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Undersea landslides probed for tsunami threat
SAN FRANCISCO, California (Reuters) -- New techniques for mapping the ocean's floor have given scientists a valuable tool for assessing the risk of catastrophic tsunamis, tidal waves that wreak death and devastation when they slam into populated coastal areas.
Catastrophes like the 1998 tsunami which killed some 2,500 people and obliterated three villages in Papua New Guinea can be better understood by probing the mechanics of submarine surface movement, scientists said Monday.
"We are at the very start of a process of generating a predictive model," David Watts, president of Applied Fluids Engineering, Inc., of Long Beach, California, told a news briefing at a meeting of the American Geophysical Union here.
The Papua New Guinea disaster has frequently been blamed on two underwater quakes which hit the area on July 17, 1998.
But new research indicates that while the earthquakes may have triggered the event, the tidal wave itself was caused by a huge undersea slump of the sea bed off-shore, a massive shift of sediment that created the 48-foot (15 metre) wall of water which surged over coastal areas.
Dave Tappin of the British Geological Survey said that examination of the sea floor with mult-beam echo sounding equipment had revealed a huge, semi-circular slope of debris which indicated that an undersea cliff had collapsed.
"This sediment slump was like a mud slide," Tappin said. "It slipped because it was shaken loose by the earthquake."
While earthquakes are blamed, either directly or indirectly, for most tsunamis, other scientists are examining the offshore landscape in other regions of the world for possible clues to other tidal wave events.
Underwater volcanic activity, and seeping water from the aquifer beneath, can also destabilize submarine cliffs, leading to the threat of landslides.
Gary Greene, a research geologist at the Monterey Bay Aquarium Research Institute in California, said that new, high resolution images of an underwater shelf off the coast of Santa Barbara indicated it, too, had partially collapsed in the past -- a possible cause for a tsunami which hit there in 1812.
"This slide moved in three different events. Each event displaced enough sediment to be capable of generating a tsunami, if the displacements occurred rapidly," he said.
The research is now focusing on a section of the shelf which remains standing, although it displays cracks which lead some scientists to think it may be potentially unstable.
Greene cautioned that scientists were still far from being able to predict specific underwater landslides. But he said improved maps of the sea floor would give scientists important clues to understanding the mechanics of tsunami generation.
Another study is focused on an undersea escarpment about 2.5 miles (4 km) off the coast of Palos Verdes, California, Jacques Locat, a geologist at Laval University in Quebec, Canada, said that it, too, showed evidence of earlier partial collapse.
A new underwater landslide here could create a tsunami which could hit Long Beach and Redondo Beach, he said.
While tsunamis are a much-feared aspect of life on the coasts of countries ranging from Japan to Turkey, their destruction can be significantly lessened by simple education of coastal communities, Watts said.
A tsunami which hit the Pacific Island of Vanuatu in 1999 was similar in both size and force to that which struck Papua New Guinea a year earlier. But because the population there had been instructed -- through something as simple as a public service video -- to head for higher ground at the first sign of possible tsunami, the death toll was far less, he said.
A hypothetical, landslide-induced tsunami headed for Long Beach would give people there as much as 24 minutes to evacuate the beaches and coastal areas, he said. "All they would have to do is walk a couple of hundred yards (metres)" Watts said. "But the problem is -- when do you start walking?"
Copyright 2000 Reuters. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.
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