Variability in Ground Motions in the San Francisco Bay Urban Area from Large Earthquakes on the San Andreas Fault

I use 3-D numerical simulations of kinematic earthquake ruptures to characterize the expected long period (T > 2.0 s) strong ground motions in the San Francisco Bay urban area from large earthquakes on the San Andreas fault. The earthquakes include the 1906 M7.9 San Francisco earthquake and hypothetical variations of the 1906 event with different hypocenters, slip distributions, and rupture speeds. The simulations use finite-elements to discretize a 250 km by 110 km by 45 km volume centered around the San Francisco Bay metropolitan area. Using the USGS 3-D geologic model and corresponding velocity model, the simulations incorporate the 3-D geologic structure, including the nonplanar geometry of the faults, the variation in material properties associated with different rock types and depth, and topography and bathymetry. The simulations suggest that much of the currently urbanized area around San Francisco Bay could be subjected to significantly stronger ground motions in the next large earthquake on the northern San Andreas fault compared with the motions in the simulation of the 1906 event. A hypocenter north of the 1906 hypocenter, which was directly off the coast of San Francisco, increases the rupture directivity for the city of San Francisco and cities around the southern half of the bay, raising the MMI one unit over much of the urban area. Alternatively, if instead of having less than average slip along the San Francisco peninsula as in the 1906 earthquake, this portion of the rupture has greater than average slip, the peninsula is subjected to significantly stronger shaking. In addition to these large-scale effects, some smaller scale effects, such as locally intense shaking in the Cupertino and Santa Rosa areas due to sedimentary basins, are present in all of the scenarios. These results corroborate previous studies that show that variations in rupture directivity and slip have a strong influence on the distribution of ground shaking in areas with complex geologic structure.