Paleoseismic Features in the Southern Terminus of the New Madrid Seismic Zone

Al-shukri, H J, Lemmer, R E, Connelly, J B, Mahdi, H H. and Egan, M A., University of Arkansas at Little Rock, Little Rock, AR 72204.

Recently conducted aerial and field surveys and Landsat analysis reveal the existence of liquefaction features (sand blows) and linear features as far as south of Marianna, Arkansas. This is more than 100 kilometers from the currently active segments of the New Madrid Seismic Zone. We believe that these possible earthquake-related features are significant for the following reasons: (1) They are at considerable distance from present-day earthquake activity. The implication of this is that they represent a new earthquake source region not previously recognized, or that they were caused by an earthquake(s) of very large magnitude in the source region of the New Madrid Seismic Zone. (2) These features appear to be very large (some ~110 meters), resembling features in the immediate vicinity of the New Madrid Seismic Zone. The implication of this is that no matter where the source region is, the ground shaking was significantly severe in order to generate them. (3) Detailed investigation of these features may have important implications for earthquake risk mapping in the central United States, they may provide important constraints on the southern terminus of the New Madrid Seismic Zone and the magnitude of the characteristic earthquake in the region. The widely distributed features resemble confirmed earthquake-related features in the immediate vicinity of the active segments of the New Madrid Seismic Zone. Based on past experience and accessibility, four features have been selected for detailed study. Three of the four possible liquefaction features were generally circular to elliptical in shape, ranged from few meters across to about 107m X 55 m. These sites contained a fine to medium grained sand, which thinned toward the margins of the features and overlie clay. The fourth site, a lineament, represents the surface expression of a possible fault that trend N56oE and is at least 1.5 kilometers long. The southeast side of this lineament is 2.75 meters higher than its northwest side. A schedule has been set for the opening of at least 5 trenches at these four sites during this fall. Logging, documenting, and sampling for carbon radioactive dating will be conducted. In at least one site, 2D GPR profiling indicates a number of discontinuities in the clay underlying the sand that resemble small dykes or faults, which possibly represent vents where the sand was ejected to the surface.