GEOPHYSICAL AND GEOLOGICAL ANALYSIS OF A POSSIBLE ACTIVE FAULT OUTSIDE THE NEW MADRID SEISMIC ZONE (NMSZ), WESTERN TENNESSEE, USA

The New Madrid Seismic Zone (NMSZ), is famous for producing of the 1811-1812 earthquake sequence including some of the largest (M ≥ 7.3) intraplate earthquakes within the North American Plate. In addition, there is geologic evidence that the NMSZ generated similar earthquake sequences during the Late Holocene. During the 1811-1812 earthquake sequence, as well as three prior events, strain apparently was distributed over three main faults within the NMSZ: the New Madrid North Fault, the Cottonwood Grove Fault, and the Reelfoot Fault. In addition, there is geological evidence to suggest that strain has been released in earthquakes along other faults outside the NMSZ proper. During a previous study, a possible fault southwest of Dyersburg, TN was hypothesized on the basis of unusual ground failure exposed in the cutbank of the Obion River and a northeast-trending lineament that passes through the site. In the summer of 2009, we conducted reflection/refraction geophysical surveying in the vicinity of the site of ground failure and across the lineament to determine if there is a fault at depth. Multiple stratigraphic layers were found. We plan on processing this data further. In addition, we logged a large compound sand blow related to earthquake-induced liquefaction and collected samples for dating the feature and thus the earthquake that caused it. Based on preliminary results of the field work and analysis of geophysical data, there is an indication that a northeast-trending right lateral strike-slip fault exists in the Obion River area. If so, the fault would be parallel to the Cottonwood Grove Fault and roughly perpendicular to the Reelfoot Fault. The shallow depth of suspected fault movement indicates that the fault was active recently, possibly around the time of major seismic activity in 900, 1450, or 1811-1812 AD. A finding of an active fault outside the NMSZ would have implications for the earthquake source model for this region.