UW study: Indian Ocean, Pacific Northwest share history of tsunamis

The discovery that the Indian Ocean has been repeatedly rocked by massive tsunamis is reverberating in the Pacific Northwest, which shares a similar record of geologic unrest.

"Both places have this history of ... catastrophes," said Seattle researcher Brian Atwater, co-author of a study published today in the journal Nature.

He and his colleagues unearthed layers of sand along the coast of Thailand that proved the deadly 2004 tsunami was not a one-time fluke. At least three of the wave surges have struck the region over the past 2,800 years, the most recent 550 to 700 years ago, he said.

Nearly a quarter of a million people perished in the 2004 tsunami, which was triggered by a magnitude-9.2 earthquake on an offshore fault.

"People were taken by surprise, because they didn't know such things were even possible," said Atwater, of the U.S. Geological Survey and the University of Washington. "The previous Indian Ocean tsunami happened before Christopher Columbus."

A similar fault lies off the coast of Washington, Oregon and Northern California. Atwater's earlier field work showed that the so-called Cascadia Subduction Zone has unleashed monster quakes and tsunamis on average every 500 to 600 years — though the intervals vary widely. The last was 300 years ago.

"This new evidence of repeated Indian Ocean tsunamis really shows that we're in the same boat as people around the Indian Ocean," Atwater said. "We can more easily picture ourselves in those tourist videos of the tsunami coming ashore."

Worst-case scenarios

Recent cores drilled from the floor of the Pacific Ocean provide a 10,000-year record of earthquakes and tsunamis on the Cascadia Subduction Zone and suggest the West Coast could be hit with waves as high as 70 to 90 feet.

Earlier estimates had put the maximum wave heights at about 50 to 65 feet, said George Priest, of the Oregon Department of Geology and Mineral Industries. Oregon is revamping its evacuation maps to account for the new worst-case scenarios.

Since the 2004 tsunami, a network of warning buoys in the Pacific Ocean has been expanded and upgraded. But the devices are most useful for distant earthquakes — in Alaska or Chile, for example — where it would take several hours for a tsunami to reach the U.S. coast.

A major slip on the Cascadia Subduction Zone could send surges crashing onto shore within 30 minutes or less. That's why people in coastal communities need to immediately head to high ground when they feel a major earthquake, said geologist Lori Dengler, of Humboldt State University in Arcata, Calif. — one of those places that would be slammed within minutes.

But when centuries pass between disasters, it can be difficult to maintain preparedness or even a collective knowledge of the threat. "We need to incorporate tsunami preparedness and awareness into our culture and we need to pass it down from generation to generation," Dengler said.

That worked in one Indonesian village, which had been hit by a localized tsunami in 1907. The elders passed the story down, and when the ground shook in 2004, everyone knew what to do. They headed for high ground, and not a life was lost.

Breakaway walls

Some parts of Washington's coast, like the Ocean Shores and Long Beach peninsulas, offer no high ground and would be extremely difficult to evacuate, particularly if roads were destroyed by the earthquake, said geologist Tim Walsh, of the Washington Department of Natural Resources.

One possible solution is to build "artificial dunes" or municipal buildings designed with breakaway walls that could survive both an earthquake and a tsunami and provide a refuge, said Walsh, who helped draw up engineering guidelines for such structures.

Tsunamis carry large amounts of sand and sediment as they rush inland, creating layers that can provide a window into the past. Led by Thai geologist Kruawun Jankaew, Atwater and colleagues from Japan and Australia spent months searching for sheltered areas along the Thai coastline where sand layers from previous tsunamis would have been preserved.

The researchers found what they were looking for on Phra Thong, a barrier island 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, though the record there only went back about 1,200 years. As in Thailand, the most recent historical layer dated to about 600 years ago — the era of Joan of Arc and the Aztec empire.

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