'They had no warning'
2004-12-27 08:16
Denver - The catastrophic death toll in Asia caused by a massive tsunami might have been reduced had India and Sri Lanka been part of an international warning system designed to warn coastal communities about potentially deadly waves, scientists say.
The warning system is designed to alert nations that potentially destructive waves may hit their coastlines within three to 14 hours.
Scientists said seismic networks recorded Sunday's massive earthquake, but without wave sensors in the region, there was no way to determine the direction a tsunami would travel.
A single wave station south of the earthquake's epicentre registered tsunami activity less than 60 centimetres high heading south toward Australia, researchers said.
The waves also struck resort beaches on the west coast of the Thailand's south peninsula, killing hundreds. Although Thailand belongs to the international tsunami warning network, its west coast does not have the system's wave sensors mounted on ocean buoys.
Most powerful earthquake
"They had no tidal gauges and they had no warning," said Waverly Person, a geophysicist at the National Earthquake Information Centre in Golden, Colorado, which monitors seismic activity worldwide. "There are no buoys in the Indian Ocean and that's where this tsunami occurred."
The tsunami was triggered by the most powerful earthquake recorded in the past 40 years.
The earthquake, whose magnitude was a staggering 9.0, unleashed walls of water more than two stories high to the west across the Bay of Bengal, slamming into coastal communities 1 600 kilometres away. Hours after the quake, Sumatra was struck by a series of powerful aftershocks.
Researchers say the earthquake broke on a fault line deep off the Sumatra coast, running north and south for about 965 kilometres or as far north as the Andaman and Nicobar islands between India and Mynamar.
"It's a huge rupture," said Charles McCreary, director of the Pacific Tsunami Warning Centre near Honolulu. "It's conceivable that the sea floor deformed all the way along that rupture, and that's what initiates tsunamis."
'Nothing to stop them'
A tsunami is not a single wave, but a series of travelling ocean waves generated by geological disturbances near or below the ocean floor. With nothing to stop them, these waves can race across the ocean like the crack of a bullwhip, gaining momentum over thousands of kilometres.
Most are triggered by large earthquakes but they can be caused by landslides, volcanoes and even meteor impacts.
The waves are generated when geologic forces displace sea water in the ocean basin. The bigger the earthquake, the more the earth's crust shifts and the more seawater begins to move.
Most tsunamis occur in the Pacific because the ocean basin is rimmed by the Ring of Fire, a long chain of the earth's most seismically active spots.
In a tsunami, waves typically radiate out in directions opposite from the seismic disturbance. In the case of the Sumatra quake, the seismic fault ran north to south beneath the ocean floor, while the tsunami waves shot out west and east.
Tsunamis are distinguished from normal coastal surf by their great length and speed. A single wave in a tsunami series might be 160 kilometres long and race across the ocean at 965 kph. When it approaches a coastline, the wave slows dramatically, but it also rises to great heights because the enormous volume of water piles up in shallow coastal bays.
And unlike surf, which is generated by wind and the gravitational tug of the moon and other celestial bodies, tsunamis do not break on the coastline every few seconds.
Because of their size, it might take an hour for another one to arrive.
Some tsunamis appear as a tide that doesn't stop rising, while others are turbulent and savagely chew up the coast. Without instrumentation, so little is known about this tsunami that researchers must wait for eyewitness accounts to determine its characteristics.
In the hours following an earthquake, tsunamis eventually lose their power to friction over the rough ocean bottom or simply as the waves spread out over the ocean's enormous surface.
The international warning system was started in 1965, the year after tsunamis associated with a magnitude 9.2 temblor struck Alaska in 1964.
- AP