NEW MADRID 1811-1812

The town of New Madrid is located in the southeast corner of Missouri, near the Mississippi River. The New Madrid seismic zone extends along the Mississippi River from the northeast corner of Arkansas up to Cairo, Illinois.
A series of powerful earthquakes occurred in the New Madrid fault zone during the winter of 1811 - 1812. The first earthquake occurred about 2 AM on December 16, 1811.
According to Bruce Bolt, the three worst earthquakes were estimated to have Richter magnitudes of 7.5, 7.3, and 7.8. Note that some sources give estimates as high as 8.7, however.
The New Madrid earthquakes were an anomaly because they occurred in the middle of the North American Tectonic Plate. Note that most major U.S. earthquakes occur along the edge of this plate. The Northridge and Loma Prieta earthquakes are examples of two recent earthquakes which occurred near the western edge of this plate.
The cause of the New Madrid earthquakes remained a topic of speculation for many years. Finally, geologists in the 1970s discovered a shallow fault in the New Madrid area. They used sound waves to make this discovery. They also found a rift valley underneath the fault and 25 miles below the surface. Rifts occur where the tectonic plate has been weakened by volcanic activity.
Furthermore, east-to-west compression of the North American plate may have been a related factor in the New Madrid earthquakes. Strain energy may have accumulated as a result of this compression.
Small earthquakes occur frequently in the New Madrid seismic zone. The last earthquake of serious consequence occurred on November 9, 1968. This earthquake was centered in southern Illinois. It was strongest in the central United States since 1895. The magnitude 5.5 shock caused moderate damage to chimneys and walls at Hermann, St. Charles, St. Louis, and Sikeston, Missouri. The earthquake was felt in portions of 23 states.
1. Bruce A. Bolt Earthquakes (Earthquakes, 4th Ed) 1999.
2. M. Levy and M. Salvadori, Why the Earth Quakes, Norton, London, 1995.
3. N. Bagnall and B. Shroeder, On Shaky Ground: The New Madrid Earthquakes of 1811-1812
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