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Indian Ocean earthquake data suggest disaster warnings too conservative

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20.05.2005

The December earthquake and tsunami that killed approximately 300,000 people in the Indian Ocean region was so powerful that no point on Earth went undisturbed, pointing to the need for more active warnings about the consequences of future events, according to University of Colorado at Boulder seismologist Roger Bilham.

Bilham offers his perspective in "A Flying Start, Then a Slow Slip," an overview of findings on the Sumatra-Andaman earthquake published in the May 20 issue of Science Magazine. The issue also includes four technical papers by other authors describing the complex rupture process of the earthquake.

"No point on Earth remained undisturbed at the centimeter level," Bilham said. "The earthquake’s uplift reduced the capacity of the Bay of Bengal and the Andaman Sea, raising sea level around the world by about .1 millimeter.

"If not for the remarkably slow plate movement at the northern end of the earthquake, there might have been much more widespread and severe damage on the coasts of India, Myanmar and Thailand," Bilham said.

Two years ago, Bilham published a study of an 1881 earthquake in the same region and predicted that a similar event could occur sometime between 2004 and 2054. Bilham didn’t anticipate the strength of the 2004 event, though, and said officials need to consider extreme worst-case scenarios as well as more probable earthquake scenarios.

"The region has a history of major earthquakes, including ones in 1833 and 1861. Regardless, there was no precedent for the complexity and magnitude of the 2004 earthquake. This should be a wake-up call that conservative seismic forecasts may not serve society well," he said.

"This earthquake happened at the worst possible time – on a very popular holiday when many people were at the beach instead of at work or in school, and at high tide in India, which increased the tsunami run-up there by one meter," Bilham said.

The Dec. 26 quake was the second largest ever recorded, and the third most fatal in human history. The energy released was equivalent to a 100 giga-ton bomb, or the amount of energy consumed in the U.S. every six months, according to Bilham. "More than 30 cubic kilometers of water were displaced by the shifting sea floor, resulting in a tsunami that traveled as far away as the Antarctic, both coasts of the Americas and even the Arctic Ocean."

Using data recorded by digital seismometers all over the world, scientists were able to determine the direction and speed of the rupturing seafloor.

"The rupture opened lengthwise at 5,000 miles per hour during the first 10 minutes of the earthquake. Seismometers in Russia and Australia recorded the event like a noisy fire engine racing northward," Bilham said. He explained that Russian seismometers recorded higher frequency sounds than those recorded in Australia, revealing a seismic Doppler effect as the sound traveled away from Australia and toward Russia.

For seismologists including Bilham, this was the first catastrophic earthquake that could be analyzed using the latest and most sensitive scientific equipment. "As a result, we will learn numerous new things about our planet, and in particular about the Pacific Northwest, where a similar earthquake could occur at any time," he said.

Scientists believe that a major earthquake and tsunami hit the Pacific Northwest around 1700 along the Cascadia Subduction Zone and that the northwest will experience major quakes, and possibly tsunamis, in the future.

Roger Bilham | Source: EurekAlert!
Further information: www.colorado.edu

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