Could We Hear a Tsunami Coming?

Naomi Brown

Research published by Usami Kadri, a postdoctoral researcher at Cardiff University, has shown that a type of sound wave called an ‘acoustic gravity wave’ could be used to detect and possibly mitigate tsunamis. These waves are formed naturally with underwater earthquakes and landslides, as well as tsunamis.

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What is an acoustic gravity wave?

A gravity wave is a wave generated in a liquid that is controlled by gravity, for example an ocean wave at the surface.  An acoustic wave, on the other hand, travels by longitudinal (lengthwise) compression.

One type of acoustic wave is a sound wave, which passes through a liquid by vibration, pushing against the particles in the fluid. Therefore, an acoustic gravity wave (AGW) is a combination of the two: a sound wave that spreads in the water layer and is governed by gravity.

Unlike surface waves, AGWs can span the entire water layer from the seafloor to the surface.  They can stretch tens to hundreds of kilometres and travel long distances in a very short time.

How could tsunami detection be improved?

Acoustic gravity waves travel at speeds close to the speed of sound in water – much faster than tsunamis. These waves also cause pressure disturbances on the seafloor, which makes them ideal for a tsunami detection system.

If two pressure sensors were placed on the seafloor in the deep ocean they could detect the acoustic gravity waves produced with an earthquake.  From this the epicentre, where the earthquake originated, could be located. Current systems detect the arrival of the tsunami so this idea could enable earlier detection.

In fact, the researcher, Usami Kadri, suggests that the Indian Earthquake in 2004 could have been detected over 3 minutes faster with AGWS. Even this could have saved many lives. He also demonstrated that by installing just 18 detection stations worldwide, all shorelines at high risk of tsunamis could be given an early alarm.

Could tsunamis be alleviated altogether?

When acoustic gravity waves interact with surface ocean waves they produce an exchange of energy. This can cause the surface ocean wave to decrease in height, also known as the amplitude, of the wave.

Kadri’s theory is that if two acoustic energy waves with carefully chosen amplitudes are emitted towards a long surface wave – a tsunami wave – there will be a distribution of energy between the three waves. This would cause the energy of the tsunami wave to dissipate.

If used against the Indian tsunami of 2004, it is predicted the height of the tsunami wave could have been decreased by 5 metres, which would have greatly reduced its impact.

Unfortunately, the technology to produce these huge, high-energy acoustic waves with good control over amplitude does not yet exist.  Therefore, the mitigation of tsunamis remains a theory at present. However, we can entertain the possibility that at some point in the future we may no longer have to face the devastation caused by tsunamis.

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