When a massive earthquake struck Nepal on April 25, it created seismic waves that travelled around the world in a matter of minutes, propagating swiftly through Earth's crust and mantle to rattle seismic stations in the US. The Nepal quake was devastating, but the fact that it was felt nearly 13,000km away is actually not unusual.
Most earthquakes strong enough to be felt near their sources also produce body waves that radiate downward through the Earth. Those body waves can travel nearly all the way around the world, so seismic instruments record the waves of distant earthquakes pretty often. In fact, seismic stations far from the source of an earthquake can help pinpoint its epicentre by triangulating with other stations.
Mapping the Interior of the Earth
Geologists rely on seismic readings from distant earthquakes to help them learn about the interior of the Earth. Seismic waves travel at different speeds depending on the type of rock they're passing through; heated rock slows the waves down, for instance. By measuring the speed of seismic waves at different points, geologists can use the changes in their speed to produce an image of the Earth's interior. It works on the same principle as radar, sonar or ultrasound imaging, but with different types of waves in a different material.
Because seismic waves from distant earthquakes travel through the mantle and the deep layers of the crust, they offer a great tool for imaging the deeper layers of Earth's interior. That's the goal of the EarthScope project, an array of seismic stations spanning the entire continental US to gather seismic data and map the deep geological structure beneath the continental US.
Here's a video of EarthScope's seismic stations picking up the Nepal earthquake.
The white dots in the animation are seismic stations, and they turn red when seismic waves move them upward and blue when seismic waves move them downward. The seismograph at the bottom of the video is from the station circled in yellow, just southwest of the Great Lakes.
EarthScope includes the Plate Boundary Observatory, a network of 1200 seismic stations around the continental US and in Alaska, which use laser strainmeters to detect small changes in the ground's shape, tiltmeters to detect changes in ground level, GPS units to detect small changes in position, and, of course, seismometers to detect even the faintest tremors.
Predicting Earthquakes and Volcanic Eruptions
The project also includes the USArray, a movable grid of 400 seismic stations which has slowly marched eastward across the US over the past decade. Geologists set up 400 closely spaced stations along the Pacific coast in 2004. For two years, they took seismic readings and gathered data to help map the deep structure of the Earth, then they moved a little further east.Now, USArray has reached the end of the line, the easternmost slice of the continental US, where it will monitor the waves from distant earthquakes until 2017.
Why do geologists care about mapping the deep structure of the Earth? It provides better insight into the processes that drive natural hazards like earthquakes and volcanoes, for one thing.
Last month, geologists combined data from USArray with seismic readings from local earthquakes around Yellowstone to produce the first complete map of the Yellowstone supervolcano's magma plumbing. That map included the discovery of a second magma reservoir, which geologists had long suspected but had been unable to locate without the deep imaging offered by distant earthquakes' body waves.
Understanding these massive subterranean structures could one day give us better insight into when the next earthquake will strike, or when a volcano will erupt. But first, we have to gather data. And that's what USArray is all about.
Picture: Idaho National Laboratory, via Wikimedia Commons