Probabilistic Tsunami Hazard Assessment (PTHA) for Crescent City, CA. Final Report for Phase I

Abstract

This demonstration Probabilistic Tsunami Hazard Assessment (PTHA) study of Crescent City, California was funded by BakerAECOM and motivated by FEMA's desire to explore methods to improve products of the FEMA Risk Mapping, Assessment, and Planning (Risk MAP) Program. The primary results, that is, the 100- and 500-year tsunami maps, are presented and discussed in Section 3 and Appendix C. These maps were generated by a signi cantly improved methodology than that of the Seaside study; the improvements include a more complete set of seismic sources (Table 1), and a more accurate method for estimating tidal uncertainty (Section 8). As expected, the inland extent and magnitude of the ooding for the 500-year tsunami far exceed that of the 100-year event; these products can now be compared with standard FEMA Flood Insurance Rate Maps (FIRMs) to determine whether these ooding levels exceed estimates of other coastal ooding hazards, such as storm surge. A nal deliverable, digital data les of the map data, have been provided to BakerAECOM, and a description of these les is given in Appendix D. The primary conclusion we have reached in the course of this study is that the maps must be used with caution because (a) there are signi cant uncertainties in the speci ation of CSZ seismic sources (Section 4 and Section 7.6) and (b) the standard 100- and 500-year maps are highly sensitive to these geophysical uncertainties and, in certain circumstances could even be misleading. Section 9.2 discusses this sensitivity in the context of a non-regulatory product that provides valuable additional insight by presenting the same probabilistic information in a di erent format that we call a p-contour map. Finally, we recommend (a) that FEMA give serious consideration to the adoption of the p-contour map as a product that supplements and aids in the practical interpretation of the same probabilistic information displayed in the standard 100- and 500-year tsunami maps, and that (b) future PTHA studies should include close collaboration with a geoscientist expert in earthquake parameterization.