Shear‐wave splitting in Quaternary sediments: Neotectonic implications in the central New Madrid seismic zone

Abstract

Determining the extent and location of surface/near‐surface structural deformation in the New Madrid seismic zone (NMSZ) is very important for evaluating earthquake hazards. A shallow shear‐wave splitting experiment, located near the crest of the Lake County uplift (LCU) in the central NMSZ, shows the presence of near‐surface azimuthal anisotropy believed to be associated with neotectonic deformation. A shallow four‐component data set, recorded using a hammer and mass source, displayed abundant shallow reflection energy on records made with orthogonal source‐receiver orientations, an indicator of shear‐wave splitting. Following rotation of the data matrix by 40°, the [Formula: see text] and [Formula: see text] sections (principal components of the data matrix) were aligned with the natural coordinate system at orientations of N35°W and N55°E, respectively. A dynamic mis‐tie of 8 ms at a two‐way traveltime of 375 ms produced an average azimuthal anisotropy of ≈2% between the target reflector (top of Quaternary gravel at a depth of 35 m) and the surface. Based on the shear‐wave polarization data, two explanations for the azimuthal anisotropy in the study area are (1) fractures/cracks aligned in response to near‐surface tensional stress produced by uplift of the LCU, and (2) faults/fractures oriented parallel to the Kentucky Bend scarp, a recently identified surface deformation feature believed to be associated with contemporary seismicity in the central NMSZ. In addition to increased seismic resolution by the use of shear‐wave methods in unconsolidated, water‐saturated sediments, measurement of near‐surface directional polarizations, produced by shear‐wave splitting, may provide valuable information for identifying neotectonic deformation and evaluating associated earthquake hazards.