The planet that hums

From New Scientist, 11 September 1999

Listen carefully and you can hear the Earth singing quietly to itself. Robert Coontz puts his ear to the ground

THEY LIVE UNDERGROUND. They are everywhere but seem to come from nowhere. They barely exist, but never leave. If sounds have shadows, they are the shadows of a sound.

Researchers call them the background free oscillations of the Earth. But last year, when a pair of Japanese geophysicists named Naoki Suda and Kazunari Nawa dredged them out of a mass of seismic data, some people called them a hum. That's a comforting thought: a mystic Om, perhaps, or just the warm, cosy sound of a planet going about its business.

Don't try to tune in, you'll never hear it, though. The Hum is far too low for human ears to detect and is so feeble that a single 5.5-magnitude earthquake can blot it out. That's just as well because, if you could hear it, the Hum might drive you mad.

"It's a very messy noise," says Hiroo Kanamori, a geophysicist at the California Institute of Technology. Messy because the Hum is not one note but fifty, crammed into less than two octaves. Their pitches range between 2 and 7 millihertz. Musically speaking, that's about sixteen octaves below middle C. Speeded up and amplified so you could hear it, the result would be a Stockhausenesque cacophony. Imagine sitting down at a piano and slamming down every note within reach, while somebody next to you does the same thing on a piano a quarter tone out of tune. "It would be like banging a trash can," Kanamori says. Endlessly.

The individual notes are pleasant enough. They are the natural tones that the Earth makes whenever something--an earthquake, a meteor, a nuclear test--sets it ringing. They are known as "free" oscillations because, like the clang of a bell or the twang of a guitar string, they keep on sounding for a while after their source is gone.

What's peculiar about the notes in the Hum is that they have no obvious source. Not earthquakes, not nuclear explosions, nothing. The vibrations triggered by cataclysmic events fade away to nothing, but the Hum continues, regardless.

So what's the cause? It is hard to tell because, like the tone of a bell, free oscillations sound much the same no matter what sets them going. The three-dimensional patterns of vibrations, known as modes, depend mainly on how big the Earth is and what it is made of, not on what excites them. So free oscillations reveal plenty about the layers of rock they pass through, but are coy about their own origins.

Looking at the particular frequencies and energies does give some clues--enough to rule out the usual Earth-shaking events. So researchers are turning to stranger ideas to explain the Earth's never-ending mantra.

Scientists knew that free oscillations ought to exist long before they managed to detect them. At the turn of the century, seismologists were already detecting ordinary seismic waves--the short, sharp shocks of earthquakes--and using them to probe the depths of the Earth. Before the First World War, physicists had proved that those relatively high-pitched seismic waves ought to set the whole surface of the planet a-tremble with patterns of lower-frequency standing waves. But the planetary plainsong eluded researchers for decades.

The problem was their equipment was too crude. Even a simple seismograph can convert the lurching motion of an earthquake into the jump of a needle. Free oscillations, however, are much more elusive. Not only do they vibrate much more slowly and more subtly than ordinary seismic waves, they are also considerably more complex: three-dimensional tangles of vibrations at scores of different frequencies and pointing in different directions. To identify them, seismologists must tease out all the components, using a procedure called Fourier analysis to separate the different frequencies. The calculations are straightforward but too tedious to undertake by hand. By the late 1950s computers had solved that problem, but seismic detectors still weren't sharp-eared enough to pick up the oscillations from normal-sized sources.

Then nature let loose a blast nobody could miss. On 22 May 1960, the most powerful earthquake ever recorded struck southern Chile. The quake, now rated at magnitude 9.5, set the Earth's interior jangling. Earth scientists scrambled to dissect the vibrations and discover what they could tell about the Earth's vibrational modes, and the elasticity and density of its interior.

In the decades that followed, seismometers grew ever more sensitive. By the 1970s and 1980s, global networks of seismic stations were monitoring the vibrations of the Earth round the clock, and any seismologist or geophysicist craving information could download it as easily as turning on a tap. Over and over again, geoscientists witnessed a classic pattern: the shriek of an earthquake striking a resounding chord of free oscillations.

