1811 - 1812 New Madrid Earthquakes

The New Madrid quakes were actually three great quakes, each of which probably measured over 8.0 on the Richter Scale. They occured from December 16, 1811 to February 7, 1812. 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. The Mississippi River flows down a great trough filled with sediment washed down from 2/3 of the continent. 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.

The USGS page on the New Madrid quakes
As usual the US Geological Survey provides extensive coverage of the quake and the seismology behind it.
The Mississippi Valley-"Whole Lotta Shakin' Goin' On"
This US Geological Survey page has good descriptions of the New Madrid quake and evaluation of risk from future quakes in the Mississippi Valley, especially in view of the fact that millions of people now live there with little appreciation of the danger from earthquakes.
Virtual Times - The Great New Madrid Earthquake
This page has links to many other sites describing the quake and discussing the seismic theories concerning it.
Evaluating Earthquake Safety in Mid-American Communities
this paper reports on research being conducted at the Mid-America Earthquake Center to explore the characteristics of seismic risk for communities in the vicintiy of the New Madrid seismic zone.