Paleoseismic Investigation of the Relationship of Earthquake-Induced Liquefaction Deposits to Deformation at a Site in the New Madrid Seismic Zone
Type of DegreeMaster's Thesis
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Geophysical, geologic, and soil surveys were conducted at a site in the New Madrid seismic zone (NMSZ) to explore the relationship of fluvial sedimentary deposits to earthquake-induced liquefaction features, such as sand blows and sand dikes. The site was selected based on satellite imagery, which showed two light-colored elongate areas that were later identified as earthquake-induced sand blows oriented parallel to the east-west flowing Pemiscot Bayou. The study was aimed at establishing (1) the factors in fluvial environments that control the consequential location of the liquefaction features, and (2) the timing of their causative earthquakes. Detail logging of ditch exposures at the site revealed crosscutting relationships of the liquefaction features to the host sediments. Radiocarbon dating of organic samples taken from the exposures suggests that the liquefaction features resulted from an A. D. 900 +/- 150 yr New Madrid event. Data from electrical resistivity tomography (ERT) surveys performed along four profiles illuminated the subsurface stratigraphy of the study site. Soil samples collected from the surface and from auger holes, along with published interpretations of depositional units found in the site vicinity, were used to relate features in the electrical resistivity profiles to depositional units of the Pemiscot Bayou. Relationships recorded in the ditch exposures provided ground truth for the ERT interpretation. Based on the ERT, satellite, and empirical data, a model was developed that explains how fine-grained deposits, such as abandoned channel, natural levee, overbank, and possibly backswamp deposits, create relatively impermeable barriers when juxtaposed with or overlain upon coarse-grained deposits such as point bar and braided stream deposits and consequently guide the upward flow of liquefied sediments toward the surface. The model explains the observation that the sand blows at the site formed along the margins of abandoned channels of the Pemiscot Bayou. Findings from the study contribute to understanding the factors in fluvial environments that control the location of liquefaction deposits and may help to predict which environments are most vulnerable to liquefaction during an earthquake.