Geophysical reconnaissance of earthquake-induced liquefaction features in the New Madrid seismic zone

Abstract

Electrical resistivity and electromagnetic induction surveys performed at a site in the New Madrid seismic zone in the central United States demonstrate the ability of geophysical instrumentation to determine the location, size and orientation of earthquake-induced liquefaction features in the subsurface. Liquefaction features, including sand blows and sand dikes, are common within the Late Pleistocene and Holocene floodplain of the Mississippi River. These features and their relationship to host sediments provide important information about historic and prehistoric earthquakes and their source parameters, such as timing, epicentral location and magnitude. Following the geophysical surveys, two excavations were made and documented for a paleoseismic study. Sediment samples of soils, Native American occupation horizons, and liquefaction features within the trenches were collected for sedimentological and archeological analyses and compared with the geophysical observations. Measurements of the heights of cotton plants growing at the site were also taken, since growth appeared to reflect variations in the texture and thickness of soils developed on fluvial deposits and sand blows. The excavations, along with sedimentological and agricultural data, provided a means for calibrating sediment characteristics with the geophysical interpretations and for developing criteria to distinguish anomalies due to facies changes from anomalies related to liquefaction features. Disruption of the local sedimentological trends by an en echelon arrangement of sand dikes and related sand blows is seen in the combined geophysical and agricultural data. Results of the surveys indicate that constant-spread resistivity profiling (Wenner array) was an effective method for locating and mapping shallow sand dikes, some with widths of less than 50 cm. The electromagnetic induction method (EM-31), while less sensitive to dike locations, was useful in characterizing depositional facies changes by their differences in electrical conductivity. Data from the study site support the interpretation that the earthquake-induced liquefaction features occurred near the boundary of a facies change, which may have constituted a zone of weakness along which excess pore fluids and sand escaped.