Geologists do not know why earthquakes occur around New Madrid, Missouri, but they agree that the Mississippi Valley's unique geology, shaped by the ancient Reelfoot rift and the Mississippi embayment, is central to the region's seismic activity. The Reelfoot rift developed 1.2 billion years ago when crustal plates pulled apart, semi-molten mantle material rose into the lower crust, then cooled, grew denser, and sank back, causing the upper crust to sag into the Reelfoot basin. That basin later filled with river sediments and became the Mississippi embayment, where the weight of those sediments may place intense stress on faults within the rift area, now called the New Madrid Seismic Zone, which includes northeast Arkansas, southeast Missouri, southwest Kentucky, and western Tennessee. In December 1811 and early 1812, this zone produced a series of the strongest earthquakes known in eastern North America, including two major shocks in December 1811, another in January 1812, and the February 1812 quake known as the hard shock, followed by many severe secondary shocks; one Louisville engineer counted 1,874 shocks between December 1811 and March 1812. The December 16, 1811 quakes, centered between New Madrid and the Big and Little Prairie settlements, destroyed the Prairie settlements, measured 8.5 and 8.3 on the Richter scale, produced a fault 28 miles long and 18 miles wide, and were felt over more than half of the United States. The strongest quake, on 7 February 1812, measured 8.8, produced a fault 46 miles long and 28 miles wide, was felt in New England and as far as South Carolina, and destroyed New Madrid, where the ground cracked in all directions, circular holes forced sand, water, and coal to the surface, and the town sank from its original height on the river bank before eventually disappearing. The surrounding region also suffered sinking land, flooding, and fissures that blocked escape. The earthquakes also transformed the Mississippi River, destroying boats and banks, creating giant swells and temporary falls, opening fissures and whirlpools called sucks, and causing channels and islands to disappear. Reelfoot Lake in western Tennessee formed when the land sank and uplift blocked streams and creeks. If a quake comparable to the 1811 events struck today, extensive damage would likely affect much of the Mississippi River valley, with liquefaction causing ground failure, landslides, fissures, sand blows, damaged bridges, and impassable roads, but geologists cannot predict when the next major quake will occur.
