Friday, May 20, 2005

December’s tsunami-triggering earthquake shook the ground everywhere on Earth’s surface — leaving the planet trembling for weeks — and displaced so much water from the Bay of Bengal and the Andaman Sea that the sea level worldwide was raised 0.004 inches.

“No point on Earth remained undisturbed,” wrote Roger Bilham of the University of Colorado in one of the first research papers on the largest quake measured by a growing array of digital seismic instruments worldwide.

The Sumatra-Andaman quake caused the planet to oscillate like a bell, which was measurable for weeks afterward, and ground movement of as much as 0.4 inches occurred everywhere on Earth’s surface — though it was too small to be felt in most areas, teams of researchers have concluded.

Data show the Dec. 26 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.

“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 at Santa Cruz.

“It is nature at its most formidable,” Mr. Lay said.

Six research teams have written papers on the quake and resulting Indian Ocean tsunami, which killed more than 176,000 people in 11 countries, that will appear in a special section of today’s issue of the journal Science. About 50,000 people are still missing and hundreds of thousands homeless.

The quake, second strongest ever recorded and the third most deadly in human history, occurred where two of the giant plates that form the surface of the Earth grind together, releasing energy equivalent to the amount consumed by the entire United States for six months.

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 sea water off in the giant wave and raised the sea level worldwide, the researchers reported.

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, which measured the oscillations, said a similar phenomenon was first noted in the 1960 quake in Chile.

The initial 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. The worldwide network of digital seismic instruments did not exist then. 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 Mr. Lay, Mr. Bilham and Mr. 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 at Berkeley. Mr. Burgmann’s article was published in Science Express, the journal’s online edition.

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