Small-strain site response of soft soils in the Sacramento-San Joaquin Delta region of California conditioned on VS30 and mHVSR

Abstract

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 ( VS30). 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 ( fp), peak amplitude ( ap), and average mHVSR amplitude over some frequency bandwidth ([Formula: see text]) 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 fp is used to model resonance effects, while a logistic function whose amplitude is correlated with [Formula: see text] 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 VS30-scaling model. Relative to a global ergodic model, the VS30-scaling model does not appreciably change the site-to-site aleatory variability ([Formula: see text]) for periods shorter than about 1.5 s but reduces [Formula: see text] by ~0.1 (natural log units) at long periods. When the mHVSR-informed model components are used, [Formula: see text] 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.