ONE Saturday morning in 2010, Jody Smith, a resident of Carolina Beach in North Carolina, was disturbed by an extraordinary booming sound. She wasn鈥檛 alone: as she rushed out into the street, she bumped into neighbours also startled by the noise. The clear blue sky ruled out thunder. Smith went back inside and posted a message on Facebook asking if others had heard it too. Within minutes she had dozens of affirmative replies, some from 25 kilometres away.
For Smith, the sound wasn鈥檛 a one-off. 鈥淚 generally hear a booming noise about once or twice a year,鈥� she says. A but failed to find any nearby human activities Athat could explain the sound, such as military activity or quarry blasts. 鈥淚鈥檝e heard these booms a couple of times myself,鈥� says Hackman. 鈥淚t really is a bit of a mystery.鈥�
The residents of North Carolina are not the only ones to have heard unexplained booming noises. Strange rumbles, whistles and blasts have been reported all over the world for centuries. In the Seneca lake region of New York state, they are called 鈥淪eneca guns鈥�; in the Italian Apennines they are described as 鈥�brontidi鈥�, which means thunder-like; in Japan they are 鈥�yan鈥�; and along the coast of Belgium they are called 鈥�mistpouffers鈥� 鈥� or fog belches.
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鈥淚n New York state, the noises are called 鈥楽eneca guns鈥�, while in Belgium they are 鈥榝og belches鈥欌��
What is going on? Some of these sounds have obvious explanations: storms, say, or the crash of ocean waves. But in many cases, like North Carolina, nobody knows what鈥檚 behind the booms. That hasn鈥檛 stopped people proposing a host of explanations. And if some of their theories turn out to be true, it could change our ideas about the extent to which the Earth itself can roar and bellow.
Mysterious booms have a long history. Moodus, a village in Connecticut, was originally named , or 鈥渢he place of bad noises鈥�. Meanwhile, in 1938, Charles Davison, an amateur seismologist, documented sounds heard throughout the UK, with descriptions ranging from traction engines to the boom of a distant cannon and the noise of an immense flock of partridges ().
Thunder is the most likely explanation for many of the booms reported. The sound is created by a rapid expansion of air, associated with the sudden increase in pressure and temperature around the lightning bolt. In coastal locations, the ocean is another plausible culprit. According to Milton Garces, an acoustics expert at the University of Hawaii, there are a number of ways in which the ocean can make deep booming sounds: a wave鈥檚 lip impacting the surface of the ocean; the air being compressed out of a wave; a giant cloud of bubbles being entrained in the waves, radiating sound; or simply by a wave crashing into the shoreline (). 鈥淪uch sounds are clarion calls to us surfers. They can easily travel a few miles inland,鈥� Garces says.
However, sounds like that heard in North Carolina are not so easy to explain. Calm weather across the region ruled out thunder, as well as the noise of a turbulent ocean.
It was this that caught the attention of (USGS) in Menlo Park, California. In a paper published last year, Hill (Seismological Research Letters, vol 82, p 619).
One explanation that Hill admits he couldn鈥檛 rule out for the North Carolina case is secret military activity, such as the sonic boom of a jet or naval cannons at sea, thanks to a nearby military base. However, he points out that people reported sounds in the region long before the base was built, or indeed supersonic flight was invented. The same goes for other reports around the world.
Meteor roar
In principle, meteors could explain the noises, as they would produce a sonic boom if they survived atmospheric entry 鈥� something that could happen without anybody spotting them. By the time the sound waves reach our ears, the visible trace they leave when entering the atmosphere would be long gone.
Still, meteors can鈥檛 explain booms that are heard every few months or years in the same region, like North Carolina. 鈥淚f you did hear a meteor explode it would probably be a one-off event,鈥� says Michael Hedlin, a geophysicist at the University of California, San Diego.
That leaves geological explanations. In some places in the world, sand dunes produce whispering, whistling and even booming sounds. Large dunes with steep leeward faces, in arid conditions, are most likely to be boomers (). Exactly how the sound is made is still poorly understood, but smooth, loosely packed, nearly spherical sand grains are required, along with very low humidity. occur in about 30 locations, including California, Egypt, China and Wales. The Carolina coast, however, is not one of them.
Perhaps the most exotic theory that Hill considered blames the noises on a giant burp of methane. Some parts of the sea floor consist of methane hydrate beds that will release gas if disrupted. This gas could ignite and explode with a booming noise when it reaches the surface 鈥� or so the theory goes. 鈥淭he problem with this idea is that the methane is unlikely to come up suddenly enough or in large enough quantities to cause explosions,鈥� says Hill.
That leaves him with just one natural culprit for the North Carolina boom and others like it: . 鈥淭he network of seismograph stations in that region is sparse and there could easily have been a very small earthquake that went unnoticed,鈥� says Hill.
