Monday, February 7, 2005
New findings super-size our tsunami threat
80-foot waves blasted Indonesia, scientists now say
Scientists studying the Indian Ocean tsunami have discovered startling evidence that the killer waves, at least for one coastal area in northern Indonesia, were much larger than earlier believed.
The new findings, of keen interest worldwide to researchers seeking insights into the mechanics of this disaster, also translate into a disturbing suggestion that the experts have significantly underestimated the magnitude of the tsunami threat in the Pacific Northwest and perhaps to coastal communities around the Pacific Ocean.
"It's just staggering," said Andrew Moore, a scientist at Kent State University who was on a research team, led by Yoshinobu Tsuji of the University of Tokyo, that recently returned from studying the wave's trail of destruction on the west coast of Sumatra.
Moore, Tsuji and their colleagues found preliminary evidence the tsunami's height along an extensive section of shoreline south of the city of Banda Aceh averaged about 80 feet above sea level, with "run-ups" on inland slopes often reaching well over 100 feet. They estimated the wave's average velocity on shore at 45 feet per second.
"It's mind-boggling to think about," said Vasily Titov, a mathematician and tsunami computer modeler at the Pacific Marine Environmental Laboratory in Seattle. The lab, operated by the National Oceanic and Atmospheric Administration, is host to some of the world's top tsunami scientists.
|Paul Joseph Brown / P-I|
|Tsunami researcher Vasily Titov watches a computer model of the global propagation of the Dec. 26 event at the Pacific Marine Environmental Laboratory at Sand Point in Seattle. He and other scientists fear that for coastal Washington, "our worst-case scenario may not be the worst-case scenario."|
These findings, Titov said, will almost certainly prompt significant revisions to basic theory and the tsunami models -- as well as a serious upgrade in the predicted size of the tsunami expected from a major quake off the coast of Washington and Oregon.
"Our worst-case scenario may not be the worst-case scenario," acknowledged Titov.
Most estimates of the Dec. 26 waves' maximum height above sea level (sometimes referred to as "flow depth" to distinguish it from onshore "run-up" heights) had put it at 30-35 feet -- primarily in parts of Indonesia nearest the undersea earthquake.
Scientists studying the Indian Ocean tsunami, which killed at least 160,000 people, had heard the reports of higher waves -- of branches and bark stripped off the first 80 feet of trees, of water marks and roof damage found at or beyond the height of six- or seven-story buildings -- but most figured these were the results of the wave running up slopes or isolated instances of "focusing."
A tsunami wave can get "focused" or boosted suddenly in one location as it comes on land by undersea canyons or other near-shore geographical features or structures that squeeze the massive surge of water into a narrower outlet -- much like a nozzle increases the force of the water coming out of a hose.
But Moore said his team's wave-height measurements were consistent over a long stretch of coastline and found well back from any slopes, indicating these were direct measurements of the overall elevation of this massive wall of water as it came on shore.
"It's just so far beyond what any of us have ever dealt with before," said Moore, who before moving East pursued tsunami research at the University of Washington.
It's also way beyond the scope of what most of the experts have been predicting will arise from the ocean depths the next time the Cascadia Subduction Zone erupts.
The Cascadia zone, encompassing western Washington and Oregon, is where two "tectonic" plates that make up the Earth's fractured crust collide. The offshore Juan de Fuca Plate runs into and dives under, or "subducts," the North American plate.
The plates move in a slow, subterranean collision at an average speed of four centimeters per year. It wasn't until the mid-1990s that it was widely recognized that Cascadia was an active subduction fault, capable of producing the planet's most massive kind of earthquake, centered some 100 miles offshore.
Since then, NOAA scientists have been working with the state Department of Natural Resources and others to create "hazard maps" that quantify the height and reach of a tsunami in certain communities at highest risk.
But the maps are based on a tsunami with a maximum wave height of about 30 feet.
"That was the best evidence available at the time," said Tim Walsh, chief hazards geologist with the state agency.
