Past earthquakes in that region haven't impacted as large an geographic area as one might expect from the energy involved. Perhaps the bedrock isn't solid enough, or the faults direct energy in other directions - I don't know.

The New Madrid zone is the one to worry about. Earthquakes are infrequent enough that building codes and inspections are not up to the job. The image on this page indicates what I'm talking about: look at the size of the affect area for a 6.8 quake! That area is full of tilt-up construction and it's likely to be a re-run of the Chinese disaster.