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Tsunami travelled 800km/h
16/01/2005 14:07 - (SA)
Colombo - Boats at sea felt only a gentle swell as the Asian tsunami passed under their hulls - but by the time the first giant wave slammed into Sri Lanka's coast it was a monstrous 5m high, scientists said on Saturday.
A survey by a 15-member tsunami research team found the wave struck with crushing force in some places, yet washed almost gently over the coast just a few hundred metres away, affected by shoreline and seabed features, the scientists said, releasing preliminary data at a seminar.
The tsunami took an hour and 50 minutes to reach Sri Lanka after it was triggered by an underwater earthquake off Indonesia's Sumatra island, said Philip Liu of Cornell University in the United States.
It took another 30 minutes to whiplash around the island to the southwestern coast, where it devastated Galle, the country's second largest city.
"If we had an adequate warning system, there would have been enough time to evacuate the coastal region," Liu said. "It was a hard lesson to learn."
Nearly 31 000 people were confirmed dead and another 6 000 were missing, putting Sri Lanka second only to Indonesia in casualties from the December 26 disaster. In all, more than 157 000 people died from the most powerful tsunami in 40 years.
Costas Synolakis, of the University of Southern California, said scientists need more information about underwater topography to map out vulnerable spots.
"We know more about the surface of Venus than we do about the sea floor," he said.
No joke
Liu, a professor of civil and environmental engineering who helped develop the Pacific Ocean tsunami warning system, led the team, which included members of the US National Science Foundation and the US Geological Survey.
The researchers have been chasing tsunamis for 12 years to study how to predict them, calculate their force and mitigate their effects.
One lesson learned, said Synolakis, is that tsunamis can hammer a coast for hours. If you escape to high ground, "never go back for at least six hours. This is no joke. It may be a difficult decision if you see people stranded", he said, but warned that more waves come with surprising speed.
The Asian tsunami travelled across deep ocean at 800km/h, the speed of an airplane, Liu said. It was more than 300km wide, and had a mild slope virtually undetectable in satellite photographs.
"If you are in a boat, you can barely feel it," he said. "You would simply go up and down over a long period of time."
It was only when the tsunami neared shallow coastal waters that the waves grew steeper until the leading one became a wall of water, its energy lethally compressed.
Not every underwater earthquake sets off a tsunami, Liu said. In the Sumatra event, one tectonic plate slipped under another, causing a vertical displacement of the sea floor.
Marks left by waves showed the highest was three to six metres high, with an average height of five metres. It went more than 1 000m inland in some places, Liu said.
Much of the damage was not caused by the wave but by the debris swept up in the roiling water - vehicles, uprooted trees and boats - he said.
- AP
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