Gary Anderson was not around to see a digger tear up the buffalo grass at his ranch near Akron, Colorado. But he was watching a few weeks later when the technicians came to dump instruments and insulation into their two-metre-deep hole. What they left behind didn't look like much: a mound of dirt and, a few paces away, a spindly metal framework supporting a solar panel. All Mr Anderson knew was that he was helping to host some kind of experiment. It wouldn't be any trouble, he'd been told, and it wouldn't disturb the cattle.
After a couple of years the people who installed it would come and take it away again. He had in fact become part of what is probably the most ambitious seismological project ever conducted. Its name is USArray and its aim is to run what amounts to an ultrasound scan over the 48 contiguous states of the US. Through the seismic shudders and murmurs that rack Earth's innards, it will build up an unprecedented 3D picture of what lies beneath North America.
It is a mammoth undertaking, during which USArray's scanner - a set of 400 transportable seismometers - will sweep all the way from the Pacific to the Atlantic. Having started off in California in 2004, it is now just east of the Rockies, stretching from Montana's border with Canada down past El Paso on the Texas-Mexico border. By 2013, it should have reached the north-east coast, and its mission end.
For something so fundamental, our idea of what lies beneath our feet is sketchy at best. It is only half a century since geologists firmed up the now standard theory of plate tectonics. This is the notion that Earth's uppermost layers are segmented like a jigsaw puzzle whose pieces - vast "plates" carrying whole continents or chunks of ocean - are constantly on the move. Where two plates collide, we now know, one often dives beneath the other. That process, known as subduction, can create forces strong enough to build spectacular mountain ranges.
In the heat and pressure of the mantle beneath Earth's surface, the subducted rock deforms and slowly flows, circulating on timescales of millions of years. Eventually, it can force its way back to the surface, prising apart two plates at another tectonic weak point. The mid-Atlantic ridge, at the eastern edge of the North American plate, is a classic example of this process in action. USArray will allow geologists to inspect Earth's internal workings right down to where the mantle touches the iron-rich core 2,900 kilometres below the surface - and perhaps even further down.
"It is our version of the Hubble Space Telescope. With it, we'll be able to view Earth in a fundamentally different way," says Matthew Fouch, a geophysicist at Arizona State University in Tempe. The combined effect of USArray's 400 seismic stations is to measure vertical and horizontal vibrations in the ground more comprehensively than ever before. Solar panels above ground provide power and, from most sites, cellular phone modems relay the seismometer data, which ends up at a co-ordinating centre in Seattle, Washington.
A total of 1,624 sites are planned, covering the 48 states. Installation work snakes up and down the map with the seasons as technicians follow the best of the weather, installing stations in the colder north in summer and in the south in winter. Where the main array reveals regions of particular interest it can be augmented by a fleeter, more flexible array of more than 2,000 smaller stations providing short-term, high-density observations.
Keeping USArray moving is no small operation. "There were quite a few people who said it couldn't be done," says Robert Busby, who manages the array for the operating consortium, Incorporated Research Institutions for Seismology, from an office in Falmouth, Massachusetts. "A few years ago, I was one of them." Each station generates plots of sound waves arriving from all directions - the acoustic calling cards of earthquakes, volcanic eruptions and even storms and ocean waves crashing on distant shores.
By assessing how long it takes vibrations to travel from an earthquake or another source to the array's various seismometers, geophysicists can deduce the properties of the intervening material. If rocks transmit sound at speeds that are unusual for their depth, they immediately become interesting: it suggests that they originated somewhere else. One area of focus will be the Reelfoot rift, a rent in Earth's fabric that extends some 200 kilometres south-west along the Mississippi valley, from New Madrid, Missouri, towards Memphis, Tennessee. More than 500 million years ago, rock began forcing its way up from the mantle beneath this area.
Had it continued it might have created a new rift valley, and ultimately a new ocean. It didn't. But that ancient drama still left its mark by creating the most seismically active area east of the Rockies. Between December 1811 and February 1812, New Madrid was the scene of a succession of huge earthquakes, one of which was powerful enough to ring church bells in Boston, Massachusetts, more than 1,500 kilometres away.
The likelihood of a repeat event around New Madrid within the next 50 years has been estimated to be between seven and 10 per cent; for a lesser, but still significant, quake the chances are between 25 and 40 per cent. As a result, the immediate region already boasts hundreds of seismometers, but further afield coverage is much sparser and knowledge of the risks correspondingly hazier. USArray should change that - and not before time. By 2011, the array will be centred over the fault zone. "That should tell us something about how stresses could build," says geophysicist Suzan van der Lee of Northwestern University in Evanston, Illinois. We will then be able to say whether New Madrid is unique, or whether other parts of the central US look similar. "That scenario is a bit scarier," she says.
USArray will provide only a snapshot in time, so will not predict when earthquakes will occur, but by telling us how the ground beneath us is structured, it might tell us which areas could be under stress, and which areas would be particularly vulnerable to shaking if an earthquake were to occur. www.newscientist.com
