Worldwide 'Love' Vibe Detected

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.

10-Light-Bulbs Mystery

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.

Related Links:

Larry O'Hanlon's blog: Earth Impacts

Planet Green

How Stuff Works: Earthquakes

Plate Tectonics in Motion