Geology Reference
In-Depth Information
Many people have experienced an earthquake, but most are
probably unaware that the shaking they feel and the dam-
age to structures are caused by the arrival of
seismic waves
,
a general term encompassing all waves generated by an
earthquake. When movement on a fault takes place, energy
is released in the form of two kinds of seismic waves that
radiate outward in all directions from an earthquake's focus.
Body waves
, so called because they travel through the solid
body of Earth, are somewhat like sound waves, and
surface
waves
, which travel along the ground surface, are analogous
to undulations or waves on water surfaces.
material they move through (Figure 8.7c). Because liquids
(as well as gases) are not rigid, they have no shear strength
and S-waves cannot be transmitted through them.
The velocities of P- and S-waves are determined by the
density and elasticity of the materials through which they
travel. For example, seismic waves travel more slowly through
rocks of greater density, but more rapidly through rocks with
greater elasticity.
Elasticity
is a property of solids, such as
rocks, and means that once they have been deformed by an
applied force, they return to their original shape when the
force is no longer present. Because P-wave velocity is greater
than S-wave velocity in all materials, P-waves always arrive at
seismic stations fi rst.
Surface waves travel along the surface of the ground, or just
below it, and are slower than body waves. Unlike the sharp
jolting and shaking that body waves cause, surface waves
generally produce a rolling or swaying motion, much like
the experience of being on a boat.
Several types of surface waves are recognized.
The two most important are Rayleigh waves and Love
waves, named after the British scientists who discovered
them, Lord Rayleigh and A. E. H. Love.
Rayleigh waves
(R-waves)
are generally the slower of the two and behave like
water waves in that they move forward while the individual
particles of material move in an elliptical path within a
vertical plane oriented in the direction of wave movement
(
Figure 8.7).
P-waves
or
primary waves
are the
fastest seismic waves and can travel through solids, liquids,
and gases. P-waves are compressional, or push-pull, waves
and are similar to sound waves in that they move material
forward and backward along a line in the same direction
that the waves themselves are moving (Figure 8.7b). Thus,
the material through which P-waves travel is expanded and
compressed as the waves move through it and returns to its
original size and shape after the wave passes by.
S-waves
or
secondary waves
are somewhat slower than
P-waves and can travel only through solids. S-waves are
shear
waves
because they move the material perpendicular to the
direction of travel, thereby producing shear stresses in the
◗
◗
Figure 8.8b
)
.
◗
Figure 8.7
Primary and Secondary Seismic Body Waves Body waves travel through Earth.
a
Undistrubed material
for reference
Undisturbed material
Surface
Undisturbed
material
Primary waves
(P-waves) compress and
expand material in the
same direction that they
travel.
b
Primary wave (P-wave)
Direction of wave movement
Secondary waves
(S-waves) move material
perpendicular to the
direction of wave
movement.
c
Wavelength
Focus
Secondary wave (S-wave)
The effect of P-and S-waves on a
surface structure.
d
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