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For the past ∼12 years the Pacific Northwest Seismic Network has been automatically detecting and locating tectonic tremor across the Cascadia subduction zone, resulting in a catalog of more than 500,000 tremor epicenters to date, which has served as a valuable resource for tremor and slip research. This manuscript presents an updated methodology for routine tremor detection in Cascadia and a new catalog of over 180,000 tremor epicenters including amplitudes detected along the subduction zone margin from 2017 to 2021. The events are detected via cross‐correlation of continuous vertical envelope data of 128 stations from northern California to northern Vancouver Island. The modified approach results in less scatter and a 55% increase in detected epicenters than previously observed, as well as a newly identified tremor source offset updip from the main tremor and slip region at the southern edge of the subduction zone. Radiated seismic energy in the 1.5–5 Hz band is used to assign epicenters an energy magnitude (MeL), which is calibrated to the ML of local earthquakes. Southern Cascadia is most active, but the highest tremor energy rates occur in northern Cascadia. Tremor in central Cascadia is systematically weaker and less frequent. Individual epicenter magnitudes range from ∼0.5–2 and spatiotemporally cluster into 1,060 swarms with cumulative MeL ranging from ∼0.8 to 3.7. The swarms reflect underlying slow slip events and occur with an earthquake‐like energy distribution with a b value ∼1. Tremor epicenters, however, follow a tapered Gutenberg‐Richter distribution with high b values, suggesting individual tremor bursts and their constituent low‐frequency earthquakes are fault‐dimension limited. Plain Language Summary: Tectonic tremor is a continuous, low‐frequency seismic signal associated with slow fault motion at major plate boundaries. In order to better understand how tremor occurs and how it relates to the underlying slow slip, scientists need a detailed catalog of tremor activity. In the Cascadia subduction zone, tremor has been routinely monitored and cataloged for the past 12 years. But, that catalog lacks information about tremor amplitude, which limits its utility and the physical insights it can provide. Here I present an update to that monitoring system to include estimates of tremor magnitudes from 2017 to present. The results identify a new tremor source and show margin‐wide trends in total radiated seismic energy and energy rates that have not been previously identified. The catalog also reveals that the magnitude distributions of underlying slow earthquakes are similar to those of regular earthquakes. Most importantly, the improved catalog continues to grow in near real‐time and serves as a valuable resource for future slow earthquake research. Key Points: Tectonic tremor is most active in southern Cascadia, strongest in northern Cascadia, and both weak and infrequent in central CascadiaNew tremor source offset from the episodic tremor and slip zone observed in northern California may be related to edge of subducting plateTremor epicenters show swarm‐like behavior, but tremor swarms have earthquake‐like Gutenberg‐Richter distribution with a b‐value ∼1 [ABSTRACT FROM AUTHOR]

Coastal subsidence, dating of plant remains and tree rings, and evidence for tsunami inundation point to coseismic activity on a sizable portion of the Cascadia subduction zone around three centuries ago. A tsunami of remote origin in 1700 C.E., probably from Cascadia, caused flooding and damage in Japan. In previous modeling, this transpacific evidence was found most simply explained by one Cascadia rupture about 1,000 km long. Here I model tens of thousands of ruptures and simulate their subsidence and tsunami signals and show that it is possible that the earthquake was part of a sequence of several events. Partial rupture of ∼400 km offshore southern Oregon and northern California in one large M ≥ 8.7 earthquake can explain the tsunami in Japan without conflicting with the subsidence. As many as four more earthquakes with M ≤ 8.7 can complete the subsidence signal without their tsunamis being large enough to be recorded in Japan. The purpose of this study is not to find a single, most likely, scenario or disprove the single‐rupture hypothesis favored by alternative evidence such as turbidites. Rather, it demonstrates that a multiple rupture sequence may explain part of the available data, and therefore cannot be discounted. Given the gaps in the presently available estimates of subsidence it is also possible that segments of the megathrust, for example from Copalis to the Strait of Juan de Fuca, did not rupture in 1700. The findings have significant implications for Cascadia geodynamics and how earthquake and tsunami hazards in the region are quantified. Plain Language Summary: There is significant evidence from geology and historical documents of a tsunami in Japan that a very large earthquake occurred in the Pacific Northwest of the U.S. on the "Cascadia Subduction Zone." To this day it has been believed that this earthquake spanned more than 1000 km from California to British Columbia. In this paper the evidence is re‐examined with new techniques to model earthquake ruptures and tsunamis. I find that it is possible to explain the geology and historical data with a "rupture sequence." That is, instead of a single very large earthquake, a series of earthquakes. This series would still include a large event in 1700 but it could be followed or preceded by as many as 4 other smaller events in the decades around 1700. This is important because whether Cascadia produces only extremely long ruptures or several shorter ones helps us to understand how to plan for the next big earthquake. Key Points: As many as five, serial ruptures may explain evidence previously ascribed to a long Cascadia ruptureThe proposed serial ruptures accord with estimates of Cascadia subsidence and Japanese tsunami heightsThese findings are based on 37,500 rupture simulations in the range M7.8–M9.6 [ABSTRACT FROM AUTHOR]

