1811-1812 Seismic Event

What many refer to as the Great New Madrid Earthquake, was actually a seismic event made up of 3 major quakes, followed by thousands of aftershocks, ranging from strong damaging quakes and diminishing to weaker quakes that lasted for years after the main shocks. The 3 main shocks were estimated at magnitudes 7.7, 7.5, and 7.7. As there were no seismograph stations in the region at that time, magnitude estimates vary widely (from 7.7 to 8.1) based on interpretation of journal accounts, damage reports, and descriptions of effects from that time.

The main shocks were felt as far away as the Gulf Coast, East Coast, and Quebec. The area of damage is estimated at 600,000 square kilometers; the quakes were felt over an area of approximately 5,000,000 square kilometers. The area of strong shaking was approximately 10 times that of the Great 1906 San Francisco Earthquake.

In addition to the main shocks, 4 aftershocks estimated at greater than 6.0 occurred between December 16, 1811 and February 1812. More than 200 aftershocks ranging from large to moderate were reported between December 1811 and March 1812. An estimated 1,800 additional aftershocks took place during the same period. Aftershocks strong enough to be felt extended through 1817. Aftershock activity continued over the next decade.

Land Changes
The effects of the quakes were extensive and dramatic. Land deformation and elevation changes, both uplift and subsidence, created major land features such as the St. Francis Sunklands in northeast Arkansas, Reelfoot Lake in western Tennessee, and temporary waterfalls on the Mississippi River. Vast tracts of land were flooded. The most severely affected area is estimated at between 78,000 and 129,000 square kilometers. Large regions of fertile land were made unfit for farming for many years afterward by landslides, sandblows, fissures, and flooding. Thousands of sandblows are still visible today.

On the Mississippi River, miles of high river embankments collapsed and entire islands were reported submerged. The river was filled with debris, navigation channels changed, and large waves were generated. Turbulence capsized boats.