Is Earth's deep hum a sea song?

By Glennda Chui
Knight Ridder Newspapers

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SAN JOSE, Calif. — Scientists think they have found the source of a mysterious hum that reverberates through Earth, too low for human ears to hear.

They once believed it came from earthquakes; a big quake will set the whole planet ringing like a bell. Even when there are no big quakes, though, the hum continues, a slow, steady slosh of waves around the planet.

Now, with instruments in California and Japan, scientists have pinpointed the source. The hum, they say, starts in the oceans, when winter storms whip the waves into a frenzy.

"These waves interact with each other to create longer waves that reach deep into the ocean, all the way to the ocean floor," said Barbara Romanowicz, a seismologist at the University of California, Berkeley.

The thumping of those waves on the bottom, like the pounding of a drum, sets the Earth vibrating in a phenomenon known as free oscillation, she said.

She and graduate student Junkee Rhie put forth their explanation in yesterday's issue of the journal Nature.

Gregory Beroza, a geophysicist at Stanford University, said he found the report convincing.

"It's just interesting, a new phenomenon that begs to be explained," he said.

The hum has fascinated scientists since its discovery six years ago by a group in Japan.

Its vibrations consist of long, slow seismic waves that raise the ground by a fraction of an inch as they go by. It takes five minutes for two of these waves to pass a given point. They put out little power, about as much as a couple of 100-watt light bulbs, Romanowicz said.
 
Although these waves crisscross the planet all the time, people cannot feel or hear them. In musical terms, the sound would be about 16 octaves below middle C.

Which may be just as well.

If the hum were shifted up into the range of human hearing, it would be a cacophony of noise, said professor Toshiro Tanimoto, a geophysicist at UC, Santa Barbara.

The tones "are never in harmony so it would be horrible, probably," he said. "More like modern music" than baroque.

To figure out where the hum starts, Romanowicz and Rhie looked at readings from about 24 seismic instruments in California and Japan.

They found that the hum originated in the northern oceans in winter and in the southern oceans during the Northern Hemisphere's summer, which is the winter storm season there.

Romanowicz said she wants to check the seismic instruments to determine if the recent string of hurricanes had any effect on the hum.

"Even though they're very strong at the surface, and have devastating consequences when they land, they're very localized. They cover only a very narrow swath," she said of hurricanes. So her hunch is that they did not — "but it has to be checked."