From California to British Columbia, the Pacific Northwest coast bears an omnipresent earthquake and tsunami hazard from the Cascadia subduction zone. Multiple lines of evidence suggests that magnitude eight and greater megathrust earthquakes have occurred ‐ the most recent being 321 years ago (i.e., 1700 A.D.). Outstanding questions for the next great megathrust event include where it will initiate, what conditions are favorable for rupture to span the convergent margin, and how much slip may be expected. We develop the first 3‐D fully dynamic rupture simulations for the Cascadia subduction zone that are driven by fault stress, strength and friction to address these questions. The initial dynamic stress drop distribution in our simulations is constrained by geodetic coupling models, with segment locations taken from geologic analyses. We document the sensitivity of nucleation location and stress drop to the final seismic moment and coseismic subsidence amplitudes. We find that the final earthquake size strongly depends on the amount of slip deficit in the central Cascadia region, which is inferred to be creeping interseismically, for a given initiation location in southern or northern Cascadia. Several simulations are also presented here that can closely approximate recorded coastal subsidence from the 1700 A.D. event without invoking localized high‐stress asperities along the down‐dip locked region of the megathrust. These results can be used to inform earthquake and tsunami hazards for not only Cascadia, but other subduction zones that have limited seismic observations but a wealth of geodetic inference. Plain Language Summary: The largest earthquakes on Earth occur along faults that develop between two tectonic plates that come into contact. Termed megathrust earthquakes, these catastrophic events are responsible for generating both strong ground‐shaking and tsunamis. The Cascadia megathrust fault straddles the Pacific coastline of North America and from evidence in both the United States and Japan, we know this fault last slipped in 1700 A.D. We have combined models of strain buildup (geodetic coupling models) with state‐of‐the‐art 3‐D computer simulations to understand the potential hazard of a future earthquake in Cascadia and show what factors might lead to the fault slipping its entire length. We compare our simulations to geologic measurements of permanent ground movement from 1700 A.D. Our results demonstrate that no matter where the earthquake is allowed to start, coupling models showing strain accumulation to the top of the fault easily leads to big earthquakes. We also look into what 1700 A.D. event may have looked like and show several scenarios that fit the geologic data very closely. This work represents the first set of 3‐D simulations that use the laws of physics to see what may control the size of future earthquakes in Cascadia. Key Points: We design the first fully dynamic 3‐D earthquake simulations based on geodetic coupling models for the Cascadia megathrustSegmentation in the stress drop is needed to produce subsidence amplitudes consistent with observed megathrust earthquakesDynamic rupture simulations demonstrate how fault friction and stress levels may control margin‐wide rupture [ABSTRACT FROM AUTHOR]

Inelastic wedge deformation likely plays an important role in the generation of tsunami and ocean acoustic waves in accretionary subduction margins. In an elastic dislocation model, whether or not the fault breaks the trench has a significant effect on seafloor deformation and resulting tsunami. However, this boundary condition is less important when significant inelastic deformation in the overriding wedge occurs, because large seafloor uplift can occur with little or no slip at the trench. Here we incorporate wedge plasticity in fully coupled dynamic rupture and tsunami simulations for a buried fault in the Cascadia subduction zone with realistic fault geometry, bathymetry, and velocity structure. A linearized Eulerian approach is verified and used to simulate gravity waves in the ocean. Our coupled models show that the inelastic deformation of wedge sediments can significantly contribute to seafloor uplift, producing tsunami heights at least twice as large as in purely elastic simulations, whilst generating weaker ocean acoustic and seismic waves. Inelastic wedge deformation is therefore an important mechanism to consider in tsunami hazard assessment in the Cascadia subduction zone. These results have important implications for tsunami generation and early warning in accretionary and other sediment‐filled margins worldwide. Plain Language Summary: Thick sediments in accretionary plate margins, such as the Cascadia subduction zone, can significantly affect tsunamigenesis and excitation of ocean acoustic and seismic waves. Due to weak strength wedge sediments can fail inelastically under dynamic stresses during an earthquake. Our fully coupled models of earthquake rupture and tsunami in the Cascadia subduction zone show that the inelastic deformation of wedge sediments produces tsunami several times larger than in purely elastic deformation models. Meanwhile, inelastic deformation reduces the excitation of most ocean acoustic and seismic waves, which poses challenges in using these waves for tsunami early warning. Inelastic wedge deformation should be incorporated into more accurate tsunami hazard assessment in the Cascadia subduction zone and other sediment‐filled margins worldwide. Key Points: Inelastic wedge deformation can significantly contribute to tsunamigenesis in the Cascadia subduction zoneA linearized Eulerian approach for modeling ocean gravity waves is verified by a semi‐analytical approachInelastic wedge deformation significantly reduces excitation of ocean acoustic and seismic waves [ABSTRACT FROM AUTHOR]

