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We evaluate Cascadia subduction ground-motion models (GMMs), considered for the 2023 US National Seismic Hazard Model (NSHM) update, by comparing observations to model predictions. The observations comprise regional recordings from intraslab earthquakes, including contributions from 2021 and 2022 events in southern Cascadia and global records from interface earthquakes. Since the 2018 NSHM update, new GMMs for Cascadia have been published by the Next Generation Attenuation (NGA)-Subduction Project that require independent evaluation. In the regional intraslab comparisons, we highlight a characteristic frequency dependence for Cascadia data, with short periods having lower ground motions and longer periods being comparable to other subduction zones. We evaluate differences in northern and southern Cascadia and find that the NGA-Subduction GMMs developed using southern Cascadia data perform better in this region than the model that did not consider these data. We compare ground-motion variability in Cascadia with the NGA-Subduction model predictions and find differences at short periods (T = 0.1 s) due to the use of global versus regional data in the development of these models. Moreover, the within-event component of aleatory variability from the GMMs overpredicts the standard deviation of Cascadia recordings at very short periods (T < 0.05 s). Using global interface earthquakes as a proxy to evaluate the Cascadia GMMs, we find long-period overprediction from a simulation-based GMM and some of the empirical GMMs. When comparing recent observations, we find a similar misfit to GMMs and the 2010 and 2022 Ferndale earthquakes. Finally, we observe different basin amplification factors arising in different subsets of the data, which indicate that differences in basin factors between empirical GMMs could arise from the data selection choices by the developers. As part of evaluating the regional basin terms, we apply basin amplification factors from the magnitude 9 Cascadia earthquake simulations to the empirical GMMs for interface earthquakes. The comparisons presented in this study indicate that the NGA-Subduction GMMs for Cascadia perform well relative to observations and older subduction GMMs. [ABSTRACT FROM AUTHOR]

The US Geological Survey National Seismic Hazard Models (NSHMs) are used to calculate earthquake ground-shaking intensities for design and rehabilitation of structures in the United States. The most recent 2014 and 2018 versions of the NSHM for the conterminous United States included major updates to ground-motion models (GMMs) for active and stable crustal tectonic settings; however, the subduction zone GMMs were largely unchanged. With the recent development of the next generation attenuation-subduction (NGA-Sub) GMMs, and recent progress in the utilization of "M9" Cascadia earthquake simulations, we now have access to improved models of ground shaking in the US subduction zones and the Seattle basin. The new NGA-Sub GMMs support multi-period response spectra calculations. They provide global models and regional terms specific to Cascadia and terms that account for deep-basin effects. This article focuses on the updates to subduction GMMs for implementation in the 2023 NSHM and compares them to the GMMs of previous NSHMs. Individual subduction GMMs, their weighted averages, and their impact on the estimated mean hazard relative to the 2018 NSHM are discussed. The updated logic trees include three of the new NGA-Sub GMMs and retain two older models to represent epistemic uncertainty in both the median and standard deviation of ground-shaking intensities at all periods of interest. Epistemic uncertainty is further represented by a three-point logic tree for the NGA-Sub median models. Finally, in the Seattle region, basin amplification factors are adjusted at long periods based on the state-of-the-art M9 Cascadia earthquake simulations. The new models increase the estimated mean hazard values at short periods and short source-to-site distances for interface earthquakes, but decrease them otherwise, relative to the 2018 NSHM. On softer soils, the new models cause decreases to the estimated mean hazard for long periods in the Puget Lowlands basin but increases within the deep Seattle portion of this basin for short periods relative to the 2018 NSHM. [ABSTRACT FROM AUTHOR]

Low-frequency earthquakes (LFEs) are small-magnitude earthquakes that are depleted in high-frequency content relative to traditional earthquakes of the same magnitude. These events occur in conjunction with slow slip events (SSEs) and can be used to infer the space and time evolution of SSEs. However, because LFEs have weak signals, and the methods used to identify them are computationally expensive, LFEs are not routinely cataloged in most places. Here, we develop a deep-learning model that learns from an existing LFE catalog to detect LFEs in 14 years of continuous waveform data in southern Vancouver Island. The result shows significant increases in detection rates at individual stations. We associate the detections and locate them using a grid search approach in a 3D regional velocity model, resulting in over 1 million LFEs during the performing period. Our resulting catalog is consistent with a widely used tremor catalog during periods of large-magnitude SSEs. However, there are time periods where it registers far more LFEs than the tremor catalog. We highlight a 16-day period in May 2010, when our model detects nearly 3,000 LFEs, whereas the tremor catalog contains only one tremor detection in the same region. This suggests the possibility of hidden small-magnitude SSEs that are undetected by current approaches. Our approach improves the temporal and spatial resolution of the LFE activities and provides new opportunities to understand deep subduction zone processes in this region.

Episodic Tremors and Slip (ETS) are highly susceptible and sensitive to external stress perturbations. The tidal and remote triggering phenomena of tremors are well-documented globally, however, the significance of the delayed triggering mechanism remains elusive. In this paper, the possibilities of the tremor modulation by the Lamb waves induced from the Hunga Tonga-Hunga Ha’apai volcanic eruption (SW Pacific) on 15 January 2022 have been explored. The increasing activity of tremors after eruption has been explored at the study region of Cascadia subduction zone where such tremor do not correlate with tidal stress perturbations or remote triggering by far-off or near-by earthquakes. The increasing tremor activities are observed during the propagation of the Lamb wave cycles (L1, L2, L3, L4) and more interestingly during inter-ETS period. From the seismic waveform analysis, we observe two coherent packets of teleseismic energy on 15 January 2022, which corresponds to arrival of surface and Lamb waves, respectively. The delayed triggering of tremors may be linked either with the teleseismic surface waves or Lamb waves from the volcanic explosion, or both. Although we cannot rule out coincidence, the delayed triggering by Lamb waves appears to be consistent with magnitude-dependent time delay and 2-D coupled pore pressure induced diffusion model.

Uplift of the overriding plate at a subduction zone denotes interseismic strain accumulation, which is subsequently released during a megathrust earthquake. Although most interseismic strain is thought to be released elastically, observations of uplifted coastal regions at subduction zones worldwide indicate that some strain may result in permanent uplift. The Grays Harbor and Willapa Bay (Washington, USA) coastal region of the Cascadia subduction zone hosts flights of marine terraces testifying to late Pleistocene rock uplift. Our new detailed mapping of the marine terraces recognizes nine new units, including estuarine and fluvial sediments. Luminescence dating, relative age based on soil maturity and terrace elevation, and an evaluation of previous ages from fossil shells collectively constrain the probable ages of three estuarine units to sea-level high stands during Marine Isotope Stages 5a, 5c, and 5e. We estimate an average uplift rate of 0.4 ± 0.1 mm/yr for the terraced estuarine units, consistent with other Pleistocene uplift and incision rates in Cascadia. When compared with observed interseismic vertical deformation, these rates suggest that about one-tenth of interseismic strain may become permanent. The values are permissible within the uncertainties of uplift based on regional estimates of interseismic vertical strain rates and of coseismic subsidence. [ABSTRACT FROM AUTHOR]

