, the distortional, and the undulatory waves, and perhaps of refractions and reflections of these (see [Fig. 448]).