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

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Date
2013-02-02Author
Gonzalez, Frank I.
LeVeque, Randall J
Adams, Loyce M.
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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.