MORE PALEOLIQUEFACTION DATA FROM SOUTHEASTERN ARKANSAS: IMPLICATIONS FOR SEISMIC HAZARDS

Two new trenches in southeastern Arkansas expose alluvium, paleosols, and vented liquefaction deposits. These trenches are in sand blow fields (Ashley County and Desha County fields) we previously documented with aerial photography, electrical conductivity surveys, and trenching. Stratigraphic relationships and soil analysis suggest multiple episodes of sand venting associated with strong ground motion. Radiocarbon and luminescence age analyses are in progress.

The Morgan trench (Ashley County field) exposed vented fine sand underlain by colluvial clayey sand that is in turn underlain by older vented fine sand. A principal sand vent (25 cm-wide) and many subsidiary dikes (<2 cm-wide) fed the two sand blows and many minor sills. At the principal vent dike, stratigraphy drops ~50 cm and 10-cm blocks of substrate are displaced upward ~45 cm in a matrix of fine sand. The Golden trench (Desha County field) exposed a vented deposit of brecciated clay clasts suspended in a fine sand matrix that was fed by several dikes (~ 5 cm wide) that extend downward through the source of the clay clasts. This clay horizon (~0.5 m below the blow) is brecciated around the primary feeder dikes, and we interpret it as the hydraulic capping unit that ruptured during ground shaking. Smaller sand dikes (<3 cm wide) fed the flanks of the principal blow and sub-blow sills (<10 cm thick). In both trenches, a weak modern soil profile is developed in the blow sand or sand/clay breccia, suggesting relatively recent (with the last 500 years(?)) liquefaction events. Underlying the blow sand deposits, weak paleosols are developed on one or more alluvial depositional events.

We identify at least two sand blow events in the Morgan trench and at least one sand blow event in the Golden trench. Together with our previous sand-blow investigations in southeastern Arkansas, a coherent picture is emerging of at least five prehistoric earthquakes affecting the area. In light of this earthquake history, we conducted a preliminary assessment of the vulnerability of communities in the study area to seismic hazards based on 2000 census data. This assessment is a first step in quantifying the socio-economic impact of strong ground motion on a local scale.