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

摘要

Determining the extent and location of surfaceear-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 S_1 and S_2 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.