Prof. Abbe gives the causes of earthquakes in a statement which says, according to views commonly accepted in geology, the solid crust of the earth consists of an unknown depth of granite and gneiss, on top of which are five or ten miles of metamorphic and and sedimetary strata.
This crust is everywhere in a state of strain, due to various kinds of stress; in other words, the outward bulgings that make the continents and the mountain ranges, or the downward bendings that have made the ocean beds, represent strains that frequently become too severe for the rock to resist. Moreover, in special localities there are upward-pressing masses of lava or other plastic material that produce great local strain. In other places the strata that ages ago were tilted up to make a mountain, are still in a state of strain, and notwithstanding the long interval that has elapse d are occasionally cracking and sliding on each"other.
These various stresses have produced the innumerable cracks that we see in the smaller beds of rock and the faults that the miner discovers in his attempt to follow up a vein of mineral ore. Even the tidal action of the sun and moon and the variations in barometric pressure and in the loads of snow and alluvium can produce appreciable effects.
Nearly all earthquakes are accompanied by a rumbling sound, due, I believe, to the small and rapid vibrations proceeding chiefly from the margins of the area over which the fault-slip producing the earthquake takes place. In some districts (Comrie in Perthshire; East Haddam in Connecticut; Pignerol in Piedmont; Meleda in the Adriatic, etc.) sounds without shocks are common during intervals which may last for several years, but slight shocks with sounds occasionally intervene, as if the sounds and shocks were manifestations, differing only in degree and the method in which we perceive them, of one and the same phenomenon.
In the great earthquakes the sound area is confined to the neighborhood of the epicenter; in moderate and slight shocks the sound area may even overlap the disturbed area. In the limiting case the disturbed area vanishes and the vibrations are perceptible only as sound.
A weather observer describes the coming of an earthquake thus:
"The shock was preceded by a rushing or hissing sound, for three or four seconds, like the wind blowing through brush. It was followed by a rumbling sound, similar to a heavy wagon on hard ground. This lasted two or three seconds. Then came this heavy, jarring shock; two shccks were felt." - " Washington Star. "