Mega-Tsunami Hit Southeast Asia 700 Years Ago

Tan Ee Lyn, PlanetArk 30 Oct 08;

HONG KONG - A mega-tsunami struck southeast Asia 700 years ago rivalling the deadly one in 2004, two teams of geologists said after finding sedimentary evidence in coastal marshes.

Researchers in Thailand and Indonesia wrote in two articles in Nature magazine that the tsunami hit around 1400, long before historical records of earthquakes in the region began.

"Tsunamis are something we never experienced before and after 2004, people thought it was something we would never experience again," Kruawun Jankaew of Thailand's Chulalongkorn University told Reuters by telephone.

"But from this, we are able to identify that the place has been hit by a mega tsunami in the past. So even though it is infrequent for this part of the world, it still happens and there is a need to promote tsunami education for coastal peoples."

The 2004 tsunami left 230,000 people either dead or missing across Asia, from Sri Lanka and India to Thailand, the Maldives and Indonesia. More than 170,000 of these victims were in Aceh province in Indonesia.

Jankaew's team studied a grassy plain on Phra Thong, an island north of Phuket in Thailand, where the 2004 tsunami reached maximum wave heights of 20 metres (65 ft) above sea level.

A separate team led by Katrin Monecke from the University of Pittsburgh looked at the sedimentary records on coastal marshes in Aceh, where the waves reached 35 metres.

They explored low areas between beach ridges called "swales" -- which are known to trap tsunami sand between layers of peat and other organic matter -- and discovered a layer of sand beneath the most recent layer (2004), from 600 to 700 years ago.

"Depending on where the depression is, it (the layer of the 1400 sand) can be 10 cm. But on higher ground, it can be two to five cm. Organic materials like bark and leaves, which contain carbon, were used for dating," Jankaew said.

The scientists are now trying to find out the scale of that catastrophe 700 years ago.

"We will look at the thickness and grain size of the sediment and we can calculate how fast the tsumani was, how far inland it went, and the floor depth," she said.

Jankaew said there are two more layers of sand under the 1400 layer but more studies would need to be done to date these.

Some experts blame the massive loss of lives in 2004 on ignorance of the region's tsunami history.

Very few people living along the coasts recognised natural tsunami warnings, such as the strong shaking felt in Aceh and the rapid retreat of ocean water from the shoreline that was observed in Thailand.

But on an island just off the coast of Aceh, most people safely fled to higher ground in 2004 because the island's oral history includes information about a devastating tsunami in 1907.

(Editing by Sanjeev Miglani)

Tides of history: Tsunami hit Indian Ocean 600 years ago
Yahoo News 29 Oct 08;

PARIS (AFP) – A massive Indian Ocean tsunami, similar in size to the behemoth that claimed a quarter of a million lives in December 2004, smashed into Thailand and Indonesia around 600 years ago, scientists believe.

The evidence comes from deposits of sand washed inland by colossal waves and preserved under layers of coastal peat, according to studies published in Thursday's issue of Nature, the London-based science journal.

The December 2004 tsunami was triggered by an earthquake of 9.2 magnitude that ripped open the Sunda Trench -- the fault that zigzags up the eastern side of the Indian Ocean -- along 1,500 kilometres (950 miles).

Around 220,000 people were killed in a dozen countries.

The two teams pored over coastal areas in Indonesia and Thailand, seeking sheets of sand that had been deposited in past tsunamis but had been left undisturbed by wind, rivers, storms, animals or humans.

They found the sedimentary treasure by drilling core samples in "swales," or dips between beach ridges, that are filled with peaty marsh.

The Indonesian site was found around two kilometres (1.2 miles) inland north of the town of Meulaboh in Aceh province, on Sumatra's northern tip, where waves up to 35 metres (113 feet) above sea level were recorded in 2004.

The Thai location was at Phra Thong Island, 124 kilometres (77 miles) north of the resort of Phuket, where the 2004 tsunami reached heights of up to 20 metres (65 feet).

The age of the sand sheets was derived from carbon-dating organic debris collected just below the deposit.

The Sumatra sand sheet was dated to 1290-1400 AD, and its apparent counterpart in Thailand to 1300-1450 AD. The clear proximity in dating suggests both deposits were left by the last big forerunner to the 2004 tsunami.

The Indonesian researchers found an older sand sheet, dated to 780-900 AD, but there was only sketchy evidence to match this in Thailand. The Thai team also found a sand sheet that was dated to around 2,200 years old.

In a commentary, Norwegian geologist Stein Bondevik said it was vital to confirm what these studies indicated -- that it may take some 600 years for stress to build on the Sunda Trench to the point that a 2004-style quake is unleashed.

Events occurring on such a slow scale had major implications for urban and coastal planning, he said.

"Inhabitants might consider the benefits of living close to the sea as greater than the risks of a catastrophic tsunami that will not return for many generations," he said.

"Also, it does not make sense to invest in and maintain a warning system for devastating tsunamis if they recur so infrequently.

"But smaller tsunamis may well happen more often, and a warning system could save lives during such event."

The Aceh team, led by Katrin Monecke of the University of Pittsburgh, note that catastrophes which occur in the distant past are prone to fade from folk memory.

In 2004, thousands of lives were saved on nearby Simeulue Island because the local population had been devastated by a tsunami in 1907, they note.

Over several generations, parents taught their children to flee to high ground if they felt the ground shake, thus providing a "natural" tsunami warning system.

Tsunami in 2004 'not the first'
Jason Palmer, BBC News 29 Oct 08;

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.

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."

"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?"