"That's already a huge, incredibly destructive wave," noted Titov. Until the Indian Ocean tsunami, he said, most experts would have thought it absurd to suggest a tsunami with an overall wave height greater than this.
But the tectonics of Cascadia and the undersea subduction fault near Sumatra that spawned the Dec. 26 tsunami are alarmingly similar, Walsh said. "We look a lot like the Indian Ocean," he said, which means they may need to incorporate some new, bigger numbers in the hazard maps.
"Obviously, this has serious implications for all of the West Coast," said Harold Mofjeld, Titov's colleague and a senior tsunami researcher at the NOAA lab. But such dramatic findings first must be confirmed and considered in the light of alternative explanations, he said.
The shoreline could have dropped significantly in elevation during the event, Mofjeld said, noting that this phenomenon of "subsidence" is often associated with such large quakes. And there's still a possibility the measurements by Tsuji and Moore were "disparities" from large-scale focusing or some other cause, he said.
A British research ship, the HMS Scott, is conducting surveys of the seafloor off Sumatra and has found evidence of massive submarine landslides. Scientists say it's also possible these undersea landslides magnified the waves.
Finding out just how similar, or different, Cascadia may be from Sumatra will require a lot more detailed information about local undersea canyons and other topographical features of the seafloor (known as "bathymetry"), Walsh said. So far, he said, the bulk of evidence indicates the similarities outnumber the differences.
"We need better bathymetry, and a lot of it is still classified because of our submarines," said Walsh, referring to the deep-sea traffic due to the U.S. Navy's Bangor submarine base headquartered south of Bremerton.
Walsh would like to see the tsunami hazard mapping project shift to getting better bathymetric and topographic data to "look for those places that are particularly vulnerable" rather than to continue the current approach of estimating the hazard community by community.
Another finding that may be of critical importance to communities farther from the coast, on the Strait of Juan de Fuca and in Puget Sound, was the Dec. 26 tsunami's ability to retain its force even when going around corners. Scientists studying the waves' behavior in Sri Lanka were stunned to see what happened on the leeward side of the island nation.
"It was fairly large in many locations on the west side, bigger than the numerical models predict," said Phil Liu, the Cornell University scientist who led an American survey team in Sri Lanka.
Given all this flood of surprising information emerging from studies of the Indian Ocean tsunami, Walsh said, officials may need to pursue a different, more comprehensive strategy assessing and seeking to mitigate the tsunami risk for the Pacific Coast.
A step in this direction locally may be taken later this week at a meeting to be held in the high-risk southwestern Washington community of Long Beach. The Coastal Tsunami Summit on Wednesday will feature tsunami experts from NOAA, the Federal Emergency Management Agency, the U.S. Geological Survey and several state agencies.
The meeting is intended to provide an overview of tsunami science, methods of hazard assessment, discussions of evacuation plans and warning systems and the National Tsunami Hazard Mitigation Program.
Though most of the new scientific findings are not on the meeting agenda, the question will likely arise: Could Long Beach and other Pacific Coast communities one day have to deal with a tsunami wave as high as a six-story building?
"That's the sixty-million-dollar question," Walsh said.
Scientists studying the impact of the Dec. 26, 2004, tsunamis in Sumatra have found evidence that they were much larger than earlier estimates, in some cases two to three times larger. If these preliminary findings are confirmed, experts may have to revise some of their fundamental assumptions about these killer waves -- including the level of threat in the Pacific Northwest.
The massive Indian Ocean tsunamis were spawned by an undersea earthquake on a series of faults known as the Sumatran Subduction Zone, where one of the Earth's massive tectonic plates known as the Indo-Australian Plate collides with and then dives or "subducts" under the South Asian Plate. The same tectonic situation exists just off the coastline of Washington, Oregon and northern California and is called the Cascadian Subduction Zone. Scientists are concerned that the same kind of tsunami produced by the Sumatran fault could one day be spawned by the Cascadian zone.
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