The fore-arc of the southern Cascadia subduction zone (CSZ), north of the Mendocino triple junction (MTJ), is home to a network of Quaternary-active crustal faults that accumulate strain due to the interaction of the North American, Juan de Fuca (Gorda), and Pacific plates. These faults, including the Little Salmon and Mad River fault (LSF and MRF) zones, are located near the most populated parts of California's north coast and show paleoseismic evidence for three slip events of several-meter scale in the past 1700 yr. However, the geodetic slip rates of these faults are poorly constrained. In this work, we analyze a new compilation of interseismic geodetic velocities from Global Navigation Satellite Systems, leveling, and tide gauge data near the MTJ to constrain present-day slip deficit rates on upper-plate faults and coupling on the megathrust. We construct Green's functions for interseismic slip deficit for discrete faults embedded in an elastic plate overlying a viscoelastic mantle. We then use a constrained least-squares inversion to determine best-fitting slip rates on the major faults and investigate slip rate trade-offs between faults. Results indicate that the LSF and MRF systems together accumulate 4-5 mm/yr of reverse-slip deficit, although their separate slip rates cannot be determined independently. Modeling of the horizontal and vertical velocities suggests that the southernmost CSZ is coupled interseismically to deeper than 25 km depth. We also find that 6-17 mm/yr of right-lateral slip deficit extends north of the MTJ and into the southern Cascadia fore-arc. These results reinforce the notion that both the southernmost Cascadia megathrust and the smaller fore-arc faults above it contribute to regional seismic hazard. [ABSTRACT FROM AUTHOR]

Because splay faults branch at a steep dip angle from the plate-boundary décollement in an accretionary wedge, their coseismic displacement can potentially result in larger tsunamis with distinct characteristics compared to megathrust-only fault ruptures, posing an enhanced hazard to coastal communities. Elsewhere, there is evidence of coseismic slip on splay faults during many of the largest subduction zone earthquakes, but our understanding of potentially active splay faults and their hazards at the Cascadia subduction zone remains limited. To identify the most recently active splay faults at Cascadia, we conduct stratigraphic and structural interpretations of near-surface deformation in the outer accretionary wedge for the ~400 km along-strike length of the landward vergence zone. We analyze recently acquired high-frequency sparker seismic data and crustal-scale multi-channel seismic data to examine the record of deformation in shallow slope basins and the upper ~1 km of the surrounding accreted sediments and to investigate linkages to deeper décollement structure. We present a new fault map for widest, most completely locked portion of Cascadia from 45 to 48°N latitude, which documents the distribution of faults that show clear evidence of recent late Quaternary activity. We find widespread evidence for active splay faulting up to 30 km landward of the deformation front, in what we define as the active domain, and diminished fault activity landward outside of this zone. The abundance of surface-deforming splay faults in the active outer wedge domain suggests Cascadia megathrust events may commonly host distributed shallow rupture on multiple splay faults located within 30 km of the deformation front.

Tsunami hazard analysis is an essential step for designing buildings and infrastructure and for safeguarding people and assets in coastal areas. Coastal communities on Vancouver Island are under threat from the Cascadia megathrust earthquakes and tsunamis. Due to the deterministic nature of current megathrust earthquake scenarios, probabilistic tsunami hazard analysis has not been conducted for the coast of Vancouver Island. To address this research gap, this study presents a new probabilistic tsunami hazard model for Vancouver Island from the Cascadia megathrust subduction events. To account for uncertainties of the possible rupture scenarios more comprehensively, time-dependent earthquake occurrence modeling and stochastic rupture modeling are integrated. The time-dependent earthquake model can capture a multi-modal distribution of inter-arrival time data on the Cascadia megathrust events. On the other hand, the stochastic rupture model can consider variable fault geometry, position, and earthquake slip distribution within the subduction zone. The results indicate that the consideration of different inter-arrival time distributions can result in noticeable differences in terms of site-specific tsunami hazard curves and uniform tsunami hazard curves at different return period levels. At present, the use of the one-component renewal model tends to overestimate the tsunami hazard values compared to the three-component Gaussian mixture model. With the increase in the elapsed time since the last event and the duration of tsunami hazard assessment, the differences tend to be smaller. Inspecting the regional variability of the tsunami hazards, specific segments of the Vancouver Island coast are likely to experience higher tsunami hazards due to the directed tsunami waves from the main subduction zone and due to the local underwater topography.

A zone of significant high-amplitude magnetic anomalies is observed without a comparable gravity high along the Cascadia margin and is spatially correlated with the low-velocity fore-arc mantle wedge. This wedge is interpreted to be serpentinized fore-arc mantle and is further considered to be the main source of the high-amplitude magnetic anomalies. To test this hypothesis, the magnetization-density ratio (MDR) is estimated along the Cascadia margin to highlight the physical characteristics of serpentinization (reduced density and increased magnetization). Interestingly, high MDR values are found only in central Oregon, where slab dehydration and fore-arc mantle serpentinization (50–60% serpentinization) are inferred in conjunction with sparse seismicity. This result may indicate either poorly serpentinized fore-arc mantle (low degree of serpentinization) or that the fore-arc mantle is deeper than the Curie temperature isotherm for magnetite in northern and southern Cascadia. This finding means that magnetic anomaly highs and serpentinized fore-arc mantle may not always be correlated in subduction zones. On the other hand, the MDR pattern suggests segmentation of the Cascadia subduction zone, which is consistent with several previous geological and geophysical observations. [ABSTRACT FROM AUTHOR]

Tsunami hazard analysis is an essential step for designing buildings and infrastructure and for safeguarding people and assets in coastal areas. Coastal communities on Vancouver Island are under threat from the Cascadia megathrust earthquakes and tsunamis. Due to the deterministic nature of current megathrust earthquake scenarios, probabilistic tsunami hazard analysis has not been conducted for the coast of Vancouver Island. To address this research gap, this study presents a new probabilistic tsunami hazard model for Vancouver Island from the Cascadia megathrust subduction events. To account for uncertainties of the possible rupture scenarios more comprehensively, time-dependent earthquake occurrence modeling and stochastic rupture modeling are integrated. The time-dependent earthquake model can capture a multi-modal distribution of inter-arrival time data on the Cascadia megathrust events. On the other hand, the stochastic rupture model can consider variable fault geometry, position, and earthquake slip distribution within the subduction zone. The results indicate that the consideration of different inter-arrival time distributions can result in noticeable differences in terms of site-specific tsunami hazard curves and uniform tsunami hazard curves at different return period levels. At present, the use of the one-component renewal model tends to overestimate the tsunami hazard values compared to the three-component Gaussian mixture model. With the increase in the elapsed time since the last event and the duration of tsunami hazard assessment, the differences tend to be smaller. Inspecting the regional variability of the tsunami hazards, specific segments of the Vancouver Island coast are likely to experience higher tsunami hazards due to the directed tsunami waves from the main subduction zone and due to the local underwater topography. [ABSTRACT FROM AUTHOR]

We compare disturbances from the historic portion of the sedimentary record from Lower Squaw Lake, Oregon, to the historic record of events from the region to (1) determine if the lake records Cascadia megathrust earthquakes, and (2) if sediment deposits can be differentiated by disturbance type. We use the sedimentological characteristics and geochemically inferred provenance of the deposits (labelled A-J) from the historic portion (post 1650 CE) of the record to discriminate between types of deposits. We show that earthquake-triggered deposits are complex and flood deposits are simpler but vary depending on flood characteristics. Disturbance deposit J dates close to 1700 CE (1680-1780 CE) through multiple approaches. This deposit suspected to result from the magnitude (M) 8.8-9.2 1700 CE Cascadia megathrust earthquake is composed of unusually well-sorted, normally graded, medium-grained silt derived from distal rocks in the upper watershed. The silt grades upward, increasing in organic content forming a long, organic-rich tail. Load structures of silt into the organic-rich sediment below suggest rapid deposition. In contrast, a deposit attributed to the ~M7.0 1873 CE intraplate earthquake is a normally graded, medium-grained, watershed-sourced silt overlain by an organic tail and preceded by a lake-wide deposit interpreted as a wall failure from an earthquake that caused the landslide dam to fail. These results suggest that inland lakes can be sensitive recorders of earthquakes, and that it is possible to discriminate between plate margin and other types of earthquakes, and floods. [ABSTRACT FROM AUTHOR]

