LONG BEACH - Fresh evidence of ancient disasters on the far side of the world should shake Washington coast citizens and leaders into planning now to survive a devastating tsunami we know will happen here sooner or later, a top University of Washington scientist urges.
The great 2004 Indian Ocean tsunami that cost a quarter-million lives seemed to strike out of the blue. Few people living along the coasts knew to heed the 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.
Last week, two teams of geologists reported that the 2004 cataclysm was only the most recent in a series of such awful events over the past 2,800 years, the most recent between 550 and 700 years ago.
Applying Asian lessons here
Brian Atwater, UW affiliate professor of Earth and space sciences and a U.S. Geological Survey geologist, was part of the team that studied tsunami history in Thailand. Related study took place in Aceh, a province at the northern tip of the Indonesian island of Sumatra where more than half the deaths from the 2004 tsunami occurred. Findings from both teams are published in the Oct. 30 edition of Nature.
Atwater is famous on the Washington coast for having discovered and proven our own long history of enormous tsunamis, the great surges of ocean water that overflow onto normally high ground following offshore earthquakes. Based on written records in Japan, Atwater and his colleagues were able to establish nearly an exact time for a January 1700 tsunami here that slammed into coast villages on the other edge of the Pacific several hours later.
In an interview last week, Atwater said that even as a geologist used to working in time frames of millions of years, it remains hard for him to wrap his mind around tsunami-generating quakes that take place only every several hundred years. The last one in the Indian Ocean happened before the time of Christopher Columbus, he noted, and this makes it difficult to educate coast-dwellers about what to watch for.
In 2004, some residents of small islands near Aceh retained an historical memory of a relatively small tsunami in 1907 and fled for higher ground, saving their lives. Our offshore Cascadia Subduction Zone resembles that near Aceh and we must learn similar life-saving instincts, he said.
"A region's tsunami history can serve as a long-term warning system," Atwater said. "Like Aceh, Cascadia has a history of tsunamis that are both infrequent and catastrophic, and that originate during earthquakes that provide a natural tsunami warning. This history calls for sustained efforts in tsunami education."
When will the next one hit here?
It's been 308 years since the last deadly tsunami here - instantly expending as much energy as the entire U.S. uses in a month - so Atwater is often asked when we might expect the next one.
"This is not a case of nature giving us a predictable cycle," he said. "If one happened tomorrow, no earth scientist would be surprised." But they would be equally unsurprised if centuries go by before Cascadia delivers its next unpleasant shock, Atwater added.
Going back 3,000 years, there are ample signs of periodic tsunamis smashing into our coast, evidence recorded in sediments around Willapa Bay and on the banks of the Lewis and Clark River near Astoria. They have happened at durations as short as a few centuries and as long as more than 1,000 years - the average is about 500 years.
"This fault is a bit like a savings account and the motion of tectonic plates is a steady income going into that account," Atwater explained. "Now and then the fault decides to go and buy something. Sometimes it spends its saved-up energy frugally and sometimes it spends lavishly. We don't which it's going to be but we know this fault is a spender."
People need to be trained to get away from the shoreline and find high ground immediately after any intense local quake, Atwater said. This means learning evacuation routes and being prepared to flee without any delay. Buildings should incorporate "vertical evacuation" designs, upper floors that can remain standing in a quake and provide places to go in low-lying areas like Long Beach.
Asia in denialEven after experiencing a huge tsunami so recently, many in the Aceh-Thailand area still can't accept that these events are something they must plan for, said Jankaew, the lead author.
"Many people in Southeast Asia, especially in Thailand, believe, or would like to believe, that it will never happen again," Jankaew said. "This will be a big step towards mitigating the losses from future tsunami events."
The team found evidence for previous tsunamis by digging pits and auguring holes at more than 150 sites on an island about 75 miles north of Phuket, a Thai tourist resort area ravaged by the 2004 tsunami. That tsunami was generated 300 miles to the west when the seafloor was warped during a magnitude 9.2 earthquake.
At 20 sites in marshes, the researchers found layers of white sand about four inches thick alternating with layers of black peaty soil. Witnesses confirmed that the top sand layer, just below the surface, was laid down by the 2004 tsunami, which ran 20 to 30 feet deep across much of the island.
Radiocarbon dating of bark fragments in soil below the second sand layer led the scientists to estimate that the most recent predecessor to the 2004 tsunami probably occurred between A.D. 1300 and 1450. They also noted signs of two earlier tsunamis during the last 2,500 to 2,800 years.
There are no known written records describing an Indian Ocean tsunami between A.D. 1300 and 1450, including the accounts of noted Islamic traveler Ibn Battuta and records of the great Ming Dynasty armadas of China, both of which visited the area at different times during that period. Atwater hopes the new geologic evidence might prompt historians to check other Asian documents from that era.
"This research demonstrates that tsunami geology, both recent and past tsunamis, can help extend the tsunami catalogues far beyond historical records," Jankaew said.