The U.S. east coast is not supposed to be subject to intense earthquakes. Most earthquakes occur along active plate boundaries. The U.S. east coast is located on a passive margin.
However, on August 23, 2011 a magnitude 5.8 earthquake occurred in Mineral, Virginia, about 70 miles southwest of Washington, D.C.
Berk Biryol is a geologist at the University of North Carolina at Chapel Hill and is the lead author on a recent article in the Journal of Geophysical Research: Solid Earth which discusses this topic.
Biryol and his co-authors suggest that the upper mantle is peeling off of the crust and sinking into the Earth. This causes the Earth's crust to thin and weaken, thus making it prone to earthquakes.