10 Janvier 2013
January 10, 2013
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
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.