The tsunami that killed 230,000 people in 2004 was the biggest in the Indian Ocean in some 600 years, and it has lessons for the Pacific Northwest, geologists said.
Geologists say the long gap between tsunamis in the Indian Ocean might explain the build-up in geologic forces that triggered the huge undersea earthquake and launched the killer waves four years ago, the journal Nature said.
In the American Northwest, too, many centuries typically pass between catastrophic quakes and tsunamis, which are likely to recur in the region, geologists at a press conference said.
They called for greater public awareness, not just in coastal towns but also among tourists.
An Oregon Department of Geology and Mineral Industries publication said major undersea quakes from the Cascadia Subduction, which runs from Vancouver Island to Northern California, happen on average every 500 to 600 years. The range of the intervals is 100-300 years to 1,000 years.
There are no reliable predictions for such quakes and tsunamis. In 1700, geologists say, a quake with an estimated magnitude of 9.0 struck, touching off waves that hit both Japan and the West Coast.
The Cascadia Subduction is a slow-moving plate nuzzling beneath the continental United States and building up tremendous pressure that periodically gives way, causing a quake and resulting tsunami.
State estimates are that in low-lying vulnerable areas such as Oregon’s Seaside or Washington’s Aberdeen, tsunami waves could wipe out entire towns.
It can take just minutes or hours for a tsunami to reach shore, depending on the location of the quake, said Lori Dengler who teaches geology at Humboldt State University in California, near the edge of the subduction.
But she said one small area in the 2004 quake had a culture of tsunami awareness, so even though the village was obliterated, all the inhabitants escaped at the first hint of the quake.
The work on the 2004 quake appears in the journal Nature. Two research teams report that by digging pits and taking core samples in Thailand and northern Sumatra, they found evidence that the last comparably large tsunami struck between the years 1300 and 1400.
In 2004 “people were taken by surprise because they didn’t even know such things were possible,” said Brian Atwater of the United States Geological Survey from the University of Washington.
But he said the article in Nature notes a rich history of tsunamis like the one in 2004, going back thousands of years.
The journal said sedimentary evidence suggests the last one of similar size there was in about 1400, long before historical records of quakes in the region began.
George Priest of the Oregon Department of Geology and Mineral Industries said a 1995 law restricts housing essential services such as hospitals from areas deemed vulnerable.
Nathan Wood of the U.S Geological Survey, who deals in risks and threats from tsunamis, said the vulnerability of risk zones, including populations and economies, must be figured in with the likelihood of a tsunami.
“Is it a one-day event or does it destroy the entire economy of a region?” he asked.
“We’ve done a great job figuring out where these zones are,” he said. “But what’s in these zones?”
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