Humming Along: Ocean waves may cause global seismic
The slow and nearly constant vibrations of Earth's
crust stem from severe winter weather over some of the
world's oceans, a new analysis of seismic data suggests.
Our planet's outer shell is constantly pulsing.
Earthquakes trigger many of these ripples, but the
ground undulates imperceptibly even on days devoid of
significant temblors. Scientists have dubbed these
persistent vibrations "Earth's hum." The largest
undulations in this seismic background noise occur at
frequencies between 2 and 7 millihertz, or once every
few minutes, says Barbara A. Romanowicz, a seismologist
at the University of California, Berkeley.
Each day, the energy driving these worldwide
oscillations is the equivalent of that released during a
magnitude 5.7 earthquake. That's more energy than can be
explained by all of the planet's small daily
earthquakes, says Romanowicz.
To search for the sources of Earth's hum, Romanowicz
and her Berkeley colleague Junkee Rhie analyzed data
gathered by seismometer networks in California and
First, the researchers discarded data collected on
days when an earthquake larger than magnitude 5.5 had
struck anywhere in the world and on days when ripples
from those quakes might still be felt somewhere. During
2000 and 2001, slightly more than 130 days were free of
such earthquake effects.
Detailed analyses of the ground motions on those days
indicate that during the winter months in the Northern
Hemisphere the vibrations seem to originate beneath the
North Pacific Ocean. When it's winter in the Southern
Hemisphere, the apparent source of the oscillations lies
in southern regions of the Atlantic and Indian Oceans.
These are times when and places where high winds and
large waves generated by severe winter weather are
common. Those phenomena may be the ultimate cause of
Earth's hum, Romanowicz and Rhie speculate in the Sept.
Pressure pulses from waves generally don't extend
more than a few dozen meters beneath the ocean surface.
However, the effects of waves that stretch out over long
distances, such as tsunamis or those spawned by major
storms, can reach all the way to the seafloor, says
Spahr C. Webb, an oceanographer at Lamont-Doherty Earth
Observatory in Palisades, N.Y. The energy pulses of
those waves then spread through Earth's crust as seismic
vibrations, he says.
Some researchers had proposed that winds and
turbulence impinging on continental landmasses were the
engines of Earth's hum, but the new analysis provides
"fairly compelling evidence" that ocean waves are a
major cause of the seismic-background noise, says
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Romanowicz, B., and J. Rhie. 2004.
Excitation of Earth's continuous free oscillations by
atmosphere-ocean-seafloor coupling. Nature
431(Sept. 30):552-556. Abstract available at http://dx.doi.org/10.1038/nature02942.
Perkins, S. 2004. Killer waves.
Science News 165(March 6):152-154. Available at
Barbara A. Romanowicz
of California, Berkeley
Department of Earth and
Spahr C. Webb
61 Route 9W, P.O. Box 1000