Cascadia Subduction Zone Earthquake Research Articles

Testing Megathrust Rupture Models Using Tsunami Deposits

AbstractThe 26 January 1700 CE Cascadia subduction zone earthquake ruptured much of the plate boundary and generated a tsunami that deposited sand in coastal marshes from northern California to Vancouver Island. Although the depositional record of tsunami inundation is extensive in some of these marshes, few sites have been investigated in enough detail to map the inland extent of sand deposition and depict variability in tsunami deposit thickness and grain size. We collected 129 cores in marshes of the Salmon River estuary in Oregon and reanalyzed 114 core logs from a 1987–88 study that mapped the inland extent of circa 1700 CE sandy tsunami deposits. The ca. 1700 CE tsunami deposit in the Salmon River estuary is easily recognized in cores ≤1 m deep in which a buried marsh peat is overlain by a well sorted sand bed with a sharp lower contact that thins and fines inland. We use tsunami deposit data and models of sandy tsunami sediment transport (using Delft3D‐FLOW) to test 15 rupture models that could represent a ca. 1700 CE earthquake. At least 12–16 m of slip offshore of the Salmon River, which results in 0.8–1.0 m of coastal coseismic subsidence, is required to match the ca. 1700 CE sand deposit's inland extent, which is consistent with models of heterogeneous megathrust slip in ca. 1700 CE. Our methods of detailed tsunami deposit mapping, combined with sediment transport modeling, can be used to test models of megathrust ruptures and their tsunamis to potentially improve earthquake and tsunami hazard assessments.

Toward multivariate fragility functions for seismic damage and loss estimation of high‐rise buildings

AbstractData‐driven models for seismic damage and loss assessment of buildings have become more common in recent years due to the availability of large repositories of recorded and synthetic ground motions coupled with structural response simulation data. This paper explores the benefits of using bivariate and multivariate fragility functions to estimate earthquake‐induced damage and economic loss in high‐rise buildings. The dataset used in this study encompasses 15,000 simulations of modern high‐rise reinforced concrete shear wall buildings ranging from eight to 24 stories which are subjected to ground motion records at five different intensity levels. The proposed functions are conditioned on average spectral accelerations and ground motion significant duration. The results indicate that bivariate fragility functions improve damage state prediction success (Brier score) by 16%, and multivariate fragility functions by 24% relative to conventional univariate functions (standard of practice). To develop multivariate functions, nominal and ordinal probit regression models are fit to the dataset. While both models yield satisfactory predictive performance, ordinal functions can lead to a 15% reduction in misclassified collapse instances, that is, the minority class. Univariate functions tend to overestimate seismic losses at lower intensity levels while underestimating them at higher intensities. These loss estimates are significantly improved when bivariate or multivariate building fragility functions are used. Given the increase in the use of physics‐based ground motion simulations and/or multi‐variate ground motion models, from which multiple intensity measures can be extracted, a shift toward a more complex representation of fragility functions, for example, multivariate curves, is necessary and inevitable. The proposed functions are used to evaluate the performance of a portfolio of modern high‐rise reinforced concrete shear wall buildings at four sites across the Seattle, Washington metropolitan area under a potential magnitude‐9 Cascadia subduction zone earthquake scenario. The results indicate that the proposed functions can be beneficial in enhancing damage state predictions and loss estimates at a regional scale.

