The tsunami that swept the Indian Ocean last December occurred because of a rare combination of geological circumstances. Its swiftness surprised and killed people many thousands of kilometers from its source, deaths that experts say could have been avoided if the region had an early warning system like the one in the Pacific.
A tsunami is a series of huge ocean waves, often called tidal waves, although tides have nothing to do with them. They are waves moving at jet speed generated most often by the sudden displacement of the sea floor by a huge earthquake, although landslides and volcanoes can also set them off.
Geologist David Applegate says the type of ground tremor that generates a tsunami is called a thrust earthquake.
"Thrust earthquakes are particularly effective at that, such as the one that occurred off of Sumatra in December, where one part of the Earth's crust suddenly moves up against the other side," he said. "That sudden movement then translates through the water column and generates the wave."
Such ocean floor thrusts occur frequently, but to generate a tsunami, they must be extremely powerful, magnitude seven or more on the Richter scale. The one in the Indian Ocean in December measured magnitude nine. The measurement combines the length of the rupture and how much it moves.
In this case, the rupture was very long, 800 kilometers, and it moved a great deal because the pressure of one side of the ocean floor against the other along the fault line had been building for centuries.
"So in the Bay of Bengal, it has been several hundred years, possibly as many as 700, since the last time there was earthquake of this magnitude," he said.
|Computer collects data of live seismic activity from Pacific Ocean as geophysicist with US National Weather Service Pacific Tsunami Warning Center, monitors computer tracking systems watching for tsunami activity |
The rarity of large Indian Ocean earthquakes might have caused a false sense of security, since the region has no tsunami early warning system like the one in the Pacific. That one has been in place since shortly after a giant wave washed over Hawaii in 1946. The Pacific system's 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 consists of several ocean bottom buoys that measure increasing pressure when the water swells and many coastline gauges that detect the height and speed of the advancing wave. 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," he said. "What we see on those gauges is not 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.
David Applegate of the U.S. Geological Survey says the December Tsunami has provided an incentive for the United States and other countries and organizations to expand warning activities into these regions.
"As part of the president's tsunami warning initiative, they are going to be deploying several dozen additional buoys throughout the Pacific, the Atlantic, and the Caribbean," he said. "Then, in the Indian Ocean, a number of countries are going to be working together through the International Oceanographic Commission and UNESCO to establish a tsunami warning system there."
Paul Whitmore says early warning can help people 30 minutes to an hour from the source of the tsunami, but it has a limitation.
"The majority of people who get killed in tsunamis are right near the coast, and our warnings may not reach those people quickly enough," he said. "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.