Site Characterization Data for Site Response Modeling in Sacramento-San Joaquin Delta Region of California

Abstract

Sites located in the Sacramento-San Joaquin River Delta region of Northern California typically have peaty organic soils near the ground surface. These soils are characteristically soft, with shear wave velocities as low as 30 m/s, and may be present to depths as great as 15 m. These unusually soft geotechnical conditions will produce significant site effects in the event of a strong earthquake, and are not expected to be reliably predicted using existing ergodic site amplification models. This study presents site characterization data related to stratigraphy, shear wave velocity (VS), and microtremor-based horizontal-to-vertical spectral ratio (mHVSR) surveys collected across the delta and archived as part of a project to develop a delta-specific site response model. Spatial variations of time-averaged shear wave velocity 30 m (VS30) across the delta region are mapped and compared to other site parameters. An analysis of 14 sites with HVSR and VS profile data shows examples of sites with and without mHVSR peaks. Sites without peaks also lack layer boundaries with large impedance contrasts within the range of the VS profile. Sites with peaks have such boundaries in some but not all cases.