Seismic hazard analyses of the Metropolitan Water District Emergency Freshwater Pathway,California

Abstract

The Sacramento-San Joaquin Delta in central California is particularly susceptible to damage in a large earthquake due to the vulnerability of the levees that protect cities, farms, and infrastructure. The Delta is located adjacent to the seismically active San Andreas fault system and is also subject to strong ground shaking from numerous other seismic sources in central California, including faults within the Delta. We performed a probabilistic seismic hazard analysis (PSHA) to provide seismic design ground motions for the Metropolitan Water District (MWD) Emergency Freshwater Pathway. We have evaluated the appropriateness of the Next Generation of Attenuation (NGA)-West2 ground motion models for use in our analyses of the Delta, evaluated shear-wave velocity (V_S) data in the vicinity of the Pathway, and performed site response analyses. The latter was performed to compute the probabilistic hazard at the top of the peat at five sites along the Pathway. The sixth site was located outside the Delta and on firm soil. The probabilistic hazard for the six sites and for a range of return periods of engineering relevance were computed in the PSHA. For a return period of 2500 years, the peak horizontal ground acceleration (PGA) values on peat ranged from 0.40g to 0.53g. The seismic sources that control the hazard at these sites vary as a function of return period and spectral frequency, but in general, the closer the sites are to faults within the San Andreas fault system, the higher the hazard.