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The EarthQuake
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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