Fault coupling is vital in determining the amount of strain that is accumulated along faults. The magnitude and location of stored elastic strain energy in highly coupled regions has important implications for understanding the full range of slip behavior at plate boundary faults, as well as earthquake and tsunami hazards. We use the temporary dense amphibious array of seismometers offered by the Cascadia Initiative to create a high‐resolution catalog of events to examine the spatio‐temporal behavior of earthquakes near the plate interface. The data show that in southern Cascadia the plate interface updip of the geodetically locked region is nearly devoid of seismicity, therefore likely highly coupled and accumulating strain. The catalog reveals events that are clustered at the downdip edge of the highly coupled megathrust that correlate in time with nearby strain transient observations. Template matching of events in the cluster using permanent stations of the Northern California Seismic Network over a 10‐year period between 2010 and 2020 indicates that this cluster is unique in space and time. Its activity only during the strain transient provides support for the utility of seismic observations in the identification of strain transients. Plain Language Summary: Great megathrust earthquakes have occurred in the Cascadia Subduction Zone. The most recent occurred in 1700, and it is expected that another will happen in the next few hundred years. Although the megathrust has hosted great earthquakes, few small interplate events have been identified along this plate boundary. Instead, a portion of the convergence between the tectonic plates is released in both periodic slow slip events and (quasi) continuous fault creep. It is essential to understand the full range of plate boundary behavior to assess the hazard risk for great earthquakes. To further this understanding, we gather data using a densely spaced temporary array of seismometers deployed in southern Cascadia in 2014–2015 to increase detection as well as determine precise earthquake locations. We identify events that are on or very near the plate boundary fault. There is some clustering of earthquakes near the plate boundary, but we find an absence of earthquakes along the shallowest segment, that indicate that the fault is likely locked. When compared to other geophysical observations in the area, it appears that the cluster of earthquakes near the plate boundary is sensitive to strain transients, demonstrating the utility of seismic observations in investigating these phenomena. Key Points: Catalog creation of 1,452 events in southern CascadiaLack of shallow offshore plate interface seismicity suggests locally high couplingTemplate matching of plate interface events reveals a cluster of seismicity sensitive to long‐term plate interface strain transients [ABSTRACT FROM AUTHOR]

Current stochastic rupture modeling techniques do not consider the potential influence of inter‐seismic coupling, a first‐order property of a megathrust, which can show correlation between areas of high coupling and areas of greater slip as seen in recent large ruptures globally. Therefore, it is reasonable to assume that it should be considered as prior information in rupture modeling. Here, we first present a mathematical formalism to introduce coupling models as prior information into stochastic rupture modeling. We then focus on how introducing slip deficit information into the stochastic rupture models influences slip distributions for the Cascadia subduction zone (CSZ). We compare rupture models created with two end member models of coupling, one with a shallow coupling and another with coupling deeper downdip. We also discuss the comparison to models created without assuming knowledge of the coupling distribution except for variation in the downdip limit of slip. Variations occur and correlate well with areas with the largest differences in slip deficit rates. The ruptures are then used for regional probabilistic tsunami hazard assessment. Overall, the tsunami amplitudes generated are much more hazardous in the northern extent of the CSZ where differences in coupling distribution are more prevalent. Models obtained from assuming a shallower downdip limit have tsunami amplitudes more similar to those from the geodetic coupling models. Although uncertainties are present in the accuracy of coupling, imposing either constraint created different hazard estimations when compared to those where no prior coupling information was used. Plain Language Summary: Uncertainties are often present when trying to determine future earthquake rupture potentials and their associated hazards. We present here a means of including a first‐order property of a megathrust, a fault's pattern of the current ability to slip (coupling), in a current rupture modeling technique often applied to hazard assessments. We model ruptures classes constrained by two different coupling models for the Cascadia subduction zone (CSZ), as well as two other classes without imposed coupling. These ruptures are then used to determine associated tsunamis and the total probabilistic tsunami hazards for the Cascadia region. Overall, the tsunami amplitudes generated are much more hazardous in the northern extent of the CSZ where differences in coupling distribution are more prevalent. Models obtained without coupling but constrained by a shallower limit of slip have tsunami amplitudes more similar to those from the geodetic coupling models. Although uncertainties are present in the accuracy of coupling, imposing either constraint created different hazard estimations when compared to those where no prior coupling information was used. Key Points: Including fault coupling produces considerable variations for stochastic slip rupture models as well as the resultant probabilistic tsunami hazardsThe coupling models produced similar patterns of tsunami hazards for the central Cascadia region and more dissimilar hazards in the northern and southern coastal Cascadia regionsExpanding seafloor observations is necessary for a more accurate understaning of the Cascadia tsunami hazard [ABSTRACT FROM AUTHOR]