Hill believes that it doesn鈥檛 take a big quake to make a noise. Small quakes with no noticeable shaking occur all the time, even far away from plate boundaries. They often barely register on seismometers. Put together, the implication is that undetected earthquakes could be sounding off near you. 鈥淓arthquake sounds may be much more widespread than most people assume,鈥� he says.
Anyone who has experienced an earthquake knows that they are far from silent. The noise most people remember, however, is the sound of shaking buildings or trembling soil, not the quake itself.
Yet earthquakes do produce sound waves, which zip ahead of the waves that shake the ground. One person who has been close enough to hear what one sounds like is Malcolm Johnston, a colleague of Hill鈥檚 at the USGS. In 2008, while deep underground in a gold mine in South Africa, he was caught at the heart of a rupturing fault that he was supposed to be studying. 鈥淚 was at a depth of 3.6 kilometres, in a little alcove within the fault zone, setting up instruments,鈥� says Johnston. Suddenly a magnitude 2 earthquake struck, and it had originated from only 20 metres away. 鈥淚 heard a noise that was like a clap of thunder, but also with high-frequency sound superimposed,鈥� says Johnston. 鈥淎s soon as I heard the noise I knew I needed to remember it. At the same time I was distracted by trying to avoid falling rocks, and knowing that if the rupture came my way I would be mincemeat.鈥�
鈥淎s I heard the booming noise, I knew that if the rupture came my way I would be mincemeat鈥�
At the surface, we often fail to hear such a noise because only the very-low-frequency sound waves produced by an earthquake reach us, and these are outside our hearing range. Audible waves with shorter wavelengths 鈥� between 20 hertz to 20 kilohertz 鈥� are more likely to be absorbed and scattered by the rocks they travel through 鈥� in the same way that you tend to hear only the bass sounds when your next-door neighbour is playing music.
However, Hill says that the audible frequencies of an earthquake can sing out from the Earth under certain conditions. For example, a shallow earthquake would increase the chances of sound waves arriving at the surface intact. As would hard, fine-grained rocks like granite, as these are less likely to scatter the frequencies we can hear. And if the waves meet a fault plane, the sound could even zip up directly to the surface (see diagram). Once there, the sound waves would be transmitted into the air. 鈥淭he ground around the person acts like a giant woofer,鈥� says Hill.
What鈥檚 more, certain weather conditions can make sound waves travel even further. For example, on cool misty mornings, when a cold layer of air is trapped underneath a warmer atmospheric blanket, sounds may reflect off the warm layer and bounce for long distances.
But could small earthquakes really go undetected by seismometers? Jonathan Lees, a geophysicist at the University of North Carolina, is sceptical. 鈥淭he instruments used to record earthquakes are very sensitive,鈥� he says. 鈥淚f there were loud noises that were not observed as earthquakes, then most likely they are something else.鈥� Still, he does concede that some of the reported booms must have natural causes.
A number of field observations support the idea that small quakes can make big noises. Back in 1975, Hill and his colleagues installed seismic stations in the Imperial Valley, California. At night, when it was very still, their microphone recorded three booming noises, all of which coincided with three earthquakes between magnitudes 2 and 3 (). More recently, Matthieu Sylvander of the Paul Sabatier University in Toulouse, France, recorded booming noises associated with small earthquakes in the French Pyrenees. 鈥淲hen the earthquake is right under your feet you hear a booming sound, whereas if it is some distance away the sound is more of a low rumble,鈥� says Sylvander. 鈥淧robably many 鈥榝unny noises鈥� can be blamed on tectonics rather than poltergeists.鈥�
North Carolina, with its regular booms, could be the place to settle the mystery once and for all. A project called EarthScope is rolling out a dense network of seismic stations 鈥� 70 kilometres apart 鈥� across the US, from west to east. It should reach North Carolina in a couple of years, and will be sensitive enough to test Hill鈥檚 earthquake theory.
We鈥檝e also begun collecting better records of noises heard during earthquakes. For example, Patrizia Tosi of the National Institute of Geophysics and Volcanology in Rome, Italy, and her colleagues ask people to report the earthquake sounds they hear via an . Using this data, they produced a that struck the town of L鈥橝quila in 2009. Many of the sounds were probably caused by the shaking of buildings, but some of the reports from within 100 kilometres of the epicentre tally with the noise expected from the earthquake itself.
Whatever caused the boom that Jody Smith heard that day in North Carolina, it appears our planet could be much more vocal than we thought. The cacophony of the modern world means that most of us tend to attribute loud noises to human activity. But the sound that you assume is the rumble of a distant truck may in fact be the voice of the Earth itself.
