Deep-ocean sound waves may aid tsunami detection

Scientists are developing a system that may help predict a tsunami by detecting sound waves that race through the deep ocean more than 10 times faster than the more destructive wave.
"Severe sea states, such as tsunamis, rogue waves, storms, landslides, and even meteorite fall, can all generate acoustic-gravity waves," said Usama Kadri, a research affiliate at Massachusetts Institute of Technology's (MIT).
"We hope we can use these waves to set an early alarm for severe sea states in general and tsunamis in particular, and potentially save lives," Kadri said.
Acoustic-gravity waves are very long sound waves that cut through the deep ocean at the speed of sound.

These lightning-quick currents can sweep up water, nutrients, salts, and any other particles in their wake, at any water depth.
They are typically triggered by violent events in the ocean, including underwater earthquakes, explosions, landslides, and even meteorites, and they carry information about these events around the world in a matter of minutes.

Researchers at MIT have now identified a less dramatic though far more pervasive source of acoustic-gravity waves - surface ocean waves.
These waves, known as surface-gravity waves, do not travel nearly as fast, far, or deep as acoustic-gravity waves, yet under the right conditions, they can generate the powerful, fast-moving, and low-frequency sound waves.

The researchers have developed a general theory that connects gravity waves and acoustic waves.
They found that when two surface-gravity waves, heading towards each other, are oscillating at a similar but not identical frequency, their interaction can release up to 95 per cent of their initial energy in the form of an acoustic wave, which in turn carries this energy and travels much faster and deeper.
This interaction may occur anywhere in the ocean, in particular in regions where surface-gravity waves interact as they reflect from continental shelf breaks, where the deep-sea suddenly faces a much shallower shoreline.
Understanding this relationship between surface-gravity waves and acoustic-gravity waves allows researchers to describe how energy is exchanged between gravity and acoustic waves, researchers said.

This energy could be vital for many marine life forms, and it could play a role in water transport and the redistribution of carbon dioxide and heat to deeper waters, thereby sustaining a healthy marine environment, they said.
Kadri calculated that if two surface waves flow towards each other at roughly the same frequency and amplitude, as they meet and roll through each other the majority of their energy - up to 95 per cent - can be turned into a sound wave, or acoustic-gravity wave.
This new understanding of wave interactions can be used for tsunami detection, researchers said.

The study was published in the Journal of Fluid Mechanics.