This article was written over a year ago, however I recently stumbled upon it and thought it was quite interesting. Most of the small earthquakes that occur in central U.S. are actually aftershocks of the bigger earthquakes that occured in the New Madrid seismic zone that hit the Midwest back in 1811 and 1812. In the middle of a continent, aftershocks tend to go on much longer. The reason for this is because aftershocks occur after an earthquake because the fault changed forces in the Earth. The aftershocks go on until the fault recovers and this takes much longer in the middle of the continent. The scientists compared this movement to the San Andreas fault where the two sides move past each other at about 1.5 inches per year, and this motion will swamp the small changes caused by past large earthquakes which will suppress aftershocks after about 10 years. In the New Madrid fault system, the two sides move more than 100 times more slowly, and it takes hundreds of years to swamp the small changes caused by large earthquakes.
http://www.sciencedaily.com/releases/2009/11/091104132652.htm
