Worldwide 'Love' Vibe Detected

April 16, 2008 -- A subtle and mysterious global hum has been detected by seismologists studying records from Earth's most boring seismic stations.

The newfound vibration is made of two-minute to five-minute, side-to-side surface seismic waves, also called Love waves. They are named for the British mathematician Augustus Edward Hough Love, who created the mathematical model of such waves in 1911.

The discovery comes 10 years after seismologists first identified louder global oscillations that resemble the ringing of a gigantic bell.

All of the planet's natural oscillations have signatures, or "modes" of vibration, depending on where and how they are created by earthquakes, ocean waves or other forces. Among the suspects for making the Love wave hum are winds, ocean waves or even the sun.

The Love wave is a mode that essentially torques the Earth's north and south hemispheres against each other. It's as if the planet is dancing the Twist, explained Rudolf Widmer-Schnidrig of the Black Forest Observatory in Wolfach, Germany, and the Institute of Geophysics at the University of Stuttgart. This gentle, faint twisting is called the "toroidal" mode.

"Standing on the surface of the Earth, you would only experience a horizontal motion left-right-left," if you were sensitive enough to feel it, Widmer-Schnidrig told Discovery News. "To picture this you imagine holding a metal or maybe rubber rod in your hands. Now twist the two ends of the rod in opposite sense and let go.

The two halves of the rod will perform small oscillatory twisting motions in opposite directions. That's what the two hemispheres of the Earth would also do."

Widmer-Schnidrig and Dieter Kurrle report their discovery in the April issue of Geophysical Research Letters.

Raucous Earth

This newfound mode is entirely different from the rowdy "spheroidal" mode, which is a far more powerful oscillation that actually warps the shape of the planet like waves on a water surface.

"At the surface you would experience an up-down motion together with a forward-backward motion," said Widmer-Schnidrig of the spheroidal mode. "Something to get seasick about."

"You can simply picture a pumpkin and an American football," said Widmer-Schnidrig. "Let a sphere (the Earth at rest) deform itself into a pumpkin, then a sphere, then an American football, then a sphere, then a pumpkin...and you have the fundamental spheroidal mode of the Earth."

The most dramatic spheroidal mode vibration detected by science to date was near the Galapagos Islands soon after the Great Sumatra-Andaman Earthquake of December 26, 2004.

The Galapagos are almost exactly on the opposite side of the Earth from the site of the 9.3 magnitude rupture, and the Earth pulsed there with such power that the surface moved up and down with the vibration about an inch every few seconds, said seismologist Rick Aster of New Mexico Institute of Mining and Technology.