October 30, 2008

Ancient Tsunami Hit Asia

Geologists have dug deep enough to find that a mega-tsunami hit southeast Asia 700 years ago rivaling the deadly one in 2004.

Two teams of geologists found sedimentary evidence in coastal marshes in Thailand and Sumatra.

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.

"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 said.

"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 findings could be used to put statistical weight behind estimates of future tsunamis.

The surge of a tsunami brings a huge amount 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.

Roger Musson of the British Geological Survey says 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."

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.

A small amount of coastal dwellers recognized 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 ran to higher ground in 2004 because the island's oral history remembers a devastating tsunami in 1907.

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 meters (65 ft) above sea level.

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

"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 tsunami was, how far inland it went, and the floor depth," she said.