LIQUEFACTION AND OTHER POSSIBLE SEISMOGENIC FEATURES SOUTHWEST OF THE NEW MADRID SEISMIC ZONE IN EASTERN ARKANSAS
Robert E. Lemmer*, Haydar J. Al-Shukri, Jeffrey B. Connelly, and Hanan H. Mahdi, University of Arkansas at Little Rock, Little Rock, AR 72204.
Aerial and surface reconnaissance has discovered earthquake-related features approximately 44 to 100 km southwest of Marked Tree, Arkansas. A detailed subsurface investigation was conducted at three sites, two near Marianna, Arkansas and one near Parkin, Arkansas. Three trenches were excavated revealing three large (>107m X 55m) elliptically-shaped liquefaction features resembling those within the NMSZ. Each of the excavations exposed a fine-medium grained sand blow deposit overlying a thick, plastic clay layer. The sand blows were fed by numerous vertical to shallow dipping sand dikes <1cm to 20cm thick. Forty-five near vertical dikes were logged at one site whereas only 3-5 dikes were exposed at a second site. A 1.5 km long linear feature near Parkin was also investigated. The lineament trends N56°E and has a ground surface that is 2.75 meters higher to the southeast. No fault was seen in the trench, however, sand and clay layers tilted to the northwest were exposed suggesting possible faulting farther to the east. A geophysical survey is planned to investigate this possibility. The size of the liquefaction features and the distance of 100 km from the active NMSZ require either a major New Madrid earthquake or another seismogenic source. In either case, the seismic hazard in the region south of the active NMSZ may be underestimated.