Observation of Shear-Wave Splitting In Quaternary Sediments of The New Madrid Seismic Zone: An Indicator of Insitu Stress Conditions?

Shear-wave splitting, induced by stress-aligned inclusions (pore spaces, fractures, faults), causes<br>shear waves to exhibit directional polarizations in response to propagation through azimuthally<br>anisotropic media. This phenomenon is manifested by differences in shear-wave velocity between<br>waves traveling parallel (S1 - fast shear wave) and perpendicular (S2 - slow shear wave) to the trend of<br>the inclusions. This investigation provides further evidence of shear-wave splitting in Quaternary<br>sediments of the Lake County uplift (New Madrid seismic zone; central United States). Preliminary<br>analysis of a multicomponent downhole data set from Ridgely, Tennessee, recorded using hammer and<br>mass sources, indicates azimuthally-controlled shear-wave splitting related to in-situ tensional stresses<br>developed across the crest of the Lake County uplift. Orientation of the split shear waves, determined<br>from particle motion plots of the three-component downhole records, shows alignment of the S1<br>polarization with the major axis of the topographic uplift, and analysis of travel-time delays between S1<br>and S2 suggests anisotropy values of 10 to 12% for the upper ~30 m of the subsurface.