Brian Atwater, University of Washington

Uncovering past disasters. Digging into Southeast Asian soils has revealed that the 2004 Indian Ocean tsunami wasn't unique.

A Disaster Spelled Out in Sand

Within a week of the 26 December 2004 Indian Ocean tsunami, geologist Kruawun Jankaew set out from Chulalongkorn University in Bangkok to survey the devastation on the southern coast of her country. What she saw shocked her. "I never thought such a thing could happen," she recalls. "In some places, we could still smell dead bodies."

But the catastrophe, which killed more than 225,000 people across Southeast Asia, also gave Jankaew and other researchers an unprecedented opportunity to understand what scars such a disaster leaves on the physical landscape. By comparing sand deposits from the tsunami to deeper layers in the soil, two independent teams of scientists now conclude that a similar killer wave struck the region 500 to 700 years ago.

The 2004 tsunami was triggered by a magnitude-9.0 earthquake near western Sumatra. As the Indo-Australian tectonic plate plunged under the Euro-Asian plate, displaced water hurtled toward the shorelines. In Thailand, the hardest hit country after Indonesia, the sea rushed 2 kilometers inland, dumping sand and marine debris such as diatom shells on the dark, peat soil.

Although historical records for the past 2 centuries mentioned no similar catastrophe, geologists reasoned that the 2004 tsunami could not have been the first such calamity from the unstable Indo-Australian/Euro-Asian fault line. So Jankaew and her team traveled to Phra Thong Island off the southwest coast of Thailand to examine sediment deposits there. They took core samples at more than 150 sites and looked for sand swaths sandwiched between peat layers that resembled the 2004 deposits.

The researchers found two sand layers embedded deep within the soil. Radiocarbon dating of dead leaves and organic waste in the sand put the shallower sand deposit at no more than 700 years old and the deeper sheet less than 2800 years old, the researchers report online today in Nature.

Meanwhile, geologist Katrin Monecke of the University of Pittsburgh at Johnston in Pennsylvania and colleagues used the same technique to date sand sheets in the Aceh region of northern Sumatra, which also was devastated by the catastrophe. There, the tsunami wave swept 1.8 kilometers inland. The researchers detected two sand sheets at sites within the soil, which radiocarbon dating showed were about 600 and 1000 years of age, the researchers also report today in Nature.

Only tsunamis could deposit such widespread sand sheets, say Jankaew and Monecke. Paleoseismologist Jody Bourgeois of the University of Washington, Seattle, who was not involved in either study, agrees. The sand layers were too distinct from the peat to have been caused by a smaller storm, she says. "This makes it more clear that 2004 was not an unusual event," Bourgeois says.

But Monecke says researchers cannot draw final conclusions about how often tsunamis recur in the area from these initial findings. Still, both teams of researchers and Bourgeois say that the findings should improve understanding of the history and patterns of tsunami disasters in the region. "I should hope that if in the future an event like this occurs, our work will help in one way or another to reduce the loss of life," Jankaew says, noting that governments should encourage education about warning signs of tsunamis and construct critical facilities such as hospitals further inland.

Posted in Earth, Asia