The Hindu : International : To watch and warn

Online edition of India's National Newspaper
Tuesday, Dec 28, 2004

International

International

To watch and warn

DENVER, DEC. 27. 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. Marine geologists recently determined that under certain conditions, the U.S. East Coast and other heavily populated coastlines also could be vulnerable.

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 km long and race across the ocean at 965 kmph. 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 does not 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.

"It was a big tsunami, but it is hard to say exactly how many waves there were or what happened," said a scientist about the latest one.

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. It is administered by the National Oceanic and Atmospheric Administration.

Member states include all the major Pacific rim nations in North America, Asia and South America, was as well as the Pacific islands, Australia and New Zealand. It also includes France, which has sovereignty over some Pacific islands, and Russia.

"Unfortunately, we have no equipment here that can warn about tsunamis," said Budi Waluyo, an official with Indonesia's Meteorology and Geophysics Agency. "The instruments are very expensive and we don't have money to buy them."

The warning system analyses earthquake information from several seismic networks, including the U.S. Geological Survey. The seismic information is fed into computer models that "picture" how and where a tsunami might form. It dispatches warnings about imminent tsunami hazards.

— AP

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