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.
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.
Interestingly, that same mega-quake produced some horizontal waves as well, says seismologist Jeffrey Park of Yale University.
"We were pretty interested in those modes because they were not what we expected," Park told Discovery News. Even the subsequent tsunamis were heard making landfall by seismometers in islands around the Indian Ocean, he said. The waves actually tilted the islands a tiny bit, which was detected. "It's pretty neat."
All this is what happens when you have a large, loud rupture setting things off -- like striking a bell with a 20-pound sledge hammer. It's noisy and messy when the seismic waves are sped up and played as audio, said Aster.
"It kind of sounds like hitting a trash can," said Aster.
Far, far away from that global cacophony, at the extreme other end of the power scale, is the newfound toroidal Love wave hum, Widmer-Schnidrig explained. This mode moves the Earth's surface a mere millionth of a meter every five minutes and dissipates less than 500 watts of energy.
"In other words, with the power needed by 10 light bulbs you could keep up the hum worldwide," said Widmer-Schnidrig. "This is unbelievable, I know. But maybe it lets you appreciate the minuscule amplitude of this oscillation."
It also explains why it has taken 10 years after the detection of Earth's roaring spheroidal ring to capture Earth's whispering twist, Widmer-Schnidrig said.
The trick to the discovery was locating four extremely quiet seismic stations and then merging the data from their most quiet periods to tease out the tiny signal. The stations are the German Black Forest Observatory (BFO), Baijiatuan in China, and the Japanese Matsushiro and Takato stations.
"It was only by finding the horizontal hum in the data of stations far from our own which enabled us to conclusively demonstrate that the observed signal is a global phenomenon and not just a local artifact," said Widmer-Schnidrig.
Making Love Waves
As for what creates the 500-watt undulations, that's a bit of a mystery.
"Perhaps winds exert shearing forces on the solid Earth...when an air mass hits a mountain range, for instance -- or perhaps long-period ocean waves hitting the undersea walls at continental shelves are generating horizontal forces," speculated geophysicist Toshiro Tanimoto of the University of California at Santa Barbara in an April 3 commentary on the discovery in Nature.
It could even be caused by the sun, said Park. Oscillations in the sun may be picked up by Earth's geomagnetic field and cause Earth to hum a solar tune.
Finding the source will take work, Widmer-Schnidrig said. The first step is to find the Love wave signals at other stations. This might help point researchers in the direction of the source. New theories are also needed to explain precisely how winds, water or the sun can produce the Love wave hum.