occur in the following order: Supposing the centre of the disturbance to be beneath the ocean, as at Lisbon, an observer on the shore might expect to experience—
1. The underground rumble, moving at the rate of 8,000 to 10,000 feet per second.
2. The shock, moving from 1,000 to 5,000 feet per second.
3. The sea-wave, moving about 528 feet per second.
4. Sound, through the air moving at the rate of 1,090 feet per second. It should be noted, however, that the velocity of the sea-wave depends on depth of water.
The vibrations of an earthquake, it is evident, differ in no respect from those produced by other causes, excepting in intensity. The jar arising from a discharge of artillery, by a carriage rolling over pavements, or slamming of heavy doors, puts in motion a series of moving waves just as truly as does the rending of rocks, or an explosion of steam or gas in a fracture thus produced. But, a question arises, What moves when the earthquake is progressing. The phenomena may be explained thus: Around the source of disturbance the rock is pressed outward in every direction as air is pressed outward around a vibrating bell, forming what is called a zone or shell of compressed rock. The extent of this compression is the width of the earthquake-wave, and depends on the force exerted and the elasticity of the rock. In each zone or shell there is always a point of maximum density—and that is where the energy of compression and the rock's elastic force are equal.
As the wave passes, another zone is formed, and the particles behind return by their elasticity to their former position. From this it is obvious that, as the wave is passing, the individual particles of the rock have first a forward and then a backward motion—a swing or excursion to and fro. The extent of this motion is the amplitude of vibration, and may be very small compared with the breadth of the wave.
Mallet found by computation that, given a certain depth of fissure, and a certain heat of steam, the expansive force would produce a wave of nine inches amplitude at the surface. His observations of the Neapolitan earthquake of 1857 show that the maximum amplitude at the surface was only 2.5 inches. In his elaborate and beautiful volume on the eruption of Vesuvius, in 1872, just published, Mr. Mallet reaffirms a statement previously made by him, that "it is the vibration of the wave itself, i. e., the motion of the wave-particles, that does the mischief, not the transit of the wave from place to place on the surface."
We understand, then, that there is motion of particles as well as a transit-wave; that the "travelling zone or shell of vibration" is a zone or shell of "elastic compression."
Fig. 9 illustrates the "shells" as they move away from B, the focus of disturbance. The transit-wave, with its interstitial vibrations, reaches the surface in the manner shown in the diagram Fig. 10.