HUMAN OCCUPATION IN A TECTONICALLY ACTIVE ALLUVIAL VALLEY, THE NEW MADRID SEISMIC ZONE OF THE MISSISSIPPI VALLEY

Humans persist living in the alluvial valleys despite recurring natural disasters, such as flooding and earthquakes. However, some sites are selected for reduced flooding risk, but not reduced seismic risk. This is probably due to greater devastation and much longer recurrence intervals between seismic events than flooding events.

Four Late Woodland and Emergent Mississippian sites were examined on abandoned Mississippi meander belt 3, ~50 km northwest of Memphis, TN. The sites were occupied millenia after the Mississippi meander belt was active 5.6 kya and the Dead Timber Lake reoccupied the abandoned Mississippi channel < 4.1 kya. The sites varied in ecology and geology. Stable carbon isotopes indicate that the northern two sites (3PO608 and 3PO610) were forested, with mixed trees and grass surrounding the southern two sites (3CT340/341). Three sites are on point bars of abandoned scroll bars and one site is on the margin of the scroll bar with the Mississippi and Dead Timber Lake abandoned channels. Because the Mississippi meander belt has aggraded above the surrounding backswamp, these sites have been subjected to minimal flooding. All sites are surface sites, though the site at the abandoned channel margin (3PO610) has a thin clay cap. Seismic events of a large enough magnitude that they produced liquefied sand (>~6.2 moment magnitude) and surface deformation is recorded in the area. Two of the sites are disrupted by liquefied sand and sand blows. There is evidence of at least 3.2 m of uplift a few km east and 1.5 m of uplift 16 km north of the sites, causing drainage disruption. Regional dating of paleoearthquakes in the New Madrid seismic zone indicates a large earthquake A.D. 900 +/- 100 years occurred in the region during site occupation A.D. 400-1400, yet there is no evidence of site abandonment at this time.