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Last Updated: Thursday, 30 September, 2004, 16:54 GMT 17:54 UK
Science pinpoints Earth's 'hum'
Earth, Nasa
The scientists say they have ruled out earthquakes as the source
Scientists believe they have pinpointed the origin of a low frequency "hum" that emanates from the Earth.

The noise - which can be picked up in the two and seven mHz (millihertz) range - occurs far below the threshold of human hearing, US researchers said.

A University of California team propose the hum is a product of storm energy being converted into other forms in the Pacific and Southern Oceans.

Details of the study appear this week in the academic journal Nature.

The daily release of energy required to generate the hum is equivalent to a magnitude 5.75 to six earthquake, say Junkee Rhie and Barbara Romanowicz of the University of California, Berkeley.

They show that the hum is not distributed over the entire surface of the globe as previously thought. Instead, they have pinpointed the source of the "noise" to the seas not the land.

But the seismologists say that this noise cannot be explained by summing the contributions of small earthquakes. So they analysed seismic waves emitted from the Earth.

These so-called Rayleigh waves were measured on two networks of seismic instruments in Japan and California. The Rayleigh waves are usually generated from medium to large magnitude earthquakes. But not in this case.

Ocean source

They filtered their data to detect and locate non-earthquake sources for these waves.

Both sets of measurements agree that the "hum" originates from the northern Pacific Ocean during the northern hemisphere winter and the Southern ocean during southern hemisphere winter.

These locations also correspond to the regions of maximum storm activity in the northern and southern winters.

Rhie and Romanowicz think some of the energy contained in powerful waves generated by ocean storms at mid-latitudes is transferred via infragravity waves to the seafloor.

Infragravity waves are probably generated in shallow water from more common types of ocean wave and are indirectly driven by winds over ocean basins.

On the seafloor, these are converted to seismic waves and it is these that generated the hum.

"The Earth's hum is generated by the interaction between atmosphere, ocean and seafloor," the researchers write in Nature.

The efficiency with which the seismic waves are generated depends on the shape and depth of the ocean floor and on the strength and persistence of the storms.

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