Japan quake could raise concerns elsewhere

[May 20, 2011]  WASHINGTON (AP) -- Scientists sifting through data from the great Japan earthquake in March are uncovering surprises that may raise concerns nearby.

Researchers led by Mark Simons of the California Institute of Technology are urging close monitoring of seismic activity in the Ibaraki region immediately south of the spot where the most recent quake occurred.

They are not predicting another quake, Simons stressed in a telephone interview. But the area where the deadly March temblor struck "was believed by many to be not likely to produce a big quake, and that was wrong." So that raises questions about other, similar regions, he said.

"We learned we have to be much more suspicious about what we know for sure, and more explicit about what we don't know," Simons said. Monitoring the region will give scientists clues to the movement of the undersea plates that slipped in the quake.

Simons' comments came as the journal Science was publishing a series of technical papers in its online edition Thursday, with data from the 9.0 magnitude Tohoku-Oki quake March 11. The quake and following tsunami are believed to have killed more than 24,500 people. Police reported last week that 15,019 were dead and 9,506 were still listed as missing. In addition, radiation leaks at the Fukushima nuclear power plant have forced 80,000 people living within a 12-mile (20-kilometer) radius of the facility to leave their homes for an indefinite time.

David Wald, a seismologist at the U.S. Geological Survey in Golden, Colo., said that turning attention now to the nearby Ibaraki region makes sense. "Adjacent segments are always a concern after a large event," he said.

"And Ibaraki is closer to Tokyo," added Wald, who was not part of Simons' team.

While the new report raises concern about the area next to the rupture zone, the issue also extends globally, said Thorne Lay, an earthquake expert and professor at the University of California, Santa Cruz.

"There are many regions for which the history of earthquake occurrence is very poorly documented," he said. "We have a long record for Japan relative to most regions, but even there we got surprised."

Lay, who was not part of Simons' team, noted that in March a large plate movement occurred far offshore and the question is whether there is similar potential for seismic activity offshore farther south, where there are larger population centers.

Sensors can measure whether strain is building up near the coast, "but it's hard to tell what's happening 100-to-150 kilometers (62-93 miles) off the coast. We have limited capabilities for doing that right now," Lay said.

A second study by Mariko Sato of the Japan Coast Guard and colleagues reported a large displacement of the Earth's crust including movement in one area of more than 20 meters (66 feet) sideways and 3 meters (10 feet) vertically in a long undersea section of the trench marking the boundary of the moving plates that caused the quake.

The March earthquake started with relatively small shocks over the first three seconds, a third paper led by Satoshi Ide of the University of Tokyo reported: "Thereafter, the earthquake quickly grew into a large event."

Following the brief initial shaking there was a deep rupture for about 40 seconds, the team reported, followed by a shallow ground movement for 60-to-70 seconds and then a continuing deep ground rupture for more than 100 seconds.

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Online:

Science: http://www.sciencemag.org/

[Associated Press; By RANDOLPH E. SCHMID]

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