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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.

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.

PUBLICATION DOWNLOADS

• Report and appendix (86 p., 51 MB PDF), including an appendix of 21 tabloid-page-size community-specific profiles for Astoria, Jeffers Garden, Warrenton, Gearhart, Seaside, Cannon Beach, and Arch Cape
• Report only (65 p., 7 MB PDF)
• Appendix community profiles sheets only (21 p., 46 MB PDF)
• Spreadsheet only (Excel spreadsheet, 71 KB) - contains all the data that are the basis for this report’s tables and figures
• Publication bundle (report, appendix, spreadsheet; 50 MB .zip file)

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.

• The total permanent resident population present on the Clatsop County coastline within a tsunami zone ranges from ~11,880 (M1) to ~19,440 (XXL1). If the temporary population is included, the local population could increase by as much ~17,380 (M1) to ~29,600 (XXL1) assuming 100% occupancy;
• The fraction of the total permanent resident population within the three tsunami zones varies widely between communities. For example, the entire community of Jeffers Garden is located within all three tsunami zones. At Gearhart, the entire (100%) community is located in the XXL1 tsunami zone, 82% is in the L1 zone, and only 44% is within M1. Conversely, Astoria, located 11 miles inside the lower Columbia River estuary, is characterized by 23%, 17%, and 11% of the resident population in the XXL1, L1, and M1 zones, respectively. These findings reflect contrasting patterns in the general shape and elevation of the Clatsop coastline, whether it is open coast versus up an estuary, tsunami travel, dispersion (loss of energy), and inundation extents between the communities as well as the distribution of permanent residents within the communities;
• All seven Clatsop County coastal communities can experience large influxes of visitors, well exceeding their local resident populations. Of note, the community of Cannon Beach can swell by ~700% to 940% (XXL1 and M1), while Seaside experiences lower increases of ~250% to 270% (XXL1 and M1) due to its larger resident population. These results demonstrate the importance of both communities as major tourist destinations with potentially large numbers of visitors located in the tsunami zones. The popularity of these communities as centers of tourism presents challenges associated with preparing such a large transient population for a CSZ earthquake and tsunami;
• Analyses of Clatsop County population demographics indicate that the countywide resident population of ≥65 years of age is ~22-23% of the total population for all three tsunami zones; this reflects ~2,250, 3,130, and 3,940 residents in the M1, L1, and XXL1 zones who are ≥65 years of age. Nevertheless, several communities have slightly more people ≥65 years of age, including Gearhart (~27%), Cannon Beach (~27%), and Arch Cape (~29%). Variations in demographics will likely impact ability to evacuate from the tsunami zone;
• The number of buildings located in a tsunami zone are greatest in Seaside, Warrenton, Gearhart, and Cannon Beach. At Seaside, the relatively small change between M1 and XXL1 is indicative of the fact that virtually the entire community is inundated by tsunami in all three scenarios, such that its exposure risk is especially high;
• Building damage caused by earthquake shaking is estimated to range from a maximum of $459 million in Astoria to a minimum of ~$8 million in Arch Cape. The large losses estimated for Astoria can be attributed to the effects of liquefaction (and lateral spreading) and landsliding. Earthquake damage losses in Warrenton and Seaside are also substantial, reaching, respectively, $347 and $362 million. Countywide damage losses caused by the earthquake are expected to exceed $1.8 billion, which equates to ~35% of the buildings damaged;
• Incorporating damage caused by the tsunami results in destruction levels for an M1 event that range from ~43% (Astoria) to 92% at Seaside; for an XXL1 event our analyses indicate 78% destruction of Arch Cape and near 100% destruction at Gearhart, Jeffers Garden, and Seaside. These data reflect the large hydraulic forces associated with the tsunami and the prevalence of light-frame construction material (i.e., wood frame) on the Oregon coast;
• Combined earthquake and tsunami damage for each tsunami zone indicates losses that range from ~$3 billion for an M1 size event, $3.87 billion for an L1 size event, and $4.92 billion for an XXL1 size event. These estimates reflect community-wide losses associated with the earthquake, combined with destruction caused by the tsunami. Note that these estimates exclude building content losses, such that the numbers may be viewed as minimal estimates;
• The destruction of buildings in coastal Clatsop county is expected to generate ~535,000 tons (M1) to ~1,133,000 tons (XXL1) of debris. This equates to ~53,000 dump trucks for M1 to as much as 110,000 dump trucks for an XXL1 event. These estimates are almost certainly on the low end, as they do not include debris associated with content from buildings (personal items, business equipment, etc.), road rip-ups, vehicles, and vegetation;
• Modeled tsunami casualties vary widely between communities. This is due to many factors, but most important is the relative distance to high ground.
  • For the M1 scenario, estimated fatalities are confined mainly to the communities of Seaside (~3,260 resident/7,080 visitor), Warrenton (~460 resident/590 visitor), and Jeffers Garden (~160 resident/0 visitor) with few fatalities in the remaining communities. Low casualties associated with the M1 scenario in the majority of the communities is indicative of the fact that high ground is located close to the population centers allowing for quick access to high ground;
  • For the maximum-considered XXL1 tsunami scenario, the number of fatalities increases dramatically, ranging from a few hundred (e.g., Jeffers Garden) to as many as 11,900 in Seaside. Of the latter, the bulk of the fatalities (67%) are likely to be from the temporary visitor population. At Seaside, the difference in fatalities between the M1 and XXL1 scenarios is ~1,500 people and is a testament to the high degree of risk observed at Seaside under all three tsunami scenarios;
  • High casualties associated with the temporary visitor population is predicated on the assumption that these facilities are at 100% occupancy.
  • The large number of potential fatalities in the communities of Seaside and Gearhart is entirely due to the significant travel distances required to reach high ground in the eastern foothills of southern Clatsop Plains.
  • These results demonstrate a need to evaluate alternative forms of high ground (e.g., vertical evacuation structures), and/or retrofitting bridges to withstand the earthquake shaking, thereby allowing for faster evacuation of the western part of Gearhart and Seaside;
• Following the effects of the earthquake and accompanying tsunami, communities can expect to have to deal with many hundreds to potentially thousands of displaced people requiring short-term shelter and care (~days to a few weeks). Hazus modeling indicates that the numbers of displaced increase significantly as one progresses from M1 (~17,690) to XXL1 (~27,530). We expect these challenges to be especially difficult at Cannon Beach, Warrenton, and Seaside; and
• Compared with fatalities, injuries from the earthquake and XXL1 tsunami were found to be relatively low, varying from ~1% to 6% of the affected community; injury ratios in Astoria (12%) and Arch Cape (15%) are highest due to having fewer deaths. Overall, our combined earthquake and tsunami Hazus modeling indicates ~1,180 injured in the M1 scenario, ~1,190 for L1, and 1,350 for XXL1.

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.