DOGAMI Open-File Report Series

[Go back] | Publications Center | DOGAMI Home

Publication Preview

Open-File Report O-20-14, Earthquake and tsunami impact analysis for coastal Tillamook County, Oregon, by Jonathan C. Allan, Fletcher E. O’Brien, John M. Bauer, and Matthew C. Williams; 121 p. report, including community-specific profiles for Manzanita, Nehalem Bay State Park, Nehalem, Wheeler, Rockaway Beach, Barview, Garibaldi, Bay City, Tillamook, Cape Meares, Oceanside, Netarts, Cape Lookout Sate Park, Sand Lake Recreation Area, Tierra Del Mar, Pacific City, and Neskowin; 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 Tillamook 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 (121 p., 102 MB PDF), including an appendix of 51 tabloid-page-size community-specific profiles for Manzanita, Nehalem Bay State Park, Nehalem, Wheeler, Rockaway Beach, Barview, Garibaldi, Bay City, Tillamook, Cape Meares, Oceanside, Netarts, Cape Lookout Sate Park, Sand Lake Recreation Area, Tierra Del Mar, Pacific City, and Neskowin
• Report only (70 p., 9.2 MB PDF)
• Appendix community profile sheets only (51 p., 93 MB PDF)
• Spreadsheet only (Excel spreadsheet, 101 KB) - contains all the data that are the basis for this report’s tables and figures
• Publication bundle (report, appendix, spreadsheet; 96 MB .zip file)
  
  

EXECUTIVE SUMMARY

This report provides an evaluation of the potential impacts of a Cascadia earthquake and accompanying tsunami in coastal Tillamook 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 Tillamook 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 Tillamook County coastline within a tsunami zone ranges from ~3,300 (M1) to ~7,200 (XXL1). If the temporary population is included, the local population could increase by as much ~18,400 (M1) to ~29,000 (XXL1) assuming 100% occupancy;
• The fraction of the total permanent resident population within the three tsunami zones varies widely between communities. For example, Nehalem Bay State Park, Cape Lookout State Park, and the Sand Lake Recreational area fall mostly within the M1 tsunami zone and are completely inundated by the L1 and XXL1 tsunami zones. The communities of Rockaway Beach, Barview, Tierra Del Mar, and Neskowin each have large numbers of people located in the XXL1 tsunami zone — respectively, 93%, 98%, 99%, and 95% of the total population. Percentage of total population within the L1 tsunami zone is 88% for Rockaway, 97% for Barview, 99% for Tierra Del Mar, and 94% for Neskowin. Percentage of total population within the M1 tsunami zone is 78% for Rockaway, 85% for Barview, 99% for Tierra Del Mar, and 90% for Neskowin. These findings reflect contrasting patterns in the general shape and elevation of the Tillamook 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 14 Tillamook County coastal communities can experience significant influxes of visitors, well exceeding their local resident populations. Of note, the community of Rockaway Beach can swell by ~540–590% (XXL1 and M1), Manzanita by ~950–1,080% (XXL1 and M1), and Pacific City by ~470–530% (M1 and XXL1). These results demonstrate the importance of these 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. Furthermore, these challenges extend to local government entities that must in turn take care of everyone (residents and visitors) after the earthquake and tsunami;
• Analyses of Tillamook County population demographics indicate that the countywide resident population of ≥65 years of age is ~29–30% of the total population for all three tsunami zones; this reflects ~1,000, 1,440, and 2,000 residents in the M1, L1, and XXL1 zones who are ≥65 years of age. Manzanita and Barview have significantly more people ≥65 years of age (~41%). Variations in demographics will likely impact ability to evacuate from the tsunami zone;
• The number of buildings located in a tsunami zone is greatest in Rockaway Beach, Pacific City, and Manzanita. At Rockaway Beach, 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 $136 million in Rockaway Beach to a low of ~$90,000 at Nehalem Bay State Park. The large losses estimated for Rockaway Beach can be attributed to the effects of liquefaction (and lateral spreading) and building engineering. Earthquake damage losses in Tillamook are also substantial and are estimated to reach $106 million. Countywide damage losses caused by the earthquake are expected to exceed $1.8 billion, which equates to ~50–60% of the buildings damaged;
• Incorporating damage caused by the tsunami results in destruction levels for an M1 event that range from ~16% (Nehalem Bay State Park) to >90% at Cape Lookout State Park, Tierra Del Mar, and Neskowin; for an XXL1 event our analyses indicate 56% destruction of Tillamook and >97% destruction at Nehalem Bay and Cape Lookout State Parks, the Sand Lake Recreation area, and the communities of Rockaway Beach, Barview, Tierra Del Mar, Pacific City, and Neskowin. These data reflect 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 ~$2.4 billion for an M1 size event, $2.8 billion for an L1 size event, and $3.3 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 Tillamook county is expected to generate ~205,000 tons (M1) to ~600,000 tons (XXL1) of debris. This equates to ~20,000 dump trucks for M1 to as much as 60,000 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 casualties are confined mainly to the communities of Rockaway Beach (~180 resident/970 visitor), Neskowin (~30 resident/180 visitor), and Cape Lookout State Park (~490 visitor) with few casualties in the other communities. Low casualties associated with the M1 scenario in the majority of the communities are indicative of the fact that high ground is located close to the population centers allowing for quick access to high ground, or the tsunami simply does not reach them;
  • For the maximum-considered XXL1 tsunami scenario, the number of casualties increases dramatically, ranging from <10 (e.g., Garibaldi, Bay City, Oceanside, Netarts and Tillamook) to as many as 2,600 in Rockaway Beach. Of the latter, the bulk of the fatalities (85%) is likely to be from the temporary visitor population. At Rockaway Beach, 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 Rockaway Beach under all three tsunami scenarios;
  • High casualties associated with the temporary visitor population are predicated on the assumption that visitor facilities are at 100% occupancy.
  • The large number of potential fatalities in the communities of Rockaway Beach and Neskowin is entirely due to the significant travel distances required to reach high ground. 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;
• 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 from M1 (~19,800) to XXL1 (~27,850). We expect these challenges to be especially difficult in three communities: Rockaway Beach, Manzanita, and Pacific City; and
• Compared with fatalities, injuries from the earthquake and XXL1 tsunami were found to be relatively low, varying from ~2% to 22% of the affected community; injury ratios in Netarts (21%), Oceanside (22%), Cape Meares (19%) and Garibaldi (18%) are highest due to having few casualties relative to the population size. Overall, our combined earthquake and tsunami Hazus modeling indicates ~1,460 injured in the M1 scenario, ~1,630 for L1, and ~1,800 for XXL1 in Tillamook County.

Although each community in coastal Tillamook 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.