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NewsAsia, 24 Jan 05
The Great Wave: Behind the tragedy, a boon for scientists PARIS : The December 26 tsunami devastated lives around the Indian Ocean's coastline but, for science at least, there was a silver lining. Satellite images, seismic sensors, even TV footage, photographs and eyewitness accounts have provided a goldmine of information, helping experts to understand more about this force of nature and how to tackle it in future. The data "will be of great use in preventing future disasters," said David Booth, senior seismologist at the British Geological Survey (BGS). "It will help to identify vulnerable areas, set up an alert system, and help to build structures and breakwaters that deflect or slow down the wave before it reaches habitation," he said in an interview from Edinburgh, Scotland. Earthquakes of the December 26 intensity, which measured nine on the Richter scale, occur on average only once every 30 or 40 years. Detailed evidence of the tsunamis they unleash is even sketchier. But in this case, earthquake sensors and orbiting monitors have supplied a wealth of data as to how the event began. And film, photos and the testimony of survivors have provided useful information about its climax, revealing beaches and building designs that were terribly at risk. A mosaic of knowledge is now building up about the tsunami-prone fault which lies off the Indonesian island of Sumatra. The earthquake "did not happen at a single point along the fault," said Booth. "It actually ruptured over a length of at least 400 kilometers (250 miles). The rupture started off the coast of Sumatra and propagated northwards at around two kilometers (1.2 miles) per second. "There was an upwards movement of the seafloor, approximately northwards. It only came up a few metres (yards), but with all that water on top, there was a massive release of energy." One worry is that the quake failed to release all the pent-up energy between these two rubbing plates of the Earth's crust. The northern and southern tips of the fault may be prone to further earthquakes, "perhaps within decades, and ... they might be even powerful enough to cause another tsunami," the British journal Nature says, quoting US and Japanese seismologists. As good luck would have it, two hours after the murderous wave radiated out to the west, north and northeast, a pair of US-French satellites, TOPEX/Poseidon and Jason-1, happened to be passing overhead. They took radar measurements of sea levels of the Bay of Bengal along a 3,000-kilometer (1,900-mile) stretch just as the tsunami's leading edge was reaching Sri Lanka and India, the British weekly New Scientist says. What the orbiting scouts saw was unique: two waves of a maximum height of about 50 centimetres (30 inches), travelling 500-800 kms (300-500 miles) apart. In between were smaller ripples about 100-200 (60-120 miles) apart. These details -- the first-ever observation of a propagating tsunami -- give vital clues as to the amount of energy released and the way it travels, is forced to slow or change direction because of underwater mountains or slopes. The 50-cm (30-inch) waves may have seemed tiny, but they represented the crest of a travelling body of water several thousand metres (thousands of feet) deep. So when the wave collided with the continental shelf, this force began to build up, sending a towering wall of water into the coast. One tragic but useful outcome of the tsunami is that it instantly showed which areas are vulnerable to big waves. By turning this data in computer models, scientists should be able to give quick and accurate advice when the planned Indian Ocean tsunami alert system gets under way next year. The risk for any alert system is of losing credibility through false alarms. By assessing the strength of a quake and the early direction of a tsunami, scientists can reduce such a risk by giving targeted warnings to those areas known to be exposed, rather than issuing a blanket alert to the whole region. Another insight gained from December 26 is the effect of natural tsunami buffers, which on many coasts have been destroyed by the tourist industry or prawn farms. "Coral reefs act as a natural breakwater and mangroves are a natural shock absorber, and this applies to floods and cyclones as well as tsunamis," said Simon Cripps, director of the Global Marine Programme at the environment group WWF Internationational. - AFP links Related articles on Tsunami and the environment |
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