Development and Incorporation of Atlantic and Gulf Coastal Plain Site Response Models in Seismic Hazard Analyses

Abstract:

Past and present research on earthquake ground motions along the Atlantic and Gulf Coastal Plains show significantly more frequency-dependent deviations in site response relative to other areas in the Central and Eastern United States (CEUS). Amplitudes and durations of ground motions are shown to be strongly correlated with sediment thickness, where greater thickness contributes to higher seismic attenuation resulting in lower ground motion intensities at short periods, but also leads to resonance of the sediment column and generally higher ground motion intensities at longer periods. While the resonance moves to longer period with increasing sediment thickness, its intensity decreases due to increasing velocity and density within the deeper part of the sediment column and thus smaller impedance contrasts at the base of the sediments. With the recent successful incorporation of basin depth corrections for select areas in seismic hazard analyses for the western United States, we move toward a similar analysis in the Atlantic and Gulf Coastal Plains by constructing a sediment thickness model and incorporating three new site response models in CEUS seismic hazard analyses. Many existing sediment thickness datasets are combined into a composite surface that delineates the bottom of Mesozoic sediments. Three Fourier-based site response models are considered with various methods for converting Fourier spectra to response spectra, the metric used in the National Seismic Hazard Model (NSHM). The new site response models are compared with observed ground motions and existing NGA-East ground motion models to examine and evaluate the benefit of the new models. Because this potential improvement to our hazard analyses for the CEUS may lead to very different estimates of hazard relative to current models, we will also need to estimate the lingering uncertainties for consideration by the user community. Our goal is to develop a roadmap for incorporating coastal plain amplification in the NSHM by the end of 2020, the deadline by which refereed articles and reports need to be published in order to be considered for inclusion in the 2023 NSHM update.