There is a strong need to model potential rupture behaviors for the next Cascadia megathrust earthquake. However, there exists significant uncertainty regarding the extent of downdip rupture and rupture speed. To address this problem, we study how the transition region (i.e., the gap), which separates the locked from slow‐slip regions, influences coseismic rupture propagation using 2‐D dynamic rupture simulations governed by a slip‐weakening friction law. We show that rupture propagation through the gap is strongly controlled by the amount of accumulated tectonic initial shear stress and gap friction level. A large amplitude negative dynamic stress drop is needed to arrest downdip rupture. We also observe downdip supershear rupture when the gradient in effective normal stress from the locked to slow‐slip regions is dramatic. Our results justify kinematic rupture models that extend below the gap and suggests the possibility of high‐frequency energy radiation during the next Cascadia megathrust earthquake. Plain Language Summary: How large, deep, and damaging a future earthquake will be depends on factors such as energy release that must be constrained by precise observations of previous earthquakes in the same area. But such data are rarely available. Instead, computer models of earthquakes guided by the laws of physics can provide us with estimates of potential ground shaking for a future event. In our study, we design two‐dimensional earthquake simulations for the Cascadia fault below the northwestern United States coast and test different hypotheses for how stress may be accumulating at depth along this fault. Our models focus on a portion of the fault referred to as the "gap." The gap physically separates a shallow region that slips during large earthquakes from a deeper region that experiences intermittent slip between large earthquakes. A gap region similar to that in Cascadia is also found in Japan, Mexico, and around other active faults worldwide. We find that our simulated rupture is able to extend to deeper regions at faster speeds given the current understanding of stress levels and earthquake fault friction in the gap. While this work represents only a first step toward understanding how stresses and friction influence how the Cascadia fault might slip, it lays the foundation for modeling more complex physics that can help scientists better predict shaking from seismic waves. Key Points: We examine dynamic source effects on along‐dip rupture propagation for a Cascadia megathrust earthquakeSimulated earthquake rupture is able to penetrate through the transition zone and reach the deeper slow‐slip regionOur results underscore the potential for a deeper downdip rupture and faster rupture speed than previously assumed in kinematic models [ABSTRACT FROM AUTHOR]

Background: The Cascadia subduction zone off the US Pacific Northwest coast produces a Force 9 earthquake once every 300 years. Cascadia Rising was a regional disaster drill modeled on such an event. Western Washington State has 5 million people and one Level I trauma center.Study Design and Methods: The blood response of the trauma center and region were modeled under the conditions laid down in the disaster scenario. The scenario assumed structural damage to the 1931 reinforced concrete building housing the trauma hospital transfusion service with loss of electricity, data services, and water. The regional blood center, in a newer building located six blocks away, suffered less disruption. The regional blood inventory is in a blood component manufacturing facility 18 miles south of downtown.Results: At best and at risk to life, the trauma center could issue universally compatible components at a rate of 60 components/hr from a damaged but still accessible transfusion service. Usable inventory would be expended in 4 hours with no clear mechanism and rules for resupply. The regional center has additional group O red blood cells and AB or A plasma to sustain that rate of usage for several more hours but no protocols for reestablishing communication or "push" resupply. Regional resources will be gone in less than a day.Conclusions: After a major Cascadia earthquake, blood resources may fail immediately, but even with luck, local resources used emergently at maximal issue rates will last 4 to 14 hours.© 2018 AABB. [ABSTRACT FROM AUTHOR]

Subduction zone megathrust faults host Earth's largest earthquakes, along with multitudes of smaller events that contribute to plate convergence. An understanding of the faulting behavior of megathrusts is central to seismic and tsunami hazard assessment around subduction zone margins. Cumulative sliding displacement across each megathrust, which extends from the trench to the downdip transition to interplate ductile deformation, is accommodated by a combination of rapid stick-slip earthquakes, episodic slow-slip events, and quasi-static creep. Megathrust faults have heterogeneous frictional properties that contribute to earthquake diversity, which is considered here in terms of regional variations in maximum recorded magnitudes, Gutenberg-Richter b values, earthquake productivity, and cumulative seismic moment depth distributions for the major subduction zones. Great earthquakes on megathrusts occur in irregular cycles of interseismic strain accumulation, foreshock activity, main-shock rupture, postseismic slip, viscoelastic relaxation, and fault healing, with all stages now being captured by geophysical monitoring. Observations of depth-dependent radiation characteristics, large earthquake slip distributions, variations in rupture velocities, radiated energy and stress drop, and relationships to aftershock distributions and afterslip are discussed. Seismic sequences for very large events have some degree of regularity within subduction zone segments, but this can be complicated by supercycles of intermittent huge ruptures that traverse segment boundaries. Factors influencing variability of large megathrust ruptures, such as large-scale plate structure and kinematics, presence of sediments and fluids, lower-plate bathymetric roughness, and upper-plate structure, are discussed. The diversity of megathrust failure processes presents a suite of natural hazards, including earthquake shaking, submarine slumping, and tsunami generation. Improved monitoring of the offshore environment is needed to better quantify and mitigate the threats posed by megathrust earthquakes globally. [ABSTRACT FROM AUTHOR]

