RADIOCARBON EVIDENCE LEADS TO BETTER EARTHQUAKE DATE ESTIMATES

DENVER - Geologists from the Universities of Arkansas and Colorado have radiocarbon dated samples from cores collected in the New Madrid Seismic Zone, documenting a better earthquake history of the Mississippi Valley to help forecast the frequency of future earthquakes.

The earthquake timeline becomes more important daily as scientists try to estimate the chances of a big tremor in the nation's heartland.

Margaret Guccione, associate professor of geosciences at the University of Arkansas, will present the researchers' findings on Wednesday, Oct. 27, at the Geological Society of America meeting in Denver.

Guccione and geologists Karl Mueller and Jocasta Champion of the University of Colorado at Boulder used radiocarbon dating techniques to determine the geologic history of Reelfoot Scarp, a geologic fold that crosses an abandoned Mississippi River channel near Tiptonville, Tenn.

The researchers study the Reelfoot Scarp because the formation of this ridge is related to large earthquakes in the New Madrid Seismic Zone.

"The geologic slate is periodically wiped clean by the river, but the river has not eroded evidence of past earthquakes for the past 2,400 years at this location" Guccione said.

The researchers took core samples from the upper 25 feet of river sediment and dug 8-foot deep trenches into the sediment to collect samples for radiocarbon dating.

Two dates from these samples indicate that the river ceased to flow through the now-abandoned channel 2,400 years ago and that the folding of the river sediment must be younger. Muddy flood deposits, dated as young as 1260-1450 AD by other workers, are about the same thickness on top of the fold as they are below the fold.

"This suggests that the scarp was not very high prior to 1260 AD and that flood waters covered the fold. But we do know that by 1811-1812, the time of the New Madrid earthquakes which rang bells in Boston and temporarily reversed the course of the Mississippi River, the fold was high enough to dam a creek, form Reelfoot Lake, and was too high to be covered by flood water," Guccione said. Much of the 27 feet of uplift must have formed in the 460-550 year interval between 1260 and 1811 AD.

Scientists have identified at least two major prehistoric quakes that shook the Reelfoot Lake area during the past 1,000 to 1,200 years. Other earthquakes probably occurred during the past 2,400 years, but they apparently did not cause as much uplift along the Reelfoot Scarp as the two most recent large earthquakes, Guccione said.

To help date the earthquakes and associated uplift, Guccione's group also examined a prehistoric archeological site located along the abandoned Mississippi River channel and the Reelfoot Scarp. A radiocarbon date confirms the archeological estimate that the site was inhabited in 900 AD.

"When native Americans lived there, the village was located on the edge of an oxbow lake. Since the village was abandoned, much of the lake filled in, forming a swamp. Historic reports confirm that the swamp was drained after it was uplifted by folding." Guccione said. "It's amazing how much a landscape can change over time."

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