As the tsunami nears shore and the ocean becomes shallower, friction with the ocean floor causes it to slow down, producing a buildup of water that can reach as much as 100 feet above sea level. When the water hits the shore, it sweeps inward with massive force, gradually slowing, but continuing inland until the ground level is higher than the wave.
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A Rare Tsunami, and a Change in Geography
The quake created the Indian Ocean's first wave of its kind in more than a century, and it moved the entire island of Sumatra 100 feet.
Catastrophe in Southern Asia
For The Record
Los Angeles Times Wednesday December 29, 2004 Home Edition Main News Part A Page 2 National Desk 2 inches; 69 words Type of Material: Correction
Indonesian earthquake -- An article in Monday's Section A said that Sunday's magnitude 9 earthquake had moved the island of Sumatra 100 feet to the southwest. The statement was based on information from the U.S. Geological Survey that subsequently was found to be incorrect. The quake changed the elevation of Sumatra and neighboring areas and is likely to have moved several smaller islands, but the exact displacements remain uncertain.
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Although a tsunami occasionally appears as a massive wave, more often it is like a fast-moving tide that keeps rising well past the normal high-water level.
Once the water reaches its peak, it recedes rapidly, often causing even more damage. In some cases, the tsunami can appear as several distinct waves, each creating its own havoc.
Sunday's tsunami began hitting coastlines about two hours after the quake. That would have been long enough to provide warnings to inhabitants if the Indian Ocean had a tsunami warning system like that in the Pacific Ocean. Unfortunately it doesn't, because scientists had underestimated the risk of a tsunami there, Synolakis said.
By midafternoon Sunday, the tsunami had run its course, said geophysicist Ken Hudnut of the U.S. Geological Survey in Pasadena. Generally, he said, the areas most heavily hit by a tsunami are those closest to the quake.
Because Sunday's quake was centered in the Indian Ocean, he added, little of its energy was directed toward the Americas.
Seismologists will use the opportunity to learn a great deal about the Earth's structure, Hudnut said. Because of the magnitude of the temblor, "the whole Earth would be ringing like a bell for a long time," he said. That effect will be like a gigantic medical CT scan, allowing researchers to study the structure of Earth's interior in detail.
