RESEARCH IN NEW MADRID SEISMIC ZONE POINTS TO POSSIBLE EARTHQUAKE THREAT

FAYETTEVILLE, Ark. — Researchers have calculated slip rates in the New Madrid Seismic Zone (NMSZ) that indicate the threat of a major earthquake in the nation’s heartland may be higher than previously reported.

Peggy Guccione, assistant professor of geosciences at the University of Arkansas, Karl Mueller and Jocasta Champion of the University of Colorado at Boulder and Keith Kelson of William Lettis and Associates report their findings in the Nov. 5 issue of Science.

In 1811-12, earthquakes shook the Midwest, reversing the course of the Mississippi River and ringing church bells in Boston. Since that time, major cities, including St. Louis and Memphis, have built up in the region, and with their growth has grown the possibility of widespread damage and loss of life if a large earthquake should occur.

The history of movement in the New Madrid Seismic Zone relates directly to the possible magnitude of major earthquakes in mid-America. Assuming a constant interval between great earthquakes, a faster slip rate implies a greater possible earthquake magnitude, Guccione said.

"The past is the key to the future," Guccione said.

The researchers focused on the Reelfoot Scarp, a geologic fold that crosses an abandoned Mississippi River channel near Tiptonville, Tenn. Scientists study the ridge because past earthquakes in the NMSZ shaped its formation.

The team measured the widths of "kink bands" in exposed trenches - sharp bends in rocks and sediment formed as material is pushed upward - and the age of the folded sediment.

They determined that the slip rate of the Reelfoot Scarp was about six millimeters per year over the past 2,300 years, totaling nearly 45 feet.

Once they had slip rates for the Reelfoot area, the researchers extrapolated their measurements using data from other researchers to calculate slip rates for the entire New Madrid zone.

The estimates of approximately 4.8 to 6.1 millimeters per year were at the high end of calculations made by another team of researchers, who published their findings in Science earlier this year. The previous researchers dismissed the likelihood of a significant quake in the next 50 years, but these new findings suggest the probability may be slightly higher.

Although no one knows the exact magnitude of the 1811-1812 earthquakes, Guccione and her colleagues estimate they were about 7.5, based on the speed of the past fault slip.

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