Earthquake event selection for regional seismic hazard analysis of levee systems

Probabilistic seismic hazard analysis is generally performed at a single site, where ground motion hazard from all of the faults that might shake the site is integrated to express the annual probability of exceedance as a function of ground motion intensity. This site-based calculation is then often repeated at many different sites to develop uniform hazard maps, expressing the ground motion intensity measure values associated with a specific annual probability of exceedance. While such maps are useful for selecting a ground motion intensity measure for structures or other infrastructure systems with geographic footprints that may be approximated as a point, they are not appropriate for application to spatially distributed infrastructure systems such as levees. The problem is that many different earthquake events contribute to the hazard at each site, and none of those events on their own will produce uniform hazard shaking levels over a spatially distributed infrastructure system. Rather, spatially distributed infrastructure systems must be analyzed using an adequate number of scenario events to reasonably represent the hazard. In this paper, we present a method for selecting an event subset from a probabilistic seismic hazard disaggregation, and adjusting the rates of occurrence of the events in the subset to recover the target hazard curve. We select the event subset based on the percent contribution to the seismic hazard at a set of locations, and then utilize a weighted least squares regression approach to identify the rates for the event subset. We illustrate the procedure for the Sacramento / San Joaquin Delta region.