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  Geotimes - April 2007 - Ocean waves drive Earth's hum

Ocean waves drive Earth's hum

Ocean waves approach the shoreline near Monterey, Calif. High-energy near-shore waves are the main source of energy for the constant seismic background noise known as Earth’s “hum.” Photograph is by Carlyle Calvin, University Corporation for Atmospheric Research.

Even in the absence of an earthquake, Earth’s crust continuously produces a mysterious background noise, or “hum.” That enigma may now be solved, as new models suggest that the hum stems from energy transferred to Earth’s crust from shallow ocean waves.

Seismic waves following an earthquake cause Earth to ring “like a bell,” or, taking into account the discordant series of tones, “like a piano dropped off a truck,” says Spahr Webb, a geophysicist at Lamont-Doherty Earth Observatory at Columbia University. But these tones continue, to a lesser degree, in the absence of large earthquakes as well, he says, “so our virtual piano is always sounding on all its strings all of the time.” The background noise composes what has become known as Earth’s hum.

Previously, researchers have suggested that atmospheric forces, pushing and pulling on Earth’s crust, have induced the hum. Still others have shown that ocean waves are capable of transferring energy into seismic waves. Now, models of the hum compared with physical ocean wave data
suggest that indeed, the hum gets most of its energy from shallow ocean waves along Earth’s continental shelf, Webb reported Feb. 15 in Nature.

To model the hum, Webb set out to calculate the hum’s energy balance. The rate of energy lost (dissipated through frictional forces) could be determined by watching how seismic waves decay following an earthquake.

But maintaining a constant hum requires that the rate of energy lost be balanced by a similar rate of energy input (such as by ocean waves). Webb combined the information to arrive a model for the expected hum spectrum.

Next, Webb set out to see if the energy contribution measured from actual waves matched the energy contribution expected in the model. Webb used existing observations from waves in the shallower water over continental shelves, where waves tend to be the most energetic, and therefore have the largest contribution to seismic energy.

The model closely matched observations, showing that energy derived from the sizable waves within the shallow waters on Earth’s continental shelf are “sufficient to explain the hum,” Webb says. By comparison, any contribution from the atmosphere would be insignificant, he says.

But Naoki Kobayashi of the Tokyo Institute of Technology says that Webb’s paper does not rule out the influence of atmosphere. Kobayashi, publishing in 2000 in Nature, found that Earth’s hum exhibited oscillation characteristics that link its source to the atmosphere, which are not recognized in Webb’s paper, he says. And, even if ocean waves are found to be the ultimate driver of the hum, the primary energy source remains in the atmosphere, where winds act to drive waves, he says.

Still, the debate should further the understanding of just what causes the hum, Kobayashi says. And that’s important, as recent work has shown that seismic background noise can be used to learn about the internal structure of Earth.

Kathryn Hansen

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