Geology, Vol. 1 [of 3]

by Thomas C. Chamberlin; Rollin D. Salisbury

A most important recent achievement is the detection and investigation of seismic tremors that appear to have come through the earth. The transmission of such waves promises to reveal much relative to the nature of the deep interior, when enough data are gathered to warrant conclusions. The rate of propagation in the central parts is found to be greater than in the outer parts, implying high elasticity within.

The amplitude of the vibrations.—From the very disastrous effects of severe earthquakes, it is natural to infer that the distinctive oscillations must have large amplitude, but in fact it is the suddenness of the vibration, rather than its length, that is effective. Instrumental investigations indicate that the oscillations, after they have left their points of origin, are usually only a fraction of a millimeter in amplitude; at most they seldom exceed a few millimeters. A sudden shock with an amplitude of 5 or 6 millimeters is sufficient to shatter a chimney. It is true that estimates assigning amplitudes of a foot or more have been made, but their correctness is open to serious doubt. It should be understood that it is the length of oscillation of the particles of the subsurface rock transmitting the vibrations that is referred to, not the movement of the free surface, or of objects on the surface. The throw at and on the surface is much greater. Just as a slight, quick tap of a hammer on a floor is sufficient to make a marble lying on it bound several inches, so a sufficiently sudden rise of the surface of the earth, though but a fraction of an inch, may project loose bodies many feet.

Fig. 447.—Illustration of the destructive effects of the Charleston earthquake, showing definite direction of throw. (W. J. McGee.)

Destructive effects.—The interpretation of the disastrous results of earthquake shocks has, therefore, its key in the suddenness and strength of rather minute vibrations of the earth-matter, but it is also dependent on the freedom of motion of the bodies affected. The rocks of the deeper zones, where the matter is sensibly continuous, transmit the seismic vibrations without appreciable disruptive effect, so far as known, though the origin of crevices has been assigned to this cause; but bodies at the surface are fractured, overturned, and hurled from their places. The reason is doubtless this: Within a great mass firmly held in place by cohesion and pressure on all sides, the forward motion of a particle develops an equal elastic resistance, and it is quickly thrown back again and the wave passes on. At the surface, where bodies are freer to move, the stroke of the vibration projects the body, and so, instead of vibratory resilience, the chief energy is converted into mass-motion. The tap of a hammer sends an almost imperceptible vibration along the floor, but this vibration may throw a glass ball, beneath which it runs, into the air. So the minute vibrations of earth-matter may travel miles from their origin through continuous substance with little result, and then so suddenly thrust a loose body on the surface, or the base of a column, or the foundation of a house, as to rack it with differential strains, or even to hurl it to destruction. So, too, earth-waves striking the sea-border may thrust the waters off shore by their sudden impact, and the reaction may develop a wave which overwhelms the coast. Such waves may doubtless arise from a sudden stroke of seismic vibrations on the sea-bottom. The great gaping fissures that sometimes open during earthquakes occur oftenest where the surface on one side is less well supported than on the other, as on a slope, or near a bluff-face or a river-channel. When in such situations the earth is once suddenly forced in the direction of least resistance, it is not always met by sufficient elastic resistance to throw it back. Sometimes, however, there is an elastic return, and the fissure closes forcibly an instant after it is opened.

Direction of throw.—Immediately above the point of origin, technically the epicentrum or epi-focal point, bodies are projected upwards. When crushing takes place in such a case, it is due to the upthrust or to the return downfall. At one side of the epicentrum the thrust is oblique in various degrees, and is usually more destructive, if not too far from the epicentrum. The destructiveness commonly increases for a certain distance from the epi-focal point, and then diminishes. Under ideal conditions, the greatest effects are found where the vibration emerges at an angle of about 45°, but various influences modify this result. Lines drawn through points of equal effect (isoseismals) are not usually regular circles or ellipses about the epicentrum, as they would be under ideal conditions. The various divergencies represent differences of effective elasticity, of surface, and of other influences. As most earthquakes originate from lines, planes, or masses, rather than points, there are doubtless differences of intensity of vibration at different points on the lines, planes, or areas of origin, and these differences introduce inequalities in propagation and in surface effects.

Fig. 448.—Illustrations of the records made by earthquake tremors after distant transmission through the earth. The four diagrams represent the same set of tremors as received at Shide, Kew, Bidston, and Edinburgh in Great Britain. The movement was from left to right. (Milne.)

Rate of propagation.—The progress of a seismic wave varies very greatly. Both experimental tests and natural observations give very discordant results. At present, they justify only the broad statement that the velocity of propagation varies from several hundreds to several thousands of feet per second at the surface. The rate seems to be greater for strong vibrations than for weak ones, and hence it is faster near the origin than farther away. The strength of a vibration dies away, theoretically, according to the inverse square of the distance from the point of origin. Practically there is to be added to this the partial destruction of the vibrations by conversion into other forms of motion.

Sequences of vibrations.—Near the source, the main shocks are apt to come suddenly and to be followed by minor tremors. At a distance there are usually “preliminary” vibrations followed by the main tremors, and these by others of gradually diminishing value. This development is assigned to different rates of propagation, and to refractions and reflections not unlike the prolongation of thunder (see [Fig. 448]). This deployment of the vibrations is notably developed in the shocks that pass long distances through the earth. The vibrations of the first phase are regarded as compressional, those of the second as distortional, while the largest oscillations which arise still later perhaps come around the surface, and may be undulatory, though their nature is not yet determined.