Saturday 3 December 2016

Fault-line stress could trigger another quake

Ian Rogers in Tokyo

Published 22/03/2011 | 05:00

Tokyo is at risk from a powerful earthquake because the recent monster shock that hit the country put more stress on a nearby geological fault line, scientists warned yesterday.

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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 March 11's 9.0 quake, said Roger Musson of the British Geological Survey.

But given the vast population -- the capital and its surroundings are home to 39 million people -- any strong tremor could be devastating.

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

Japan is located on the Ring of Fire, an arc of volcanos and fault lines spanning the Pacific Basin, and is regularly hit by earthquakes.

But before the recent quake -- the largest to hit the country since records began 130 years ago -- few geologists considered Japan to be a strong candidate for a 9-plus earthquake, said Andrew Moore, of Earlham College in Richmond, Indiana.

But there is mounting evidence that Japan has been struck by several severe quakes in the last 3,500 years -- most in the northern reaches of the country.

Sand deposits indicate that several quakes have spawned 30ft-high waves that slammed into the northern island of Hokkaido, he said, the most recent in the 17th century.

Similar deposits underlie the city of Sendai -- the area rocked 10 days ago -- with the most recent from an 869 tsunami that killed 1,000 people and washed more than 2.5 miles inland.

And even weaker quakes that hit Tokyo in the past have caused significant damage.

But the most recent tremor changed the coastal landscape -- and not just above sea level. It created a trench in the sea floor 240 miles long and 120 miles wide as one tectonic plate dived 30 feet beneath another.

While that relieved stress at the breaking point, it appears to have piled pressure on to adjacent segments, said Brian Atwater, a geologist with the US Geological Survey. That added strain could now trigger a strong, deadly aftershock on Tokyo's doorstep.

Irish Independent

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