Investigating the various geophysical events that occur deep inside our planet is no easy task. Researchers are particularly interested in what is happening some 3,000 kilometres below the surface, but getting a clear picture of these processes requires more than a little ingenuity.

Alexander Hutko and his colleagues at the University of California, Santa Cruz, solved the problem by tracking the energy waves triggered by earthquakes. The results, detailed on page 333, offer a three-dimensional picture of events occurring near Earth's core.

Earth is divided broadly into three components: the crust, mantle and core. The crust and the upper part of the mantle together make up a region known as the lithosphere: broadly speaking the part of Earth that forms the tectonic plates. These plates ‘float’ on the rest of the mantle, which allows them some degree of movement — earthquakes occur when these plates grind against one another.

Hutko and his team were interested in what happens to slabs of the lithosphere that are driven deeper into the Earth when the tectonic plates bump into one another. To find out, the researchers began by analysing how seismic waves bounce off different layers and structures inside the planet.

“A simple way to think about this is as if you were standing on a shore and someone on the opposite side throws a stone, and you observe the waves to determine where the stone landed,” says Hutko.

The team homed in on a region in the Pacific Ocean off the west coast of Central America, situated between an earthquake-prone area of South America and a dense network of recording stations in the western United States. The earthquake data, which had been collected since 1991, were analysed using techniques developed by the oil-exploration industry about two decades ago to get detailed pictures of structures just below Earth's surface. “We have only recently had enough data to achieve this kind of resolution,” says Hutko.

Hutko and his team fed the data into a computer program that “spits out a bunch of numbers”, he explains. From these numbers, they were able to create two-dimensional images corresponding to slices of the deep Earth. By putting several slices next to one another, the team built up a three-dimensional picture. “It is amazing to go from simple records of the ground moving side-to-side to getting snapshots of the middle of the Earth,” Hutko says.

The team found that slabs of Earth's lithosphere, once part of the ocean floor, had sunk all the way down to the boundary between the core and the mantle. “Others had suspected that the base of the mantle was a graveyard of slabs, but no one knew how deep they went,” notes Hutko.

He now plans to travel to Japan to collect and analyse data beneath the western Pacific Ocean. “It is an exciting prospect as this region has one of the densest high-quality networks of recording stations in the world. The sheer volume of data is amazing,” he says.