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.
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REFERENCES
Please send comments and questions to Tom
Irvine at: tomirvine@aol.com