Geology Reference
In-Depth Information
far away a seismic station is from an earthquake. The
greater the difference in arrival times between the two
waves, the farther away the seismic station is from the
earthquake. Three seismographs are needed to locate the
epicenter.
amounts of the energy stored in rocks and thus avoid a
large earthquake and the extensive damage that typically
results.
Earth has an outer layer of oceanic and continental crust
below which lies a rocky mantle and an iron-rich core
with a solid inner part and a liquid outer part.
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Intensity is a subjective, or qualitative, measure of the
kind of damage done by an earthquake. It is expressed
in values from I to XII in the Modifi ed Mercalli Intensity
Scale.
The Richter Magnitude Scale measures an earthquake's
magnitude, which is the total amount of energy released
by an earthquake at its source. It is an open-ended scale
with values beginning at 1. Each increase in magnitude
number represents about a 30-fold increase in energy
released.
The seismic-moment magnitude scale more accurately
measures the total energy released by very large earth-
quakes.
The destructive effects of earthquakes include ground
shaking, fi re, tsunami, landslides, and disruption of vital
services.
Seismic risk maps help geologists in determining the like-
lihood and potential severity of future earthquakes based
on the intensity of past earthquakes.
Earthquake precursors are changes preceding an earth-
quake and include seismic gaps, changes in surface eleva-
tions, and fl uctuations of water levels in wells.
A variety of earthquake research programs are underway
in the United States, Japan, Russia, and China. Studies
indicate that most people would probably not heed a
short-term earthquake warning.
Because of the tremendous energy involved, it seems
unlikely that humans will ever be able to prevent earth-
quakes. However, it might be possible to release small
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Studies of P- and S-waves, laboratory experiments, com-
parisons with meteorites, and studies of inclusions in vol-
canic rocks provide evidence for Earth's internal structure
and composition.
Density and elasticity of Earth materials determine the
velocity of seismic waves. Seismic waves are refracted
when their direction of travel changes. Wave refl ection
occurs at boundaries across which the properties of rocks
change.
Geologists use the behavior of P- and S-waves and the
presence of the P- and S-wave shadow zones to esti-
mate the density and composition of Earth's interior,
as well as to estimate the size and depth of the core and
mantle.
Earth's inner core is probably made up of iron and nickel,
whereas the outer core is mostly iron with 10-20% other
substances.
Peridotite, an igneous rock composed mostly of ferro-
magnesian silicates, is the most likely rock making up
Earth's mantle.
Oceanic crust is composed of basalt and gabbro, whereas
continental crust has an overall composition similar to
granite. The Moho is the boundary between the crust and
the mantle.
The geothermal gradient of 25°C/km cannot continue to
great depths; within the mantle and core, it is probably
about 1°C/km. The temperature at Earth's center is esti-
mated to be 6500°C.
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Important Terms
discontinuity (p. 214)
earthquake (p. 190)
elastic rebound theory (p. 190)
epicenter (p. 193)
focus (p. 193)
geothermal gradient (p. 217)
intensity (p. 198)
Love wave (L-wave) (p. 197)
magnitude (p. 200)
Modifi ed Mercalli Intensity Scale
(p. 198)
Mohoroviˇi ´ discontinuity (Moho)
(p. 216)
P-wave (p. 196)
P-wave shadow zone (p. 214)
Rayleigh wave (R-wave) (p. 196)
refl ection (p. 212)
refraction (p. 212)
Richter Magnitude Scale (p. 200)
seismograph (p. 192)
seismology (p. 192)
S-wave (p. 196)
S-wave shadow zone (p. 215)
tsunami (p. 206)
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