Geology Reference
In-Depth Information
Intensity is a qualitative assessment of the damage
done by an earthquake.
The Richter Magnitude Scale and Moment Magni-
tude Scale are used to express the amount of energy
released during an earthquake.
Great hazards are associated with earthquakes, such
as ground shaking, fi re, tsunami, and ground failure.
Efforts by scientists to make accurate, short-term
earthquake predictions have thus far met with only
limited success.
Geologists use seismic waves to determine Earth's
internal structure.
Earth has a central core overlain by a thick mantle
and a thin outer layer of crust.
Earth possesses considerable internal heat that
continuously escapes at the surface.
kind of animal on which Earth rested. In Japan, it was a giant
catfi sh; in Mongolia, a giant frog; in China, an ox; in South
America, a whale; and to the Algonquin of North America,
an immense tortoise. A legend from Mexico holds that earth-
quakes occur when the devil, El Diablo, rips open the crust so
that he and his friends can reach the surface.
If earthquakes are not the result of animal movement
or the devil ripping open the crust, what does cause earth-
quakes? Geologists know that most earthquakes result from
energy released along plate boundaries, and as such, earth-
quakes are a manifestation of Earth's dynamic nature and the
fact that Earth is an internally active planet.
Why should you study earthquakes? The obvious reason
is that they are destructive and cause many deaths and in-
juries to the people living in earthquake-prone areas. Earth-
quakes also affect the economies of many countries in terms
of cleanup costs, lost jobs, and lost business revenues. From
a purely personal standpoint, you someday may be caught
in an earthquake. Even if you don't live in an area subject to
earthquakes, you probably will sooner or later travel where
there is the threat of earthquakes, and you should know what
to do if you experience one. Such knowledge may help you
avoid serious injury or even death.
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At 5:54 a.m., on May 27, 2006, a 6.3-magnitude earthquake
struck Java, Indonesia. When the earthquake was over, more
than 6200 people were dead, at least 38,600 were injured, more
than 127,000 houses were destroyed, and approximately 451,000
structures were damaged. The amount of economic destruc-
tion infl icted by this earthquake is estimated to be a staggering
$3.1 billion. All in all, this was a disaster of epic proportions.
Yet it was not the fi rst, nor will it be the last major devastating
earthquake in this region or other parts of the world.
Earthquakes, along with volcanic eruptions, are mani-
festations of Earth's dynamic and active makeup. As one of
nature's most frightening and destructive phenomena, earth-
quakes have always aroused feelings of fear and have been
the subject of myths and legends. What makes an earthquake
so frightening is that when it begins, there is no way to tell
how long it will last or how violent it will be. Approximately
13 million people have died in earthquakes during the past
4000 years, with about 2.7 million of these deaths occurring
during the last century (Table 8.1). This increase in fatalities
shows that the rapid rise in numbers of humans living in haz-
ardous conditions has trumped our improved understanding
of how to build and live safely in earthquake-prone areas.
Geologists defi ne an
earthquake
as the shaking or trem-
bling of the ground caused by the sudden release of energy,
usually as a result of faulting, which involves the displace-
ment of rocks along fractures (we discuss the different types
of faults in Chapter 10). After an earthquake, continuing ad-
justments along a fault may generate a series of earthquakes
known as
aftershocks
. Most aftershocks are smaller than the
main shock, but they can still cause considerable damage to
already weakened structures.
Although the geologic definition of an earthquake is
accurate, it is not nearly as imaginative or colorful as the
explanations many people held in the past. Many cultures
attributed the cause of earthquakes to movements of some
Based on studies conducted after the 1906 San Francisco
earthquake, H. F. Reid of The Johns Hopkins University pro-
posed the
elastic rebound theory
to explain how energy is
released during earthquakes. Reid studied three sets of mea-
surements taken across a portion of the San Andreas fault
that had broken during the 1906 earthquake. The measure-
ments revealed that points on opposite sides of the fault had
moved 3.2 m during the 50-year period prior to breakage in
1906, with the west side moving northward (
Figure 8.1).
According to Reid, rocks on opposite sides of the San An-
dreas fault had been storing energy and bending slightly for
at least 50 years before the 1906 earthquake. Any straight line,
such as a fence or road that crossed the San Andreas fault, was
gradually bent because rocks on one side of the fault moved
relative to rocks on the other side (Figure 8.1). Eventually, the
strength of the rocks was exceeded, the rocks on opposite sides
of the fault rebounded or “snapped back” to their former
undeformed shape, and the energy stored was released as earth-
quake waves radiating outward from the break (Figure 8.1).
Additional fi eld and laboratory studies conducted by Reid
and others have confi rmed that elastic rebound is the mech-
anism by which energy is released during earthquakes.
The energy stored in rocks undergoing deformation
is analogous to the energy stored in a tightly wound watch
spring. The tighter the spring is wound, the more energy is
stored, thus making more energy available for release. If the
spring is wound so tightly that it breaks, then the stored en-
ergy is released as the spring rapidly unwinds and partially
regains its original shape.
Perhaps an even more meaningful analogy is simply
bending a long, straight stick over your knee. As the stick
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