Risk‐targeted seismic evaluation of functional recovery performance in buildings

AbstractPast earthquakes have highlighted the impact of prolonged downtime on community recovery. The introduction of performance objectives expressed in terms of recovery time has been identified as a key goal for the next generation of building codes. Recent development of downtime estimation frameworks now allows for the discussion of how to use recovery time estimates to (1) establish recovery‐targeted performance and (2) assess the functional recovery performance of buildings designed with modern code provisions. In this paper, we employ methods adapted from life‐safety provisions to evaluate building functional recovery performance using downtime simulation outputs. A scenario‐based, an intensity‐based and a risk‐targeted analysis of functional recovery performance objectives are presented. The functional recovery performance of a series of modern residential reinforced concrete shear wall archetype buildings, ranging from eight to 24 stories in height, at a site in downtown Seattle, Washington, are then assessed. Results of the scenario‐based assessment of an M9 Cascadia Subduction zone earthquake, comparable to an intensity‐based assessment at the 975‐year intensity level, show that the probability of achieving functional recovery within four months is close to zero, while the likelihood of achieving functional recovery by one year is approximately 60%. The results of the risk‐targeted assessment of the archetype buildings show a 50% probability in 50 years of the building downtime to functional recovery exceeding one month, a 43% in 50‐year probability of the downtime exceeding four months and an 8% in 50‐year probability of downtime exceeding one year. Finally, the expected annual downtime is approximately three days for all building heights considered. Disaggregation was used to identify intensity levels that contribute most of the downtime risk. The risk of downtime exceeding one month to one year is dominated by frequent, low‐intensity earthquakes (e.g., 100‐year return period). The risk of exceeding longer recovery times is dominated by less frequent, higher intensity earthquakes (i.e., return periods greater than 975 years). While the results of the analysis are sensitive to assumed damage thresholds and impeding factor delays triggered at low intensity levels, the findings suggest that reinforced concrete shear wall buildings designed following current building codes have a high risk of lengthy downtimes. Ultimately, the proposed methods and results illustrate the importance of introducing recovery‐based provisions into the next generation of building codes.

Impacts of a Cascadia Subduction Zone Earthquake on Water Levels and Wetlands of the Lower Columbia River and Estuary

AbstractSubsidence after a subduction zone earthquake can cause major changes in estuarine bathymetry. Here, we quantify the impacts of earthquake‐induced subsidence on hydrodynamics and habitat distributions in a major system, the lower Columbia River Estuary, using a hydrodynamic and habitat model. Model results indicate that coseismic subsidence increases tidal range, with the smallest changes at the coast and a maximum increase of ∼10% in a region of topographic convergence. All modeled scenarios reduce intertidal habitat by 24%–25% and shifts ∼93% of estuarine wetlands to lower‐elevation habitat bands. Incorporating dynamic effects of tidal change from subsidence yields higher estimates of remaining habitat by multiples of 0–3.7, dependent on the habitat type. The persistent tidal change and chronic habitat disturbance after an earthquake poses strong challenges for estuarine management and wetland restoration planning, particularly when coupled with future sea‐level rise effects.

Spectral acceleration basin amplification factors for interface Cascadia Subduction Zone earthquakes in Canada’s 2020 national seismic hazard model

Canada’s 2020 national seismic hazard model (CSHM 2020) provides hazard estimates for interface Cascadia Subduction Zone (CSZ) earthquakes in southwestern Canada using four ground motion models (GMMs) with equal weights. Two out of the four GMMs were derived using data primarily from subduction earthquakes in Japan so their use in CSHM 2020 includes a “Japan-to-Cascadia” factor to account for local site conditions. Despite this regional factor, the GMMs do not explicitly consider the amplification effects from the Georgia sedimentary basin below Metro Vancouver. This study benchmarks ground motion shaking from a suite of 30 physics-based simulations of M9 CSZ earthquakes, which explicitly account for basin effects for periods exceeding 1 s, to corresponding estimates from interface GMMs in CSHM 2020. Using this comparison, this study proposes site-specific and period-dependent basin amplification factors for a range of sites located within the Georgia sedimentary basin. The average basin amplification factors at the deepest basin location reach values as high as 2.24 and 6.29 at a 2 s period with respect to basin-edge and outside-basin reference locations, respectively. We evaluate the proposed factors by comparing them against long-period amplifications observed in empirical strong motion recordings from the 2001 M6.8 Nisqually earthquake. A framework to incorporate the proposed basin amplification factors in the calculation of CSHM 2020 uniform hazard spectra (UHS) is also proposed. The modified UHS ordinates at the deepest basin location reach values that are 58% to 271% higher, at a 2 s period, than non-basin UHS estimates when amplification factors are calculated with respect to basin-edge and outside-basin reference locations, respectively.

