Massachusetts Institute of Technology (MIT) researchers found a new source of acoustic-gravity waves that can help predict tsunamis: surface-gravity graves. The research team believes these are more reliable sources of acoustic-gravity waves.
Acoustic-gravity waves are fast-moving, long sound waves that travel deep across the ocean at the speed of sound. Ocean events such as landslides, explosions, underwater earthquakes and even meteorites trigger acoustic-gravity waves.
The MIT researchers found that surface ocean waves are more persistent sources of acoustic-gravity waves. Also known as surface-gravity waves, these are the waves that can be observed from the shores or from a boat.
While surface-gravity waves do not travel as fast as acoustic-gravity waves, the former can create fast-moving, powerful and low-frequency sound waves at the right circumstances.
The research team created a new theory: when two surface-gravity waves traveling towards one another are moving at different frequencies, the waves' interaction can give off nearly 95 percent of their original energy as an acoustic wave, which can travel deeper and much faster than the two surface-gravity waves.
The new equation takes into consideration higher nonlinear terms, gravity and compressibility. According to linear theory, when surface-gravity waves pass one another, they go on their own ways as if nothing ever happened. But when nonlinear effects are studied, the scene becomes more complicated. The two waves exchange energies and sometimes create new waves.
"Here, at specific frequency ranges, gravity waves can actually produce an acoustic wave that has completely different properties – and that is amazing," said assistant professor Usama Kadri and MIT's department of mathematics' research associate.
This kind of wave interaction can happen anywhere in the vast ocean, especially in the areas where surface-gravity waves collide due to continental shelf breaks. This is where the deep areas of the ocean meet shallower shores.
The new wave equation allowed the researchers to study both gravity and acoustic waves. The researchers hope to use the waves to predict tsunamis and potentially save lives. It can be effective as an early alarm system.
The study was published in the Journal of Fluid Mechanics.
Photo : Sönke Biehl | Flickr