fault movement. The composition of geologic materials between these points is a major factor in transmitting the energy to buildings and other structures on the Earth’s surface. Earthquake strength is recorded by a seismograph and is described using either the Richter Scale, which is a measure of the intensity of energy produced by an earthquake, or the Modified Mercalli Scale, which describes earthquake intensity by the damage that results.
While geologists often refer to the midwest as the “stable midcontinent,” because of its lack of major crustal movements, there are two regions of active seismicity, the Nemaha Ridge and the New Madrid Fault Zone. The Nemaha Ridge in Kansas and Nebraska, associated with the Humboldt Fault, is characterized by numerous small earthquakes that release stresses before they build to dangerous levels. The area is not considered a threat to Iowa.
The New Madrid Fault Zone, on the other hand, has a greater destructive potential. It is located along the valley of the Mississippi River, from its confluence with the Ohio River southward, and includes portions of Illinois, Kentucky, Tennessee, Missouri, Arkansas, and Mississippi. The Earth’s crust in the midcontinent is older, and therefore thicker, cooler, and more brittle than that in California, for example. Consequently, earthquake shock waves travel faster and farther in the Midwest, making quakes here potentially more damaging than similar sized events in other geologic settings. The strongest historic earthquakes in North America occurred as a series of four shocks along the New Madrid Fault Zone between Dec. 16, 1811, and Feb. 7, 1812, and were centered near the town of New Madrid in the boot-heel area of Missouri. Based on historic eyewitness accounts, scientists have estimated the intensity of the earthquakes using the Modified Mercalli Scale and estimated an equivalent Richter magnitude of 8.3 to 8.7. These devastating earthquakes were felt from the Atlantic seaboard to the Rockies and felled trees, opened fissures, destroyed log buildings, erupted sand and water, created Reelfoot Lake in western Tennessee, and reportedly caused the Mississippi River to temporarily reverse flow in places. Because of the low population density, fatalities were relatively few, with most occurring on or near the Mississippi River.
Earthquake prediction is an inexact science. Even in areas that are well monitored with instruments, such as California’s San Andreas Fault Zone, earthquakes are only rarely predicted by scientists. As in the case of the Dec. 3, 1990, prognostication, there are always people who will profess some special knowledge of natural events that transcends traditional science and who will attract media attention. The public will do well to look at the credentials of those who make such dramatic statements and to seek authoritative sources for dependable information.