We infer a ~ 2,700-year history of Cascadia megathrust and other earthquakes from two small mountain lakes located 100 km inland of the coast near the California/Oregon border. We use the characteristics of disturbance deposits in the historic portion of the sediment cores from the lower lake to identify a deposit from the 1700 CE Cascadia earthquake (deposit J). This deposit is composed of light-coloured silt (indicating it is enriched in watershed-sourced sediment), without visible mica grains (which would indicate a lake bedrock source), organic grading of the deposit tail, and a basal contact with evidence of rapid loading. Seven deposits downcore have some of the characteristics of deposit J. An age-depth model suggests that the five deposits most similar to deposit J (including deposit J) are temporal correlatives to the largest margin-wide marine turbidite event deposits from Goldfinger et al., 2012, (T1, T2, T3, T4, T5 and T6), whereas the two deposits with some of the characteristics are potential correlatives of smaller turbidites T5a and T5b. Other thinner deposits are temporal correlatives of T2a and T3a and other smaller deposits of uncertain origin. Lake core physical property data can be correlated to those from other regional lake records and offshore cores. These results suggest that small Cascadia lakes with sufficient sedimentation rates (~ 1–2 cm/decade) with mixed clastic and organic sedimentation may be good recorders of earthquakes, that subduction earthquake deposits are different from those from other types of earthquake deposits and deposits from other types of disturbances, such as floods. [ABSTRACT FROM AUTHOR]

Seismic wave amplitudes have tremendous sensitivity to subduction structure; however, they are affected by attenuation, scattering and focusing, and have therefore been sparsely used compared with traveltimes. We measure and model teleseismic body wave amplitudes recorded at a dense broadband array in the Washington Cascades. These data show anomalous amplitude variations with complex azimuthal dependence at frequencies as low as 0.05 Hz, accompanied by significant multipathing. We demonstrate using spectral‐element numerical simulations that focusing of the teleseismic wavefield by the Juan de Fuca slab is responsible for some of the amplitude anomalies. The focusing effects can contaminate the apparent differential attenuation measurements and produce at least 20% of the inferred attenuation signal. Our results indicate that the amplitudes are sensitive to the subducting slab geometry and subduction structure, and can be used to refine seismic images. Ubiquitous and consistent amplitude anomalies are observed along the arc, suggesting that the Juan de Fuca slab may be continuous from Canada to northern California. Plain Language Summary: Seismic wave amplitudes can provide critical information on subsurface structure; however, they have been sparsely used compared with traveltimes. Here, we measure and model teleseismic body wave amplitudes recorded from distant earthquakes at a dense seismic array in the Washington Cascades. We observed unexpectedly large variations in amplitudes, by a factor of two, for long‐period signals where usually amplitudes do not vary over small distances. Based on the waveform modeling, we conclude that these amplitude changes are caused by a focusing effect, in which the high‐wavespeed subducting Juan de Fuca plate as a lens to amplify some signals. Similar focusing effects are observed along the Cascadia arc, mapping a continuous subducting Juan de Fuca plate. Key Points: Anomalous amplitude variations with complex azimuthal dependence at 20 s period are observed by the imaging Magma Under mount St. Helens arrayFocusing of teleseismic wavefield by Juan de Fuca slab is responsible for the observed amplitude anomaliesFocusing can account for ≥20% of the apparent differential attenuation measurements [ABSTRACT FROM AUTHOR]

The boundary between the overriding and subducting plates is locked along some portions of the Cascadia subduction zone. The extent and location of locking affects the potential size and frequency of great earthquakes in the region. Because much of the boundary is offshore, measurements on land are incapable of completely defining a locked zone in the up‐dip region. Deformation models indicate that a record of seafloor height changes on the accretionary prism can reveal the extent of locking. To detect such changes, we have initiated a series of calibrated pressure measurements using an absolute self‐calibrating pressure recorder. A piston‐gauge calibrator under careful metrological considerations produces an absolutely known reference pressure to correct seafloor pressure observations to an absolute value. We report an accuracy of about 25 ppm of the water depth, or 0.02 kPa (0.2 cm equivalent) at 100 m to 0.8 kPa (8 cm equivalent) at 3,000 m. These campaign survey‐style absolute pressure measurements on seven offshore benchmarks in a line extending 100 km westward from Newport, Oregon from 2014 to 2017 establish a long‐term, sensor‐independent time series that can, over decades, reveal the extent of vertical deformation and thus the extent of plate locking and place initial limits on rates of subsidence or uplift. Continued surveys spanning years could serve as calibration values for co‐located or nearby continuous pressure records and provide useful information on possible crustal deformation rates, while epoch measurements spanning decades would provide further limits and additional insights on deformation. Plain Language Summary: The Cascadia subduction zone has produced large earthquakes and tsunamis whose potential size and interval is affected by the amount and distribution of locking between the tectonic plates. A large portion of the subduction zone is offshore, where typical land‐ and satellite‐based methods are ineffective at measuring crustal changes. Seafloor water pressure observations can be used to measure height changes, but pressure gauges inherently drift at rates typically greater than the expected vertical seafloor deformation rates. The absolute self‐calibrating pressure recorder (ASCPR) measures the true, absolute, sensor‐independent seafloor pressure by addressing and correcting sources of error caused by the internal piston gauge calibrator and recording pressure gauges. The accuracy of our measurements is about 25 ppm of the water depth, or about 2.5 cm of height per 1,000 m of water. Campaign survey‐style measurements using the ASCPR at seven benchmarks off the coast of Newport, Oregon from 2014 to 2017 establish a long‐term record of absolute measurements that can be referenced by studies decades or more in the future or can estimate and correct drift of nearby continuous pressure gauges. Continued measurements can provide insights on seafloor deformation and thus locking in Cascadia. Key Points: Campaign‐style surveys of absolute calibrated seafloor pressure measurements were made in the Cascadia subduction zone from 2014 to 2017These sensor‐independent measurements act as long‐term, absolute reference values that can be used for future vertical deformation studiesWe document and quantify the sources of error in the technique with a total uncertainty of ∼25 ppm, or ∼0.25 kPa per 1,000 m water depth [ABSTRACT FROM AUTHOR]

