Estimating Site Response with Recordings from Deep Boreholes and HVSR: Examples from the Mississippi Embayment of the Central United States

Recordings of weak-motion S waves at two deep vertical seismic arrays in the northern Mississippi embayment (i.e., vertical seismic array Paducah [VSAP] and the Central United States Seismic Observatory [CUSSO]) were used to estimate empirical site responses using ratios of surface-to-bedrock transverse-component amplitude spectra TFT. The mean TFT curves were also compared with theoretical transfer functions derived from Thomson-Haskell propagator matrices. The results were comparable, indicating that mean spectral ratios, calculated from few (10) events at local and regional distances, represent empirical linear S-wave transfer functions for weak-motion SH waves at these sites. At both sites, the largest amplifications implied by the theoretical responses and the observed S-wave spectral ratios occur at frequencies higher than the sites' fundamental frequencies. These spectral ratios were used to evaluate the suitability of surface S-wave horizontal-to-vertical spectral ratios HVs for estimating the empirical site transfer function. The mean S-wave HVs curves are similar to the mean TFT spectral ratios for frequencies below approximately the fifth natural frequency at each site; for higher frequencies, vertical-component amplification from incident SV waves reduces HVs relative to TFT. Therefore, HVs curves at these sites reflect the SH-wave transfer functions for low frequencies. We also observed that HVs curves from ambient noise recordings do not estimate the SH-wave transfer function at these deep borehole sites for frequencies higher than the fundamental.