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Site Response in the Illinois Basin From S-Wave H/V and Spectral-Element Modeling

Session: Advances in Upper Crustal Geophysical Characterization II

Type: Oral

Date: 4/21/2021

Presentation Time: 02:15 PM Pacific

Description: 

In engineering seismology, ground-motion site response in the frequency band of concern—i.e., 0.1 – 10 Hz—is determined from the dynamic parameters of near-surface, unlithified sediments. Frequency-specific amplifications observed in this band have also been attributed to impedance contrasts from stratigraphic boundaries beneath the sediment-bedrock contacts in global basins, however, 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 used to estimate site responses at sites with strong underlying subsurface impedance contrasts. HV_S calculated at EarthScope flexible arrays that traversed the Illinois Basin reveal a nearly continuous band of spectral amplifications within the band of engineering concern, approximately 1 Hz to 4 Hz, that correlates with the top of a Pennsylvanian-aged formation. This unit contains numerous low-density coal beds among interbedded shales and sandstones, and the frequencies of the HV_S peaks are inversely related to the depths of the formation’s base. Using borehole logs and published crustal models, we constructed a 2D earth model of the sedimentary basin and underlying crystalline crust along the trend of a linear array of selected stations. We used the SpecFEM2D spectral-element algorithm and multiple scenario earthquakes distributed in our earth model to calculate mean HV_S and basin/no-basin spectral ratios. We also modeled simplified crustal and basin earth structures (e.g., one-layer basin). Our results suggest that the entire basin contributes to frequency-specific amplifications, but the Pennsylvanian-aged formation is needed to explain the spectral-ratio peaks observed in the approximately 1 – 4 Hz band. Thus, deeper (10² to 10³ m below the surface) stratigraphic layers in the Illinois Basin are important for linear site response for engineering purposes.

Presenting Author: Seth Carpenter

Student Presenter: No

Authors