Geoscience Reference
In-Depth Information
Earthquakes and
Tsunami as Hazards
CHAPTER 10
overlaps completely the two shadow zones produced
by refraction of P-waves at the core-mantle boundary.
The spatial distribution and time separation between
the arrivals of P- and S-waves at a seismograph station
can be used to determine the location and intensity of
an earthquake. Three stations receiving both P- and
S-waves are necessary to determine the exact position
of any epicenter.
Earthquakes also generate several different long or
L-waves trapped between the surface of the Earth and
the crustal layers lower down. These waves do not
transmit through the mantle or core, but spread slowly
outwards from the epicenter along the surface of the
globe. Their energy is dissipated progressively from
the focal point of the earthquake. One type of L-wave,
the Rayleigh wave, behaves similarly to an ocean wave,
while another type, the Love wave, literally slithers
back and forth through the crust. This latter type of
wave is responsible for much of the damage witnessed
during earthquakes, and produces the shaking that
makes it impossible to stand up. Long waves are the
slowest of all seismic waves, taking about 20 minutes to
travel a distance of 5000 km along the Earth's surface.
All three types of waves, because they travel differently
through dissimilar rock densities and states of matter,
have been used to delineate the structure of the Earth's
core, mantle, and crust.
TYPES OF SHOCK WAVES
(Hodgson, 1964; Holmes, 1965; Whittow, 1980)
Earthquakes are shock waves that are transmitted from
an epicenter, which can extend from the surface to
700 km beneath the Earth's crust. Earthquakes
generate a number of types of waves, illustrated in
Figure 10.1. A primary or P-wave is a compressional
wave that spreads out from the center of the earth-
quake. It consists of alternating compression and
dilation, similar to waves produced by sound traveling
through air. These waves can pass through gases,
liquids and solids, and undergo refraction effects at
boundaries between fluids and solids. P-waves can thus
travel through the center of the earth; however, at the
core-mantle boundary they are refracted producing
two shadow zones, each 3000 km wide, without any
detectable P-waves on the opposite side of the globe.
The second type of wave is a shear or S-wave, which
behaves very much like the propagation of a wave
down a skipping rope that has been shaken up and
down. These waves travel 0.6 times slower than
primary waves. While the velocity of a primary wave
through Earth depends upon rock density and com-
pressibility, the rate of travel of a shear wave depends
upon rock density and rigidity. Shear waves will travel
through the mantle, but not through the Earth's rigid
core. Thus, there is a shadow zone on the opposite side
of the Earth that does not record S-waves. This zone
