Page last updated at 18:00 GMT, Wednesday, 29 October 2008

Tsunami in 2004 'not the first'

By Jason Palmer
Science and Technology reporter, BBC News

Tsunami before/after (AP)
Before and after photos show the sediment coverage after the 2004 tsunami

The Indian Ocean tsunami in 2004 was not the first of its size to hit the region, according to new research.

Two international collaborations have sampled the sediments in Thailand and Sumatra to examine tsunami history.

At both sites, there was evidence of sediment laid down by a large tsunami between 600 and 700 years ago, pre-dating written and oral records.

The findings, reported in Nature, could be used to put statistical weight behind estimates of future tsunami.

The surge of a tsunami brings with it a great deal of sediment that rushes inland; the bigger the tsunami, the deeper and further inland the layer of sediment it leaves behind.

In locations where those deposits aren't disturbed by wind or running water, they can be used as a historical record of these powerful events after more layers are added.

Lake sediment sampling (Widjo Kongko)

The study of these layers in coastal regions has revealed instances of tsunami elsewhere in the world, including a prehistoric event that inundated the Shetland and Orkney islands off Scotland.

To investigate the tsunami record in the Indian Ocean basin, two research groups took core samples that capture the layers of sediment below the surface.

One group, led by Kruawun Jankaew of Chulalongkorn University in Thailand, sampled 150 sites on Phra Thong, a barrier island off the west coast of Thailand. Another group headed up by Katrin Monecke, now at the University of Pittsburgh, sampled 100 sites in the Aceh region in the north of Sumatra.

In both locations, a deep sandy layer was found beneath the surface, matching the top layer of sand left from the 2004 tsunami. By using radiocarbon dating to estimate the age of the buried sand layer, both teams found that they came from 600-700 years ago.

Dr Monecke's team also found evidence of a deeper sandy layer, with a corresponding age of about 1,200 years, suggesting a "recurrence time" for large tsunami of around 600 years.

Those buried layers occurred as far inland as those on the surface, suggesting that the tsunami that deposited them centuries ago was of roughly the same size as the 2004 event.

The team in Thailand found some suggestion of the 1,200 year-old layer and more substantial evidence for layers corresponding to around 2,000 years ago.

Timely results

Though the earthquakes that drive tsunami don't happen predictably, the results, reported in the journal Nature, suggest that another tsunami of that scale will not occur in the near future.

Roger Musson of the British Geological Survey says that the findings are "not only interesting but useful because from a point of view of understanding the hazard. It's important to know what the recurrence time is."

Kruawun Jankaew (Brian Atwater)
The samples showed a number of sandy deposits correlated to tsunami

"Geological data is increasingly being used to back up forecasts of how likely there is to be large earthquakes in the future."

From Dr Monecke's point of view, that kind of information can serve as a basis for tsunami education in the region. That, she says, could contribute to policy decisions in the near term.

"For coastal planners I think it's very important to know this," Dr Monecke told BBC News.

"We saw that whole villages were being relocated 10km [6 miles] inland, and these people are mainly fishermen.

"You have to balance this; would it be better to be that far away so that if in a few generations, another tsunami hits, and they are that far away, or would it be better to stay at the coastline and be prepared for it?"

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