An interdisciplinary agent-based evacuation model: integrating the natural environment, built environment, and social system for community preparedness and resilience

Abstract. Previous tsunami evacuation simulations have mostly been based on arbitrary assumptions or inputs adapted from non-emergency situations, but a few studies have used empirical behavior data. This study bridges this gap by integrating empirical decision data from surveys on local evacuation expectations and evacuation drills into an agent-based model of evacuation behavior for two Cascadia subduction zone (CSZ) communities that would be inundated within 20–40 min after a CSZ earthquake. The model also considers the impacts of liquefaction and landslides from the earthquake on tsunami evacuation. Furthermore, we integrate the slope-speed component from least-cost distance to build the simulation model that better represents the complex nature of evacuations. The simulation results indicate that milling time and the evacuation participation rate have significant nonlinear impacts on tsunami mortality estimates. When people walk faster than 1 m s−1, evacuation by foot is more effective because it avoids traffic congestion when driving. We also find that evacuation results are more sensitive to walking speed, milling time, evacuation participation, and choosing the closest safe location than to other behavioral variables. Minimum tsunami mortality results from maximizing the evacuation participation rate, minimizing milling time, and choosing the closest safe destination outside of the inundation zone. This study's comparison of the agent-based model and the beat-the-wave (BtW) model finds consistency between the two models' results. By integrating the natural system, built environment, and social system, this interdisciplinary model incorporates substantial aspects of the real world into the multi-hazard agent-based platform. This model provides a unique opportunity for local authorities to prioritize their resources for hazard education, community disaster preparedness, and resilience plans.

Evaluation of hazard brochures using topic viewing durations: Application to tsunami evacuation brochures.

This study describes a novel method of assessing risk communication effectiveness by reporting an evaluation of a tsunami information brochure by 90 residents of three Pacific coast communities that are vulnerable to a Cascadia Subduction Zone earthquake and tsunami-Commencement Bay, Washington; Lincoln City, Oregon; and Eureka, California. Study participants viewed information that was presented in DynaSearch, an internet-based computer system that allowed them to view text boxes and tsunami inundation zone maps. DynaSearch recorded the number of times each text box or map was clicked and the length of time that it was viewed. This information viewing phase was followed by questionnaire pages assessing important aspects of tsunami hazard and sources of tsunami warnings. Participants gave the longest click durations to what to do in the emergency period during earthquake shaking and in its immediate aftermath before a tsunami arrives-topics that should be displayed prominently in tsunami brochures and emphasized in talks to community groups. The smallest adjusted click durations were associated with advance preparations for a tsunami-topics that can be posted on websites whose URLs are printed in the brochures.

Toward Models of Impact and Recovery of the US Western Grid from Earthquake Events

A Cascadia Subduction Zone (CSZ) earthquake will cause widespread damage to numerous lifelines and infrastructure along the northern US west coast. The goal of the presented research is to provide a bottom up estimate of the impact on and subsequent recovery of a Cascadia Subduction Zone earthquake on the US western grid to supplement and enhance the expert opinion estimates provided to date. The scope is limited to only consideration of shaking damage to utility substation equipment components of a power system model. The analysis utilizes probabilistic models of damage and recovery for substation power system assets, along with graph techniques for modeling connectivity, and Monte Carlo quasi steady state power flow solutions. The results show that a conservative estimate of the initial damage and loss of load is approximately 4000 MW, with a recovery estimate of 230 days.

