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New Madrid quakes weaker than thought

Pasadena, Calif. -- Earthquakes that rumbled in the U.S. midsection nearly 200 years ago weren't nearly as powerful as previously thought, an expert says.

Susan Hough, a seismologist with the U.S. Geological Survey, says the three main temblors associated with the New Madrid quakes of 1811-12 registered no more than a 7-magnitude, not the 7.7 that had been the standard estimate. That means the quakes, which made the Mississippi River flow south to north and created Tennessee's Reelfoot Lake were only 1/20th as powerful as thought, The (Memphis) Commercial Appeal reported Monday.

Hough presented her findings at a meeting of the Seismological Society of America last month in Portland, Ore.

Since there were no measuring instruments at the time of the quakes, Hough had a team of international experts independently assign "intensity values" to historical accounts of the quakes. That information was then submitted to computer analysis.

"When you do that, across the board the magnitudes are lower," said Hough, who is based in Pasadena, Calif.

Chris Cramer, a research associate professor with the Center for Earthquake Research and Information at the University of Memphis, questioned the accuracy of magnitudes based on the historical accounts, the newspaper reported.

"There is a little bit of a disconnect," Cramer said. "Scientifically, you cannot say what she is stating is true."

Copyright 2010 United Press International, Inc. (UPI).

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