Meanwhile, between earthquakes, the Earth hummed away unnoticed.

The vibrations were there, all right; they were just extremely subtle. Rudolf Widmer-Schnidrig, a German geophysicist at the Scripps Institution of Oceanography in California, calculates that the power of the Hum is a mere 500 watts worldwide--barely enough to run five ordinary light bulbs. Even so, by the 1980s seismic instruments were perfectly capable of detecting it, and they did. Background free oscillations were plainly visible, for example, in the noise plots researchers used to gauge the quality of seismometers. But geophysicists paid the oscillations no more heed than the background hiss of a vinyl record.

The Hum almost came to light in the late 1980s, when a team at the Massachusetts Institute of Technology noticed that the Earth was oscillating even when there had been no earthquakes to set it in motion. The investigators decided that the vibrations must be due to "slow" or "silent" earthquakes, mysterious seismic events that were thought to release energy gradually, without any faults rupturing. Unable to pin down where the supposed slow quakes were taking place, however, the MIT researchers lost interest. The Hum never crossed their minds.

Then, in 1997, Suda and Nawa came on the scene and turned things upside down. Instead of starting with oscillations and looking for earthquakes to explain them, they looked between the earthquakes for oscillations they couldn't explain. Suda, a seismologist then at Nagoya University, and Nawa, then working on his doctorate under Suda's supervision, took their inspiration from a little-noticed paper by Naoki Kobayashi, a theorist at the Tokyo Institute of Technology. Kobayashi predicted that the Earth's atmosphere ought to excite free oscillations in the Earth. Suda and Nawa set about finding them.

Nawa had just spent a year at Japan's Syowa Station in Antarctica, tending a device called a superconducting gravimeter. The instrument had been installed to look for a controversial hour-long oscillation of the Earth's core, but it could also pick up shorter-period vibrations. Suda suggested that Nawa check its records for evidence of unexplained free oscillations. Meanwhile, Suda combed through archived data from seismic stations around the world. Then they started crunching numbers.

"It's actually not that sophisticated, which is why those of us who didn't do this can all be moderately embarrassed," says Duncan Agnew, a geophysicist also at the Scripps Institution of Oceanography. "You take the stations with the lowest noise. You take the days when there are no earthquakes. For each day, you take a Fourier transform of the data, which shows the distribution of energy with different frequencies. And then you simply add up all the days."

The result was a jagged graph showing a series of "spectral peaks", the frequencies at which the Earth oscillated in the lulls between large earthquakes. Nawa and Suda then subtracted everything that they could account for by known sources, including a theoretical estimate of the effects of earthquakes small enough to slip through the seismic net. They wound up with a residue of faint vibrations with no known source: the Hum. Nawa and Suda announced their results in 1998, and other researchers quickly confirmed them. The vibrations, it turned out, had been buzzing in their ears all along.

"The mystery is, where do they come from?" says Göran Ekström, a geophysicist at Harvard University. Ekström and most other geophysicists hope they have an underground source that might reveal something new about the depths of the Earth: slow earthquakes, the rumbling of tectonic plates or some exotic seismic process in a little-studied part of the Earth, such as oceanic fracture zones--places where the seafloor is being ripped apart in a complicated pattern of faults.

Earthquakes, an early favourite, started to lose their lustre on closer examination. When an earthquake strikes, it pounds out a chord made of frequencies from all the vibrational modes at the same time. In the Hum, by contrast, individual "notes" constantly drop out and reappear--a different style of music. For a while, deep-earth enthusiasts took heart from a strange signal in Nawa's Antarctic recordings. The gravimeter picked up oscillations with periods as long as 54 minutes--too long, in theory, to have been produced near the surface of the Earth. But those signals have not shown up in any other data, and Suda now thinks they must have come from a source at or near Syowa Station, perhaps buildings shuddering in the wind.

