Thursday November 4 2:02 PM ET

Quake Could Hit Mississippi Valley

By PAUL RECER AP Science Writer

WASHINGTON (AP) - Researchers say a fault in the center of the country is creeping toward a major earthquake and should be considered a serious threat sometime during the next 500 years or so.

A new study of the New Madrid seismic zone on the Missouri-Tennessee border is moving at the rate of about six millimeters a year, enough to produce a 7.2 magnitude earthquake every 500 years, or a 7.5 magnitude quake every 1,000 years.

``Our evidence shows that the New Madrid seismic zone is indeed a threat,'' said Karl Mueller, a geologist at the University of Colorado, Boulder, and the first author of study appearing Friday in the journal Science.

``For the first time we can see how fast the earthquake engine is running and how long it takes to build up energy for a quake,'' he said.

A fault system called New Madrid extends through Arkansas, Missouri, Tennessee, Kentucky and Illinois. It jolted the continent with a series of quakes in 1811 and 1812 that were powerful enough to ring bells as far away as Boston.

Based on his new research, Mueller estimated the magnitude of those 19th century quakes at about 7.5, powerful enough to do major and extensive damage to any nearby city. At the time, the area of the earthquake was sparsely populated, with few manmade structures to be affected, but aftershocks from that series of jolts are still occurring.

Mueller and his co-authors from the University of Arkansas and from a Walnut Creek, Calif., company, William Lettis and Associates, analyzed the motion of the New Madrid fault by digging deep trenches along one section. This revealed folding in sediments caused by the movement along the underlying fault. By dating the deformed sediments and then measuring the amount of folding, the researchers could estimate how much the fault had moved over many centuries.

The slip rate, said Mueller, is about six millimeters a year, or about 45 feet over the last 2,300 years. Eventually, that slippage will build up a crustal strain that will be suddenly released, causing an earthquake, he said.

Mueller said there are no major cities in the immediate vicinity of Lake County, Tenn., where the fault measurements were taken, but he believes a major quake there would be enough to collapse levees along the nearby Mississippi River and to cause some damage as far away as Memphis.