25 January 2005

Behind the tsunami tragedy

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.

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.

Wealth of data
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 kilometres. The rupture started off the coast of Sumatra and propagated northwards at around two kilometres per second.

"There was an upwards movement of the seafloor, approximately northwards. It only came up a few metres, 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.

Further earthquakes
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 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-kilometre 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, travelling 500-800kms apart. In between were smaller ripples about 100-200kms apart.

By turning all the 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.


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