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”.
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,
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
Journal reference: Nature (vol 431, p 552)