Analysis of Near-Surface Shear Wave Velocity Data for Site Response Applications in the Sacramento–San Joaquin Delta Region of California

ABSTRACT Sites located in the Sacramento–San Joaquin River Delta region of California typically have peaty organic soils near the ground surface. These soils are characteristically soft with shear wave velocities (VS) as low as 20 m/s and thicknesses as great as 12 m. These unusually soft site conditions produce significant site response effects that are not reliably predicted using existing ergodic site amplification models. To develop a region-specific site response model that captures the unique behaviors of peaty organic soils in the Delta, accurate site characterization is needed. This paper presents site characterization data with emphasis on near-surface VS profiles collected across the Delta and develops two region-specific VS-related models. The first is a regionally calibrated model to predict the time-averaged shear wave velocity in the upper 30 m (VS30), which is a common input parameter to ergodic site amplification models, conditioned on peat thickness (tp). Peat thickness has been mapped throughout the Delta and is, therefore, a useful proxy for VS30. This VS30 proxy model significantly reduces bias and uncertainty compared with existing proxy-based models not calibrated for peaty soils. The second model, also dependent on peat thickness, was developed to extrapolate relatively shallow VS profiles to deeper and firmer reference conditions for use in ground response analyses and other applications. Results of ground response analyses for a deep and truncated-and-extrapolated VS profile are used to verify the effectiveness of the extrapolation model. Although the models are developed specifically for the Delta, the framework used to develop these models can be applied anywhere.