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Earth's 'hum' springs from stormy seas

18:00 29 September 04 news service

An enigmatic humming sound made by the Earth may be caused by the planet’s stormy seas, suggests a new analysis.

Japanese seismologists first described the Earth’s humming signal in 1998. It is a deep, low-frequency rumble that is present in the ground even when there are no earthquakes happening. Dubbed the “Earth’s hum”, the signal had gone unnoticed in previous studies because it looked like noise in the data.

“People aren’t usually that interested in looking at the noise, they want to get rid of it,” explains Barbara Romanowicz, a geologist from the University of California at Berkeley, US. “But this is an unusual phenomenon, it’s very intriguing.”

No-one was sure what source of energy could be causing the constant vibrations, which have a frequency of just a few millihertz – well below the limits of human hearing. The Japanese team suggested that variations in atmospheric pressure might drum on the surface of the ground, giving rise to the vibrations, but Romanowicz was not convinced.

“From the beginning, I had a hunch that the oceans might be involved” says Romanowicz, “but then I had to prove it somehow”.

Sloshing around

She and colleague Junkee Rhie collected data from networks of seismometers in California, US, and Japan. They worked out the direction that the hum signal was travelling on each of 60 earthquake-free days Earth experienced in one year. Using the directions measured at the two distant networks, they could trace the seismic signal back to its source.

During January and March, the hum came mainly from the North Pacific Ocean. Then the source swapped to the southern oceans around Antarctica, before shifting north again in October. Therefore, the hum appears to follow winter in each hemisphere, when ocean storms are at their worst.

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Romanowicz now plans to use computer models to work out exactly how water sloshing around in the ocean basins can transfer its energy to Earth to create the rumble. Others in the field say they will only be convinced when this link has been proved.

“The situation is not clear. It is possible that both the oceanic and atmosphere turbulence are exciting the vibrations,” says Philippe Lognonne from the Paris Geophysical Institute in Paris, France.

If Romanowicz’s mechanism is the right one, it will mean disappointment for scientists who thought similar vibrating signals would be seen on other planets, creating a kind of symphony in space. Dry planets would hum only if their atmospheres could set up oscillations.

Journal reference: Nature (vol 431, p 552)


Jenny Hogan


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