Making pictures of the Earth's insides is more work than photographing stars. USArray is doing it two different ways—by listening to the natural seismic noises of earthquakes and by making artificial seismic waves from explosions.
Listening for earthquakes, or natural-source seismology, involves setting up permanent seismographic stations and waiting for quakes. It's rather expensive and takes a long-term commitment by a university or the government. Natural-source studies are our best tool for examining the Earth's mantle and core. They don't work as well for studying the crust, though, except in highly seismic regions like Southern California or in aftershock zones of large earthquakes.
The crust is usually studied with active-source seismology, where specially equipped heavy trucks or shots of explosives produce tiny "earthquakes" on demand. The seismic waves are recorded by temporary seismic stations. By moving sources and stations along a straight line on the ground and combining the data from hundreds of shots, we can build profiles of the crust just like ultrasound images of an unborn child.
The idea behind USArray is to get hundreds of identical, top-grade seismographs and lay them out in a big grid about the size of Texas. The network sits there for a year or two while scientists set off explosions to their hearts' content, mapping the crust and mantle down to 100-kilometer depths in unprecedented detail. The network also listens for natural earthquakes from all over the world, allowing us to look at interesting things like the core-mantle boundary. Then the machines are moved to another part of the country for another sojourn, like a doctor moving a stethoscope. After about ten years of this, the whole lower 48 states and Alaska will be covered.
At the same time, the Advanced National Seismic Network, the government's set of permanent seismographic stations, is being beefed up. And all of its stations will give us better data than before—the national "Earth telescope" has a clearer lens—because having the crust so well mapped lets seismologists distinguish local effects from distant ones.
The program began with the first grid covering California. Getting the Canadian and Mexican national seismic networks involved would give coverage for the whole continent, and a group is working on seafloor seismographs to extend USArray around the continent's edges. Canada's counterpart, POLARIS, is a scattered network of solar-powered stations that's already in use prospecting for diamonds and monitoring Canadian seasmic zones. European scientists are preparing a similar effort to be called Euroarray.
The authors of the USArray proposal are doing public education and outreach wherever the seismographs go. Teachers at all levels have something to show their students. So do the media—and with so many seismographs deployed, every city and town in America has a local angle. It's a decade-long scientific road show, coming to a place near you.
USArray is just one of four geoscience projects making up the EarthScope initiative. Read more here.
PS: If USArray works out like the Hubble Telescope, that would be a wonderful thing. Hubble is the most important telescope since Galileo's spyglass of 1610, and Hubble pictures have brought beauty and enlightenment to the rest of us too. For example, visit Fleur Helsingor's art + music gallery/concert hall for artworks based on Hubble images.