"It follows from these considerations that the rocks which form the
earth's crust over the surface of the continents and the islands, or
beneath the bed of the ocean, must have a lessening acreage year
by year. These rocks must therefore submit to compression, either
continuously or from time to time, and the necessary yielding of the
rocks will in general take place in those regions where the materials
of the earth's crust happen to have comparatively small powers of
resistance. The acts of compression will often, and perhaps generally,
not proceed with uniformity, but rather with small successive shifts,
and even though the displacements of the rocks in these shifts be
actually very small, yet the pressures to which the rocks are subjected
are so vast that a very small shift may correspond to a very great
terrestrial disturbance.
"Suppose, for instance, that there is a slight shift in the rocks on
each side of a crack, or fault, at a depth of ten miles. It must be
remembered that the pressure ten miles down would be about thirty-five
tons to the square inch. Even a slight displacement of one extensive
surface over another, the sides being pressed together with a force of
thirty-five tons on the square inch, would be an operation necessarily
accompanied by violence greatly exceeding that which we might expect
from so small a displacement if the forces concerned had been of more
ordinary magnitude. On account of this great multiplication of the
intensity of the phenomenon, merely a small rearrangement of the
rocks in the crust of the earth, in pursuance of the necessary work of
accommodating its volume to the perpetual shrinkage, might produce an
excessively violent shock, extending far and wide.
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