Scientists discover a historical record of massive tsunamis

In a discovery that holds lessons for the Northwest coast of the United States, scientists have found the deadly Indian Ocean tsunami in 2004 was not a one-time fluke.

Sand layers buried along the coast of Thailand show at least three of the massive waves struck the region over the past 2,800 years, the most recent between 550 and 700 years ago.

"The previous, major Indian Ocean tsunami probably happened before Christopher Columbus," said University of Washington geologist Brian Atwater, a co-author of the study published today in the journal Nature. "People knew nothing about it, and that lack of information contributed to the great loss of life in 2004."

Nearly a quarter of a million people were killed in Indonesia, Thailand and nine other nations when a magnitude 9.2 earthquake on an offshore fault triggered the tsunami Dec. 26.

A similar fault lies off the coast of Washington, Oregon and California. Atwater was the first to discover evidence of past earthquakes on that fault, piecing together a geologic record that shows major shaking every 500 to 600 years. The most recent, accompanied by a tsunami that pounded the Pacific Northwest and traveled all the way to Japan, was about 300 years ago.

"The message is that people have to prepare for these catastrophes that don't happen very often," said Atwater, who also holds a joint appointment with the U.S. Geological Survey. "It's a reminder of our own situation."

Other recent studies off the Northwest coast hint that the region may be vulnerable to even larger tsunamis than previously believed, though experts are still debating that data.

Led by Thai geologist Kruawun Jankaew, Atwater and colleagues from Japan and Australia spent months searching for sheltered areas along the Thai coastline where layers of sand swept in by earlier tsunamis would have been preserved. "We flailed around a lot, looking in mostly the wrong places," Atwater said. Mangrove swamps, which seemed promising at first, were riddled with crabs. "They burrow through everything and mix it up."

The researchers found what they were looking for on Phra Thong, a barrier island about 75 miles north of the resort town of Phuket, which was ravaged in the 2004 tsunami. Phra Thong was hard-hit, too, with 20- to 30-foot waves washing completely across the island in some places. The scientists dug pits in low-lying swales, and discovered white bands of sand, interspersed with dark layers of peat. Radiocarbon dating of organic material in the peat gave approximate dates of the past tsunamis.

Another team of scientists working on the Indonesian island of Sumatra found similar sand layers.

Much closer to the epicenter of the earthquake, Sumatra was one of the first places to be slammed by the 2004 tsunami, and even three years later the devastation was widespread, said University of Pittsburgh geologist Katrin Monecke, who coordinated the project.

Much of the Sumatran coast is swampy, and Monecke and her team had to wade through mucky bogs and battle leeches in their search for suitable sites to collect sediment cores. They found evidence of at least two previous tsunamis, the oldest about 1,000-1,200 years ago.

The most recent major tsunami appears to have hit about 600 years ago, the era of Joan of Arc and the Aztec empire. That tsunami, or series of tsunamis, was probably the same event that laid down the sand layers Atwater and his team found in Thailand, Monecke said.

It's good news for the Indian Ocean region — and the Pacific Northwest — that major tsunamis appear to be relatively rare events, Monecke said. But when the collective memory fades, so, too, does awareness of what to do when the next tsunami hits.

The people of one Indonesian island, hit by a localized tsunami in 1907, kept that memory alive through their oral history and traditions. So when the 2004 earthquake struck, people knew to run for high ground, and most survived, Atwater pointed out.

Many coastal communities in Washington and Oregon have developed evacuation plans and tsunami warning systems. The National Oceanic and Atmospheric Administration beefed up a network of tsunami detection buoys, but the system is of little benefit to much of the Northwest coast: A tsunami triggered by an offshore quake could reach some areas within 30 minutes.

Sandi Doughton: 206-464-2491 or sdoughton@seattletimes.com