We're approaching the bicentennial anniversary of the largest earthquake and swarm east of the Rocky Mountains, at least in recent history. Coincidentally, Oklahoma has just had it's largest earthquake in the state's history. Bicentennial of the New Madrid Earthquake Sequence December 1811–February 1812 http://pubs.usgs.gov/gip/134/ The seismic activity can abruptly and dramatically change anywhere in the world, as was witnessed in the Canterbury region around Christchurch, NZ on Sept 4, 2010, although one might expect such activity in that part of the world which sits astride the boundary of the Australia and Pacific plates. The middle of the US has been considered stable, so strong earthquakes are rather surprising. It will be interesting to see what follows the recent earthquakes in Va and Ok.