Characterizing the Earthquake Ground Shaking Hazard in the Sacramento-San Joaquin Delta, California

The effects of earthquakes may be the most significant natural hazard that can impact the Delta levees, whose potential failure would be catastrophic to California's economy. A probabilistic seismic hazard analysis (PSHA) was performed to characterize the ground shaking hazard that could contribute to levee failure in the Delta. Based on a seismic source model that builds upon the characterizations of the major faults in the San Francisco Bay region from the Working Group on California Earthquake Probabilities and several newly recognized faults, a PSHA was performed for key sites throughout the Delta and probabilistic hazard maps were developed. Of particular significance are several seismic sources within the Delta, which were characterized as part of this study including the northern and southern Midland fault and two areal zones of deformation, which we call the Montezuma Hills and Thornton Arch zones. These potential seismic sources have been the subject of very little research and thus there is greater uncertainty in their characterization compared to the better studied major faults in the Bay region. Ground motion attenuation relationships from the Next Generation Attenuation (NGA) Project were used in the PSHA. Quasi-time-dependent hazard in 2005, 2050, 2100, and 2200 was calculated. The time-dependent behavior of the major faults was adopted from the WGCEP, which weighted both time-dependent and independent models in their forecasts. The hazard was calculated for a firm soil site condition (average shear-wave velocity in the top 30 m [V_s30] = 1100 ft/sec). For a return period of 500 years, the time-dependent probabilistic peak horizontal accelerations ranged from 0.2 to 0.4 g. Time-independent (Poissonian) hazard for the Delta differs little from the time-dependent hazard because the hazard is generally being controlled by the local Delta faults. The hazard as expects is higher at sites closer to the major faults.