The Queen Charlotte triple junction/Explorer microplate region offshore British Columbia, Canada, is marked by poorly understood and rapidly evolving microplate tectonics. Although the region hosts abundant seismicity, it has received relatively scant attention in recent years due to its remote, offshore location. We use the Regressive ESTimator (REST) algorithm to generate a new catalog of automatically detected earthquakes from 1995 to 2021, which, when merged with the existing Geological Survey of Canada catalog, yields the most extensive seismicity data set offshore British Columbia to date. We apply double‐difference relocation to these events and perform stress inversions using moment tensors for subregions within the study area. Our results confirm and extend previous models of microplate deformation processes. We suggest the Revere‐Dellwood‐Queen Charlotte fault system has evolved as a NW‐migrating, pull‐apart system between Haida Gwaii and the Explorer ridge that obeys global length/width scaling and whose bathymetric expression is influenced by volcanism plausibly induced by interaction with the Kodiak‐Bowie hotspot. Seismicity within the Explorer microplate is dominated by prominent, northeast‐trending lineations that emanate from the Sovanco fracture zone and parallel the Nootka fault zone. Alignment of these features with spreading structures that bound the microplate suggests that its breakup is controlled primarily by a strength fabric inherited at spreading ridges. Stress inversions are dominated by near‐vertical intermediate compressive stress reflecting the dominance of strike‐slip faulting. Stress varies systematically between transpression to the north along southern Haida Gwaii and seafloor spreading to the south along the Juan de Fuca ridge. Plain Language Summary: Offshore British Columbia, the Pacific, North America, and Juan de Fuca tectonic plates meet in a broad zone called a triple junction. As the plates adjust to changing relative plate motions, their present‐day deformation is recorded by seismicity. We expand the existing catalog of earthquakes offshore with an automatic earthquake detection algorithm and estimate their locations. We also analyze dominant stress patterns to provide insight into how the triple junction and Explorer plate are changing and moving. Extension between overlapping faults under prevailing compression gives rise to the observed volcanism and seismicity patterns. The length of overlap relative to the width follows a well established scaling. Along the Revere‐Dellwood fault, the seismicity pattern transitions from concentrated earthquakes along the fault with some compression at the continental margin, to more distributed seismicity and associated volcanism in the overlap region between it and the Queen Charlotte fault. Structures generated at the oceanic spreading ridges may control deformation as evidenced by our earthquake locations. Within the Explorer plate, the locations reveal NE‐trending lineations aligned with ridges generated by seafloor spreading. Seismicity provides a means to analyze deformation in the Explorer plate where the seafloor is obscured by heavy sedimentation. Key Points: Merged manual and automatic catalogs yield 18,441 double‐difference‐relocated earthquakes in the triple junction and Explorer regionsSeismicity distribution explained by migrating pull‐apart mechanism that yields insights into triple junction evolutionAlignment of seismicity in Explorer microplate with bordering ridges suggests breakup controlled by ocean‐spreading fabric [ABSTRACT FROM AUTHOR]

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 M 9 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 M 6.8 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. [ABSTRACT FROM AUTHOR]

This study presents new stochastic source models for the Cascadia subduction earthquakes in the Pacific Northwest, which can trigger massive tsunamis along the shoreline of Vancouver Island. An extensive set of 5,000 stochastic source models is generated for the moment magnitude ranges between 8.1 and 9.1, and regional tsunami hazard simulations are performed at the grid resolution of 270 m. The results from the stochastic tsunami simulations are characterized by evaluating the regional tsunami hazard metric that is based on the geometric mean of the maximum wave heights along the Vancouver Island coast. Subsequently, using the probability distribution of the regional tsunami hazard parameter, representative source models are identified by capturing the average as well as rare rupture cases and then detailed tsunami hazard results, such as maximum wave height maps and wave profiles at specific locations, are examined. Numerical results highlight the directivity effects of tsunami generation and wave propagation on tsunami hazards along the Canadian Pacific coast and the earthquake source characterizations in terms of fault geometry and earthquake slip distribution. The developed source models and tsunami simulation results serve as the first step for performing probabilistic tsunami hazard analysis for the Cascadia subduction zone. [ABSTRACT FROM AUTHOR]

Abstract Several tectonic processes combine to produce the crustal deformation observed across the Cascadia margin: (1) Cascadia subduction, (2) the northward propagation of the Mendocino Triple Junction (MTJ), (3) the translation of the Sierra Nevada–Great Valley (SNGV) block along the Eastern California Shear Zone–Walker Lane and, (3) extension in the northwestern Basin and Range, east of the Cascade Arc. The superposition of deformation associated with these processes produces the present-day GPS velocity field. North of ~ 45° N observed crustal displacements are consistent with inter-seismic subduction coupling. South of ~ 45° N, NNW-directed crustal shortening produced by the Mendocino crustal conveyor (MCC) and deformation associated with SNGV-block motion overprint the NE-directed Cascadia subduction coupling signal. Embedded in this overall pattern of crustal deformation is the rigid translation of the Klamath terrane, bounded on its north and west by localized zones of deformation. Since the MCC and SNGV processes migrate northward, their impact on the crustal deformation in southern Cascadia is a relatively recent phenomenon, since ~ 2 –3 Ma.

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. [ABSTRACT FROM AUTHOR]

This paper presents the use of tsunami evacuation drills within a coastal community in the Cascadia Subduction Zone (CSZ) to better understand evacuation behaviors and thus to improve tsunami evacuation preparedness and resilience. Evacuees' spatial trajectory data were collected by Global Navigation Satellite System (GNSS) embedded in mobile devices. Based on the empirical trajectory data, probability functions were employed to model people's walking speed during the evacuation drills. An Evacuation Hiking Function (EHF) was established to depict the speed–slope relationship and to inform evacuation modeling and planning. The regression analysis showed that evacuees' speed was significantly negatively associated with slope, time spent during evacuation, rough terrain surface, walking at night, and distance to destination. We also demonstrated the impacts of milling time on mortality rate based on participants' empirical evacuation behaviors and a state-of-the-art CSZ tsunami inundation model. Post-drill surveys revealed the importance of the drill as an educational and assessment tool. The results of this study can be used for public education, evacuation plan assessment, and evacuation simulation models. The drill procedures, designs, and the use of technology in data collection provide evidence-driven solutions to tsunami preparedness and inspire the use of drills in other types of natural disasters such as wildfires, hurricanes, volcanoes, and flooding. [ABSTRACT FROM AUTHOR]

Slow 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. Plain Language Summary: Subduction zones, where tectonic plates converge, are responsible for the world's largest earthquakes and present significant seismic risk to the surrounding regions. The Cascadia Subduction Zone, which extends from northern California to Vancouver Island, has not produced a great earthquake since 1700, suggesting that strain has been accumulating for over 300 years. Previous studies suggest that the shallow (<30 km depth) portion of the fault has been coupled, allowing strain to accumulate, and slow slip events (SSEs), a form of slow strain release that does not produce the shaking that typically occurs during an earthquake, have been detected on the fault at greater depths. Here, we test the assumption that fault coupling is restricted to the shallow portion of the fault interface and examine the role that SSEs play in the strain accumulation and release cycle. Because the magnitude of great earthquakes is related to the area of the fault that ruptures, gaining a better understanding of the interaction between SSEs and coupling is crucial to understanding the seismic hazard posed by the Cascadia Subduction Zone. Key Points: We estimate pure interseismic velocities reflecting subduction zone coupling by removing slow slip effects from GPS time seriesPure interseismic coupling may extend deeper than 30 km, providing a source of stress released by slow slip eventsSlow slip events below 30 km depth on the Cascadia Subduction Zone partially release deep interseismic slip deficit [ABSTRACT FROM AUTHOR]

