They were largest earthquakes in the history of the contiguous United States. Estimated at over Magnitude 8.0 on the Richter Scale, they were felt over most of the Eastern United States, as much as 1,000 miles from the epicenter, just west of the Mississippi. This is an unusual place for earthquakes, since it occured in the middle of the North American Plate. Most earthquakes are on the edges of tectonic plates, as they push or slide by other plates. The New Madrid zone, however, seems to represent a weak point in the North American Plate.
Nobody seems to understand exactly how that causes quakes.
Seismic activity continues in the New Madrid Fault Zone and scientists believe that another large earthquake will someday strike that area again. While this area was sparsely populated in 1812, today there are millions of people throughout the Midwest who are at risk from a large earthquake. Many people rate this at the top of a hazards list in the United States. This page has links to many other sites describing the quake and discussing the seismic theories concerning it.