A new kind of ocean wave

A new kind of ocean wave has been detected. The Hawaii-2 Observatory, which sits on the sea floor between Hawaii and California, observes waves of many varieties. Some are acoustic waves that alternately expand and compress water as they propagate through the ocean at the speed of sound in water. Others are Rayleigh waves that are triggered by earthquakes and propagate as horizontal and vertical motions of Earth’s crust, including the sea floor. Researchers have now detected a “coupled” acoustic and Rayleigh wave that swaps energy across the interface at the ocean’s floor. Propagating at the sound velocity of water, the wave both induces motion of the sea-floor sediments and creates regions of expansion and compression in the water. The new wave requires that the Rayleigh wavelength be shorter than the water’s depth and that the shear velocity at the interface not exceed the water’s sound velocity. The researchers speculate that similar modes might occur at the air–soil interface. (R. Butler, C. Lomnitz, Geophys. Res. Lett. 29 , 57, 2002 http://dx.doi.org/10.1029/2002GL014722 .)