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Chapter 10: A Violent Pulse: Earthquakes
Feature Articles
The Rest of the Story: The Mystery of the New Madrid Seismic Zone
by Elizabeth Lane MasonMost earthquakes occur at plate boundaries, but in 1811 and 1812, three magnitude 8 earthquakes struck the interior of the North American plate near New Madrid, Missouri. Since then, geologists have struggled unsuccessfully to solve the mystery of the New Madrid seismic zone. However, new research has identified a likely culprit: the melting of an enormous glacial ice sheet at the end of the last ice age.
Geologists have long been aware that the anomalous seismic zone located at the corner between Missouri, Arkansas, Tennessee, and Kentucky lies directly above a failed rift zone. Near the end of the Precambrian, upwelling magma beneath the crust threatened to break apart the North American plate. Before the rift occurred, the upwelling stopped. But the process had introduced a weakness in the plate. The existence of this weakness alone doesn't cause earthquakes, however.
During the last ice age, much of the North American plate was covered by a thick sheet of ice that reached as far south as Illinois. Though the ice didn't extend all the way to New Madrid, the weight of the sheet caused the earth to flex in that area. When the ice melted between 8,000 and 19,000 years ago, the removal of all that weight allowed the crust to rise to its original position. This rebound may be what is triggering the earthquakes in the failed rift zone.
The crustal rebound process is extremely slow. In fact, North America won't completely bounce back for another 10,000 years. This means that seismicity in the New Madrid area will also continue well into the future.
Evidence of liquefaction preserved in the sediments of the New Madrid seismic zone shows that major earthquakes of magnitude 7 or higher have occurred every 200 to 900 years for the past 1200 years. If the trend continues, this heavily populated area of the Midwest may be due for another big one any day now. Because the interior of the plate consists of old, rigid crust, seismic waves will travel far and a major earthquake there would affect a relatively large area. In 1812, the region was sparsely populated, but today it encompasses several large cities including St. Louis and Memphis. Compounding the danger, very few structures in the area were built to withstand an earthquake.
Scientists hope that if the deglaciation theory is correct, models of glacial melting and crustal rebound will help them more accurately predict if large earthquakes will strike the New Madrid seismic zone in the near future.
REFERENCES
Grollimund, B. and Zoback, M.D., 2001, Did deglaciation trigger intraplate seismicity in the New Madrid seismic zone?: Geology v.29, no. 2, p. 175-178.
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