The New Madrid seismic zone is located in the northern part of what has been called the Mississippi embayment. The Mississippi embayment is a broad trough filled with marine sedimentary rocks about 50-100 millions years old and river sediments less than 5 millions years old. The upper 30 meters of sediment within the embayment includes sand, silt, and clay deposited by the Mississippi, Ohio, St. Francis, and White Rivers and their tributaries over the past 60,000 years. Wisconsin valley train deposits formed during the glacial period from 10,000-60,000 years ago, and the Holocene meander belt deposits were laid down during the past 10,000 years. In the winter of 1811 and 1812, the New Madrid seismic zone generated a sequence of earthquakes that lasted for several months and included three very large earthquakes estimated to be between magnitude 7 and 8. The three largest 1811-1812 earthquakes destroyed several settlements along the Mississippi River, caused minor structural damage as far away as Cincinnati, Ohio, and St. Louis, Missouri, and were felt as far away as Hartford, Connecticut, Charleston, South Carolina, and New Orleans, Louisiana. In the New Madrid region, the earthquakes dramatically affected the landscape. They caused bank failures along the Mississippi River, landslides along Chickasaw Bluffs in Kentucky and Tennessee, and uplift and subsidence of large tracts of land in the Mississippi River floodplain. One such uplift related to faulting near New Madrid, Missouri, temporarily forced the Mississippi River to flow backwards. In addition, the earthquakes liquefied subsurface sediment over a large area and at great distances resulting in ground fissuring and violent venting of water and sediment. One account of this phenomena stated that the Pemiscot Bayou "blew up for a distance of nearly fifty miles."
After the earthquake [of 1811-1812] moderated in violence, the country exhibited a melancholy aspect of chasms, of sand covering the earth, of trees thrown down, or lying at an angle of forty-five degrees, or split in the middle. The Little Prarie settlement was broken up. The Great Prarie settlement, one of the most flourishing before on the west bank of the Mississippi, was much diminished. New Madrid dwindled to insignificance and decay; the people trembling in their miserable hovels at the distant and melancholy rumbling of the approaching shocks.
The Mississippi embayment is underlain by Paleozoic sedimentary rocks up to 570 millions years old. The Paleozoic rocks are underlain by even older rocks that appear to have been deformed about 600 million years ago when the North American continent almost broke apart. During the process of continental rifting, a deep valley formed that is bounded by faults and known as the Reelfoot rift. The Reelfoot rift is identified today as a subsurface system of fractures and faults in the earth's crust. New Madrid seismicity is spatially associated with the Reelfoot rift and may be produced by movement on old faults in response to compressive stress related to plate motions.
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