Scientists challenge accepted theory on where quakes occur

January 10, 2013

By SHIGEKO SEGAWA/ Staff Writer

Even slowly shifting tectonic plates can cause major earthquakes, something seismologists have previously considered impossible, two scientists said.

According to accepted seismological theory, seismic events occur when stress builds up in areas where two tectonic plates are fixed against each other and causes them to undergo a large slip.

It has been believed that little stress builds up in slow-slip regions, where plates slowly move against each other.

Hiroyuki Noda, a scientist with the Japan Agency for Marine-Earth Science and Technology, and a colleague at the California Institute of Technology jointly conducted computer simulations of earthquake cycles over millenniums.

They showed that areas where plates slide against each other only slowly during normal times may undergo seismic slip when major movement caused by a large earthquake in a nearby fixed region propagates at high speeds toward the slow-slip region.

The numerical results also showed that a slow-slip region can turn into a fixed region under certain circumstances.

Before the Great East Japan Earthquake struck in March 2011, seismologists believed that little stress was accumulating and no large earthquake would likely occur in the areas around its hypocenter beneath the Japan Trench, where the oceanic plate is only slowly diving beneath the continental plate.

The devastating magnitude-9.0 temblor, which involved major slippage along that part of the plate boundary, was received as proof against the accepted seismological view.

"Plate motions are complicated," Noda said. "There is a need to evaluate risk carefully on the basis of long-term records, including records of tsunami deposits."

The research results were published online on Jan. 9 in Nature, a British scientific journal.

By SHIGEKO SEGAWA/ Staff Writer
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