“What’s the mechanism?”
Tectonic Setting of the August 2011 Virginia Earthquake
http://www.earth.northwestern.edu/people/seth/research/VA2011/
Eventually, rifting starts within a continent, and the cycle starts again. Often the new rifting starts near the site of the earlier rifting, because continents don’t heal well. The eastern U.S. is a nice example. The Appalachian Mountains formed in a continental collision that closed an earlier Atlantic Ocean about 300 million years ago. Since then, the present Atlantic Ocean opened during the past 200 million years. This process left lots of fossil faults, old weak zones where earthquakes can - and do - happen. The Virginia earthquake probably happened on one of these.
No, no, no, and no! It is all the Chinese suddenly deciding to walk in the same direction all at the same time. . . in lock-step.
Or it's Aliens.