We employ an automatic earthquake detection algorithm to seismic waveforms recorded between the years 2000 and 2020 in southwest British Columbia. 32,121 events which possess at least three paired P‐ and S‐wave arrival times are located, compared to 21,538 seismic events in the existing Geologic Survey of Canada catalog. This augmented catalog is employed for double‐difference seismic tomography across Vancouver Island, with particular focus on the Nootka Fault zone (NFZ). The NFZ is a transform boundary that separates the Juan de Fuca and Explorer plates in a zone of distributed left‐lateral strike‐slip faulting. Tomographic results indicate that a double seismic zone exists within the NFZ that parallels Vp/Vs structure typically observed in subduction zones. Specifically, a dipping high Vp/Vs layer is underlain by reduced Vp/Vs material. Structural complexities are revealed by three Mw > 6 events and their aftershocks. The 2004 Mw 6.3 and 2011 Mw 6.4 events are interpreted to reside southeast of the NFZ within the Juan de Fuca plate at depths as shallow as 20 km, and the 2014 Mw 6.6 event is interpreted to reside within oceanic lithosphere of the NFZ at depths >35 km. Vp/Vs structure further indicates that underthrust oceanic lithosphere of the Explorer plate extends to 20 km depth beneath Brooks Peninsula. Our results support the hypothesis that the NFZ represents a structurally independent slice of oceanic lithosphere that exhibits (a) a more northerly trajectory than typically depicted, and (b) increased plate curvature when compared to either the Juan de Fuca or Explorer plates. Plain Language Summary: A newly determined earthquakes catalog is leveraged to study seismic velocity structure in British Columbia. The Nootka Fault zone (NFZ), the boundary that separates the Juan de Fuca and Explorer oceanic plates, is interpreted as a structurally complex region based on earthquake locations and a diagnostic velocity structure observed in subduction zones around the world. In particular, landward‐dipping layering in P‐to‐S velocity ratio parallels earthquake concentrations to depths >30 km beneath the continental slope and suggests that the NFZ comprises a structurally independent sliver of oceanic material. Three M > 6 earthquakes from 2004, 2011, and 2014 and their associated aftershocks provide constraints that support a recent re‐definition of the NFZ – one with a more northerly trajectory that intersects Vancouver Island south of Brooks Peninsula. To the north and south of the NFZ, more shallowly dipping Explorer and Juan de Fuca plates are interpreted from the tomographic model, respectively. Supporting evidence for a steeply dipping NFZ is considered from independent studies offshore Vancouver Island, and we posit that the 2014 Mw 6.6 event may have reactivated a plate‐bending fault in response to increased plate curvature. Key Points: A new seismic event catalog is compiled for southwest British ColumbiaTomographic results suggest the Nootka Fault zone represents a structurally independent region of oceanic lithosphereVp/Vs structure indicates that underthrust Explorer plate is present beneath Brooks Peninsula [ABSTRACT FROM AUTHOR]

The Cascadia Trench is stretching along the convergent plate boundaries of Pacific Plate, North America Plate and Juan De Fuca Plate. It is an important geomorphological structural feature in the north-east Pacific Ocean. The aim of the paper is to analyse the geomorphology of the Cascadia Trench west of Vancouver Island (Canada and USA) using the GMT cartographic scripting toolset. The unique geomorphological feature of the Cascadia Trench is that the thick sediment layer completely obscures the subduction zone and abyssal hills. This results in the asymmetric profile in the cross-section of the trench. Bathymetric data were extracted from the GEBCO 2019 dataset (15 arc-second grid), sediment thickness by the GlobSed dataset. Due to the dominance of high sedimentary rate and complexity of the tectonic processes and geologic settings, Cascadia Trench develops very specific asymmetric geomorphic shape comparing to the typical V-form. The results of the geomorphic modelling show that eastern side of the trench has a gentle curvature (slope: 35.12°), partially stepped, due to the tectonic movements and faults. The opposite, oceanward side is almost completely leveled. The trench is narrow with maximal depth at the selected segment -3489 m and for the whole dataset -6201 m. The most repetitive depth is in a range -2500 to -2400 m (267 samples) and -2500 to -2600 m (261 samples). The bottom is mostly flat due to the high sedimentation rates indicating the accumulative leveling processes. Marine free-air gravity anomalies along the Cascadia Subduction Zone are characterized by weakly positive values (20 mGal) increasing rapidly in the zone of the continental slope (>200 mGal), which is associated with a decrease in thickness of the Earth’s crust.

Within the fore‐arc of the Cascadia Subduction Zone, there are significant along‐strike differences in the orientation of splay faults, sediment consolidation, and fault roughness. Here, we use dynamic rupture simulations of megathrust earthquakes on different realizations of a fault system that incorporate fore‐arc properties representative of offshore Oregon and Washington to estimate how splay faults may behave in future megathrust earthquakes in Cascadia. While splay faults were activated in all of our simulations, splay orientation is a primary control on slip amplitude. Seaward vergent faults accommodate significant amounts of slip resulting in large seafloor uplift and significantly larger tsunami amplitudes. For example, our median tsunami heights including splay faults are about a factor of two larger than those that did not include splay fault deformation. We suggest that there is an urgent need to revisit existing approaches to tsunami hazard assessment in Cascadia to include the influence of splay faults. Plain Language Summary: The Cascadia subduction zone has hosted many great earthquakes in the past. Due to the potential of generating great earthquakes, this zone poses a huge earthquake and tsunami hazard. The main fault (or Cascadia megathrust) responsible for generating great earthquakes is about ∼1,000‐km long and is inferred to have variation in the geometrical and mechanical properties. One such difference is the difference between properties of the "forearc" region. The "forearc" is the shallow subsurface region on the landward side adjacent to the Cascadia megathrust. The orientation of the secondary faults (i.e., splays) is different in the forearc for the offshore Washington and Oregon region. Similarly, the consolidation of sediments within the forearc is also different for the Washington and Oregon region. In this study, we performed earthquake and tsunami simulation to understand how the difference in these forearc properties affects the tsunami hazard of the region. We found that the tsunami hazard in the Washington region is higher as compared to the Oregon region due to higher magnitude slip on the splay and megathrust. We find that this difference is mainly due to the difference in the orientation of splay faults. Key Points: Dynamic earthquake rupture simulations of the Cascadia Subduction Zone (CSZ) offshore Washington and Oregon region to investigate updip rupture behaviorLarger tsunami amplitudes observed for the Oregon region suggest a large tsunami hazard in central and southern CascadiaDifferent orientations of the splays within the CSZ forearc result in differences in the amount of slip and tsunami heights [ABSTRACT FROM AUTHOR]

Abstract Shear‐wave splitting observations of SKS and SKKS phases have been used widely to map azimuthal anisotropy, as caused by the occurrence of olivine, to constrain the dominant directions of upper mantle deformation. While SK(K)S splitting measurements at individual seismic stations are often averaged before interpretation, it is useful to consider additional information, for example, based on the variation of splitting parameters with azimuth due to the non‐vertical incidence of core‐phases. These constraints in theory enable a differentiation between various types of olivine and may allow us to infer otherwise poorly known upper mantle parameters such as stress, temperature, and water content. In this study, we predict the azimuthal variation of splitting parameters for A‐, C‐, and E‐type olivine fabrics and match them with observations from the High Lava Plains, Northwestern Basin and Range, and Western Yellowstone Snake River Plain in the Pacific Northwest US. This helps to constrain the amount of water in the upper mantle in the back‐arc of the Cascadia subduction zone, known for its consistent E‐W oriented seismic anisotropy, and particularly large splitting delay times. Our investigation renders the C‐type olivine mechanism improbable for this location; A‐ and E‐type fabrics match the observations, although differentiating between them is difficult. However, the agreement of the amplitude of backazimuthal variation of the fast orientation, plus the potential to explain large splitting delay times, suggest the occurrence of E‐type olivine, and thus the likely presence of a moderately hydrated upper mantle beneath Cascadia's back‐arc.

Southwestern British Columbia (BC) is exposed to the highest seismic hazard in Canada. Ocean Networks Canada (ONC) has developed an Earthquake Early Warning (EEW) system for the region. The system successfully utilizes offshore cabled seismic instruments in addition to land-based seismic sensors and integrates displacement data from Global Navigation Satellite Systems (GNSS). The seismic and geodetic data are processed in real-time onsite at 40 different stations along the coast of BC. The processing utilizes P-wave and S-wave detection algorithms for epicentre calculations as well as incorporation of geodetic and seismic displacement data into a Kalman filter to provide magnitude estimates. The system is currently in its commissioning phase and has successfully detected over 60 earthquakes since being deployed in October 2018. To increase the coverage of the EEW system, we are in the process of incorporating detection parameters from neighbouring networks (e.g., the Pacific Northwest Seismic Network (PNSN)) to provide additional information for future event notifications.

