Earthquakes and Floods

FAYETTEVILLE, Ark. — A University of Arkansas geologist studied four archeological sites in eastern Arkansas and determined that although the inhabitants may have experienced one or two earthquakes during the period of habitation, they escaped the frequent flooding of the Mississippi River.

Margaret Guccione, professor of geosciences in the J. William Fulbright College of Arts and Sciences, will report her findings Wednesday, Oct. 25, at the Geological Society of America conference in Philadelphia.

Guccione studied four sites in eastern Arkansas along the planned route of Interstate 555 between Jonesboro and Memphis. She and graduate student John Horn worked with two archeological companies, Spears Professional Environmental & Archeological Research Service Inc. and Mid-Continental Research Associates, to examine the sites. Guccione and Horn looked for evidence of the physical and biological environment before, during and after the time of occupation.

While working at the sites, Guccione noticed little evidence for flooding of the sites by the Mississippi River, but some evidence for surface deformation and earthquakes. A stream, called “Dead Timber Lake,” looked as though it had flowed uphill. Guccione and Jackson Cothren, assistant professor of geosciences, used survey-grade global positioning equipment to determine the current height of the Dead Timber stream at six locations. The GPS studies showed that along the stream, the channel and its banks slope both downstream and upstream, with a small lake in the area between the opposing slopes.

“Streams don’t flow uphill. The stream goes 'uphill’ because there has been some deformation since the channel formed about 4,000 years ago,” indicating possible shifting along faults beneath the earth’s surface and the occurrence of earthquakes,” Guccione said.

In addition to the deformation, the researchers also found evidence of sand blows that are caused by earthquakes in the site excavations.

“This area is part of the New Madrid Seismic Zone and is affected by earthquakes,” Guccione said.

Past studies have shown that at least two earthquakes occurred in the region during the time that American Indians occupied the study sites — between A.D. 500 and 1450. Despite the evidence of disruption at two of the sites, there is no evidence that the earthquakes caused the inhabitants to abandon them. In these instances, the earthquake that caused the most damage to the archeological deposits occurred after the archeological villages were abandoned by the occupants.

Why would they choose to live in this earthquake-prone area? Guccione’s work offers an explanation. Cores taken by the researchers at the sites show no evidence of clay deposition that would be expected if periodic flooding occurred.

“The image of the Mississippi valley is that if you get a flood, most of it will be underwater,” Guccione said. However, these sites are located along an old meander bend of the Mississippi, and the heightened banks allowed protection from flooding.

“They chose these site locations because they didn’t flood,” Guccione said.