Continent-spanning seismic array images geological structures

This sounds like an extension - on a grand scale - of the methods used by geophysics companies to find oil. But it goes well beyond that. 

The station is one of hundreds deployed in a giant array that currently stretches from the St. Lawrence Valley to Virginia. The array’s purpose is not simply to detect earthquakes but to use them to visualize what is happening below the surface. Since earthquake vibrations are affected by changes in the composition and density of the material they travel through, it’s possible to combine data from many stations to map the three-dimensional structure of solid rock.