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Open-File Report O-23-08, Earthquake and Tsunami Impact Analysis for Coastal Curry County, Oregon

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Open-File Report O-23-08, Earthquake and Tsunami Impact Analysis for Coastal Curry County, Oregon, by Jonathan C. Allan and Fletcher E. O'Brien; 75 p. report, including data tables and community-specific profiles.


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 Curry County, Oregon, 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 Curry County. The analyses presented here include an assessment of the number of people, businesses, and critical facilities located in three Cascadia tsunami inundation zones (M1, L1, and XXL1). XXL1 represents the maximum-considered inundation scenario given our knowledge of the Cascadia Subduction Zone (CSZ). Large (L1) and Medium (M1) tsunami zones reflect smaller earthquake and tsunami scenarios that are more likely to occur than XXL1. L1 captures 95% of the uncertainty in tsunami modeling (there is a ~5% chance that the tsunami could exceed the L1 tsunami zone), whereas the M1 scenario captures 78% of the uncertainty (there is a ~22% chance that the tsunami could exceed the M1 tsunami zone).

A major focus of this study is to provide improved estimates of local population demographics in each community to better understand evacuation challenges that could affect different population groups, as well as socioeconomic impacts associated with a CSZ earthquake and 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 coastal Curry County.

We used previously developed physical models of a CSZ earthquake and tsunami, “Beat the Wave” (BTW) tsunami evacuation modeling, and the recently published Federal Emergency Management Agency (FEMA) Hazus Tsunami Model to develop standardized damage loss estimates for each community, as well as estimates of injuries, fatalities, and displaced population. From the building damage losses, we estimated the amount of debris generated. Our population model improves upon previous studies by providing spatially detailed estimates of permanent and temporary populations — the latter quantifying numbers of visitors, which vary widely throughout the calendar year. The tsunami injury and fatality modeling evaluates a nighttime (2 AM) evacuation scenario, which assumes people are in their homes/hotels/campgrounds at the time of the event (as opposed to on the beach or walking around town). We also maximize visitor occupancy by assuming all hotels/second homes/campgrounds are at capacity, to fully quantify potential impacts to permanent and temporary residents. Our major findings include the following:

