A New Kind of Ocean Wave Has Been Discovered

A new kind of ocean wave has been discovered by geophysicists in the US and Mexico (Rhett Butler, IRIS Consortium, rhett@iris.edu and Cinna Lomnitz, UNAM, cinna@prodigy.net.mx).

At the Hawaii-2 Observatory, an unmanned research laboratory sitting on the seafloor between Hawaii and California, ocean waves of many varieties are observed.

Some are acoustic waves, underwater cousins of sound waves in the air, and consisting of pressure waves that alternately expand and compress water as they propagate through the ocean at the speed of sound in water.

Others are Rayleigh waves, seismic waves that propagates near the surface of the earth. Triggered by earthquakes, Rayleigh waves propagate as horizontal and vertical motions in the sediments and underlying crust.

Researchers have now detected a new kind of wave created by seismic events, for example, a 6.2-magnitude earthquake 10 km below the Pacific Ocean in June 2000.

The newly discovered wave, the researchers have concluded, is a "coupled" acoustic and Rayleigh wave that swaps energy above and below the seafloor.

Propagating at the sound velocity of water, the wave both induces horizontal and vertical motions in the seafloor sediments and creates regions of expansion and compression in the water. This coupled wave, the researchers found, carries more energy than conventional deep-Earth waves observed at the Hawaii-2 Observatory. (Butler and Lomnitz, Geophysical Research Letters, 24 May 2002.)