Page last updated at 07:24 GMT, Wednesday, 27 May 2009 08:24 UK

An old flame in the Ring of Fire

By Richard Black
Environment correspondent, BBC News, Japan

Tonankai image
The 1944 Tonankai earthquake devastated the region

Every hundred years or so, an unwelcome visitor arrives on the south-eastern coast of Honshu, Japan's largest island.

Heralded by a tremendous shaking of the ground, the sea sucks back before rushing onto the shore with devastating force, bringing destructive surges nine or 10 metres high.

The activity continues for maybe a year maybe two; then the Earth calms, apparently at ease, and the inhabitants of the Kii Peninsula and the surrounding lands - shorn of some of their fellows, now - can return to their fields and rebuild their houses.

Welcome to the Pacific "Ring of Fire"; welcome to the Nankai Trough.

Here, about 60km off the Japanese coast under the currently balmy Pacific Ocean, two of the Earth's tectonic plates are colliding.

At a rate of 4cm per year, the Philippine Sea plate is being forced underneath the Eurasian plate, on which sits most of Japan.

Chikyu diagram
The ship will sample "cores" of rock from the sites where plates slip.

Where their surfaces are lubricated by a mixture of water, soft sediments and heat, they flow smoothly over one another. But in the Nankai Trough, they snag; simple friction at the plate boundary prevents the Philippine Sea plate from disappearing quietly.

Every century or so, the stresses in the subduction zone become too much. The plates relieve their tension in a sudden rip of energy: the Eurasian plate edge bucks upwards, setting the Earth rippling, and sending a hugely energetic column of water hurtling across the ocean.

Earth story

Of all the seismological hot-spots around the Pacific Rim, none has been as well documented as the Nankai Trough.

"We have just a phenomenal record of earthquakes here," says Harold Tobin from the University of Wisconsin-Madison.

"The coastal population in this part of Japan has given us a 1,400-year recorded history of when big quakes took place, and when big tsunamis came inland and caused damage."

The written record is supplemented by investigations in what has become known as palaeoseismology.

Land uplifted by quakes contains records of its previous elevation - traces of marine life, now suspended metres above the sea.

Tsunamis splash material from the sea floor into fields, creating sandy bands between the more conventional layers of soil and peat.

All of this data can be used to reconstruct the timing and the scale of previous events. And what the record here shows is that the Nankai Trough has regularly brought earthquakes estimated at more than magnitude eight - sometimes just one, sometimes two or three separated by intervals of between a single day and a few years.

Nankai locator map

Events that occurred here before the last century are now given appellations corresponding to eras in Japanese history: the Meio earthquake of 1498, the Keicho quake of 1605, the Hoei quake of 1707.

But although the events are all big in seismological terms, they are not all alike.

"In 1944, we had a very great earthquake, and almost 100 years before that, again a great earthquake," says Gaku Kimura from the University of Tokyo.

"But 400 years before that, we had not such a big earthquake but a very serious tsunami.

"In this case, it means a much slower slip along the plate boundary - so it's a difficult issue, and very interesting."

Sometimes the trough springs apart just once, along its whole length; at other times, half of it un-snags, creating more stress in the adjoining section, generating a second quake there.

These are issues that carry practical implications, with Nankai's next destructive burst probably only a few decades away.

At fault

Professor Kimura has been able to get to grips with the issue at first hand as one of the first chief scientists on the NanTroSEIZE (Nankai Trough Seismogenic Zone Experiment) project - the most ambitious attempt yet to unravel the inner workings of a subduction zone.

Over its scheduled six years of operation, scientists will drill down right into the earthquake-generating zone and try to understand exactly what is happening at the plate boundary and what that means for the production of quakes and tsunamis.

Already, the drills have penetrated the accretionary prism, the region above the plate boundary itself where sediment carried westward on the Philippine Sea plate is scraped off by the Eurasian plate.

Rock cores from NanTroSEIZE project
Rock cores from the project serve as a history of seismic activity

This region is what actually contacts the water, so how it behaves during an earthquake is clearly going to determine whether a tsunami results, and if so, how powerful it will be.

"Statistically, the great earthquakes in the world - not all cases, but many cases - are associated with the development of such an accretionary prism," says Professor Kimura.

"And the Nankai Trough is an example - so is Sumatra."

Fire warnings

The 2004 Sumatran earthquake and the tsunami that followed are still very much in living memory - as, just barely, are the most recent upheavals in the Nankai Trough.

The Tonankai and Nakaido quakes in 1944 and 1946 respectively each caused about 1,000 fatalities, with a total of about 100,000 houses swept away.

Both quakes induced tsunamis, the first being felt as far away as Alaska.

On board the research ship Chikyu as this year's NanTroSEIZE field season gets underway, the expedition's co-chief project scientist Harold Tobin is looking forward to unravelling the mechanics of this geologically complex and socially important region.

"Another quake here would be pretty devastating," he says.

"The coastal region is much more populated now.

"And although there are much better tsunami warning systems now and I hope that would prevent some loss of life, I don't see how there could not be some loss of life."

The Japanese government, which provides the major part of the funding for NanTroSEIZE, estimates there is a 60% chance of a quake of 8.1 magnitude or greater here within the next 30 years.

The Ring of Fire may have its embers banked for now, but the flames will inevitably rise again.

Perhaps by understanding the patterns of the past and the mechanics of the present, it may be possible at least to give some warning of who is going to feel the heat, and when.

Richard.Black-INTERNET@bbc.co.uk



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