By Memorie Yasuda

Tsunamis Are Most Common in the Pacific Ocean

Most tsunamis are caused by large undersea earthquakes. These earthquakes generally take place in areas where a geologic process called "subduction" occurs. Subduction zones are the places where thick, mostly horizontal slabs of the earth's crust, called tectonic plates, dive under neighboring plates. Subduction is similar to the way rubber conveyor belts disappear under the edge of a grocery store checkout counter, though it doesn't operate quite as smoothly. The subducting tectonic plates often stick to the overriding plate as it moves downward. This activity causes the buildup of stress until the subducting plate is eventually released and lurches forward, generating earthquakes and releasing tremendous amounts of energy. When the earthquake causes a vertical displacement of the seafloor, a tsunami results, with the magnitude and extent of the tsunami waves depending on the size of the vertical displacement and the length of the fault rupture. The earthquake displacement effect can be thought of as similar to suddenly tilting the floor of a bathtub, generating a wave that travels to the edges of the basin.

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During the recent Sumatra earthquake, two plates lurched toward each other just offshore of the island of Sumatra, which is part of the nation of Indonesia. At the subduction zone, colliding plates slipped 35 feet past each other across a large surface area 750 miles long by 100 miles wide, ripping rock from south to north and occurring over nine minutes. The earthquake released the energy of a magnitude nine earthquake and caused the Indian Ocean tsunami. In Sumatra, such earthquakes occur every several hundred years. Similar activity takes place in an area between northern California and Washington state called the Cascadia Subduction Zone.

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