March 21, 2011

Geologists Warn Of More Stress Near Tokyo After Earthquake

Geologists say the earthquake that rocked Japan on March 11 has altered the earth's surface and loaded more stress on a fault line much closer to Tokyo.

Roger Musson of the British Geological Survey said that the structure of the tectonic plates and fault lines around the city makes it unlikely that Tokyo would be hit by a quake anywhere near the intensity of the 9.0-magnitude one that struck a few weeks ago.

However, any trembler could be devastating given the 39 million people who live in and around Tokyo.

"Even if you've got, let's say, a 7.5, that would be serious," the seismologist said in a statement.

Japan is located on the Ring of Fire and is regularly hit by earthquakes.

Andrew Moore of Earlham College in Richmond, Indiana told The Associated Press (AP) that few geologists considered Japan to be a strong candidate for a 9-plus earthquake before last week.

However, there is mounting evidence that Japan has been struck by several severe quakes in the last 3,500 years.  Moore said sand deposits indicate that several quakes have spawned 30-foot-high waves that slammed into the northern island of Hokkaido.

Similar deposits underlie the city of Sendai with the most recent from an 869 A.D. tsunami that killed 1,000 people and washed more than 2.5 miles inland.

Eric Fielding of NASA's Jet Propulsion Laboratory said that the March 11 earthquake changed the coastal landscape.  He said in a statement that it created a trench in the sea floor 240 miles long and 120 miles wide as one tectonic plate drove 30 feet beneath another.

Brian Atwater, a geologist with the U.S. Geological Survey, told AP that the quake appears to have piled pressure onto adjacent segments.

He said that added strain could now trigger a strong, deadly aftershock on Tokyo's doorstep.

It is common for this to take place after strong earthquakes, and took place after the 2004 mega-earthquake and tsunami off Indonesia that killed 230,000 people in a dozen nations.

An 8.6-magnitude earthquake erupted farther down the fault line three months later and killed 1,000 people on sparsely populated Nias island.

"But it's difficult to say," said Atwater. "There are good examples of such stresses leading to other earthquakes, big earthquakes, and there are good examples of that not happening."

Scientists are studying the March 11 earthquake and ongoing seismic activity to determine where new strains might be building.

"When the main shock is this big, you get a football-shaped region where aftershocks are fair game. It extends in all directions," including toward Tokyo, USGS seismologist Susan Hough and other experts said.

However, they said that it is hard to keep up.

"We are drinking from a fire hose here. The input data keeps changing and augmenting," Ross Stein, of the USGS, wrote in an email to AP.


Image Caption: Ground rupture caused by the Sendai Earthquake 2011. Credit: Danny Choo/Wikipedia (CC BY-SA 2.0) 


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