DOGAMI Open-File Report Series

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Open-File Report O-20-10, Earthquake and tsunami impact analysis for coastal Clatsop County, Oregon, by Jonathan C. Allan, Fletcher E. O'Brien, John M. Bauer, and Matthew C. Williams; 86 p. report, including community-specific profiles for Astoria, Jeffers Garden, Warrenton, Gearhart, Seaside, Cannon Beach, and Arch Cape; data tables spreadsheet.

Reissued 12-11-2020
This reissue supersedes the publication originally released 10-29-2020. The reissue includes a spreadsheet containing data that are the basis for the report’s tables and figures. Appendix community profile Figures C and D are revised to subtract 10 mins from the tsunami wave arrival time, which then determines the travel distance threshold.

WHAT'S IN THIS REPORT?
This report evaluates a Cascadia subduction zone earthquake (Mw 9.0) and tsunami (M1, L1, and XXL1 scenarios) affecting coastal Clatsop County, Oregon, in order to understand the degree of potential destruction, including building losses, debris generated, fatalities and injuries, and estimated numbers of the displaced populations. The goal is to help coastal communities prepare for this inevitable disaster.

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EXECUTIVE SUMMARY

This report provides an evaluation of the potential impacts of a Cascadia earthquake and accompanying tsunami in coastal Clatsop County. The analyses presented here include an assessment of the numbers of people, businesses, and critical facilities located in three Cascadia tsunami inundation zones (M1, L1, and XXL1). Furthermore, our analyses evaluate local population demographics in each community in order to better understand potential evacuation challenges that could affect different population groups, as well as socioeconomic impacts associated with a Cascadia subduction zone (CSZ) earthquake and resultant tsunami. The results and analyses presented here reflect a comprehensive effort to document the likely effects the next great earthquake and tsunami will have on Clatsop County.

We used previously developed physical models of a CSZ earthquake and tsunami, “Beat the Wave” tsunami evacuation modeling, and the recently published FEMA Hazus Tsunami Model to develop standardized loss estimates for each community, including injuries, fatalities, and building damage. From the latter we estimated the amount of debris generated from the building damage. Our population model improves upon previous studies by providing spatially detailed estimates of permanent and temporary populations—the latter quantifying numbers of visitors and second-home owners, which vary widely throughout the calendar year. The tsunami injury and fatality modeling evaluates a nighttime (2 AM) evacuation scenario (maximizing visitor occupancy), quantifying impacts to permanent and temporary residents.

Although each community in coastal Clatsop County has unique circumstances and challenges, as supported by the results of this study, our results unequivocally demonstrate that in every community, injuries and fatalities from a tsunami can be minimized if people evacuate on foot toward safety as soon as possible and travel as fast as possible.