Limits to coseismic landslides triggered by Cascadia Subduction Zone earthquakes

Landslides are a significant hazard and dominant feature throughout the landscape of the Pacific Northwest. However, the hazard and risk posed by coseismic landslides triggered by great Cascadia Subduction Zone (CSZ) earthquakes is highly uncertain due to a lack of local and global data. Despite a wealth of other geologic evidence for past earthquakes on the Cascadia Subduction Zone, no landslides have been definitively linked to such earthquakes, even in areas otherwise highly susceptible to failure. While shallow landslides may not leave a lasting topographical signature in the landscape, there are thousands of deep-seated landslides in Cascadia, and these deposits often persist for hundreds of years and multiple earthquake cycles. Synthesizing newly developed inventories of dated large deep-seated landslides in the Oregon Coast Range, we use statistical methods to estimate the proportion of these types of landslides that could have been triggered during past great Cascadia Subduction Zone earthquakes. Statistical analysis of high-precision dendrochronology ages of landslide-dammed lakes and surface roughness-dated bedrock landslides reveal Cascadia Subduction Zone earthquakes may have triggered 0–15 % of large deep-seated landslides in the Oregon Coast Range over multiple earthquake cycles. Our results refine estimates from previous studies and further suggest that coseismic triggering accounts for a small fraction of the total deep-seated bedrock landslides mapped in coastal Cascadia. However, if the real rate of coseismic landslide triggering during CSZ earthquakes is near our estimated upper bound for the 1700 CSZ earthquake, we estimate up to 2400 coseismic large deep-seated landslides could occur in the Oregon Coast Range in a single earthquake. These findings suggest Cascadia is consistent with global observations from other subduction zones and that coseismic landslides may still represent a serious geohazard in the region.

Recovery Rate of a Salt Marsh From the 1700 CE Cascadia Subduction Zone Earthquake, Netarts Bay, Oregon

AbstractSince the 1700 CE Cascadia Subduction Zone earthquake and associated coseismic subsidence and tsunami, vegetated intertidal habitats have reestablished across Pacific Northwest estuaries, yet timescales and mechanisms of recovery are uncertain. We investigated the timescale of salt marsh reestablishment in Netarts Bay, Oregon following the 1700 CE earthquake using a combination of excess 210Pb, 14C, stratigraphic constraints, and Bayesian age‐depth modeling. Coseismic subsidence lowered the area to low/mid marsh, which persisted for 200 years before transition to modern high marsh. The modern high marsh now appears in dynamic equilibrium with modern sea level rise. In addition to serving as a methodological proof of concept for dating the past 300 years, these results provide insight into intertidal morphodynamic response to large perturbations along tectonically active margins.

Modeling Regional and Local Resilience of Infrastructure Networks Following Disruptions from Natural Hazards

This paper presents a framework to evaluate the regional and local resilience of infrastructure networks following disruptions from natural hazards. Herein, the regional resilience of a network relates to the accessibility of a community within a larger network, whereas the local resilience concerns the ability of a network to provide its intended service within the boundaries of a community. Using this framework, a methodology is developed to demonstrate its application to a road and highway transportation network disrupted by ground shaking and inundation under a Cascadia Subduction Zone earthquake and tsunami scenario. The regional network extents encompass the entire coast of the US state of Oregon. Embedded within this regional network are 18 local networks associated with coastal communities. Regional and local connectivity indexes are defined to identify the initial damage and then track the postdisaster recovery of the transportation network, i.e., evaluate the network resilience. The study results identify the attributes that lead to a regionally or locally resilient network and highlight the importance of considering local infrastructure networks embedded within larger regional networks. It is shown that without regional considerations, the time to recover may be severely underpredicted. The methodology is further used as a decision support tool to demonstrate how mitigation options impact the transportation network’s resilience. The importance of strategically considering mitigation options is emphasized as some communities see significant reductions in time to recover, whereas others see little to no improvement.