The Mendocino Triple Junction region is the most seismically active part of the Cascadia Subduction Zone. The northward moving Pacific plate collides with the subducting Gorda plate causing intense internal deformation within it. Here we show that the stress field rotates rapidly with depth across the thrust interface from a strike-slip regime within the subducting plate, reflecting the Pacific plate collision, to a thrust regime in the overriding plate. We utilize a dense focal mechanism dataset, including observations from the Cascadia Initiative ocean bottom seismograph experiment, to constrain the stress orientations. To quantify the implications of this rotation for the strength of the plate boundary, we designed an inversion that solves for the absolute stress tensors in a three-layer model subject to assumptions about the strength of the subducting mantle. Our results indicate that the shear stress on the plate boundary fault is likely no more than about ∼50 MPa at ∼20 km depth. Regardless of the assumed mantle strength, we infer a relatively weak megathrust fault with an effective friction coefficient of ∼0 to 0.2 at seismogenic depths. Such a low value for the effective friction coefficient requires a combination of high fluid pressures and/or fault-zone minerals with low inherent friction in the region where a great earthquake is expected in Cascadia. [ABSTRACT FROM AUTHOR]

We identify and describe submarine channels, submarine landslides, and three unusual erosional features on the toe of the Cascadia accretionary wedge near Willapa Canyon, offshore Washington, USA. We use new high-reso lution multibeam bathymetric data and chirp sub-bottom and multichannel seismicreflection profiles. Composite data sets were generated from the Cascadia Open-Access Seismic Transects (COAST) cruise and from the site survey cruise for the Cascadia Initiative. This high-resolution data set has illuminated geomorphic features that suggest that this section of the margin underwent largescale erosion event(s) which likely occurred during the latest Pleistocene. Three unusual features imaged superficially resemble slope failures of the landward-vergent frontal thrust ridge but are distinguished from such failures by (1) complete or near-complete incision of the crest of the frontal thrust, anti clinal ridge, and piggyback basin; (2) the lack of semi-coherent blocky landslide debris; (3) asymmetrical incision of feature floors to levels well below the abyssal plain; and (4) connections to the main Willapa Deep-Sea Channel by likely co-genetic but now barely active paleochannels. We conclude that the unusual geomorphic features were likely created by massive turbidity currents created by the Missoula glacial-lake outburst flood events. The floods directed massive sediment volumes through the Willapa Submarine Canyon System, eroding a broad swath of the accretionary wedge and either cutting through or causing slope failure of the frontal thrust. Turbidity-current modeling on a bathymetric reconstruction supports our hypothesis that a large-volume flow like the Missoula floods could have inundated the paleodrainage system and created the unique features we imaged. [ABSTRACT FROM AUTHOR]

GPS velocity fields in the Western US have been interpreted with various physical models of the lithosphere-asthenosphere system: (1) time-independent block models; (2) time-dependent viscoelastic-cycle models, where deformation is driven by viscoelastic relaxation of the lower crust and upper mantle from past faulting events; (3) viscoelastic block models, a time-dependent variation of the block model. All three models are generally driven by a combination of loading on locked faults and (aseismic) fault creep. Here we construct viscoelastic block models and viscoelastic-cycle models for the Western US, focusing on the Pacific Northwest and the earthquake cycle on the Cascadia megathrust. In the viscoelastic block model, the western US is divided into blocks selected from an initial set of 137 microplates using the method of Total Variation Regularization, allowing potential trade-offs between faulting and megathrust coupling to be determined algorithmically from GPS observations. Fault geometry, slip rate, and locking rates (i.e. the locking fraction times the long term slip rate) are estimated simultaneously within the TVR block model. For a range of mantle asthenosphere viscosity (4.4 x 1018 to 3.6 x 1020 Pa s) we find that fault locking on the megathrust is concentrated in the uppermost 20 km in depth, and a locking rate contour line of 30 mm yr-1 extends deepest beneath the Olympic Peninsula, characteristics similar to previous time-independent block model results. These results are corroborated by viscoelastic-cycle modelling. The average locking rate required to fit the GPS velocity field depends on mantle viscosity, being higher the lower the viscosity. Moreover, for viscosity ≲ 1020 Pa s, the amount of inferred locking is higher than that obtained using a time-independent block model. This suggests that time-dependent models for a range of admissible viscosity structures could refine our knowledge of the locking distribution and its epistemic uncertainty. [ABSTRACT FROM AUTHOR]

