Earthquakes are the
result of movement within the earth. They in turn produce effect at the surface
that may be disastrous to man and his buildings. The most general accepted theory of
earthquakes (seismic) activity is the rupture of rock by faulting.
Shock waves are
released by an earthquake at the underground site of the faulting, which is
called focus. Earthquakes are classified as shallow, intermediate, and deep.
The nearest place on the surface is the epicenter.
Three main types of
earthquake wave are known, although these wave types generate many other kinds
as they travel through the earth and interact with one another and with the
rock boundaries (discontinuities). P waves are compressing wave that move
through the earth’s interior from the point of origin. S waves are shear wave
that also go through the body of the earth from the point of origin. L waves are surface waves that begin
near the ground and travel just below it.
The instrument that
records earthquakes is a seismograph. A seismogram is the permanent record that is produced.
The intensity of an
earthquake is determined by the amount of damage done. The actual energy
released by an earthquake is measured as a magnitude.
Although an
earthquake may happen anywhere in the world, most of those that now occur are
concentrated around the pacific Ocean and the Mediterranean Sea, across central
Asia, down the Mid-Atlantic Ridge, and in separate areas in eastern Africa and eastern Asia.
Tsunamis are
so-called tidal waves, but they are due to earthquakes, usually originating at
the bottom of the ocean. They may travel more than 600 miles an hour across the open sea,
piling up when they reach a shallow shoreline.
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