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Earth hums while making 'Love' waves

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, called Love waves.

They are named after the UK mathematician Professor 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, says Dr 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 says.

"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 journal Geophysical Research Letters.

Raucous earth

This newfound mode is entirely different from the rowdy spheroidal mode, which is a far more powerful oscillation that 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," says Widmer-Schnidrig of the spheroidal mode, "something to get seasick about."

"You can simply picture a pumpkin and an American football," he says.

"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 26 December 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 2 centimetres every few seconds, says seismologist Professor Rick Aster of New Mexico Institute of Mining and Technology.

Horizontal waves too

Interestingly, that same mega-quake also produced some horizontal waves, says seismologist Professor Jeffrey Park of Yale University.

"We were pretty interested in those modes because they were not what we expected," Park says.

Even the subsequent tsunamis were heard making landfall by seismometers in islands around the Indian Ocean, he says.

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 9 kilogram sledge hammer.

It's noisy and messy when the seismic waves are sped up and played as audio, says Aster.

"It kind of sounds like hitting a trash can," he says.

Mystery of the 10 light bulbs

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 explains.

This mode moves the earth's surface a mere millionth of a metre every 5 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," says 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, he says.

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, 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 artefact," says 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," says geophysicist Professor Toshiro Tanimoto of the University of California, Santa Barbara in a recent commentary in the journal Nature.

It could even be caused by the sun, says 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 says. The first step is to find the Love wave signals at other stations, which might help point researchers in the direction of the source.

New theories are also needed to explain precisely how wind, water or the sun can produce the Love wave hum.