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Sumatra Quake Shook Earth's Total Surface

December's great Sumatra-Andaman earthquake -- the most powerful in more than 40 years and the trigger of a devastating tsunami -- shook the ground everywhere on Earth's surface. Weeks later the planet was still trembling.


The quake resulted from the longest fault rupture ever observed -- 720 miles to 780 miles, which spread for 10 minutes, also a record. A typical earthquake's duration would be 30 seconds.


The December quake was the first of its size to be measured and studied by the new worldwide array of digital seismic instruments.


Those results are starting to come in, with a special section of a half-dozen research papers on the quake appearing in Friday's issue of the journal Science.


"This is really a watershed event. We've never had such comprehensive data for a great earthquake because we didn't have the instrumentation to gather it 40 years ago," said Thorne Lay, professor of Earth sciences and director of the Institute of Geophysics and Planetary Physics at the University of California, Santa Cruz.


"It is nature at its most formidable," Lay said in a statement.


The earthquake and resulting tsunami, which swept across the Indian Ocean, killed more than 176,000 people in 11 countries and left about 50,000 missing and hundreds of thousands homeless.


The quake occurred where two of the giant plates that form the surface of the Earth grind together.


At that spot the Eurasian plate was being pulled downward by the descending Indo-Australian plate. The quake released the edge of the Eurasian plate, which sprang up, lifting the ocean floor and sending the seawater off in the giant wave that killed so many, the researchers reported.


They said the higher sea floor displaced so much water from the Bay of Bengal and the Andaman Sea that sea level worldwide was raised 0.004 inch.


"No point on Earth remained undisturbed," wrote Roger Bilham of the University of Colorado.


Indeed, ground movement of as much as 0.4 inch occurred everywhere on Earth's surface, though it was too small to be felt in most areas.


And the temblor "delivered a blow to our planet" that was felt for weeks, noted a team of researchers led by Jeffrey Park of Yale University.


His group calculated that the quake caused the planet to oscillate like a bell, at periods of about 17 minutes, which they were able to measure for weeks afterward. A similar phenomenon was first noted in the 1960 quake in Chile.


The initial Dec. 26 Sumatra quake is estimated to have had a magnitude of 9.1 to 9.3 and a second quake to the south on March 28 registered 8.6.


By comparison, the 1960 Chile earthquake was magnitude 9.5 and the 1964 Alaska earthquake was magnitude 9.2. California's 1989 Loma Prieta earthquake had a magnitude of 6.9.


Among the other findings reported in the various papers:


* In Sri Lanka, more than 1,000 miles from the epicenter, the ground moved nearly 4 inches.


* The rupture spread from south to north, resulting in a Doppler effect in instruments measuring it. Seismometers in Russia recorded the quake at a higher frequency because it was moving toward them, while those in Australia measured a lower frequency as it moved away.


* When the surface waves from the Sumatra quake reached Alaska they triggered a swarm of 14 local earthquakes in the Mount Wrangell area.


In addition to Lay, Bilham and Park, the lead authors of the articles were Charles J. Ammon of Pennsylvania State University, Michael West of the University of Alaska and Roland Burgmann of the University of California, Berkeley. Burgmann's article was published in Science Express, the journal's online edition.


(China Daily via agencies May 20, 2005)





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