In 2004 , the provincial government of British Columbia in Canada initiated a seismic retrofit program to quantify the seismic risk and to implement a seismic mitigation plan over 750 school buildings located in high seismic hazard regions of the province. The current phase of the program focuses on conducting a series of full‐scale shake‐table tests of a retrofitted two‐story wood‐frame school building prototype as part of the performance‐based Seismic Retrofit Guidelines (SRG). This paper presents the numerical modeling and seismic assessment of the tested building. A detailed two‐dimensional model and a computationally efficient three‐dimensional model were developed. By performing nonlinear dynamic analyses, both models predicted the building responses accurately when compared with the experimental tests in terms of roof drift, energy dissipation, and global hysteresis behavior. The retrofitted school building showed a large overstrength that mainly came from the unblocked walls. Empirical equations based on the FEMA P‐807 guidelines and a simplified method developed in the SRG, called Toolbox Approach were investigated for estimating its overstrength and ultimate capacity. Finally, following a performance‐based tri‐hazard probabilistic methodology proposed in the SRG, the seismic risk of the building was assessed by performing incremental dynamic analysis using 30 ground motion records that were selected based on a multiple event‐based conditional mean spectrum (CMS) approach. The results indicated that the subduction motions posed higher probability of drift exceedance and collapse risk of the retrofitted school building than crustal and subcrustal motions. [ABSTRACT FROM AUTHOR]

Paleo-earthquakes along the Cascadia subduction zone inferred from offshore sediments and Japan coastal tsunami deposits approximated to M9+ and ruptured the entire margin. However, due to the lack of modern megathrust earthquake records and general quiescence of subduction fault seismicity, the potential megathrust rupture scenario and influence of downdip limit of the seismogenic zone are still obscure. In this study, we present a numerical simulation of Cascadia subduction zone earthquake sequences in the laboratory-derived rate-and-state friction framework to investigate the potential influence of the geodetic fault locking on the megathrust sequences. We consider the rate-state friction stability parameter constrained by geodetic fault locking models derived from decadal GPS records, tidal gauge and levelling-derived uplift rate data along the Cascadia margin. We incorporate historical coseismic subsidence inferred from coastal marine sediments to validate our coseismic rupture scenarios. Earthquake rupture pattern is strongly controlled by the downdip width of the seismogenic, velocity-weakening zone and by the earthquake nucleation zone size. In our model, along-strike heterogeneous characteristic slip distance is required to generate margin-wide ruptures that result in reasonable agreement between the synthetic and observed coastal subsidence for the AD 1700 Cascadia Mw∼9.0 megathrust rupture. Our results suggest the geodetically inferred fault locking model can provide a useful constraint on earthquake rupture scenarios in subduction zones. This article is part of the theme issue 'Fracture dynamics of solid materials: from particles to the globe'. [ABSTRACT FROM AUTHOR]

Several tectonic processes combine to produce the crustal deformation observed across the Cascadia margin: (1) Cascadia subduction, (2) the northward propagation of the Mendocino Triple Junction (MTJ), (3) the translation of the Sierra Nevada–Great Valley (SNGV) block along the Eastern California Shear Zone–Walker Lane and, (3) extension in the northwestern Basin and Range, east of the Cascade Arc. The superposition of deformation associated with these processes produces the present-day GPS velocity field. North of ~ 45° N observed crustal displacements are consistent with inter-seismic subduction coupling. South of ~ 45° N, NNW-directed crustal shortening produced by the Mendocino crustal conveyor (MCC) and deformation associated with SNGV-block motion overprint the NE-directed Cascadia subduction coupling signal. Embedded in this overall pattern of crustal deformation is the rigid translation of the Klamath terrane, bounded on its north and west by localized zones of deformation. Since the MCC and SNGV processes migrate northward, their impact on the crustal deformation in southern Cascadia is a relatively recent phenomenon, since ~ 2 –3 Ma. [ABSTRACT FROM AUTHOR]

Long‐term slow slip events have been observed at several subduction zones around the globe, where they play an integral part in strain release along megathrust faults. Nevertheless, evidence for long‐term slow slip has remained elusive in the Cascadia subduction zone. Here we conduct a systematic analysis of 13 years of GNSS time series data from 2006 to 2019 and present evidence of at least one low‐amplitude long‐term slow slip event on the Cascadia subduction zone, with the possibility of others that are less resolved. Starting in mid‐2012, a 1.5‐year transient is observed in southern Cascadia, with a group of coastal GNSS stations moving ∼2 mm to the west. The data are modeled as a Mw 6.4 slow slip event occurring at 15–35 km depth on the plate interface, just updip of previously recognized short‐term slow slip and tremor. The event shares many characteristics with similar long‐term transient events on the Nankai subduction zone. However, the total fault slip amplitude is an order‐of‐magnitude smaller in Cascadia when compared to large events elsewhere, making long‐term slow slip detection challenging in Cascadia. While there are other westward long‐duration transients in the refined data set, the surface displacements are below the level of the noise or are limited spatially to a few neighboring stations, making interpretation unclear. Plain Language Summary: In subduction zones, the "locked zone" represents a shallow portion of the plate boundary where the two plates do not move past each other except for during large megathrust earthquakes. Deeper on the plate interface, the plates are still locked, but periodically slip past each other during slow earthquakes. These slow earthquakes, or slow slip events (SSEs), last on the order of week‐to‐years and can release the same amount of energy as a Mw 7 earthquake. Studying the location and frequency of SSEs in subduction zones is important for understanding the seismic cycle, as SSEs can influence the locked zone and the timing of megathrust earthquakes. Long‐term SSEs, which last on the order of months‐to‐years, are observed at numerous subduction zones around the world. However, these events have not yet been observed in the Cascadia subduction zone. Therefore, this study analyzes GNSS time series data to look for signs of long‐term SSEs in Cascadia. Our results reveal evidence for a long‐term SSE in southern Cascadia and potential smaller events along other parts of the margin. These findings are important, as they help illuminate where long‐term slow slip is occurring in Cascadia and how this slip is impacting the locked zone. Key Points: Analysis of 13 years of GNSS time series data in Cascadia reveals evidence for long‐term slow slipThe largest and best resolved long‐term slow slip event occurs in southern Cascadia, has a Mw of 6.4, and spans 1.5 yearsLong‐term slow slip events in Cascadia are relatively small in comparison to most events documented on other subduction zones [ABSTRACT FROM AUTHOR]

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.

