The New Madrid seismic zone affects Illinois, Indiana, Missouri, Arkansas, Kentucky, Tennessee, and Mississippi. It is a 150-mile fault that stretches southward from Cairo, Illinois, through New Madrid, and down into Marked Tree, Arkansas. It also includes Reelfoot Lake in western Tennessee.
Through high-resolution imagery, geologists have determined the fault zone is mechanically weaker than the adjacent geologic structures. The weak rock extends down as much as 120 miles beneath the surface, actually reaching the mantle.