New Madrid Seismic Zone

Location

See text

References

(selected) Fuller 1912, Penick 1981, Stauder 1982, McKeown and Pakiser 1982, Gori and Hays 1984, Nuttli 1990, Stewart and Knox 1993

Description

Although it is neither within Illinois nor a structural feature, the New Madrid Seismic Zone must be mentioned here as a subject of public interest and concern. The earthquakes that shook the central Mississippi Valley in 1811 and 1812 were probably the most powerful experienced in the conterminous United States since the start of European settlement. Nuttli (1990) estimated that five separate events had surface-wave magnitudes of 8.0 or greater. That compares with 7.1 for the 1989 San Francisco Bay area earthquake. The New Madrid Seismic Zone is continuously active and hundreds of tremors are recorded yearly by modem seismographs. Most of these are too small to be felt. Quakes large enough to cause damage occur at infrequent intervals.

When earthquake epicenters are plotted on a map, a narrow zone of intense activity is seen to trend from northeastern Arkansas to the northwest comer of Tennessee, where it jogs north-northwest to New Madrid, Missouri.

From New Madrid the zone turns again northeast and terminates near Cairo, Illinois. The complete zone of intense activity is the New Madrid Seismic Zone. Away from the New Madrid Zone epicenters are scattered in a seemingly random pattern across a large area, which includes approximately the southern one-third of Illinois. The scattered quakes are mostly too small to detect except by instruments, but an occasional event attains a magnitude of 5.0 to 6.0, which is large enough to cause localized damage.

The New Madrid Seismic Zone overlies the Reelfoot Rift, a great fault-bounded trough that formed during the Cambrian Period. Since Cambrian time faults in the rift have been reactivated repeatedly under various tectonic stress fields. Because of these repeated movements the earth's crust in the New Madrid area is greatly weakened. Today, plate-tectonic movements subject the central United States to compressive stress oriented from east-west to east-northeast to west-southwest (Sbar and Sykes 1973, Zoback and Zoback 1980, Nelson and Bauer 1987). Stress builds in the New Madrid area until it is suddenly released by slippage along faults, producing earthquakes. The greater the amount of energy stored before release, the larger the quake. Because the New Madrid Seismic Zone has been observed for less than 200 years, the recurrence interval for destructive quakes similar to those of 1811 and 1812 is a matter of speculation.