Should Site Response Be Incorporated into Central and Eastern US Hazard Maps?

Session: Forthcoming Updates of the USGS NSHMs: Hawaii, Conterminous U.S. and Alaska [Poster]
Type: Poster
Date: 4/28/2020
Time: 08:00 AM
Room: Ballroom
Description: 

Frequently in the central and eastern U.S. (CEUS), unlithified soils overlie stiff albeit weathered bedrock layers and S-wave resonance dominates site response in these conditions. The primary site-response parameter pair fundamental frequency, f₀, and base-mode amplification (typically the maximum), A₀, calculated from simplified physics-based expressions using average one-layer-over-bedrock earth models were compared with 1-D linear, viscoelastic S-wave full-resonance site responses. Tests conducted using S-wave velocity profiles at 11 seismic stations in the central and eastern U.S. showed the simplified expressions approximate f₀ and A₀ in most cases. Also, S-wave H/V observations were compared with the full-resonance responses at these sites. The theoretical and empirical results were similar, particularly for f₀; S-wave H/V estimates A₀ when a correction factor is applied. Observations at a few sites within the Illinois Basin also revealed at least one large, unmodeled impedance contrast much deeper than the sediments. These peaks indicate the importance of collecting empirical site response estimations. They also indicate the need to investigate deeper velocity structures in CEUS basins to account for potential important contributions to site responses for engineering purposes.

Results show that at most sites the parameter pair f₀ and A₀ capture primary site-resonance characteristics. Also, the large variabilities in f₀ and A₀ at sites with the same NEHRP site class demonstrate that site factors based on Vs30 may not reliably account for the effects of S-wave resonances in the CEUS. Thus, accurate characterizations of site response uncertainties in CEUS ground-motion models may not be possible using this proxy. Therefore, and because accurate site response quantification requires site-specific investigations, we suggest it is more appropriate to consider site response separately and that site response not be included in CEUS seismic hazard maps.

Presenting Author: Seth Carpenter

Authors