Just a quick pieces cut and paste from here and there (as quick as dial-up allows, that is):
THE NEW MADRID FAULT SYSTEM EXTENDS 120 MILES SOUTHWARD from the area of Charleston, Missouri, and Cairo, Illinois, through New Madrid and Caruthersville, following Interstate 55 to Blytheville and on down to Marked Tree, Arkansas. It crosses five state lines and cuts across the Mississippi River in three places and the Ohio River in two places.
THE GREAT NEW MADRID EARTHQUAKE OF 1811-1812 was actually a series of over 2000 shocks in five months, five of which were 8.0 or more in magnitude. Eighteen of these rang church bells on the Eastern seaboard. The very land itself was destroyed in the Missouri Bootheel, making it unfit even for farmers for many years. It was the largest burst of seismic energy east of the Rocky Mountains in the history of the United States and was several times larger than the San Francisco quake of 1906.
It's been some time (10 or 15 years), but I recall reading some concerns about it and that a large enough quake would impact much of Midwest and Canada....