  • The total permanent resident population present on the Curry County coastline within a tsunami zone ranges from ~1,970 (M1) to ~6,940 (XXL1) (Table 3 1). If the temporary population is included, the local population could increase to ~7,050 (M1) to ~16,690 (XXL1) assuming 100% occupancy.
  • The fraction of permanent residents within the three tsunami zones varies considerably between communities (Table 3 1). These variations reflect contrasting patterns in the general shape and elevation of the county coastline, whether it is open coast versus up an estuary; inundation extents; and the distribution of permanent residents within the communities. Notable observations: 
    • For the M1 scenario, the community with the largest number of people in the tsunami zone include Gold Beach (34%). For the L1 scenario, 52% of the Gold Beach population is located in the tsunami zone, this increases to 76% of the population in the XXL1 tsunami zone.
    • Brookings has few (~1%) people in the M1 and L1 tsunami zones and 9% of its population in the XXL1 tsunami zone.
    • Port Orford and Nesika Beach have relatively few people (~3-5%) in the M1 tsunami zones. This increases to ~20% for the L1 scenario, and 58% and 76% respectively in the XXL1 zone. 
  • All five communities and the single major campground distributed along the Curry County coastline can experience significant influxes of visitors, well exceeding their local resident populations. Of note, the population of Harbor can swell by ~3000% to 323% (M1 to XXL1), and Gold Beach by ~290% to 215%. The popularity of these communities as centers of tourism present challenges associated with preparing such a large transient population for a CSZ earthquake and tsunami.
  • An understanding of how population demographics are geographically distributed within each tsunami zone can provide an insight into those communities that may experience evacuation challenges. We use people over 65 years of age as a proxy for those who may experience increased evacuation difficulty (reduced evacuation travel speeds). Analyses of the Curry County population demographics indicate that the countywide resident population of ≥65 years of age is ~39% of the total population for all three tsunami zones; this indicates there are ~630 residents in the M1, ~1,180 in L1, and ~2,590 in the XXL1 (Table 3 2). Nevertheless, two communities have significantly more people ≥65 years of age: Harbor (68%), followed by Brookings (~40%). Variations in demographics will likely impact ability to evacuate from the tsunami zone.
  • The number of buildings located in a tsunami zone is a useful metric for determining exposure to the tsunami hazard. Communities with particularly high exposure to the tsunami hazard (Table 3‑4) include: Gold Beach at 33% (M1), 51% (L1) and 75% (XXL1); Nesika Beach at 78% (XXL1); and Port Orford at 58% (XXL1).
  • Building damage caused by earthquake shaking is estimated to range from a maximum of $1 billion in Brookings to a low of tens of thousands in the various campgrounds (Table 3 4). The large losses estimated for Brookings can be attributed to factors such as the severity of earthquake shaking in southwest Oregon, building engineering (age and type of structure), and the effects of liquefaction (and lateral spreading). Earthquake damage losses sustained by the many buildings located in unincorporated (“other” category) Curry County are also substantial and are estimated to reach ~$730 million. Countywide damage losses caused by the earthquake are expected to exceed $2.7 billion.
  • An M1 event could yield damage levels that range from ~75% at Brookings to ~96% at Harbor. Damage caused by the XXL1 tsunami reveals destruction levels of >88% in all five communities; complete destruction occurs at the Humbug Mountain State Park campground. These findings can be attributed to the powerful 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 ~$2.9 billion for an M1 size event, $3 billion for an L1 size event, and $3.3 billion for an XXL1 size event (Table 3 4). These estimates reflect communitywide 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 minimum estimates.
  • The destruction of buildings in coastal Curry County is expected to generate ~172,000 tons (M1) to ~585,000 tons (XXL1) of debris (Table 3‑4). This equates to ~17,200 dump trucks for M1 to as much as 58,500 dump trucks for an XXL1 event (assuming dump truck capacity of ~10 yd3). 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 (injuries and fatalities) vary widely between communities (Table 3‑6). This is due to many factors, but the most important is the relative distance to high ground. We estimate that, combined, countywide fatalities from the tsunami could reflect ~1,390 (M1), ~3,040 (L1) and ~9,350 (XXL1).
    • Low casualties associated with the M1 scenario observed in Port Orford, Nesika Beach and Brookings is indicative of the fact that these communities are either built on high ground (marine terraces) or because high ground is located close to the population centers, allowing for quick access.
    • For the XXL1 tsunami scenario — the largest-considered — the potential for significant fatalities is apparent for Gold Beach (~3,800), the “other" category in Curry County (~1,980), Harbor (~1,560), and Port Orford (~1,140). Overall, the bulk of the fatalities (>54%) are likely to be from the temporary visitor population.
    • High casualties associated with the temporary visitor population is predicated on the assumption that these facilities are at 100% occupancy.
    • These results demonstrate a need to evaluate alternative forms of high ground (e.g., vertical evacuation structures), thereby allowing for faster evacuation. 
  • Following the earthquake and accompanying tsunami, communities will have to deal with many hundreds to potentially thousands of displaced people requiring immediate short-term shelter and care (for days to a few weeks), after which many people are likely to be evacuated from the coast. Hazus modeling indicates that the number of displaced people is significantly higher in the XXL1 scenario (~7,300) compared to the M1 scenario (~5,660) (Table 3 8). We expect large numbers of displaced people to severely challenge the following communities: Gold Beach, Harbor, and Brookings. Furthermore, an estimated 2,100 people outside of community urban growth boundaries (UGB) and unincorporated boundaries will require shelter and care. 
  • Compared to fatalities, injuries from the earthquake were found to be moderately low. Overall, our combined earthquake and tsunami Hazus modeling indicates ~750 critically injured (Levels 2 and 3 in Table 2 4) for the M1 scenario, ~1,180 for L1, and ~1,150 for XXL1 in Curry County. Access to hospital services will be essential for those with critical injuries, especially those with level 3 injuries.

Although each community has unique circumstances and challenges, 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.