Summary: Studies of recorded ground motions and simulations have shown that deep sedimentary basins can greatly increase the intensity of earthquake ground motions within a period range of approximately 1–4 s, but the economic impacts of basin effects are uncertain. This paper estimates key economic indicators of seismic performance, expressed in terms of earthquake‐induced repair costs, using empirical and simulated seismic hazard characterizations that account for the effects of basins. The methodology used is general, but the estimates are made for a series of eight‐ to 24‐story residential reinforced concrete shear wall archetype buildings in Seattle, WA, whose design neglects basin effects. All buildings are designed to comply with code‐minimum requirements (i.e., reference archetypes), as well as a series of design enhancements, which include (a) increasing design forces, (b) decreasing drift limits, and (c) a combination of these strategies. As an additional reference point, a performance‐based design is also assessed. The performance of the archetype buildings is evaluated for the seismic hazard level in Seattle according to the 2018 National Seismic Hazard Model (2018 NSHM), which explicitly considers basin effects. Inclusion of basin effects results in an average threefold increase in annualized losses for all archetypes. Incorporating physics‐based ground motion simulations to represent the large‐magnitude Cascadia subduction interface earthquake contribution to the hazard results in a further increase of 22% relative to the 2018 NSHM. The most effective of the design strategies considered combines a 25% increase in strength with a reduction in drift limits to 1.5%. [ABSTRACT FROM AUTHOR]

Southwest British Columbia has the potential to experience large‐magnitude earthquakes generated by the Cascadia Subduction Zone (CSZ). Buildings in Metro Vancouver are particularly vulnerable to these earthquakes because the region lies above the Georgia sedimentary basin, which can amplify the intensity of ground motions, particularly at medium‐to‐long periods. Earthquake design provisions in Canada neglect basin amplification and the consequences of accounting for these effects are uncertain. By leveraging a suite of physics‐based simulations of M9 CSZ earthquakes, we develop site‐specific and period‐dependent spectral acceleration basin amplification factors throughout Metro Vancouver. The M9 simulations, which explicitly account for basin amplification for periods greater than 1s, are benchmarked against the 2016 BC Hydro ground motion model (GMM), which neglects such effects. Outside the basin, empirical and simulated seismic hazard estimates are consistent. However, for sites within the basin and periods in the 1‐5 s range, GMMs significantly underestimate the hazard. The proposed basin amplification factors vary as a function of basin depth, reaching a geometric mean value as high as 4.5 at a 2‐s period, with respect to a reference site located just outside the basin. We evaluate the impact of the M9 simulations on tall reinforced concrete shear wall buildings, which are predominant in the region, by developing a suite of idealized structural systems that capture the strength and ductility intended by historical seismic design provisions in Canada. Ductility demands and collapse risk conditioned on the occurrence of the M9 simulations were found to exceed those associated with ground motion shaking intensities corresponding to the 975 and 2475‐year return periods, far exceeding the ∼500‐year return period of M9 CSZ earthquakes. [ABSTRACT FROM AUTHOR]

Recent seismic results from the Cascadia Initiative indicate that heterogeneity in the oceanic asthenosphere affects subduction dynamics. Accurate characterization of the oceanic upper mantle is thus necessary to fully understand subduction processes, including the behavioral segmentation of the megathrust. A key challenge is integrating onshore and offshore datasets, which span large variations in near‐surface features that teleseismic body wave tomography is ill‐posed to resolve. Here, we perform a series of P and S forward modeling predictions to better understand the relative contribution of elevation, crustal thickness, offshore sedimentation, and near‐surface velocity structure to teleseismic delay times. Crustal thickness and elevation variations dominate the signal, contributing ∼1 s of delay time difference for P‐waves (roughly double for S). We test several inversion strategies to account for near‐surface features, identifying potential artifacts and causes of imaging errors. Undamped station statics are found to absorb mantle structures and introduce low‐velocity artifacts beneath the forearc. Our preferred inversion strategy utilizes a three‐dimensional starting model (including elevation) of the upper 50 km and heavily damped station statics, which we find leads to better resolution of mantle structure, particularly at asthenospheric depths. These insights guide inversions of observed delay times from the Cascadia subduction zone and Juan de Fuca plate system. We present a new onshore‐offshore S model and an updated P model. Major features are common to both models, including localized subslab low‐velocity anomalies, along‐strike variations in slab structure, and offshore heterogeneity, while regional differences may reflect changes in Vp/Vs. Key Points: Isotropic onshore‐offshore teleseismic body wave tomography of the Cascadia subduction zone; Updated P‐ and new S‐wave modelsForward modeling quantifies the effects of crustal thickness, elevation, sediments, and velocity variations on the shore‐crossing datasetWe explore the impacts of inversion strategy choices, identify imaging artifacts, and outline a preferred methodology [ABSTRACT FROM AUTHOR]

Coastal regions around the Pacific Ring of Fire share the risk of massive earthquakes and tsunamis. Along with their own political-economic, cultural and biophysical contexts, each region has their own history and experiences of tsunami disasters. Coastal areas of Washington State in the U.S. are currently at risk of experiencing a tsunami following a massive Magnitude 9 (M9) earthquake anticipated in the Cascadia Subduction Zone (CSZ). Looking ahead to consider adaptive planning in advance of a tsunami following this M9 event, this paper explores how lessons from recent megaquake- and tsunami-related experiences of risk-based planning and relocation in coastal areas of Japan and Chile could inform anticipatory action in coastal Washington State. Based on a comparison of earthquake and tsunami hazards, social factors, and the roles of government, this paper outlines a framework to compare policy contexts of tsunami risk-based planning and relocation in three Ring of Fire countries, including factors shaping the possible transfer of approaches between them. Findings suggest some aspects of comparative significance and commonalities shared across coastal communities in the three countries and at the same time highlight numerous differences in governance and policies related to planning and relocation. Although there are limitations to the transferability of lessons in disaster adaptive planning and anticipatory action from one national/regional context to another, we believe there is much more that Washington and the Pacific Northwest can learn from Japanese and Chilean experiences. In any context, risk reduction policies and actions need to garner political support in order to be implemented. Additional case study research and detailed analysis is still needed to understand specific lessons that may be applied to detailed risk-based planning and relocation programs across these different national contexts. [ABSTRACT FROM AUTHOR]

We infer a history of three great megathrust earthquakes during the past 2000 years at the Nehalem River estuary based on the lateral extent of sharp (≤3 mm) peat-mud stratigraphic contacts in cores and outcrops, coseismic subsidence as interpreted from fossil diatom assemblages and reconstructed with foraminiferal assemblages using a Bayesian transfer function, and regional correlation of 14C-modeled ages for the times of subsidence. A subsidence contact from 1700 CE (contact A), sometimes overlain by tsunami-deposited sand, can be traced over distances of 7 km. Contacts B and D, which record subsidence during two earlier megathrust earthquakes, are much less extensive but are traced across a 700-m by 270-m tidal marsh. Although some other Cascadia studies report evidence for an earthquake between contacts B and D, our lack of extensive evidence for such an earthquake may result from the complexities of preserving identifiable evidence of it in the rapidly shifting shoreline environments of the lower river and bay. Ages (95% intervals) and subsidence for contacts are: A, 1700 CE (1.1 ± 0.5 m); B, 942–764 cal a BP (0.7 ± 0.4 m and 1.0 m ± 0.4 m); and D, 1568–1361 cal a BP (1.0 m ± 0.4 m). Comparisons of contact subsidence and the degree of overlap of their modeled ages with ages for other Cascadia sites are consistent with megathrust ruptures many hundreds of kilometers long. But these data cannot conclusively distinguish among different types or lengths of ruptures recorded by the three great earthquake contacts at the Nehalem River estuary.

