A shrinkage of the earth, in the course of the incessant
adjustment between the interior and the exterior, will take place by
occasional little jumps at this particular centre. The fact that there
is this weak spot at which small adjustments are possible may provide,
as it were, a safety-valve for other places in the same part of
the world. Instead of a general shrinking, the materials would be
sufficiently elastic and flexible to allow the shrinking for a very
large area to be done at this particular locality. In this way we may
explain the fact that immense tracts on the earth are practically free
from earthquakes of a serious character, while in the less fortunate
regions the earthquakes are more or less perennial.
"Now, suppose an earthquake takes place in Japan, it originates a series
of vibrations through our globe. We must here distinguish between the
rocks--I might almost say the comparatively pliant rocks--which form
the earth's crust, and those which form the intensely rigid core of the
interior of our globe. The vibrations which carry the tidings of the
earthquake spread through the rocks on the surface, from the centre of
the disturbance, in gradually enlarging circles. We may liken the spread
of these vibrations to the ripples in a pool of water which diverge from
the spot where a raindrop has fallen. The vibrations transmitted by
the rocks on the surface, or on the floor of the ocean, will carry the
message all over the earth. As these rocks are flexible, at all
events by comparison with the earth's interior, the vibrations will be
correspondingly large, and will travel with vigor over land and under
sea.
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