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

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 (V-S), 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 (V-S30) across the delta region are mapped and compared to other site parameters. An analysis of 14 sites with HVSR and V-S 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 V-S profile. Sites with peaks have such boundaries in some but not all cases.