Netarts Bay is the setting of one of the largest concentrations of late prehistoric Native American settlements on the tectonically active Oregon coast. A prehistoric site (35TI74) exposed by sea cliff erosion in 1998 at the south end of the Netarts littoral cell contained a stratigraphic record of activity by prehistoric Native Americans and interbedded paleotsunami deposits. The nearfield paleotsunamis were produced by great Cascadia earthquakes (Mw8.5 ± 0.5) emanating from the Cascadia Subduction Zone offshore, as previously documented in episodically buried tidal marsh surfaces at the south end of Netarts Bay and in many other estuaries in the region. The cultural deposits at 35TI74 reflect multiple reoccupations by Native Americans at this barrage creek flood plain site between paleotsunami runup events (≥7.5 m elevation) at 1.3 ka, 1.1 ka, 0.8-0.9 ka, and 0.3 ka. The conditions that led to the exposure of 35TI74 are traced to erosion of a protective beach dune ramp at the south end of the Netarts littoral cell, placing the site within reach of progressive storm-surf erosion. The diminishing geoarchaeological record at 35TI74 represents a microcosm of the sea cliff erosional processes that threaten vulnerable prehistoric shoreline archaeological sites along much of the Pacific Northwest Coast. [ABSTRACT FROM AUTHOR]

Bathymetric data reveal abundant submarine landslides along the deformation front of the northern Cascadia margin that might have significant tsunami potential. Radiocarbon age dating showed that slope failures are early to mid-Holocene. The aim of this study is the analysis of slope stability to investigate possible trigger mechanisms using the factor of safety analysis technique on two prominent frontal ridges. First-order values for the earthquake shaking required to generate instability are derived. These are compared to estimated ground accelerations for large ( M=5 to 8) crustal earthquakes to giant ( M=8 to 9) megathrust events. The results suggest that estimated earthquake accelerations are insufficient to destabilize the slopes, unless the normal sediment frictional resistance is significantly reduced by, for example, excess pore pressure. Elevated pore pressure (overpressure ratio of 0.4) should significantly lower the threshold for earthquake shaking, so that a medium-sized M=5 earthquake at 10 km distance may trigger submarine landslides. Preconditioning of the slopes must be limited primarily to the mid- to early Holocene as slope failures are constrained to this period. The most likely causes for excess pore pressures include rapid sedimentation at the time of glacial retreat, sediment tectonic deformation, and gas hydrate dissociation as result of ocean warming and sea level rise. No slope failures comparable in size and volume have occurred since that time. Megathrust earthquakes have occurred frequently since the most recent failures in the mid-Holocene, which emphasizes the importance of preconditioning for submarine slope stability. [ABSTRACT FROM AUTHOR]

Stratigraphic, sedimentologic (including CT 3D X-ray tomography scans), foraminiferal, and radiocarbon analyses show that at least six of seven abrupt peat-to-mud contacts in cores from a tidal marsh at Talbot Creek (South Slough, Coos Bay), record sudden subsidence (relative sea-level rise) during great megathrust earthquakes at the Cascadia subduction zone. Data for one contact are insufficient to infer whether or not it records a great earthquake—it may also have formed through local, non-seismic, hydrographic processes. To estimate the amount of subsidence marked by each contact, we expanded a previous regional modern foraminiferal dataset to 174 samples from six Oregon estuaries. Using a transfer function derived from the new dataset, estimates of coseismic subsidence across the six earthquake contacts vary from 0.31 m to 0.75 m. Comparison of subsidence estimates for three contacts in adjacent cores shows within-site differences of ≤0.10 m, about half the ±0.22 m error, although some estimates may be minimums due to uncertain ecological preferences for Balticammina pseudomacrescens in brackish environments and almost monospecific assemblages of Miliammina fusca on tidal flats. We also account for the influence of taphonomic processes, such as infiltration of mud with mixed foraminiferal assemblages into peat, on subsidence estimates. Comparisons of our subsidence estimates with values for correlative contacts at other Oregon sites suggest that some of our estimates are minimums and that Cascadia's megathrust earthquake ruptures have been heterogeneous over the past 3500 years. [ABSTRACT FROM AUTHOR]

A stratigraphic synthesis of dozens of deep-sea cores, most of them overlooked in recent decades, provides new insights into deep-sea turbidites as guides to earthquake and tsunami hazards along the Cascadia subduction zone, which extends 1100 km along the Pacific coast of North America. The synthesis shows greater variability in Holocene stratigraphy and facies off the Washington coast than was recognized a quarter century ago in a confluence test for seismic triggering of sediment gravity flows. That test compared counts of Holocene turbidites upstream and downstream of a deep-sea channel junction. Similarity in the turbidite counts among seven core sites provided evidence that turbidity currents from different submarine canyons usually reached the junction around the same time, as expected of widespread seismic triggering. The fuller synthesis, however, shows distinct differences between tributaries, and these differences suggest sediment routing for which the confluence test was not designed. The synthesis also bears on recent estimates of Cascadia earthquake magnitudes and recurrence intervals. The magnitude estimates hinge on stratigraphic correlations that discount variability in turbidite facies. The recurrence estimates require turbidites to represent megathrust earthquakes more dependably than they do along a flow path where turbidite frequency appears limited less by seismic shaking than by sediment supply. These concerns underscore the complexity of extracting earthquake history from deep-sea turbidites at Cascadia. [ABSTRACT FROM AUTHOR]

