Map of the Central U.S. showing the pattern of Modified Mercalli Intensities that might be expected from a 7.7M earthquake that occurs along the New Madrid Seismic Zone.  Source:  USGS Click here to view the Modified Mercalli Intensity Scale.

The New Madrid seismic zone today generates about 200 tiny quakes annually, but it also let loose a magnitude 4.1 quake in February 2005 and a magnitude 4.0 quake in June 2005. The U.S. Geological Survey says there is a 9-in-10 chance of a magnitude 6 or 7 temblor occurring in this area within the next 50 years.

The fault structure under the New Madrid region is a "failed rift" created by the opening of the ocean that later became the Atlantic Ocean 650 to 600 million years ago, Forte said.

That activity also caused rifts in the St. Lawrence, Saguenay and Ottawa river valleys in Canada, where there is similar mid-continental quaking, he said. Another set of faults far from the boundaries of the North American Plate are associated with the Keweenawan Rift, a 1240-mile-long rift in the area surrounding Lake Superior.