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Sensors can stop tsunami scares

By David Adam— © Guardian Newspapers Limited 2004

LONDON, DEC. 31. A tsunami early warning system in the Indian Ocean can reduce the number of false alarms that triggered the panic in India on Thursday.

Not all sub-sea earthquakes trigger giant waves. The key is monitoring the motion of the water, not the shaking of the ground.

The most hi-tech feature of the much-heralded warning system in the Pacific is a series of pressure sensors on the seabed. These detect changes in the weight of the water above them caused by shifts in sea level as small as one cm, and send updates through cables to buoys on the surface. The information is transmitted by satellite to a control station in Hawaii.

In November last, the system was able to give the all clear after a magnitude 7.5 earthquake struck deep beneath the ocean off the Alaskan coast. A tsunami warning was issued within 25 minutes of the quake, and then withdrawn an hour later when a pressure sensor hundreds of miles to the south indicated the resulting wave was only two cm high. When it rolled into the harbour in Hilo, Hawaii, a few hours later it raised water levels by just 21cm. Each sensor costs about $200,000 and the data from the Indian Ocean can be relayed by existing satellites.

Dave Tappin, of the British Geological Survey, said sensors would have to be carefully placed near the most likely earthquake zones. "It's still possible that a wave would miss all the buoys and there would be no warning."

Coastal tidal gauges, the cornerstone of the Pacific system, are more reliable but only register dangerous changes in water levels when the leading edge of a tsunami strikes land.

Several countries around the Indian Ocean had discussed linking their tidal gauges with high-speed broadband cables before the disaster. Establishing and maintaining such systems is crucial. A seismograph on the Indonesian island of Java failed to warn officials in Jakarta before the recent disaster because it was disconnected during an office move in 2000.

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