Now geophysicists are considering the possibility that the Hum could be generated above ground--and has little to do with their beloved rocks. Take the oceans. For seismologists listening for earthquakes, the pounding of surf along the world's coastlines is a constant annoyance. As waves crash onto the shore they create a 6-10 second thrum that can drown out the crackle of slipping faults. Some of that energy might excite the longer-period modes that make up the Hum. At present, though, oceanic sources look like a long shot. The smart money seems to be on Kobayashi's original bet, the atmosphere.

Could thin air really pack enough punch to turn the Earth into a huge aeolian harp? Easily, says Toshiro Tanimoto, from the University of California, Santa Barbara, a key proponent of the atmospheric-excitation hypothesis. The atmosphere receives enough energy from the Sun to keep the Earth humming thousands of times over.

In Tanimoto's opinion, the humming starts with drumming, the constant throb of fluctuating atmospheric pressure all over the Earth. When air pressure rises, the atmosphere presses down slightly harder on the ground or sea beneath it. When the pressure drops, the surface gently rebounds. In other words, the world is like a gong being constantly buffeted by countless soft rubber mallets. And at any given moment, some of them will be tapping at the right frequencies to excite the modes that make up the Hum.

Tanimoto has worked out exactly how energy from the atmosphere could be converted into the oscillations Suda and Nawa observed. His model predicts that the sounding of the global gong ought to vary over the course of a year, peaking in winter, when atmospheric pressure is highest and the airy mallets hit hardest. To test that prediction, Tanimoto analysed readings from 15 exceptionally quiet seismic stations scattered around the globe. By adding together spectral peaks from many years' worth of records, he amplified the vibrations until he could see subtle changes in their intensity. At each station Tanimoto checked, the Hum grew about 10 per cent louder between December and February and between June and August--winter in the northern and southern hemispheres respectively.

That twice-yearly rise in volume is the clincher, he says. "Processes in the solid Earth cannot possibly explain seasonal variations. There may be some slow movements of the Earth, but they don't happen in a seasonal fashion." And Suda has recently found evidence that the Hum also varies over the course of a day--further support for a source above ground.

An air-driven hum would be ho-hum for geophysicists, because it probably could not tell them anything they haven't already learned from the louder, cleaner signals of earthquakes. But even if continuous free oscillations turn out to be of no earthly use, they may have unearthly ones. After all, if the Hum starts in the atmosphere, then other planets with atmospheres ought to hum, too, and some researchers think background free oscillations could be just the ticket for studying their interiors. That's particularly likely to be true of a cool, tectonically dead planet such as Mars. Marsquakes are thought to be rare, but the Martian hum, if it exists, will always be turned on--faint, but available.

Philippe Lognonné, a geophysicist at the Institute of the Physics of the Earth in Paris, is in charge of coordinating the experiments for the first mission to explore Mars's geology. The Netlander mission, due to be launched in 2005, will place four seismic stations on Mars. Broadband seismometers will record a wide range of vibrations, including those likely to be found in a Martian hum, and relay the information back to Earth for one Martian year (about two Earth years).

To get some idea of what to expect, Lognonné and François Forget, an atmospheric scientist at the Pierre and Marie Curie University of Paris, are creating computer models of the Martian atmosphere and the free oscillations it might kick up inside the planet. Though the air on Mars is much thinner than that on the Earth, Lognonné says, the violent winds that tear across the Red Planet's surface ought to set Mars ringing, too--possibly as loudly as the Earth does. And with less background noise to interfere, the vibrations may be easier to detect. If so, they could give valuable information about the planet's mantle, about which next to nothing is known.

It's possible, of course, that Mars doesn't hum at all. The background free oscillations on Earth may turn out to come from the oceans, which Mars lacks, or from some subterranean process unique to our planet. But even if researchers never put the Hum to a practical use, its small, persistent whisper is a reminder that there are still mysterious things going on right under their noses. "Whatever the explanation is, we'll learn from it," Ekström says. "And until we do, it's fun to speculate."

Robert Coontz is a freelance writer based in Waukesha, Wisconsin