The Hindu : National : Earthquake magnitude was 9.3

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Friday, Feb 11, 2005

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Earthquake magnitude was 9.3

NEW DELHI, FEB. 10. The earthquake that caused the deadly tsunami was three times more powerful than previously reported, according to U.S. geologists. They warned that the danger of a local tsunami generated by a large aftershock in the region still "remains."

Latest analysis of seismograms from the December 26 Sumatra earthquake has led to revision of its magnitude to 9.3 from the previously reported 9.0, scientists at Northwestern University reported. The revised value makes it second only to the 1960 Chile earthquake "and explains in part why the tsunami was so destructive," said Seth Stein and Emile A. Okal of the Department of Geological Sciences .

The additional energy release occurred by slow slip along the 1,200-km long fault delineated by aftershocks, making the rupture zone much larger than previously thought from analysis of shorter period waves, they wrote in their website.

Because the entire rupture zone slipped, strain accumulated from subduction of the Indian plate beneath the Burma microplate has been released, leaving no immediate danger of a comparable tsunami being generated by an earthquake on this segment of the plate boundary. "However, the danger of a local tsunami generated by a large aftershock or a comparable ocean-wide tsunami resulting from a great earthquake on segments to the south remains," the scientists cautioned.

They said these results come from analysing the earth's normal modes — ultra long vibrations by which the earth rings like a bell (or more precisely rattles like a garbage can) for days and even weeks after such a gigantic earthquake.

U.S. scientists at the Woods Hole Oceanographic Institution (WHOI) have also revised the quake magnitude upward.

"I think the earthquake rupture zone is almost certain to be much longer than what was suggested by the short-period seismogram recording," said Jian Lin, a geophysicist at the WHOI.

He said the earthquake rupture zone consisted of a 450-600 km long southern segment, which radiated seismic waves that were recorded by seismogram stations worldwide; and a northern segment that is just as long as, or slightly longer than, the southern segment. "This northern segment is associated with abundant aftershocks, but did not seem to have radiated short-period seismic waves as the southern segment did. Thus the best explanation is that this northern segment may have ruptured as a `slow' earthquake," he added. "If the above interpretation is correct, the combined earthquake energy of the December 26, 2004 earthquake could be significantly larger than the short-period magnitude of 9.0," he said. — PTI

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