Site Response from Deep Stratigraphic Layers in the Illinois Basin from S-wave H/V Observations and Spectral-Element Modeling

Abstract:

In engineering seismology applications, ground-motion site response in the frequency band of concern—i.e., 0.1 to 10 Hz—is determined from the dynamic parameters of near-surface, unlithified sediments. In several global basins, however, frequency-specific amplifications observed in this band have been attributed to impedance contrasts from stratigraphic boundaries beneath the sediment-bedrock contacts, suggesting S-wave resonance between those horizons and the surface. The horizontal-to-vertical ratios of local-earthquake S-wave amplitude spectra (HV_S) have been shown to estimate site response at sites with strong underlying subsurface impedance contrasts and has been used to identify basin resonances. At several permanent seismic stations in the Illinois Basin, U.S.A., HV_S peaks at frequencies from ~1 Hz to ~4 Hz, associated with the basin response, are of magnitudes similar to or greater than those from resonance within the soil overburden. We calculated HV_S at more than 300 temporary and permanent broadband stations in and around the basin and formed a 3D grid by interpolating the HV_S(f) curves spatially and in frequency, i.e. HV_S (lon, lat, f). Consistent with the non-interpolated HV_S observations from the linear Wabash Valley flexible array, a slice through the grid along the array and parallel to the f-axis reveals spatially continuous HV_S peaks with an arcuate, concave-up (frequency increases upwards) shape from ~1 Hz to ~4 Hz. This feature correlates with the top of a Pennsylvanian formation containing numerous coal beds among interbedded shales and sandstones. A continuous band of lower frequency (~0.4 Hz to ~0.7 Hz) HV_S peaks extends beneath the entire basin, suggesting additional resonances from a deeper impedance contrast that underlies the entire basin. Using available borehole logs and crustal models in the literature, we built a ~500 km-long 2D earth model of the sedimentary basin and underlying crystalline crust along the trend of the Wabash Valley array and modeled HV_S using the spectral-element method in SpecFEM2D. Our results suggest that the deeper (10² to 10³ m below the surface) stratigraphic layers in the Illinois Basin make important contributions to site response at frequencies of engineering concern.