A tsunami is a series of huge ocean waves that can travel at high speed, making survival for people near its origin less likely than those farther away. Yet Sunday's Indian Ocean tsunami killed people many thousands of kilometers from the source, deaths that experts say could have been avoided if the region had an early warning system like the one in the Pacific.
Tsunamis are commonly called tidal waves, but tides have nothing to do with them. They are underwater waves most often generated by the sudden displacement of the sea floor caused by an earthquake.
Tsunamis move at the speed of a jet, 600 to 1000 kilometers per hour. Even at that speed, however, Sunday's tsunami is estimated to have taken two hours to reach Sri Lanka from its epicenter off the northwestern Sumatra coast, another hour to arrive at India, and three more hours to get to the east African coast.
Yet the giant waves surprised and killed people at all these distant locations. U.S. Geological Survey geophysicist Waverly Person told NBC television's "Today" show interviewers that is because the region lacks a system of water sensors that can warn of an impending tsunami.
"Had they had tide gauges installed, many of these people that were farther away from the epicenter could have been saved because they would have been able to track the waves and tell the people along the coast area to move off the beach and give them an approximate time the waves were going to hit. They couldn't tell them how high they were going to be, but at least they could say, 'This is the approximate time they will hit your area, so move away from the coasts,'" Mr. Person says.
Such an early warning system has been in place for the Pacific Ocean since shortly after a tsunami washed over Hawaii in 1946. Its Hawaiian headquarters is now supplemented by warning centers in Russia and Japan, and a regional network focusing on Alaska and the U.S. west coast. The system monitors hundreds of sea bottom sensors that detect earthquakes and swelling water and many more coastline gauges that measure the height and speed of a tsunami.
At the Tsunami Warning Center in Alaska, Paul Whitmore says the coastal gauges provide vital information.
"It gives us an idea of the severity of a tsunami," Mr. Whitmore says. "What we see on those gauges isn't necessarily the highest wave, but we can take the results of what we see on those gauges and put it into tsunami models to determine maybe how big the wave will be or if there are other places it will be more severe."
Mr. Whitmore says the network can issue tsunami warnings within 10 minutes of an earthquake, much faster than the hour or more it took a decade ago.
The United Nations Educational, Scientific, and Cultural Organization, which oversees the Pacific tsunami warning system, says its International Tsunami Information Center has been involved in activities outside the Pacific in recent years because South Pacific, Indian Ocean, and Mediterranean countries have been asking for help developing warning programs for their areas.
But Waverly Person of the U.S. Geological Survey says the rarity of such events in the Indian Ocean is probably the reason a system was not in place before Sunday's tragedy.
"They probably have experience some of what you call local tsunamis, but not anything of this magnitude. It may have been that they thought, 'Since we haven't had a history of many big tsunamis, we don't need this warning system,'" Mr. Person says.
Experts note that the Pacific tsunami warning system is not foolproof, especially for coastal dwellers near the epicenter. Paul Whitmore says they must learn to be alert.
"The existing warning system can help those that are, say, 30, 40 minutes, an hour away from the tsunami. We can get messages to those people," Mr. Whitmore says. "But the majority of people who get killed in tsunamis are right near the coast, and our warnings may not reach those people quickly enough. So the best thing to do as far as saving lives is education of those near the coast to know that if they feel a strong earthquake, they need to get inland or to high ground and not wait for a warning."
Mr. Whitmore also advises never go to the beach to watch a tsunami, for no one can outrun one.