Nuke test sensors could hear tsunamis coming

16:30 11 March 2011

Debora MacKenzie, Brussels correspondent

At the moment it's a matter of guesswork whether a submarine earthquake is going to produce a tsunami. But a treaty banning nuclear tests - or at least, trying to - may change that, if the treaty doesn't die first.

The Comprehensive Nuclear Test-Ban Treaty was signed in 1996 but has not yet come into effect, mostly because the US has not yet ratified it - though President Obama wants to.

Meanwhile a worldwide network of seismographs and other sensors designed to detect nuclear blasts listens for any tests. But it also gives fast, reliable warnings of earthquakes. The Comprehensive Nuclear Test-Ban Treaty Organization monitoring centre in Vienna, Austria, spotted today's Japanese quake, and alerted Indonesia and other governments to the quake off Sumatra that caused the 2004 Boxing Day tsunami. And it can also do other tricks too.

The system includes arrays of sensors at 60 sites across the world that listen for the low boom of atmospheric blasts. They are tuned to infrasound - frequencies under 20 hertz (cycles per second), the lowest humans can hear. Bombs aren't all they can hear - they also pick up volcanoes, and even rogue waves in the north Atlantic.

In 2009 the CTBT's scientists reported that their station on the Indian Ocean island of Diego Garcia had picked up infrasound from big quakes in Sumatra in 2004, and Mantawi and Nias Island in 2005. The first two produced tsunamis; Nias didn't. The first two also made a characteristic groan at 60 hertz near their epicentres. Nias didn't.

Milton Garces of the University of Hawaii at Manoa thinks this might be used to make a tsunami warning system. The infrasound signals generated by the tsunami-making quakes travel at up to 330 metres per second, while the tsunamis themselves propagate at about 260 metres per second. So the sound signal should arrive at a monitoring station before the wave hits.

He recorded these sounds from the 2004 Boxing Day quake. The first sound is the earth moving. The second is an echo of the sound of the earth that travelled through the ocean. The third is the signal generated by the production of a tsunami. No one knows yet what process gives rise to the noise.

Further research is under way. But the infrasound network being used to do it is under threat: the longer the CTBT remains unratified, the less enthusiastic governments are to keep funding it.

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