New type of ocean wave detected

Taking measurements between Hawaii and California

By Dr David Whitehouse BBC News Online science editor

A new type of ocean wave has been found. It hugs the sea bed and constantly exchanges energy between the sea floor and the water just above it.

It was discovered by Cinna Lomnitz of the University of Mexico and US oceanographer Rhett Butler using data from a unique sea floor seismic observatory in the Pacific.

Professor Lomnitz was alerted to the possible existence of so-called coupled waves by the disastrous earthquake in Mexico City in 1985.

Destructive waves rippled through mud layers beneath the city causing many buildings to collapse. The waves found spreading through the oceans following earthquakes are similar.

Bed of sand

As a resident of Mexico City, Professor Cinna Lomnitz witnessed the destruction caused by the 1985 earthquake.

Hawaii-2 was installed in 2000

More than 400 high-rise buildings were destroyed; more than 10,000 people died.

Professor Lomnitz believes that one reason why the destruction was so widespread was because Mexico City is built on a bed of sand that allows an unrecognised type of seismic wave to travel with little hindrance.

"It is a coupled wave - that is, two waves that constantly share energy. Such waves are known from many areas of physics. For instance, a radio signal couples with electric waves in a radio antenna," Professor Lomnitz told BBC News Online.

He suspected that such waves could travel along the sea floor and he contacted oceanographer Dr Rhett Butler after he had seen the quality of data that the recently installed Hawaii-2 seafloor observatory was producing.

Earthquake protection

The observatory was established in 2000 and is positioned on the Pacific floor, half-way between Hawaii and California. It is connected to an obsolete trans-Pacific telephone cable.

Coupled waves in the water and sea bed

The station houses a series of scientific instruments that monitor sea floor and water movements.

Lomnitz and Butler found evidence for the new type of ocean wave. They classify it as a form of the so-called Rayleigh wave that propagates along the surface of solids.

Technically, the wave is a coupled acoustic and Rayleigh wave that constantly exchanges energy between waves in the seafloor and the water above it.

"Wave coupling is a fascinating phenomenon," Professor Lomnitz said. "Their time has come now that we understand them better than we did in the past."

As well as providing a fascinating insight into how energy travels through the oceans, the new discovery may help Professor Lomnitz find ways to protect his home city against a future earthquake.

"The problem with Mexico City is unfortunately more complex than the transmission of energy across the ocean, but I believe it is the same physics."