Web Issue 2108 September 30 2004   
Revealed: secret of the deep on why the world is humming
MARTYN McLAUGHLIN September 30 2004

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IT is a gentle humming noise that has baffled and irritated scientists in equal measure. But researchers yesterday pinpointed this constant deep drone, which is below the range of human hearing, as the product of an Earth "orchestra" consisting of the atmosphere, the oceans and the sea floor.
They believe the sound is probably caused by the conversion of storm energy to oceanic waves. This energy is then transferred to the sea floor, interacts with the shape of the ground, and sets up vibrations in the Earth called Rayleigh waves, which create the noise.
The hum represents the release of considerable amounts of energy, equivalent to an earthquake of magnitude 5.75 to six on the Richter scale, and has been consistently identified in seismic records in the frequency range of two to seven millihertz.
Junkee Rhie and Barbara Romanowicz, from the University of California, used seismic stations to locate sources of very long waves of energy on the Earth's surface. They found the hum originated mainly in the North Pacific during northern winters, and in the South Atlantic during southern winters.
In the journal Nature, they wrote: "Our results show that the ocean plays a key role in the excitation of the Earth's hum."
Japanese seismologists discovered the humming signal in 1998. Ms Romanowicz added: "People aren't usually that interested in looking at the noise, they want to get rid of it. But . . . It's very intriguing."