Evaluation and Application of Probabilistic Tsunami Hazard Analysis in California

Abstract:

The California Geological Survey (CGS) and URS Corporation are in the process of generating tsunami hazard map products for land-use planning and construction through the California Seismic Hazard Mapping Act (Public Resources Code, sec 2690 et seq.). Similar to seismic hazard zonation, these products for land-use and development decision-making are typically based on a probabilistic analysis, and require information on the frequency of occurrence through a probabilistic tsunami hazard analysis (PTHA). In Phase 1 of CGS’s work, the California PTHA Work Group was established to evaluate the results of PTHA demonstration projects in Crescent City and Huntington Beach. The results of this Phase 1 review of the two independent analyses indicate PTHA’s can be developed with recommended improvements in source characterization, PTHA methods, and numerical model use. An immediate significant improvement is to align the characterization of the Cascadia Subduction Zone PTHA with the seismic characterization of the National Seismic Hazard Map Program of the USGS. In addition to applying PTHA to land-use planning and the two demonstration projects, CGS and the CA-PTHA Work Group identified other potential applications for various PTHA risk levels (ARP = Average Return Period), including flood insurance (100 and 500 year ARP), building codes (2,500 year ARP), and emergency response planning (1000 year ARP or larger). CGS is working with URS Corp., the California Office of Emergency Services, and FEMA on a Phase 2 plan to produce a single set of reliable and consistent PTHA maps for multiple risk levels and work with various end-users to determine how to use the maps. The California PTHA and the results of the Work Group review are also proposed to be used by the U.S. National Tsunami Hazard Mitigation Program to develop guidelines for production in other coastal states.