none 10.1177/87552930231217165 Wiley Wiley (John Wiley & Sons) 311 157173703 5867 2026012503564800433 10.1002 2026-01-25T12:24:09Z 2024-01-19T00:08:28Z 13 Earthquake Spectra Earthquake Spectra 8755-2930 1944-8201 02 2024 40 1 Tristan E Buckreis Department of Civil & Environmental Engineering University of California Los Angeles Los Angeles CA USA https://orcid.org/0000-0002-5128-9313 Jonathan P Stewart Department of Civil & Environmental Engineering University of California Los Angeles Los Angeles CA USA https://orcid.org/0000-0003-3602-3629 Scott J Brandenberg Department of Civil & Environmental Engineering University of California Los Angeles Los Angeles CA USA https://orcid.org/0000-0003-2493-592X Pengfei Wang Department of Civil & Environmental Engineering Old Dominion University Norfolk VA USA https://orcid.org/0000-0002-2844-4557 Sites located in the Sacramento‐San Joaquin Delta region of California typically have peaty‐organic soils near the ground surface, which are characteristically soft, with shear wave velocities as low as 30 m/s. These unusually soft geotechnical conditions, which are outside the range of applicability of existing ergodic site amplification models, can be anticipated to produce significant site effects during earthquake shaking. We evaluate site response for 36 seismic stations in the Delta region using non‐ergodic methods with low‐amplitude ground motion data. We model first‐order site effects using a period‐dependent relation conditioned on the 30 m time‐averaged shear wave velocity ( V S30 ). Relative to extrapolations of global ergodic models, this Delta‐specific model provides lower and higher levels of amplification for short and long periods, respectively. While the local model provides unbiased predictions for Delta sites as a whole, it smooths over site‐specific effects such as resonant peaks. Microtremor‐based horizontal‐to‐vertical spectral ratios (mHVSRs) were measured for 34 sites, from which additional site parameters such as peak frequency ( f p ), peak amplitude ( a p ), and average mHVSR amplitude over some frequency bandwidth () are derived. Sites with prominent mHVSR peaks are found to often exhibit site resonance effects, while sites without prominent peak features generally do not. A modified Ricker wavelet pulse function conditioned on f p is used to model resonance effects, while a logistic function whose amplitude is correlated with is used to model general broadband amplification that transitions to zero at long periods. These mHVSR‐informed models are implemented as additive components to the V S30 ‐scaling model. Relative to a global ergodic model, the V S30 ‐scaling model does not appreciably change the site‐to‐site aleatory variability () for periods shorter than about 1.5 s but reduces by ~0.1 (natural log units) at long periods. When the mHVSR‐informed model components are used, is reduced by ~0.05 to 0.1 for short‐to‐intermediate periods (up to ~2 s). A model for nonlinear effects, derived from nonlinear ground response analyses, will be presented in a separate paper. 02 2024 02 2024 230 260 10.1177/87552930231217165 Department of Water Resources https://doi.org/10.13039/100004813 4600012415 https://creativecommons.org/licenses/by/4.0/ http://doi.wiley.com/10.1002/tdm_license_1.1 2 10.1177/sage-journals-update-policy journals.sagepub.com true Department of Water Resources https://doi.org/10.13039/100004813 4600012415 https://creativecommons.org/licenses/by/4.0/ https://journals.sagepub.com/page/policies/text-and-data-mining-license 10.1177/87552930231217165 https://onlinelibrary.wiley.com/doi/10.1177/87552930231217165 http://sage.cnpereading.com/paragraph/article/?doi=10.1177/87552930231217165 https://onlinelibrary.wiley.com/doi/pdf/10.1177/87552930231217165 https://onlinelibrary.wiley.com/doi/pdf/10.1177/87552930231217165 https://onlinelibrary.wiley.com/doi/full-xml/10.1177/87552930231217165 https://pubs.geoscienceworld.org/earthquake-spectra/article/40/1/230/633726/small-strain-site-response-of-soft-soils-in-the 10.1785/BSSA0820010505 PEER reports 2015‐07, June Al Atik L 2015 10.1193/070913EQS197M 10.1785/0120010235 10.1785/BSSA0870030710 10.1785/0120090400 Bulletin of the Seismological Society of America Boore DM 1057 30 3 2014 NGA‐West2 equations for predicting PGA, PGV, and 5%‐damped PSA for shallow crustal earthquakes 10.1193/1.1585791 10.1785/BSSA0660020467 The Loma Prieta, California Earthquake of October 17, 1989 Strong Ground Motion (Professional Paper 1551‐A) Borcherdt RD A77 1994 10.1193/072113EQS209M 10.1201/9781315139470 BuckreisTE(2022)Customization of path and site response components of global ground motion models for application in Sacramento‐San Joaquin Delta region of California. 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