ABSTRACT

Site response at many locations in the central and eastern United States is a resonance‐based phenomenon, which site factors based on the VS30 proxy may not reliably capture. Alternative factors, fundamental frequency, f0, and fundamental‐mode amplification, A0, calculated from simplified expressions were evaluated against 1D linear, viscoelastic SH‐wave full‐resonance (FR) site responses. Tests were conducted using S‐wave velocity profiles to bedrock at 11 seismic stations. The results showed that simplified expressions approximate FR f0 and A0 at most stations. However, at two sites with intermediate‐depth strong impedance contrasts, the simplified approach underestimates A0 by 40%.

In addition, FR f0 and A0 were compared with weak‐motion earthquake S‐wave horizontal‐to‐vertical (H/V) spectral ratios. Measuring f0 and A0 from S‐wave H/V for our comparisons required considering multiple observations at the regional scale. The first S‐wave H/V peaks occurred between 1 and 3  Hz at all seven stations in the Illinois basin (IB), which is a much lower f0 than expected for five of these sites. Thus, we used the first S‐wave H/V peaks at f>3  Hz at these five sites in our comparisons. The S‐wave H/V peaks we evaluated indicate that S‐wave H/V can approximate FR f0. However, although the A0 measured from S‐wave H/V is positively correlated with theoretical A0, it overestimated FR A0 by more than 40% at all but three sites, indicating that additional study is needed to determine the appropriate use of A0 from S‐wave H/V. The observed, unmodeled amplifications between 1 and 3  Hz are of similar magnitudes as those from the shallower sediment layers at most stations, which highlights the importance of collecting empirical site‐response estimations. Furthermore, the amplifications indicate the need to investigate deeper velocity structures in the IB to account for site responses within the frequency band of engineering interest.

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