The 1811 or 1812 New Madrid Earthquake, is one of the largest succession of earthquakes, including the most intensive ever indirectly inferred (not recorded) in the contiguous United States, beginning with an initial pair of very large earthquakes on December 16th, 1811 plus aftershocks and other large related quakes separated by a succession of smaller aftershock quakes with the largest event classified as a Mega-quake of greater than 8.0 on the Richter scale occurring on February 7, 1812. It got its name from its primary location in the New Madrid Seismic Zone, near New Madrid, Louisiana Territory (now Missouri), where a stretch of land five miles deep spanning from Arkansas to Illinois shifted and slipped. The fault is believed to generate a slip every 250-400 years.
This earthquake was preceded by three other major quakes: two on December 16, 1811, and one on January 23, 1812. These earthquakes destroyed approximately half the town of New Madrid. There were also numerous aftershocks in the area for the rest of that winter with research indicating a series of some 2,000 earthquakes overall that affected the lands of what would become eight of today's heartland states of the United States..
There are estimates that the earthquakes were felt strongly over roughly 130,000 square kilometers (50,000 square miles), and moderately across nearly 3 million square kilometers (1 million square miles). The historic 1906 San Francisco earthquake, by comparison, was felt moderately over roughly 16,000 square kilometers (6,000 square miles).
Some sections of the Mississippi River appeared to run backward for a short time. Sandblows were common throughout the area, and their effects can still be seen from the air in cultivated fields. Church bells were reported to ring in Boston, Massachusetts and sidewalks were reported to have been cracked and broken in Washington, D.C. There were also reports it toppled chimneys in Maine.
The Reelfoot Rift goes back about 750 million years, to when the entire landmass of the earth constituted a single supercontinent, designated now as Rodinia. At the time a constructive fault zone began to form, now called the Reelfoot Rift, but it failed, and the zone became inactive.
About 550 million years later, at the time of the supercontinent called Pangaea, the fault zone again became active but no longer functioned as a constructive plate and remains in the same condition today. The earthquakes are therefore traced to seismic activity 5 to 25 kilometers (3-15 mi) below the crust of the earth.
The old riverbed, however, still defines state lines along the river, resulting in numerous small jogs where the borders divert away from the river and incorporate small exclaves of land on the opposite side of the river from the rest of their state.
The zone remains active today. In recent decades minor earthquakes have continued. New forecasts estimate a 7 to 10 percent chance, in the next 50 years, of a repeat of a major earthquake like those that occurred in 1811-1812, which likely had magnitudes of between 7.5 and 8.0. There is a 25 to 40 percent chance, in a 50-year time span, of a magnitude 6.0 or greater earthquake.
Understanding of this earthquake zone is growing slowly in comparison to awareness of the San Andreas fault.
A few states have joined forces and founded a special institute for their earthquake zone, to prepare as well as possible for a major earthquake. The Mississippi River will probably present one of the incalculable problems. A few emergency funds for earthquake victims have been founded. Measures are also being ordered to mitigate any natural disaster resulting from an earthquake; thus in the construction of dams, bridges, and highways, earthquake safety is particularly being taken into account.