On Aug. 23, 2011, a magnitude-5.8 earthquake struck near Mineral, Va., shaking the Piedmont region and damaging several historic buildings in Washington, D.C. The quake caught many people by surprise because the eastern U.S. lies in the interior of the North American Plate, more than 1,500 kilometers from the nearest plate boundary. In a new study, researchers peering beneath the southeastern portion of the North American Plate may have found an explanation for why parts of the region experience more quakes than expected.