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Last Updated: Wednesday, 12 October 2005, 11:06 GMT 12:06 UK
A-bomb system can warn of tsunami
By Richard Black
Environment Correspondent, BBC News website

Tsunami seismogram
The tsunami's unique "chirp" could help scientists predict its course
Monitoring stations set up to detect atomic explosions could help predict the path of a tsunami, research shows.

Californian scientists have analysed sound waves produced in the Indian Ocean by last December's Asian tsunami.

Writing in the journal Geophysical Research Letters, the team says the tsunami produced a "unique" signal.

This indicates, they say, that stations set up to implement the A-bomb test ban treaty could be involved in the new Indian Ocean tsunami warning system.

Nuclear ban

Adopted by the United Nations in 1996, the Comprehensive Test Ban Treaty seeks to eliminate experimental nuclear explosions

In this frequency range, the hydrophones detect a unique signal
Roger Bowman
Although it has not been ratified by sufficient countries to bring it into force, a preparatory commission of the treaty organisation (CTBTO) based in Vienna has already established a global network of monitoring stations that could detect and pinpoint tests.

These work in several ways. Some measure seismic activity - movements in the ground - while others look for infrasound, very low frequency soundwaves in the atmosphere.

A third set use hydrophones, underwater soundwave transducers, which are similar to the detector component of submarine sonar systems.

On 5 Jan, CTBTO reported that it had "...recorded the earthquake west of northern Sumatra, Indonesia, on 78 of its waveform monitoring stations within seconds to minutes of the event on 26 December, 2004.

"Of the 78 stations, 71 were using the seismic, six the hydroacoustic and one the infrasound technologies," it said in a statement.

Jeffrey Hanson and Roger Bowman from the Science Applications International Corporation (Saic) in San Diego have now analysed the signals coming from the hydroacoustic detectors, or hydrophones.

Tell-tale chirp

"After the quake on 26 December, all geophysical researchers were looking for signals in their data," Roger Bowman told the BBC News website.

Seismogram image
"Triads" of detectors can pinpoint the earthquake's direction
"One of the common ways was to make spectrographs - looking at how the spectrum of sound waves developed over time - and in this we saw the unique signal."

The two researchers describe the unique signal found on spectrograph plots recorded by Indian Ocean hydrophones as a "chirp".

What it means is that low-frequency vibrations are arriving before those of higher frequencies, producing a distinctive upward curving slope.

"In this frequency range - and these are very low frequencies, well below 1Hz - this is a unique signal," said Dr Bowman.

Most of the hydrophones are arranged in sets of three, called triads.

This enables them to pick up with considerable precision the direction from which the tsunami is coming, meaning that using several different arrays, the location of the earthquake which caused it could be determined easily, and projections made about which areas would also be at risk.

Predict and survive

In the weeks following the 26 December earthquake, moves began to establish an integrated warning system for Indian Ocean tsunami, replicating the Pacific Ocean system co-ordinated from Hawaii.

A number of monitoring stations were envisaged, using a mix of water pressure sensors on the ocean floor, tidal gauges, and seismometers, perhaps augmented by satellite observations.

A Thai official is silhouetted by a map in the new warning centre on the outskirts of Bangkok (AP)

On 20 January, the United Nations agreed to oversee the scheme, and despite some early political squabbling, elements are now being built.

Could the nuclear test-ban stations add anything to what is already envisaged? Roger Bowman believes so, though there may be political obstacles.

"Until this earthquake killed 200,000 people, the data was only made available to the CTBTO itself and to state signatories," he said, "and not to any hazard-warning organisation.

"I think there is going to be a loosening of data restrictions for this purpose, and I think the kind of data interpretation we have done could be folded into a hazard warning system."


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