A Community Impact Scale for Regional Disaster Planning with Transportation Disruption

This paper proposes a simple analytical scheme and associated qualitative impact scales that capture the spatially varying effects of a regional disaster. Large-scale disasters that affect many towns and cities pose particular challenges for emergency response planning. For example, disruption to transportation systems can impede regional supply chains of critical goods, thereby exacerbating the impacts suffered locally in communities. Conventional metrics of disaster severity, such as number of casualties or intensity of ground shaking, do not adequately capture how community impacts and needs may vary across the affected region, and they do not typically consider regional transportation disruption. Using a series of impact scales, the approach in this paper captures essential attributes of three broad components related to community impacts from a regional disaster—local disaster impacts in a community, regional transportation disruption to the community, and the community’s coping capacity—and aggregates them to an overall metric of community impact. The approach can be implemented with widely varying degrees of data availability, as demonstrated in two case applications. Both cases involve an M9 Cascadia subduction zone earthquake affecting a broad region of coastal British Columbia, Canada. The first application illustrates how in a pre-event planning situation, modeled results can be used to anticipate which communities are at greatest risk, and to help prioritize mitigation and emergency response planning. The second case demonstrates how in the immediate aftermath of a disaster, the approach can be used with limited information to help prioritize response and recovery activities.

Measuring the destruction and recuperation of the natural gas pipeline system at Oregon's Critical Energy Infrastructure Hub.

Portland has become a hot spot for geological discussions over the last few years. The event that has everyone talking, and preparing for, is the Cascadia Subduction Zone earthquake. Cascadia is categorized as a catastrophic natural, seismic event, where the Juan De Fuca plate and the North American plate subduct off the Northwest coast, causing a violent response in the zone between the two plates. Geological history has shown us that every 200-300 years, a major seismic event occurs in this area of the Pacific; in current disaster discussions in the emergency management industry, where this author has resided for the last 6 years, it is not a matter of if, but a matter of when. The Cascadia event will cause a massive earthquake, affecting millions of lives and costing just as much if not more. Currently, there is no model that exists to equate for all of the following: damages to pipes, restoration, and disruption, and maximum thresholds for utility usage. The models and plans discussed in this paper will cast a needed spotlight for establishing a new model combining these elements and more. It is vital that efforts be made to calculate, beyond a base percentage rate of recovery, what Oregon and the region will lose and what it will take regain a "back to business" level of operations, economically, structurally, and environmentally. Many calls were placed with nonprofit energy stewards to determine these statistics, but no current information was available, or easy to attain, further emphasizing a need for real-time utility data.

Age of the late Holocene Bonneville landslide and submerged forest of the Columbia River Gorge, Oregon and Washington, USA, by radiocarbon dating

AbstractThe late Holocene Bonneville landslide, a 15.5 km2 rockslide-debris avalanche, descended 1000 m from the north side of the Columbia River Gorge and dammed the Columbia River where it bisects the Cascade Range of Oregon and Washington, USA. The landslide, inundation, and overtopping created persistent geomorphic, ecologic, and cultural consequences to the river corridor, reported by Indigenous narratives and explorer accounts, as well as scientists and engineers. From new dendrochronology and radiocarbon dating of three trees killed by the landslide, one entrained and buried by the landslide and two killed by rising water in the impounded Columbia River upstream of the blockage, we find (1) the two drowned trees and the buried tree died the same year, and (2) the age of tree death, and hence the landslide (determined by combined results of nine radiocarbon analyses of samples from the three trees), falls within AD 1421–1455 (3σ confidence interval). This result provides temporal context for the tremendous physical, ecological, and cultural effects of the landslide, as well as possible triggering mechanisms. The age precludes the last Cascadia Subduction Zone earthquake of AD 1700 as a landslide trigger, but not earlier subduction zone or local crustal earthquakes.

Cascadia Subduction Zone Residents’ Tsunami Evacuation Expectations

The U.S. Pacific Northwest coast must be prepared to evacuate immediately after a Cascadia Subduction Zone earthquake. This requires coastal residents to understand the tsunami threat, have accurate expectations about warning sources, engage in preimpact evacuation preparedness actions, and plan (and practice) their evacuation logistics, including an appropriate transportation mode, evacuation route, and destination. A survey of 221 residents in three communities identified areas in which many coastal residents have reached adequate levels of preparedness. Moreover, residents who are not adequately prepared are willing to improve their performance in most of the areas in which they fall short. However, many respondents expect to engage in time-consuming evacuation preparations before evacuating. Additionally, their estimates of evacuation travel time might be inaccurate because only 28–52% had practiced their evacuation routes. These results indicate that more coastal residents should prepare grab-and-go kits to speed their departure, as well as practice evacuation preparation and evacuation travel to test the accuracy of these evacuation time estimates. Overall, these results, together with recommendations for overcoming them, can guide CSZ emergency managers in methods of improving hazard awareness and education programs. In addition, these data can guide transportation engineers’ evacuation analyses and evacuation plans.

