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."