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Massive Earthquake in Asia a Geological Rarity

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WASHINGTON - Geologists describe Sunday's powerful earthquake in the Indian Ocean as a once-in-a-generation event.

The U.S. Geological Survey says the 9.0 magnitude earthquake that triggered the devastating tsunami, or tidal wave, occurred at a depth of 10 kilometers, and was centered west of the Indonesian island of Sumatra, some 1,600 kilometers northwest of Jakarta.

"This is actually the fifth-largest [seismic] event since 1900. It is the largest since the Prince William Sound earthquake in Alaska of 1964. It is pretty rare," said Julie Martinez, a USGS geophysicist in the western U.S. state of Colorado.

Geologists say the earth's crust is not a seamless skin. Rather, it is fractured into seven major plates, and more than a dozen lesser ones. These plates are not stationary; they move and collide with one another, creating what are known as faultlines. Along some faultlines, one plate's edge will sink beneath another. In other cases, the plates rub and slip sideways along the faultline.

Julie Martinez says Sunday's earthquake occurred along the so-called Andaman Thrust, a faultline that runs between several plates in the eastern Indian Ocean.

"This earthquake is on a quite [active] seismic zone. It is on the Andaman Thrust. The plates are colliding. It is the collision of the Australia and Indonesia plates with the Burma plate. With this size of an earthquake, it caused a rupture of about a thousand kilometers along the Andaman thrust," she explained.

Ms. Martinez says the tidal wave spawned by the quake reached as far away as Somalia's coastline, but in a much-weakened form. A series of aftershocks have been recorded, including one registering 7.3 on the Richter scale that jolted parts of Bangladesh. Minor aftershocks have been detected as far away as the U.S. states of Alaska and California.

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