The Role of Slow Slip Events in the Cascadia Subduction Zone Earthquake Cycle

AbstractSlow slip events (SSEs) detected on the Cascadia Subduction Zone interface at 30–50 km depth imply a release of accumulated strain. However, studies of interseismic deformation in Cascadia typically find coupling on the upper 30 km of the interface, which is generally accepted as defining the seismogenic zone. Estimates of coupling using net interseismic velocities (including SSE effects) and restricting coupling to the shallow interface may underestimate slip deficit accumulation at depths >30 km. Here, we detect reversals in GPS motion as indications of SSEs, then use SSE displacements to estimate cumulative slow slip from 2007 to 2021. We calculate pure interseismic velocities, correcting for SSE displacements, and use them to constrain an elastic block model, estimating slip deficit on the subduction interface down to 50 km. By evaluating slip deficit and slow slip independently, we examine SSEs’ effect on interseismic strain accumulation, and the effect of inter‐SSE slip deficit and slow slip on vertical deformation of the forearc. We find that moderate to high coupling extends to 40 km depth, and while shallow coupling is consistent with previous estimates of the seismogenic zone, a deeper region of slip deficit beneath the Olympic Peninsula may be partially (61%) relieved aseismically by SSEs. Patterns of surface uplift suggest that complete relief of deep coupling over multiple decades may be accomplished by time‐varying rates of aseismic slip.

From information to public preparedness in the Cascadia Subduction Zone: Examining risk communication outcomes in Metropolitan Portland, OR

A Cascadia Subduction Zone (CSZ) earthquake will cause widespread damage along the Pacific Northwest (PNW) coast of the United States. It is, therefore, crucial to understand how to reduce future impacts across this region and assess current levels of household preparedness. Here, we examine whether decades of risk and preparedness campaigns have established protective knowledge and promoted hazard adjustments for residents of the Portland, Oregon metropolitan (PDX) region, an area where risk and preparedness campaigns have been increasing over the last decade. We also explore how PDX preparedness levels compare to those in other PNW regions using analogous data from multiple surveys over a 20-year time interval and assess PDX respondent’s perceptions of risk information sources. We find that 63% of PDX residents do not intend to “Drop, Cover, and Hold On” (DCHO) when earthquake shaking starts and that, although residents are generally aware of earthquake hazards in the area, they are less aware of the specific risks for their homes. Furthermore, PDX residents’ preparedness levels seem to be comparable to those recently surveyed in Washington, but somewhat less than those surveyed two decades ago in California, though more comprehensive data are needed to verify these results. We also find that people rate risk information provided by emergency managers and their websites as more accurate, understandable, relevant, and new than other sources. Our results suggest that significant gaps remain in translating broad awareness of the CSZ earthquake into personal knowledge and preparedness. This work provides guidance to PDX emergency educators for more targeted messaging and provides methods to measure preparedness variables in other regions for future comparisons. By paying close attention to preparedness gaps, local officials can use their limited resources more effectively to develop strategies to inform their communities and improve preparedness before a major earthquake strikes.

