Most earthquakes occur along prominent faults at plate boundaries. Earthquakes along the San Andreas fault, or the one earlier this year in Japan are examples of that. Today’s earthquake in Virginia is an intraplate quake, one that occurs in the middle of the plate, not at the edge. (The boundary of the North American Plate is in the middle of the Atlantic). Although these earthquakes are not as common as the others, they also are not unexplainable. They are caused by stresses that are concentrated along ancient zones of weakness in the rocks. The Central Virginia Seismic Zone is one such zone. That part of North America was once a plate boundary (back when the Appalachian Mountains were formed), which left many ancient faults in the area. I know a small earthquake occurred in that area in 2009 and I think also in 2003. Today’s quake was just bigger than those, so it made the national news.


Another thing about eastern earthquakes that is interesting is that they usually affect a larger area than similar magnitude quakes in western North America. This has to do with the structure and composition of the rocks. East coast rocks are older, colder, and tend to be less fractured, which allows seismic waves to travel through them with greater ease.