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Japan's 'Big One' hit, but not where it was expected

This looks like the Big One for Japan — but it's in the wrong place, seismically and bureaucratically.

Japanese geologists have long forecast a huge earthquake along a major fault line southwest of Tokyo and have poured enormous resources into monitoring the faint traces of strain building in that portion of the Earth's crust. They have predicted the amount of property damage and the number of landslides. They have even given the conjectured event a name: the Tokai Earthquake.

But now the largest recorded earthquake in Japan's history, measured at a stunning magnitude 8.9, has hit far to the north, some 231 miles northeast of Tokyo.

The epicenter was 80 miles off the coast of Sendai province on Honshu, Japan's largest island. It caused a rupture 186 miles long and 93 miles wide 15 miles beneath the sea floor. The epicenter was also beneath the sea floor near a major boundary between two plates of the Earth's crust. At the boundary is a subduction zone, a place where one tectonic plate dives beneath the other, forming a deep trench. Although subduction zones, including this one, are known to cause earthquakes — and there was a significant temblor of magnitude 7.9 just two days ago near Friday's event — there is no record of such a "mega-quake" along this portion of the fault.

Scientists Friday said the event has once again humbled them, reinforcing a growing sense that the field of seismology needs to ditch some of its presumptions about major earthquakes.

"It took place in a stretch of the coast of Japan that was not considered prone to mega-earthquakes," said Emile Okale, a Northwestern University geophysicist reached in Tahiti, where he was preparing to evacuate in advance of the tsunami generated by the Japan quake. He said the huge Sumatra earthquake six years ago that generated the devastating tsunami along the rim of the Indian Ocean also happened on what was presumed to be a relatively quiescent stretch of a subduction zone.

"It's really just a kind of guessing game, and Mother Nature never really puts up with those guessing games," said seismologist Dave Wald of the U.S. Geological Survey's National Earthquake Information Center in Golden, Colo.

Susan Hough, a USGS seismologist in Pasadena, Calif., noted that the recent 6.1 magnitude earthquake in Christchurch, New Zealand, happened on an unmapped fault and caught scientists somewhat by surprise.

"We do tend to focus on the expected events. We're going to get blindsided by unusual events. Sumatra was not a common event. This one is not common. Christchurch was not common. But uncommon events happen," Hough said. "The analog that's worrisome is Boston. Put a 6.1 under Boston. You have all that unreinforced masonry."

Scientists develop hazard maps showing where earthquakes are most likely to occur, but the field remains fraught with controversy over the extent to which seismic events can be forecast. Seismic hazard maps for Japan, including one on the U.S. Geological Survey Web site, show a relatively modest earthquake hazard — by Japan's standards — for the region off the coast of Sendai.

Japan's 'Big One' hit, but not where it was expected 03/11/11 [Last modified: Friday, March 11, 2011 10:30pm]
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