The New Madrid fault zone is the result of something that happened over 600 million years ago, when the pre-Pangea super-continent of Rhodina was breaking apart. When supercontinents break apart they do so in a 3-prong process. Ultimately one of the three prongs "fails" and spreading continues along the remaining two. This is what happened there.
It is also what is currently going on in the area of the Great Rift Valley in Africa, the Red Sea, and the Gulf of Aden.
The resulting rift system failed to split the continent, but has remained as an aulacogen (a scar or zone of weakness) deep underground, and its ancient faults appear to have made the Earth’s crust in the New Madrid area mechanically weaker than much of the rest of North America.
This relative weakness is important, because it would allow the relatively small east-west compressive forces associated with the continuing continental drift of the North American plate to reactivate old faults around New Madrid, making the area unusually prone to earthquakes in spite of it being far from the nearest tectonic plate boundary.[16]
Since other ancient rifts are known to occur in North America but not all are associated with modern earthquakes, other processes could be at work to locally increase mechanical stress on the New Madrid faults.[17][18]
It has also been suggested that some form of heating in the lithosphere below the area may be making deep rocks more plastic, which would concentrate compressive stress in the shallower subsurface area where the faulting occurs.[19][20]
Earthquakes in the New Madrid and Wabash Valley seismic zones
Potential for future earthquakes:
In a report filed in November 2008, the U.S. Federal Emergency Management Agency warned that a serious earthquake in the New Madrid Seismic Zone could result in “the highest economic losses due to a natural disaster in the United States,” further predicting “widespread and catastrophic” damage across Alabama, Arkansas, Illinois, Indiana, Kansas, Kentucky, Mississippi, Missouri, Oklahoma, Texas, and particularly Tennessee, where a 7.7 magnitude quake would cause damage to tens of thousands of structures affecting water distribution, transportation systems, and other vital infrastructure.[21]
The earthquake is expected to also result in many thousands of fatalities, with more than 4,000 of the fatalities expected in Memphis alone.
The potential for the recurrence of large earthquakes and their impact today on densely populated cities in and around the seismic zone has generated much research devoted to understanding in the New Madrid Seismic Zone.
By studying evidence of past quakes and closely monitoring ground motion and current earthquake activity, scientists attempt to understand their causes and recurrence intervals.
In October 2009, a team composed of University of Illinois and Virginia Tech researchers headed by Amr S. Elnashai, funded by the Federal Emergency Management Agency (FEMA), considered a scenario where all three segments of the New Madrid fault ruptured simultaneously with a total earthquake magnitude of 7.7.
The report found that there would be significant damage in the eight states studied – Alabama, Arkansas, Illinois, Indiana, Kentucky, Mississippi, Missouri, and Tennessee – with the probability of additional damage in states farther from the NMSZ.
Tennessee, Arkansas, and Missouri would be most severely impacted, and the cities of Memphis, Tennessee, and St. Louis, Missouri, would be severely damaged. The report estimated 86,000 casualties, including 3,500 fatalities, 715,000 damaged buildings, and 7.2 million people displaced, with two million of those seeking shelter, primarily due to the lack of utility services. Direct economic losses, according to the report, would be at least $300 billion.[22]