ARCHEOLOGY, GEOLOGY HELP SCIENTISTS DATE MOVEMENT IN NEW MADRID SEISMIC ZONE

TORONTO - To help geologists form a timeline for a forecasted major earthquake in the nation's heartland, geologists from the Universities of Arkansas and Colorado are dating earthquakes and calculating rates of movement in the New Madrid Seismic Zone using archeological sites along the Mississippi River.

The timeline becomes more important daily, as scientists estimate there's a 90 percent chance of an earthquake of magnitude 6.0 or higher by the year 2040.

"The past is the key to the present," said UA professor Margaret Guccione, explaining that establishing the rate of movement and the frequency of past quakes will help people make decisions about current risk management practices.

Guccione presents her team's findings today (Oct. 27) at the Geological Society of America meeting in Toronto.

The researchers' work supports the idea that at least two periods of seismic activity occurred between A.D. 800 and 1812. During the winter of 1812, three quakes destroyed the town of New Madrid, Mo., rang church bells in Boston and disrupted the flow of the Mississippi River.

Guccione, University of Colorado geologists Karl Mueller and Jocasta Champion and Tennessee Department of Environment and Conservation, Division of Archeology archeologist William Lawrence examined an American Indian archeological site by a geologic fold called the Reelfoot scarp. This fold crosses an abandoned Mississippi River channel near Tiptonville, Tenn.

The abandoned channel, like the present Mississippi river channel, was originally straight, but relentless erosion caused the channel to form a loop or meander bend. Eventually the river cut off the loop to take a straighter path and deserted the old channel, which became an oxbow lake.

Guccione and her colleagues examined an American Indian archeological site, dated approximately A.D. 800, which is located along the bank inside the meander's curve. The village must have been established after the river formed the meander loop and was probably founded after the loop was cut off to form an oxbow lake.

Tribes typically located their sites near such oxbow lakes, Guccione said.

"It's a safer location with benefits," she said. The people could fish, hunt and maintain transportation routes, but were not in as much danger of flooding as they would have been along the Mississippi River, she said.

However, at some point the earth moved, lifting the village area above the water table and draining that part of the oxbow. It is unlikely that the village would have been built next to a dry lake bed, which means the scarp probably formed later - after A.D. 800, Guccione said.

Guccione's team's approach differs from traditional geological practices because they selected their site with dating in mind, she said. Geologists working in the New Madrid Seismic Zone typically study deformed layers of river deposits and try to determine how they were deformed. Finding organic material to date the deformation is a bonus, but is not the primary reason for choosing the study location.

"What they haven't done is looked at the broader picture of the Mississippi River," she said.

The researchers cored the upper eight meters of river sediment and dug trenches three meters into the sediment to collect samples for radiocarbon dating. These dates, including some from the archeological site, will provide a more accurate picture of the timing of seismic events, Guccione said.

The picture promises to be dramatic. The land at Reelfoot appears to have risen four meters in 1,000 years - quickly in geologic time, Guccione said. Only part of that can be accounted for in the 1811-12 quakes, she said.

The researchers received funding from the U.S. Geological Survey, which since 1990 has intensified its efforts to study the New Madrid Seismic Zone.

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