New wave equation developed for faster tsunami detection

Researchers from Massachusetts Institute of Technology (MIT) have identified a new and more reliable source of acoustic-gravity waves that may help scientists detect an upcoming tsunami early.

Acoustic-gravity waves are very long sound waves that cut through the deep ocean at the speed of sound. These lightning-quick currents are typically triggered by violent events in the ocean, including underwater earthquakes, explosions, landslides and even meteorites.

The researchers have now identified a less dramatic though far more pervasive source of acoustic-gravity waves: surface ocean waves such as those that can be seen from a beach or the deck of a boat.

These waves, known as surface-gravity waves, do not travel nearly as fast, far or deep as acoustic-gravity waves but under the right conditions, they can generate the powerful, fast moving, and low-frequency sound waves.

"Severe sea states such as tsunamis, rogue waves, storms, landslides and even meteorite fall, can all generate acoustic-gravity waves," said Usama Kadri, visiting assistant professor and a research affiliate in MIT's department of mathematics.

"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 added.

The researchers have developed a general theory that connects gravity waves and acoustic waves.

They found that when two surface-gravity waves, heading toward each other, are oscillating at a similar but not identical frequency, their interaction can release up to 95 percent 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.

Kadri derived a wave equation that includes compressibility and gravity as well as higher-order nonlinear terms.

The newly derived wave equation allowed Kadri to study the behaviour of both acoustic and gravity waves.

Kadri and his colleague Triantaphyllos Akylas, professor of mechanical engineering at MIT, have published their results in the Journal of Fluid Mechanics.

According to them, the results may help scientists connect interactions between not only surface and deep ocean waters but also with the atmospheric forces that affect surface waves.