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Friday, Jan. 11, 2013

3/11 study finds clues to why faults thought 'stable' may suddenly slip

AFP-Jiji

PARIS — Seismologists said Wednesday they have found clues as to why the March 11, 2011, Great East Japan Earthquake occurred on a fault previously deemed to be of little threat.

The findings, published Wednesday in the journal Nature, have repercussions for the country's earthquake strategy and for other locations, including California's notorious San Andreas fault, with a similar seismic profile, they said.

Hiroyuki Noda of the Japan Agency for Marine-Earth Science and Technology and Nadia Lapusta of the California Institute of Technology based their findings on a computer model of the March 2011 mega-quake, which triggered tsunami that killed about 19,000 people and wrecked the Fukushima No. 1 nuclear plant, sparking the world's worst atomic crisis in a generation.

The 9.0-magnitude earthquake struck off Tohoku in part of the so-called Japan Trench, where the Pacific plate ducks beneath the Okhotsk plate, on which the Japanese archipelago lies.

This area of the Japan Trench had been generally considered to be stable, as it was a "creeping" segment, meaning any movement of the plate there was smooth and regular.

A commonly accepted theory says this steady movement prevents stress from building up to the point where the fault rips open — rather like a safety valve on a steam engine.

But Noda and Lapusta suggest fault segments that have long-term, stable "creep" in fact weaken when a nearby section ruptures. And if the fault is infiltrated by hot geological fluids, this acts as a lubricant, helping a big slip to occur.

"Steadily creeping fault segments are currently considered to be barriers to earthquake rupture. Our study shows they may join large earthquakes, amplifying seismic hazard," said Noda.

The authors said they hope their work will be factored into Japan's earthquake awareness program. Some experts have accused the program of focusing obsessively on the risk to Tokyo.

The findings also have implications for risk assessment for the San Andreas, which also has a creeping segment regarded as a blocker for big earthquakes, said Noda.

"But whether it always acts as a barrier or can join a great earthquake is not a trivial question," he warned.



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