Ground Motion Hazard and Scenario Design Earthquakes Including Source Rupture Effects,Case in Study: City of San Francisco

Abstract

Using a probabilistic approach, a directivity model, and the fault and seismicity database of the California Geological Survey, one-second spectral accelerations for a site in the city of San Francisco are computed for several fault rupture types. Five rupture scenarios were investigated. Of these, two cases involve random distribution of hypocenters and two are “limiting cases,” resulting in the lower-bound and the upper-bound ground motions at the site. Deaggregation of hazard in terms of magnitude, distance, epsilon, and directivity for the rupture scenarios studied reveals that three scenario events dominate ground motion hazard at the site. Expressed in terms of modal values of the hazard parameters, these are: (1) M∼7.8, R∼7 km , (2) M∼7.5, R∼12 km , and (3) M∼7.3, R∼7 km . The relative significance of these scenario events varies mostly with rupture type and to lesser degrees with site condition and return period. The first scenario event is a repeat of the 1906 San Francisco earthquake. The second represents the impact of the San Gregorio fault and North-Coast and Offshore segments of the San Andreas fault, and the third reflects the seismicity mainly on the Peninsula and the Santa Cruz Mountains segments of the San Andreas fault.