Humming along: ocean waves may cause global seismic noise.

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 Japan.

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. 30 Nature.

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 Webb.