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Scientists Reexamine Huge December Earthquake

March 30, 2005

DENVER (AP) — Researchers at a U.S. university have recrunched data from December’s devastating earthquake in Sumatra and found it was even more powerful than previously believed, but other scientists suggest their findings are premature.

A study by seismologists at Northwestern University increases the intensity of the Dec. 26 earthquake from a magnitude 9.0 to a whopping magnitude 9.3. That’s about three times more powerful than initial estimates.

However, the new magnitude has yet to be accepted by the U.S. Geological Survey – regarded as one of the world’s leading authorities for such information – and the international earthquake research community. Several other studies to recalibrate the earthquake’s magnitude aren’t completed yet.

If the Northwestern calculations are later confirmed, it would make the undersea Sumatra-Andaman earthquake the second-most powerful ever recorded. The quake and the tsunami waves it generated already are considered to be among the world’s worst natural disasters with an estimated 300,000 people killed or missing.

“There is currently no firm consensus on the ‘correct’ magnitude of this earthquake,” although it is likely to change from 9.0, said geophysicist Stuart Sipkin of the USGS Earthquake Information Center in Golden, Colo.

Details of the Northwestern recalibration appear in Thursday’s issue of the journal Nature.

In a second Nature study, Chinese researchers recalculated the rupture of the Sunda Trench where the earthquake occurred off the west coast of Sumatra.

By analyzing high-frequency energy radiation released by the earthquake, geophysicists at the University of Science and Technology at Hefei concluded the rupture extended northward along the fault for 720 miles (1,200 kilometers), or nearly twice as long as initial assessments.

Other scientists said the new, bigger numbers – both the quake size and the rupture length – could be good news for Indonesia and the rest of the region. The more energy an earthquake releases, the more it reduces the danger of another catastrophic event along the same segment of a fault, perhaps for hundreds of years.

But it doesn’t mean that other segments won’t slip with devastating consequences – as seen in Monday’s magnitude 8.7 earthquake in another segment of the Sunda Trench to the south off Nias Island.

Seismologists believe the Sunda’s southern stretch was in the most immediate danger of rupturing within weeks of the Dec. 26 quake, and Monday’s earthquake was virtually coupled to the larger quake three months ago.

Sumatra is one of the world’s most seismically active regions. Off its west coast lies a subduction zone where plates of the Earth’s crust grind and dive, generating great disturbances of the earth and seas.

Scientists say even greater uncertainty persists in adjacent faults extending northward to Mynamar, Bangladesh and India, where hundreds of millions of people live in the shadow of the Himalayas.

“Long sections of the enormous thrust fault along which India is diving down beneath the Himalayas have not failed for centuries,” said California Institute of Technology seismologist Kerry Sieh, “and they are only one to three fault lengths away from the 2004 rupture.”

Researchers commonly revise earthquake assessments as more data becomes available from instruments around the world. So it’s normal for scientists to revise their take on the December event.

But Northwestern seismologists Seth Stein and Emile Okal said they were concerned that conventional surface seismic measurements “dramatically underestimated” the size of the earthquake.

This earthquake broke slowly as it ruptured northward along the Sunda Trench, so they recalculated the new magnitude after analyzing the very low-frequency seismological signal generated by the quake.

USGS officials said magnitude 9.0 quakes are so rare that no others have been measured using the same method and scientists don’t know whether other giant earthquakes behave the same way.

For example, Sipkin said other measurements show the magnitude 9.2 Alaska earthquake in 1964 was larger than the Sumatra quake. But slow-slip measurements did not exist then, and comparing the different data from the two earthquakes now is “like comparing apples and oranges,” he said.

“Changing the magnitude of the Sumatra earthquake to 9.3 would not only be premature at this time,” Sipkin said, “but would place it in the wrong position on the list of largest earthquakes.”

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