PALEOSEISMIC FEATURES IN THE SOUTHERN PART OF THE NEW MADRID SEISMIC ZONE
Connelly, Jeffrey B., Hanan H. Mahdi, Robert E. Lemmer, Marilyn Egan, and Haydar J. Al-Shukri, University of Arkansas at Little Rock, Little Rock, AR 72204.
In the past 25 years our understanding of earthquake hazards in the New Madrid seismic zone (NMSZ) has been profoundly changed by results of paleoseismic investigations. Numerous large liquefaction features resembling those within the NMSZ have been discovered 90 to 100 km south-southwest of Marked Tree, Arkansas. Following an aerial reconnaissance survey of east central Arkansas, a detailed subsurface investigation was conducted at three sites, two near Marianna, Arkansas (Nancy 1 and Nancy 2) and one near Parkin, Arkansas (Parkin 1). One trench was excavated at Nancy 1 and two trenches were excavated at Nancy 2. Each of the excavations exposed a fine-medium grained sand blow deposit overlying a thick, plastic clay layer. The sand blows are fed by numerous vertical to shallow dipping sand dikes <1cm to 20cm thick. Forty-five near vertical dikes were logged at Nancy 1 whereas only 3-5 more gently dipping dikes were exposed in the trenches at Nancy 2. Parkin 1 was also trenched to investigate a 1.5 km long linear feature. The lineament trends N56°E and has a ground surface that is 2.75 meters higher to the southeast. No fault was observed in the trench; however, sand and clay layers tilted to the northwest were exposed suggesting possible faulting. The size of the liquefaction features and the distance (100 km) from the active NMSZ require either a major New Madrid earthquake or a more local seismogenic source. In either case, the seismic hazard in the region may be underestimated.