Quake triggered devastating tsunami

Process of water wall is efficient and fast

December 27, 2004|By Thomas H. Maugh II | Thomas H. Maugh II,LOS ANGELES TIMES

The magnitude 9.0 earthquake that struck off Indonesia yesterday morning moved the island of Sumatra about 100 feet to the southwest, pushing up a gigantic mass of water that collapsed into a tsunami that devastated shorelines around the Indian Ocean and the South China Sea.

The quake was the largest since a magnitude 9.2 temblor struck Prince William Sound, Alaska, in 1964 and was one of the biggest ever that scientists recorded. It triggered the first tsunami in the Indian Ocean since 1883, civil engineer Costas Synolakis of the University of Southern California said.

Yesterday's temblor, which occurred off Sumatra's northwestern tip in an active geological region, ruptured an estimated 600-mile-long stretch of the Earth beneath the Indian Ocean. The quake caused one side of the fault to slide past the other. The huge tsunami waves caused most of the damage and more than 13,000 deaths.

Tsunamis - often, and inaccurately, called tidal waves - are unlike anything else that occurs in the ocean. They are most often created by earthquakes, but can also be triggered by events including an underwater landslide or a meteor impact.

The sudden movement of Sumatra and the undersea acreage southwest of it caused a mass of water to build up well above sea level. As the water collapsed back down to sea level, it created a disturbance that affected water hundreds and thousands of miles away.

A normal wave, created mainly by wind, affects the top 30 feet of the ocean, at most, and moves very slowly. A tsunami, in contrast, affects the entire water column from surface to sea floor and can reach very high speeds. The deeper the ocean, the faster the tsunami travels. In open ocean, the tsunami moves upward of 500 mph, with the entire column of water moving up and down. However, because the ocean is so deep, initial movement of the surface is slight.

This process is very efficient, and the tsunami can travel vast distances. In 1960, a tsunami created by magnitude 9.5 earthquake off the central Chilean coast struck shores all around the Pacific, even as far away as Japan, where 200 people were killed by the surge of water.

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 great force, continuing inland until the ground level is higher than the wave.

Although the tsunami occasionally appears to be a huge 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. Yesterday's tsunami began hitting coastlines about two hours after the quake. That would have been long enough to provide warning to inhabitants if the Indian Ocean had a tsunami warning system like that in the Pacific Ocean.

The Los Angeles Times is a Tribune Publishing newspaper.

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