Disaggregating UCERF3 for Site-Specific Applications

Abstract

We present methods to simplify the use of the Uniform California Earthquake Rupture Forecast 3 (UCERF3) in site-specific seismic hazard analyses. UCERF3 defines the “state-of-practice” for hazard assessments in California and underlies the current USGS National Seismic Hazard Map (NSHM). UCERF3 contains many more ruptures than previous models and fault-to-fault connectivity in the model makes ruptures less readily incorporated into conventional hazard analyses. We demonstrate a decomposition of the UCERF3 model using site-specific subsection magnitude-frequency distributions (S3MFDs) that reduces computational demands while still including every rupture. The decomposed hazard can be used to evaluate site-specific fault source epistemic uncertainties. We illustrate using spectral acceleration at 1 Hz at three trial sites, one near a single dominant fault, one near Long Beach, where many low slip-rate faults contribute, and one in San Jose, where fault hazard is concentrated in a few high slip-rate faults.