A Rare Tsunami, and a Change in Geography
The magnitude 9.0 earthquake that struck off Indonesia on Sunday morning moved the entire island of Sumatra about 100 feet to the southwest, pushing up a gigantic mass of water that collapsed into a tsunami and 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 recorded by scientists. It triggered the first tsunami in the Indian Ocean since 1883, civil engineer Costas Synolakis of USC said.
For the record:
12:00 a.m. Dec. 29, 2004 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.
Sunday’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, much like seismologists expect the San Andreas fault to do when the “Big One” hits California.
The massive tsunami waves, not ground shaking, 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.
In Sunday’s case, 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. But because the ocean is so deep, initial movement of the surface is very slight. Someone on a boat in the area wouldn’t notice it.
This process is very efficient, and the tsunami can travel vast distances. In 1960, a tsunami created by a 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 massive force, gradually slowing, but continuing inland until the ground level is higher than the wave.
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.