Saturday 10 December 2016

Effect was like 'dropping the Isle of Wight into the ocean'

Richard Alleyne and Aislinn Laing

Published 12/03/2011 | 05:00

It may have struck close to the north east coast of Japan but in less than a day the impact of the "superquake" will have been felt across the entire Pacific Ocean basin.

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The biggest earthquake on record to strike Japan sent waves rippling across the sea and crashing on shores as far as Chile, more than 10,000 miles away.

Travelling at more than 500mph, the tsunami -- Japanese for 'harbour wave' -- left Hawaii, the Pacific atolls and Australia in its wake.

Nearly a day later, it smashed against the rocks and sands of north and south America.

The epicentre of this natural disaster -- about 80 miles off the north east coast of Japan -- lies at the heart of a region known as the Pacific Ring of Fire, a 30,000 mile stretch of fault lines and volcanoes stretching in a horseshoe from New Zealand, north through Alaska and back down to Chile.

The shifting of the plates that meet at Japan released energy thought to be the equivalent of 1.5 billion tonnes of explosives.

This movement caused an underwater wave to form, that ripples out in all directions.

Dr Simon Boxall of the University of Southampton's Oceanography department, said: "It is like dropping a great big boulder into the sea -- a boulder the size of the Isle of Wight.

"The splash is huge and creates a wave around 200 miles long, that ripples out in all directions at 500mph.

"In deep water it's not very big, so shipping isn't affected but when it hits shallow water, the front of the wave slows down, effectively causing water to pile up behind it.

Impact

"This is why we see waves changing from a foot high to 30 to 40-feet high."

Dr Boxall said islands with a diameter of 60 miles or more, including any underwater sediment surrounding them, will slow down the waves and thus experience a larger tsunami impact.

The waves hit land at Sendai city with enough force to pick up cars and knock buildings but this is unlikely to be the case thousands of miles away.

Dr Ken McCaffrey, Reader in the Department of Earth Sciences, Durham University, said the waves would lose power the further they travelled.

"Waves will reach the US Coast, but they will not be anywhere near as high as Japan, and therefore will not go inland as much," he said.

The disaster is unlikely to cause as much damage as the Sumatra tsunami of 2004 because it is about half as powerful. Japan also has the world's most advanced warning systems. Up to 1,000 motion sensors around the islands are able to detect the first signs of a tremor and allow alerts to be issued nationwide. The system then sends an automatic warning broadcast on TV, radio and through mobile phones.

All TV and radio stations switch immediately to official earthquake coverage, informing the public of the risks.

The warning system is supported by strict building laws which also prevent wide-scale damage in Japan's heavily populated cities, with skyscrapers designed to sway but not crack.

Since 1952, the country also has a tsunami warning service.

There are six centres connected to 300 sensors situated across Japan's islands -- and 80 sea sensors -- which monitor seismic activity. An alert can be issued within three minutes. (© Daily Telegraph, London)

Irish Independent

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