New Madrid quakes of 1811-12 still felt in retrospect as experts meet in Memphis

By Tom Charlier
Memphis Commercial Appeal
Posted April 14, 2011

As a major conference on earthquakes opened in Memphis Wednesday, the 500 international experts attending turned their focus away from recent disasters in Japan, New Zealand, Chile and Haiti and looked back 200 years.

Their attention was locked on the 1811-12 sequence of quakes that shook the New Madrid fault zone north of Memphis, creating Reelfoot Lake and triggering landslides and other upheavals along the Mississippi River. It's an event that remains the subject of intense speculation and scientific interest even today.

Presentations at the annual meeting of the Seismological Society of America included a paper reporting that the notorious quakes probably weren't nearly as powerful as previous estimates of magnitude 7.7 or greater.

When independent experts reviewed historical accounts, they assigned the three main temblors that occurred between Dec. 16, 1811, and Feb. 7, 1812, a maximum magnitude of about 7.0, said author Susan E. Hough, research seismologist with the U.S. Geological Survey.

"The numbers that came out were lower," Hough said in an interview, adding that historical accounts of the quakes' effects in St. Louis, the only major city of the region at the time, "weren't that bad."

Determining the magnitude of the 1811-12 quakes might seem trivial, said Chris Cramer, research associate professor at the University of Memphis Center for Earthquake Research and Information, but it is "really key" to assessing how well buildings will hold up in future temblors.

The SSA, which moves its annual meetings among cities, chose Memphis this year largely because of the upcoming bicentennial of the New Madrid quakes.

The event at the Cook Convention Center, which lasts through Friday, has attracted seismologists from around the world.

The meeting follows a series of major quakes worldwide -- most recently the magnitude 9.0 Tohoku temblor and resulting tsunami off Japan that killed more than 13,000 people.

Although the New Madrid events might seem distant, the Japan quake highlights the importance of learning about previous seismic activity in the fault zone, said Thomas L. Holzer, engineering geologist with the USGS. Japanese experts "basically underestimated" the potential magnitude of temblors in the area where the Tohoku event occurred, he said.

Archaeological studies indicate the New Madrid zone has produced quakes of around magnitude 7 every 500 years or so. Quakes that strong can be devastating near populated areas, as evidenced by last year's 7.0 temblor that killed more than 200,000 in Haiti and the 6.3 quake that killed at least 148 people in Christchurch, New Zealand, in February.

Christchurch is "a very good analogy" to Memphis, Cramer said.

"There are a lot of big buildings that are vulnerable here," he said.

That point was underscored in another paper presented Wednesday estimating that a magnitude 6.4 to 6.9 quake near Memphis could cause up to $130 billion in damage to private property and businesses, while losses from a 7.7-magnitude event could surpass $250 billion. But at least 65 percent of damage would be covered by insurance, according to the paper by M.L. Zoback.

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