The Juan de Fuca (JDF) plate system is pivoting and breaking apart as a result of resistance to subduction. At the northern end, the Explorer (EXP) plate moves independently of the JDF plate along the Nootka Fault Zone (NFZ), which forms an unstable triple junction with the JDF ridge and the Sovanco Fracture Zone. We trace the subsurface extension of the NFZ using tomography by combining results from a new nearshore study with a previous ocean‐bottom study and other geophysical constraints. Cross sections from the offshore NFZ through central Vancouver Island suggest a strong fold in the subducting lithosphere (∼40°). Near the deformation front, the NFZ is defined by microseismicity that extends into oceanic mantle with low VP/VS (<1.7) values. This anomaly extends to the NE below the continental shelf with a more northerly trajectory than common NFZ depictions through the seismicity concentration off Nootka Island. Tremor locations and receiver functions indicate that this more northerly trajectory persists across Vancouver Island. We propose a revised tectonic evolution for the NFZ that leads to (i) an EXP microplate confined to shallow depths below the convergent margin and which underthrusts North America no farther landward than Brooks Peninsula and (ii) a northern edge of Cascadia subduction that cuts between Brooks Peninsula and Nootka Island. The plate fold offshore Nootka Island and abundant seismicity there and at Brooks Peninsula are ascribed to stress concentrations on either side of the NFZ associated with deformation at the edges of more competent JDF, EXP, and North American plates. Key Points: Geophysical evidence redefines Nootka Fault Zone/Juan de Fuca plate edge across Vancouver IslandPlate fold develops over ≤50 km within northern Juan de Fuca plate edge off Nootka IslandProposed tectonic configuration confines Explorer plate to shallow depths at/seaward of Vancouver Island coast [ABSTRACT FROM AUTHOR]

We determine late Holocene (past 4 kyr) vertical land motion (VLM) rates from relative sea level observations along the coastline of western North America and compare these to contemporary (decadal‐scale) rates inferred from Global Positioning System data. The residual rates (contemporary minus late Holocene) indicate uplift at most locations, which likely reflects short‐term signals associated with the Cascadia subduction earthquake cycle and processes that were recently activated. Regarding the latter, we model and remove the signals associated with ground water extraction and twentieth‐century glacier retreat, which have a significant influence in, respectively, California and southern British Columbia. We interpret the remaining signal as being dominated by interseismic deformation associated with the locking of the Cascadia megathrust. A preliminary comparison of this signal with output from a model of interseismic deformation indicates good agreement at most locations within a range of key model parameter values. This agreement is particularly encouraging as the locking model is based on the inversion of only horizontal land motion observations. Considering the data‐model fit at all locations, preference was found for models featuring nearly full locking to a relatively shallow depth (<30 km), although the locking state preference is variable along the coast and not strong. The best fitting parameter set varies considerably from site to site, and no set of parameter values was able to capture the residual VLM rates in northwestern Vancouver Island. Our study indicates that the residual VLM data provide useful constraints for megathrust locking models of Cascadia subduction. Key Points: Contemporary and late Holocene rates of vertical land motion are quantified along the western coast of North AmericaContemporary minus Holocene rates are interpreted in terms of recent surface loading and interseismic deformationVertical land motion observations provide useful constraints on locking models of the Cascadia subduction zone [ABSTRACT FROM AUTHOR]

Episodic Tremor and Slip, or ETS, is a well‐recognized phenomenon in the Cascadia subduction zone. Constraining the precise location of geodetically measured slip during ETS is important for clarifying the relationship between slip and tremor during ETS, the location of ETS slip relative to the strongly locked zone, and the role of ETS in the overall slip budget. Here I present a new method for separating Global Navigation Satellite System time series in Cascadia into ETS and inter‐ETS components, deriving long‐term average ETS velocities for each site. These velocities are then inverted for a time‐averaged ETS slip rate on the plate interface. I find that ETS and its role in the slip budget are highly variable and segmented along strike. This inversion represents the highest‐resolution image of the ETS zone derived from geodetic data to date. Slip correlates well with tremor locations, and a possible second updip ETS zone is detected. Key Points: The role of Episodic Tremor and Slip in the overall slip budget in Cascadia is highly variable along strike Episodic Tremor and Slip slip rates in southern Cascadia exceed the expected plate convergence rateA possible second updip Episodic Tremor and Slip zone is weakly detected [ABSTRACT FROM AUTHOR]

We present seafloor pressure records from the Cascadia Subduction Zone, alongside oceanographic and geophysical models, to evaluate the spatial uniformity of bottom pressure and optimize the geometry of sensor networks for resolving offshore slow‐slip transients. Seafloor pressure records from 2011 to 2015 show that signal amplitudes are depth‐dependent, with tidally filtered and detrended root‐mean‐squares of <2 cm on the abyssal plain and >6 cm on the continental shelf. This is consistent with bottom pressure predictions from circulation models and comparable to deformation amplitudes from offshore slow slip observed in other subduction zones. We show that the oceanographic component of seafloor pressure can be reduced to ≤1‐cm root‐mean‐square by differencing against a reference record from a similar depth, under restrictions that vary with depth. Instruments at 100–250 m require depths matched within 10 m at separations of <100 km, while locations deeper than 1,400 m are broadly comparable over separations of at least 300 km. Despite the significant noise reduction from this method, no slow slip was identified in the dataset, possibly due to poor spatiotemporal instrument coverage, nonideal deployment geometry, and limited depth‐matched instruments. We use forward predictions of deformation from elastic half‐space models and hindcast pressure from circulation models to generate synthetic slow‐slip observational records and show that a range of slip scenarios produce resolvable signals under depth‐matched differencing. For future detection of offshore slow slip in Cascadia, we recommend a geometry in which instruments are deployed along isobaths to optimize corrections for oceanographic signals. Plain Language Summary: Slow‐slip earthquakes, a special class of earthquake in which slip along a fault occurs over periods of days to years without producing shaking, have increasingly been found to occur in the subsea segments of subduction zones worldwide. There is evidence that these offshore slow‐slip earthquakes can occur within the same region where large, damaging earthquakes are generated and that they may precede and potentially trigger these events. Offshore slow‐slip earthquakes cause the seafloor to move vertically by up to a few centimeters, which can be detected by measuring the pressure on the seafloor if the effects of ocean tides and circulation can be corrected for. In this study, we search for evidence of slow‐slip earthquakes in seafloor pressure data from the Cascadia Subduction Zone, off the U.S. and Canadian west coast, and assess how to best reduce oceanographic signals in these data in order to reliably observe slow‐slip deformation. No evidence for these events is seen in Cascadia pressure data from 2011 to 2015. Using a combination of observational data and model simulations, we show that oceanographic signals can be largely eliminated from seafloor pressure measurements if instruments are placed in rows of constant depth. A network of 22 instruments would be sufficient to monitor for large offshore slow‐slip earthquakes offshore central Oregon, but smaller events would be difficult to detect with this method. By increasing our ability to detect these slow‐slip earthquakes, we can better understand subduction zone processes and their relation to large, damaging megathrust earthquakes. Key Points: In Cascadia, differencing tidally filtered, depth‐matched seafloor pressure records can reduce oceanographic noise to less than 1‐cm RMSModels of offshore SSEs in Cascadia suggest that a network of depth‐matched sensors at 30‐km spacing can detect Mw 6.4 and larger eventsNo offshore slow‐slip earthquakes are detected in the available Cascadia seafloor pressure data from 2011 to 2015 [ABSTRACT FROM AUTHOR]

Propagator wakes within the Juan de Fuca plate represent zones of oceanic crust affected by dynamic readjustments of the spreading centers. They are traditionally mapped from distortions of magnetic anomalies and have been previously interpreted as zones of denser than regular oceanic crust from gravity analysis along several seismic lines in 2-D approximation. In this paper, we utilized a 2.75-D modeling assumption that allowed us to incorporate the effects of the nearby seamounts that were not accounted for in the previous 2-D study. Our analysis reveals that the positive gravity effect of those geological objects was misinterpreted as a need for higher crustal density within propagator wake zones. When seamounts are included in the model, the crust of propagator wakes requires lower densities than the surrounding oceanic crust to explain the observed gravity field. We postulate that the lower densities of propagator wakes relate to faulting during the readjustment of the spreading centers, suggesting that they represent zones of weaker crust. • Propagator wakes on the Juan de Fuca plate have a lower density than the surrounding crust. • 2.75-D gravity modeling reveals low density nature of propagator wakes. • Propagator wakes may contain a higher amount of faults than the surrounding crust. [ABSTRACT FROM AUTHOR]