Large Earthquake Closer to Tokyo May Be Possible, Study Says

The earthquake that shook Japan in March, triggering a tsunami that left more than 24,000 people dead or missing, was bigger than seismologists expected and may indicate another large temblor closer to Tokyo is possible, U.S. researchers said.

Before the 9-magnitude quake on March 11, the buildup of tension along a fault had suggested a smaller tremor, scientists led by Mark Simons, a professor of geophysics at the California Institute of Technology in Pasadena, wrote yesterday in Sciencexpress, the online version of the journal Science.

The so-called Great East Japan Earthquake happened on a fault known as the Japan Trench megathrust, where the Pacific Plate sinks below the plate on which Japan sits at about 8 centimeters (3 inches) a year, the researchers said. During the quake, some parts of the Pacific Plate slipped as much as 60 meters, which is as much as three times more than the temblor that rocked Chile in February last year.

“The extent to which the 2011 earthquake was unexpected suggests that we should consider the potential for similar large events elsewhere on the Japan Trench megathrust,” Simons and colleagues wrote. “An earthquake similar to the 2011 event is possible offshore Ibaraki and Fukushima just south of the most recent event.”

Sea Mountain

The researchers hypothesized that a mountain on the seabed of the sinking plate may have prevented it from slipping for as long as 1,000 years, leading seismologists to the false conclusion that it was sliding slowly under the Japan plate without building up quake-generating tension. Instead, the mountain may have been limiting the amount of slip on neighboring areas and accumulating tension, they wrote.

An area south of where the quake occurred that hasn’t ruptured may hold the potential for another large temblor, the researchers wrote.

Scientists should make “a concerted effort to determine the extent to which this area is stuck with the potential for generating a large earthquake or whether it is creeping, thereby posing less of a threat,” Simons said in an e-mail.

A 6-magnitude earthquake struck off the coast east of Tokyo at 9:46 a.m. today, according to the U.S. Geological Survey. It was the biggest aftershock to hit the country since May 14, when a 6.2-magnitude temblor struck further north.

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