Large coseismic slip was thought to be unlikely to occur on the shallow portions of plate-boundary thrusts, but the 11 March 2011 Tohoku-Oki earthquake [moment magnitude (Mw) = 9.0] produced huge displacements of ~50 meters near the Japan Trench with a resultant devastating tsunami. To investigate the mechanisms of the very large fault movements, we conducted high-velocity (1.3 meters per second) friction experiments on samples retrieved from the plate-boundary thrust associated with the earthquake. The results show a small stress drop with very low peak and steady-state shear stress. The very low shear stress can be attributed to the abundance of weak clay (smectite) and thermal pressurization effects, which can facilitate fault slip. This behavior provides an explanation for the huge shallow slip that occurred during the earthquake. [ABSTRACT FROM AUTHOR]

Characterizations of tsunami hazards along the Cascadia subduction zone hinge on uncertainties in megathrust rupture models used for simulating tsunami inundation. To explore these uncertainties, we constructed 15 megathrust earthquake scenarios using rupture models that supply the initial conditions for tsunami simulations at Bandon, Oregon. Tsunami inundation varies with the amount and distribution of fault slip assigned to rupture models, including models where slip is partitioned to a splay fault in the accretionary wedge and models that vary the updip limit of slip on a buried fault. Constraints on fault slip come from onshore and offshore paleoseismological evidence. We rank each rupture model using a logic tree that evaluates a model's consistency with geological and geophysical data. The scenarios provide inputs to a hydrodynamic model, SELFE, used to simulate tsunami generation, propagation, and inundation on unstructured grids with <5-15 m resolution in coastal areas. Tsunami simulations delineate the likelihood that Cascadia tsunamis will exceed mapped inundation lines. Maximum wave elevations at the shoreline varied from ~4 m to 25 m for earthquakes with 9-44 m slip and Mw 8.7-9.2. Simulated tsunami inundation agrees with sparse deposits left by the A.D. 1700 and older tsunamis. Tsunami simulations for large (22-30 m slip) and medium (14-19 m slip) splay fault scenarios encompass 80%-95% of all inundation scenarios and provide reasonable guidelines for land-use planning and coastal development. The maximum tsunami inundation simulated for the greatest splay fault scenario (36- 44 m slip) can help to guide development of local tsunami evacuation zones. [ABSTRACT FROM AUTHOR]

Gravimeter records of three recent megathrust earthquakes are used to investigate the splitting of the fundamental spheroidal mode 0S2. Benefiting from recent improvements in the design and performance of superconducting gravimeters and using multitapers for the spectral analysis we obtain singlet frequencies with smaller and more robust error bars than previous studies. Based on forward modelling experiments we can show that the errors of the singlet frequencies are small enough to enable discrimination between physically plausible 3-D density models. We present an estimate of the rotational splitting parameter of 0S2, which provides a linear integral constraint on the spherically symmetric mass density distribution much like the Earth's mass or the Earth's moment of inertia do and whose relative error is smaller than that of the Earth's mass. [ABSTRACT FROM PUBLISHER]

Abstract: Quantitative estimates of land-level change during the giant AD 1700 Cascadia earthquake along the Oregon coast are inferred from relative sea-level changes reconstructed from fossil foraminiferal assemblages preserved within the stratigraphic record. A transfer function, based upon a regional training set of modern sediment samples from Oregon estuaries, is calibrated to fossil assemblages in sequences of samples across buried peat-mud and peat-sand contacts marking the AD 1700 earthquake. Reconstructions of sample elevations with sample-specific errors estimate the amount of coastal subsidence during the earthquake at six sites along 400 km of coast. The elevation estimates are supported by lithological, carbon isotope, and faunal tidal zonation data. Coseismic subsidence at Nehalem River, Nestucca River, Salmon River, Alsea Bay, Siuslaw River and South Slough varies between 0.18 m and 0.85 m with errors between 0.18 m and 0.32 m. These subsidence estimates are more precise, consistent, and generally lower than previous semi-quantitative estimates. Following earlier comparisons of semi-quantitative subsidence estimates with elastic dislocation models of megathrust rupture during great earthquakes, our lower estimates for central and northern Oregon are consistent with modeled rates of strain accumulation and amounts of slip on the subduction megathrust, and thus, with a magnitude of 9 for the AD 1700 earthquake. [Copyright &y& Elsevier]

