deep-seated fracture in the Earth's crust trending southwest-northeast through Arkansas, Missouri, Tennessee, and Kentucky, U.S. Lying in the central area of the North American Plate, the rift is about 45 miles (70 km) wide and 190 miles (300 km) long. The deep fracture is overlaid by thick layers of rock, which in turn are overlaid by deep, unstable alluvial material relating to the Mississippi, Missouri, and Ohio rivers.

On Dec. 16, 1811, and Jan. 23 and Feb. 7, 1812, a series of three earthquakes (earthquake)—the largest in recorded American history—occurred near the frontier town of New Madrid, Mo. (epicentre 36.6° N 89.6° W), each measuring greater than magnitude 8.0. (Milder aftershocks occurred daily for more than a year.) The first shock was felt from Canada to New Orleans and as far away as Boston, Mass., and Washington, D.C. In the end, some 3,000 to 5,000 square miles (7,800 to 13,000 square km) were visibly scarred with the effects—causing such topographical changes as fissures, landslides, and upheavals, the creation and destruction of lakes and swamps, and the wasting of forests.