According to an article posted on the United States Geological Survey web site, in 1995 scientists estimated the probablility of a magnitude 6 to 7 earthquake occuring in the New Madrid earthquake zone within the next 50 years is higher than 90%.

 

The eighteen largest earthquakes in the United States

Recent Earthquakes - with real-time maps

The Mississippi Valley-"Whole Lotta Shakin' Goin' On''