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 Earthquake.
The mere mention of the word induces thoughts of crumbling houses
and cracking ground, collapsing bridges and exploding gas lines. All
this chaos is caused by great tremors just beneath the Earth's surface.
Earthquakes occur with little warning, and usually last just moments,
but have the potential to leave behind great devastation. Not all
earthquakes cause the damage mentioned earlier in this article. The
fact is that many earthquakes occur throughout the world on a daily
basis, but of those, only a hand full cause catastophic damage. The
dilemma is that no one can definitively predict how strong an earthquake
will be. This is why it is important to know the procedures to protect
yourself during EVERY earthquake in your area.
 So
what exactly is it that causes an earthquake?
The Earth's outer layers are made up of plate-like formations. These
plates fit up against each other almost like pieces of a puzzle would
fit together. The point at which two or more plates meet is called
a fault. Faults may range in length from a few millimeters to thousands
of kilometers. Most faults produce repeated displacements over geologic
time. The plates have a tendency to shift and move against each other
at these fault lines. During an earthquake, the rock on one side of
the fault suddenly slips with respect to the other. This sudden shift
causes energy waves to radiate through the surface of the earth, which
causes the shaking associated with eartquakes. This shaking can last
from a few seconds up to a few minutes, depending on the severity
of the earthquake.
Earthquakes
generally occur in the crust or upper mantle, which ranges from the
earth's surface to about 800 kilometers deep (about 500 miles). The
point at which an earthquake occurs, is called its epicenter. This
is where its tremors are the strongest. An earthquake's epicenter
is determinded by studying the earth's vibration with a device called
a seismograph. A series of seismograms are obtained from strategically
placed seismographs throughout the world. The seismogram with the
strongest reading, indicates the approximate viscinity of the earthquake.
An
earthquake may be accompanied by aftershocks and foreshocks. Foreshocks
are earthquakes which precede larger earthquakes in the same location.
Aftershocks are smaller earthquakes which occur in the same general
area during the days to years following a larger event or "main
shock." As a general rule, aftershocks represent minor readjustments
along the portion of a fault that slipped at the time of the main
shock. The frequency of these aftershocks decreases with time. Historically,
deep earthquakes (>30km) are much less likely to be followed by
aftershocks than shallow earthquakes. (Univ. of Washington).
One of the most popular and active fault lines
in the United States is the San Andreas Fault (pictured above). This
fault is located in California, and is where the Pacific Plate and
the North American Plate meet.The Pacific Plate consists of most of
the Pacific Ocean floor and the California Coast line. The North American
Plate comprises most the North American Continent and parts of the
Atlantic Ocean floor.The San Andreas Fault is more than 650 miles
long and extends to depths of at least 10 miles. Many other smaller
faults like the Hayward (Northern California) and the San Jacinto
(Southern California) branch from and join the San Andreas Fault Zone.
The Pacific Plate grinds northwestward past the North American Plate
at a rate of about two inches per year. Parts of the San Andreas Fault
system adapt to this movement by constant "creep" resulting
in many tiny shocks and a few moderate earth tremors. In other areas
where creep is NOT constant, strain can build up for hundreds of years,
producing great earthquakes when it finally releases.
 Earthquakes
are measured using a mathematical formula, called the Richter
Scale. Essentially, this groups earthquakes according to their
magnitude. The scale places the weakest earthquakes at a rating of
3 or less, but the scale does not have a ceiling. Although an earthquake's
strength has the ability to be infinite, the strongest, most devastating
earthquakes experienced on earth have reached the 8-9 rating on the
Richter Scale. The largest recorded earthquake in the United States
was a magnitude 9.2 that struck Prince William Sound, Alaska on Good
Friday, March 28, 1964. The
largest recorded earthquake in the world was a magnitude 9.5 (Mw)
in Chile on May 22, 1960.
Information
obtained from United
States Geological Survey
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