Effects of the Georgia Sedimentary Basin on the Response of Modern Tall RC Shear-Wall Buildings to M9 Cascadia Subduction Zone Earthquakes

Tall residential RC shear wall buildings (RCSW), which are predominant in Metro Vancouver, have the potential to experience large magnitude earthquakes generated by the Cascadia Subduction Zone (CSZ). Furthermore, the region lies above the Georgia sedimentary basin, which can amplify the intensity of ground motions at medium to long periods and the resulting damage in tall structures. This study provides insights into the effects of the Georgia sedimentary basin amplification on (1) spectral accelerations associated with magnitude 9 (M9) CSZ earthquakes, (2) resulting force- and deformation-controlled actions in modern tall RCSW buildings, and (3) ensuing earthquake-induced repair costs and times. To this end, we leveraged a suite of physics-based ground motion simulations of a range of M9 CSZ earthquake scenarios, which explicitly consider basin effects, and benchmarked these scenarios against a range of seismic hazard intensities, which neglects basin effects. While the M9 simulations have an estimated 500-year return period, at deep basin sites their spectra exceed the 2,475-year hazard in the 1–3 s period range. Nonlinear dynamic analysis results under probabilistic seismic hazard estimates result in negligible collapse risk. In contrast, collapse risk conditioned on the occurrence of the M9 motions results in probabilities as high as 15%. Additionally, seismic demands from the M9 simulations at deep basin sites result in earthquake-induced repair costs and times that exceed those associated with the 2,475-year hazard level.

Collapse risk of pre-Northridge tall steel moment-resisting frames in the Seattle basin during large-magnitude subduction earthquakes

The Pacific Northwest has the potential to experience large-magnitude earthquakes generated by the Cascadia Subduction Zone, which is located approximately 100 km from the city of Seattle. Tall buildings in Seattle are particularly vulnerable to these earthquakes because the city lies above a deep sedimentary basin, which can amplify the intensity of earthquake ground motions at long periods. This paper evaluates the response of an archetype 1970s 50-story steel moment-resisting frame office building in Seattle under 30 simulated scenarios of a magnitude-9 (M9) Cascadia Subduction Zone earthquake with explicit consideration of basin effects. The archetype is assumed to have fracture-prone welded connections consistent with those identified following the 1994 Northridge earthquake. The probability of collapse conditioned on the occurrence of the M9 scenarios, which have a return period of approximately 500 years, is 30%. The annualized collapse risk of the archetype building is also assessed considering all sources (i.e., crustral, intraslab and interface earthquakes) that contribute to the seismic hazard through a multiple stripe analysis procedure. The results indicate a 50-year collapse risk of 6.9% when basin effects are neglected and 10.5% when basin effects are considered. These results exceed the 1% in 50-year target implicit in modern seismic design standards by a factor of 10. These high collapse risks are largely driven by (i) deep sedimentary basin effects and (ii) the expected brittle behavior of fracture-prone welded beam-to-column connections. The simulations of the performance of the building under the M9 scenarios outside of the basin or with ductile beam-to-column connections result in a negligible probability of collapse.

Assessing local impacts of the 1700 CE Cascadia earthquake and tsunami using tree-ring growth histories: a case study in South Beach, Oregon, USA

Abstract. We present an investigation of the disturbance history of an old-growth Douglas-fir (Pseudotsuga menziesii) stand in South Beach, Oregon, for possible growth changes due to tsunami inundation caused by the 1700 CE Cascadia Subduction Zone (CSZ) earthquake. A high-resolution model of the 1700 tsunami run-up heights at South Beach, assuming an “L”-sized earthquake, is also presented to better estimate the inundation levels several kilometers inland at the old-growth site. This tsunami model indicates the South Beach fir stand would have been subjected to local inundation depths from 0 to 10 m. Growth chronologies collected from the Douglas-fir stand shows that trees experienced a significant growth reductions in the year 1700 relative to nearby Douglas-fir stands, consistent with the tsunami inundation estimates. The ±1–3-year timing of the South Beach disturbances are also consistent with disturbances previously observed at a Washington state coastal forest ∼220 km to the north. Moreover, the 1700 South Beach growth reductions were not the largest over the >321-year tree chronology at this location, with other disturbances likely caused by climate drivers (e.g., drought or windstorms). Our study represents a first step in using tree growth history to ground truth tsunami inundation models by providing site-specific physical evidence.