Group A, Poster #241, Ground Motions

Fourier-Based Site Response of Southern California Sedimentary Basins

Jeff Bayless, Jonathan P. Stewart, Chukwuebuka C. Nweke, & Scott J. Condon
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Poster Presentation

2022 SCEC Annual Meeting, Poster #241, SCEC Contribution #12507 VIEW PDF
This ongoing study updates the Bayless and Abrahamson (2019; BA19) ground motion model for Fourier Amplitude Spectra (FAS) by incorporating two frequency-dependent site amplification model components, both regionalized for southern California. The first component is a linear and nonlinear Vs30-scaling model inferred from southern California earthquake recordings, as described in SCEC Report #19097. The second is the sediment depth scaling component (also called basin depth scaling elsewhere) which is updated as follows. Nweke et al. (2022) provided a similar southern California site amplification model for response spectra utilizing Vs30 and depth, with the depth component conditioned on typ...e of geomorphic province: basins, basin edges, valleys, and mountain/hills. In this study, the same provinces are adopted, and the Nweke et al. (2022) procedure is followed for the FAS using NGA-West2 data and data recorded in Southern California since the conclusion of NGA-W2 (Nweke et al., 2022; This procedure involves undertaking ground motion residual analyses using the BA19 model as the reference model.

Preliminary results show that the low-frequency (long period) FAS site terms behave similarly to those for long period response spectra in Nweke et al. (2022). If the geomorphic province categorization is ignored, the site terms are zero-centered and scale with differential basin depth as in BA19, which was developed for all of California. The site terms are partitioned by geomorphic province and analyzed further, and at low frequencies, sites categorized as Basin and Basin Edge have positive mean amplification, and those categorized as Valley and Mountain provinces both have negative mean amplification. Additionally, minimal de-amplification is observed for Basin sites with shallower than expected basin depths. This study will conclude by comparing results with Nweke et al. (2022), and by developing a parametric model for the sediment depth scaling conditioned on southern California geomorphic province. The sediment depth scaling model will be combined with the Vs30-scaling model to complete the southern California regional update of BA19.