An earthquake is manifested as a shaking of the ground resulting from a
series of shock waves generated following the brittle failure of rocks
within the earth's crust or upper mantle. The failure comes about due to
the build up of stress which occurs because of the constant movement of
blocks of the earth's crust known as the lithospheric plates. Failure
occurs at a point, or in a fairly small zone, known as the focus with
the epicentre being the point on the earth' surface directly above this
focus. However, once failure has occurred, movement may persist along a
zone of weakness - known as a fault - for a considerable distance,
occasionally as much as 1000 km).
Many earthquakes occur each year, on average greater than 800,000, but
most are small and not felt by humans. A severe earthquake, with a
magnitude of greater than 8.0, can be expected every 8 to 10 years. Yet,
a significant number of smaller earthquakes, which are still capable of
destruction, occur each year.
Earthquakes show a marked spatial distribution. The vast majority are
located within narrow zones which correspond to the boundaries of the
plates. These plates are in continuous movement relative to each other,
thought to be driven by convective processes in the earth's mantle, the
region of rocks beneath the crust which are heated to the point of
becoming soft or plastic,An earthquake can be scary. It can make the
ground shake so hard that lamps inside houses swing from the ceiling or
crash to the floor. Earthquakes also can make buildings fall apart. Some
earthquakes can cause the land to open up and swallow whole roads.
Earthquakes under the ocean can create a huge wave called a tsunami.
Scientists use the theory of plate tectonics to explain earthquakes.
Imagine the earth as a large ball with a hot center called a core.
Wrapped around the core is another layer called the mantle. On top of
the mantle is the earth's crust, which is the ground we walk on ,
Scientists say the earth's crust is made up of 10 to 25 plates. These
plates are gigantic pieces of rock that make up our continents and ocean
floors. Although we can't feel it, these plates are always moving around
the globe. Earthquakes can occur when two plates collide , Scientists
call the two plates that grind against each other under Alaska the
Pacific Plate and the North American Plate. The Pacific Plate is forced
under the North American Plate, causing both plates to crack and break.
Sometimes their movement causes earthquakes.
When the Pacific Plate is pushed down into the earth's mantle, changes
occur that cause melting. Melted rock in the mantle is called magma.
Magma can rise back up to the earth's surface to create a volcano, or a
chain of volcanoes.
More earthquakes occur in Alaska than in all other places in the United
States combined. Three of the world's six largest earthquakes happened
in Alaska, About 120 earthquakes are recorded every week in Alaska by
scientists in the Alaska Earthquake Information Center at the
Geophysical Institute of the University of Alaska Fairbanks
If scientists could predict earthquakes the way weather forcasters can
predict rainy days, then many lives and homes would be saved from
destruction. Scientists at the Geophysical Institute and other places
around the world are working hard to find signs that can foretell
earthquakes. A story in the Geophysical Institute Quarterly tells about
one sign scientists hope to use to help them forecast earthquakes in the
future , Predicting the next major earthquake may depend more on finding
quiet ground than on listening for rumbles and shakes , New research
shows that periods of "seismic quiescence"---times when there are far
fewer small earthquakes in an area than normal---sometimes precede the
world's strongest and most damaging earthquakes.
Geophysical Institute Professor Max Wyss, who holds the Wadati Chair in
Seismology, is developing new methods to test the hypothesis that major
earthquakes often occur after the ground has been unusually quiet for
months or years , Much of Earth shakes with a constant number of small
earthquakes created by the continual movement of plates around the
globe. The theory of plate tectonics suggests that these semi-rigid
plates move independently on a viscous underlayer in Earth's mantle.
When the plates grind against each other, the contact can cause
earthquakes or volcanic eruptions , "When there is a sudden decrease of
earthquakes in an area, it suggests something unusual is happening,"
Wyss said , Wyss hopes that identifying periods of seismic quiescence in
areas that normally shake with small tremor will someday help scientists
forecast moderate and large earthquakes around the world. Periods of
quiet activity already have been observed before the occurrence of large
earthquakes in California, Hawaii, Turkey, Utah, Japan, Italy, Armenia,
and the Pacific.
In a recent study, Wyss and Seismologist Walter Arabasz, director of the
University of Utah Seismograph Station, searched for periods of seismic
quiescence before seven large earthquakes recorded in Utah from
1974-'96. Quiet periods preceded at least three of those earthquakes ,
To find out why large earthquakes don't always follow seismic
quiescence, Wyss researched earthquakes in Japan, where much has been
published in support of the quiescence hypothesis. There, Wyss found
that seismically quiet periods precede significant earthquakes only when
sufficient underground stress has accumulated , Wyss studied three areas
around Tokyo that have been experiencing seismic quiescence for three
years. Each of the areas has experienced large earthquakes in the past ,
Within the areas, Wyss mapped and tested asperities, or hard spots,
which can supply the source of energy for an earthquake. He assigned
each asperity a "b-value" as a measure of the accumulation of
underground stress. The measuring of b-values can help seismologists
rule out other reasons for a decrease in the rate of small earthquakes
caused by physical factors, such as ground water movement.
High b-values mean it is unlikely the ground contains enough stress or
energy to produce an earthquake. Low b-values mean the ground has great
potential for producing an earthquake , All three of the areas of
seismic quiescence near Tokyo contained asperities with high b-values.
Consequently, Wyss predicts there is less than a 20 percent chance of
major earthquakes occurring in those areas any time soon. Wyss calls
areas containing seismic quiescence that do not produce major
earthquakes, "false alarms."
"False alarms do not disprove the hypotheses. They just mean the
hypothesis doesn't work in this case," Wyss said. "Scientists who
predict the weather experience false alarms quite frequently."
Wyss has made progress in confirming his hypothesis, but he is careful
to say it is not ready yet to be used as a prediction tool. He advocates
using multiple methods of prediction because earthquakes follow a
variety of patterns.
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