The catastrophic death toll in Asia caused by a massive tsunami (search) 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.

More than 15,000 people in India and Sri Lanka were among the more than 20,900 killed after being hit by walls of water triggered by a tremendous earthquake early Sunday off Sumatra (search).

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 epicenter registered tsunami activity less than 2 feet 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.

The northern tip of the earthquake fault is located near the Andaman Islands (search), and tsunamis appear to have rushed eastward toward the Thai resort of Phuket (search) on Sunday morning when the community was just stirring.

"They had no tidal gauges and they had no warning," said Waverly Person, a geophysicist at the National Earthquake Information Center (search) in Golden, Colo., 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 (search), slamming into coastal communities 1,000 miles 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 600 miles or as far north as the Andaman and Nicobar islands between India and Mynamar.

"It's a huge rupture," said Charles McCreery, director of the Pacific Tsunami Warning Center (search) near Honolulu. "It's conceivable that the sea floor deformed all the way along that rupture, and that's what initiates tsunamis."

Tsunamis as large and destructive as Sunday's typically happen only a few times in a century.

A tsunami is not a single wave, but a series of traveling 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 miles.

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. Marine geologists recently have 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 100 miles long and race across the ocean at 600 mph. 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.

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

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 (search).

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

However, India and Sri Lanka are not members.

"That's because tsunamis are much less frequent in the Indian Ocean," McCreery said.

"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 analyzes 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, including predictions how fast the waves are traveling and their expected arrival times in specific geographic areas.

As the waves rush past tidal stations in the ocean, bulletins updating the tsunami warning are issued. Other models generate "inundation maps" of what areas could be damaged, and what communities might be spared.

Not all earthquakes generate tsunamis. The warning center typically does not issue warnings for earthquakes below magnitude 7.0, which are still unusually powerful events.