A Preliminary Model of Firm Foundation Acceleration Hazard in the San Francisco Bay Area

The results of a study of earthquake peak horizontal component acceleration hazard is presented for the nine counties of the San Francisco Bay Area. The main objective of the study is to develop an interim and simple estimate of the regional earthquake acceleration hazard using an extended hazard algorithm that allows a more complete consideration of recently published regional earthquake source parameters. Complexities of source directivity, soft foundation conditions, and site topography are not considered. Results are presented in the form of contour maps of acceleration on rock or stiff soil with a probability of exceedance of 10% for the next (as of 1990) 20 and 50 years. As would be expected, the locations of greatest earthquake shaking hazard in the Bay Area are near major active faults. When characteristic earthquake size, characteristic earthquake recurrence interval, and time of occurrence of the last earthquake along specified fault segments (if known) are explicitly used in the source model for larger earthquakes, the hazard computed depends significantly on these parameters. If smaller earthquakes are not constrained to major faults, but are considered to occur randomly throughout the Bay Area, significant high-frequency motion hazard at points away from the immediate vicinity of active faults comes from these smaller, randomly located earthquakes. All results show considerable variation of peak accelerations over areas generally treated as homogeneous in conventional engineering design practice for seismically active regions.