← Back to Sessions

Identifying Site Resonance in the Central and Eastern U.S. Using HVSR: Insights From Ambient-noise and Earthquake S-wave Energy Sources and Implications for Site Characterizations

Description: 

Earthquake S waves can become trapped, or resonate, between the free surface and high-impedance basal layers, strongly contributing to site response at site-specific frequencies. Strong S-wave resonances have been observed in the central and eastern United States (CEUS), where Vs30 has been shown to be an unreliable proxy for site effects and where stiff bedrock layers underlie slow, unlithified sediments at many locations. We evaluated the ability of single-station horizontal-to-vertical spectral ratios (HVSR) to estimate fundamental-mode site resonance frequencies (f₀) and amplifications (A₀) by comparison with the amplifications and frequencies of the first peaks calculated using 1D linear analyses. Our analyses relied on S-wave velocity profiles and recordings of ambient noise and earthquakes from more than 20 seismic stations and borehole arrays in the CEUS, with site conditions ranging from bedrock to thick (>100 m) soils. Consistent with previous studies, we found that f₀ is estimated well by HVSRs produced from both earthquake S-wave and ambient-noise energy sources. In contrast, HVSR estimates A₀ less reliably, although the empirical measurements derived from both energy sources correlate positively with the predicted values. Improved site characterizations, particularly of the bedrock S-wave velocities, may yield improved correlations. At stations in sedimentary basins, many HVSR curves reveal resonance frequencies much lower than those predicted by the 1D analyses, indicating both that deeper sedimentary layers influence HVSRs and that HVSRs developed in such geological settings must be interpreted with care for engineering use. We also found that although both types of HVSRs record peaks from multiple strong impedance contrasts, S-wave HVSRs reveal higher-mode resonance peaks, which are absent from the ambient-noise HVSRs. Thus, we encourage simultaneously interpreting site HVSRs from both earthquakes and ambient noise to identify site resonances and to gain additional insights into the subsurface structures that bear on site response.

Session: Station Installations and Site Conditions, a Quest for Improved Strong Motion Database [Poster]

Type: Poster

Date: 4/16/2025

Presentation Time: 08:00 AM (local time)

Presenting Author: Seth

Student Presenter: No

Invited Presentation: 

Poster Number: 60

Authors