Study traces Earth's hum to planet's troubled water

Sound: Scientists believe storm-ravaged seas are the source of the constant yet inaudible terrestrial tune.

Medicine & Science

October 04, 2004|By Michael Stroh | Michael Stroh,SUN STAFF

Talk about rock music.

In 1997, Japanese scientists poring over readings from gravity and seismic sensors around the globe made a startling discovery: The planet is humming to us.

You can't hear this terrestrial tune. Its notes sound roughly 16 octaves below middle C on the piano - well below the threshold of human hearing. Even if it were audible, "it's not going to be baroque-type music," says geophysicist Toshiro Tanimoto.

It's more like the sound of someone banging on a garbage can, scientists say.

Still, it's there. And now, after years of speculation, California researchers think they may have figured out why.

Employing techniques similar to those used to detect clandestine atomic blasts, the scientists have traced the hum to storm-ravaged regions of the northern and southern oceans where some of the choppiest waters on the planet are found.

"I've been thinking about this mystery for a long time," said co-author Barbara Romanowicz, director of the University of California's Berkeley Seismological Laboratory.

The hum's source, she says, also suggests an explanation: the noise is created by the force of the roiling waters pummeling the sea floor.

Romanowicz and graduate student Junkee Rhie report their findings in the current issue of the journal Nature.

Scientists have known since the 1960s that the Earth can ring like a bell after an earthquake, atomic blast or other violent event - reverberating for days after the seismic source has stopped. These so-called free oscillations can raise and lower the ground by as much as a centimeter.

But scientists always assumed that the Earth eventually fell silent.

So it came as a surprise when Japanese geophysicists Naoki Suda and Kazunari Nawa in 1997 researched seismic records and noticed that the Earth was constantly quivering, even in the absence of earthquakes or other seismic events. The planet, in other words, is humming.

The sound is exquisitely subtle. With a frequency between 2 and 7 millihertz, its cumulative global energy would power no more five 100-watt light bulbs, according to one calculation.

For this reason, the source of the hum has proven elusive. Over the years scientists have proposed everything from exotic "slow" earthquakes to aberrations in the Earth's core. "People were scratching their heads how to explain this," Romanowicz says.

It was also unclear whether the hum was occurring everywhere or in a particular spot.

To find out, Romanowicz and Rhie turned to a pair of sensitive seismic arrays planted across Japan and California. The instruments revealed that the hum had two sources: it was confined to a broad swath of the northern Pacific Ocean during the winter and then showed up in the southern oceans during the summer.

Checking satellite and other data, the researchers discovered that these two spots had something in common: violent storms and towering waves.

Romanowicz says this suggested that energy from the atmosphere - wind, for example - was being transferred into the ocean and ultimately the sea floor, causing the Earth to quiver.

Toshiro Tanimoto of the University of California at Santa Barbara says the Berkeley team's data may have finally nailed the mystery. "For me, it's convincing," he says.

Tanimoto, who has proposed a similar explanation for the humming phenomenon, says that the hum holds a larger lesson for geologists and other scientists.

"The solid Earth is regarded as this massive thing not affected by liquid and gasses," he says. "Now it's becoming clear that the whole thing is a system, and changes in one can affect the other."

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