Types Of Seismic Waves
Seismic waves are
elastic waves that propagate in solid or fluid materials. They can be divided into
body waves that travel through the interior of the materials;
surface waves that travel along surfaces or interfaces between materials; and
normal modes, a form of standing wave.
Body waves
There are two types of body wave,
P-waves and
S-waves (both body waves). Pressure waves or Primary waves (
P-waves), are
longitudinal waves that involve
compression and
rarefaction
(expansion) in the direction that the wave is traveling. P-waves are
the fastest waves in solids and are therefore the first waves to appear
on a
seismogram.
S-waves, also called shear or secondary waves, are
transverse waves
that involve motion perpendicular to the direction of propagation.
S-waves appear later than P-waves on a seismogram. Fluids cannot support
this perpendicular motion, or
shear, so S-waves only travel in solids. P-waves travel in both solids and fluids.
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Surface waves
The two main kinds of surface wave are the
Rayleigh wave,which has some compressional motion, and the
Love wave,
which does not. Such waves can be theoretically explained in terms of
interacting P- and/or S-waves. Surface waves travel more slowly than
P-waves and S-waves, but because they are guided by the surface of the
Earth (and their energy is thus trapped near the Earth's surface) they
can be much larger in amplitude than body waves, and can be the largest
signals seen in earthquake seismograms. They are particularly strongly
excited when their source is close to the surface of the Earth, as in a
shallow earthquake or explosion.
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Normal modes
The above waves are traveling waves. Large earthquakes can also make the Earth "ring" like a bell. This ringing is a mixture of
normal modes
with discrete frequencies and periods of an hour or longer. Motion
caused by a large earthquake can be observed for up to a month after the
event.
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The first observations of normal modes were made in the 1960s as the
advent of higher fidelity instruments coincided with two of the largest
earthquakes of the 20th century - the
1960 Great Chilean Earthquake and the
1964 Great Alaskan Earthquake. Since then, the normal modes of the Earth have given us some of the strongest constraints on the deep structure of the Earth.
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