Abstract: Changes in species assemblages of intertidal foraminifera can be used to estimate the amount of earthquake-related subsidence during plate-boundary earthquakes at the Cascadia subduction zone. The accuracy and precision of foraminiferal methods in paleoenvironmental reconstruction is underpinned by the relations between contemporary taxa and their environment, which are used to calibrate fossil foraminiferal assemblages in sediment sequences. A contemporary training set of surface sediment samples from five intertidal marshes along the Oregon coast was used to determine foraminiferal distributions and prevailing environmental control(s) along elevational transects. Dominant taxa include Balticammina pseudomacrescens, Trochamminita irregularis, Haplophragmoides wilberti, Trochammina inflata, Jadammina macrescens and Miliammina fusca. Unconstrained cluster analysis and detrended correspondence analysis was used to identify two elevation-dependent faunal zones: Faunal Zone I (upland, high marsh, middle marsh) dominated by Balticammina pseudomacrescens, Haplophragmoides wilberti and Trochammina inflata, and Faunal Zone II (low marsh and tidal flat) dominated by Miliammina fusca. Site-specific differences in assemblages at three marshes enabled further subdivision of Faunal Zone I. Zone Ia is based on one or more of Balticammina pseudomacrescens, Trochammina inflata, Trochamminita irregularis and Haplophragmoides sp., and Zone Ib on Jadammina macrescens, Haplophragmoides sp., Trochammina inflata and Miliammina fusca. Canonical correspondence analysis (CCA) and partial CCA of the training set from the Nehalem River marsh transect was used to infer that the zonation of foraminifera is elevation-dependent (39% of explained variance). A transfer function was developed to reconstruct sudden changes in relative sea-level during plate-boundary earthquakes in Oregon. The results indicate a robust performance of the transfer function (r jack 2 =0.82) with the error estimate (RMSEPjack =0.20m) comparable to local and regional transfer functions from other temperate marshes. To illustrate the potential of the technique, the transfer function was applied to reconstruct subsidence during the AD 1700 earthquake using at Alsea Bay, Oregon. The reconstruction (0.18±0.20m) is less than half the subsidence estimate of Nelson et al. [2008. Great-earthquake palogeoesy and tsunamis of the past 2000years at Alsea Bay, central Oregon coast, USA. Quaternary Science Reviews, 27, 747–768] using their foraminiferal transfer function, perhaps because of differences in taxonomy and the species relationship to elevation. [Copyright &y& Elsevier]

Abstract: The basaltic Tseax flow is the product of one of only two eruptions in western Canada during the last thousand years. Reinterpretations of 14C and paleomagnetic data indicate that Tseax volcano last erupted between 1668 and 1714 CE. This date straddles that of the Cascadia megathrust earthquake of 26 January 1700, whose rupture lay 450 km to the south. Hence, the largest recent earthquake in northwest North America may have rejuvenated an existing magmatic system and produced this isolated flow. Although the flow is chemically uniform there are significant textural differences between the early and late parts of the flow. It is proposed that both magmatic components were contained within a steep conduit. Gas produced by degassing of magma in the lower part of the conduit ascended, heated magma in the upper part, coarsening plagioclase, and then continued to the surface along fissures. This stable configuration was disrupted by the Cascadia earthquake: dilatation widened the conduit and enabled both magmas to rise to the surface along existing fissures. [Copyright &y& Elsevier]

Seismic hazard assessments for a Cascadia subduction zone earthquake are largely based on the rupture area predictions of dislocation models constrained by geodetic and geothermal data; this paper tests the consistency of the models for the 1700 great Cascadia earthquake with compiled coastal coseismic subsidence as estimated from paleoelevation studies. Coastal estimates have large uncertainties but show a consistent pattern. Greatest coseismic subsidence (∼1–2 m) occurred in northern Oregon/southern Washington; subsidence elsewhere was ∼0–1 m. Elastic dislocation models constrained by interseismic geodetic and thermal data are used to predict the coseismic subsidence for two likely strain accumulation periods of (i) 800 and (ii) 550 yr of plate convergence and for uniform megathrust slip of 10, 20, 30, and 50 m. The former two models provide a better and equally good fit; predicted subsidence is in broad agreement with marsh estimates. Discrepancies exist, however, at the ends of the subduction zone. In the south, misfit may be due to breakup of the Gorda plate. The discrepancy in the north may be explained if the 1700 event released only part of the accumulated strain there, consistent with long-term net uplift in excess of eustatic sea-level rise. The coseismic slip magnitude, estimated by comparing uniform slip model predictions with marsh coseismic subsidence estimates, is consistent with the M 9 earthquake indicated by Japanese tsunami records. The coseismic slip was greatest in northern Oregon/southern Washington, declining to the north and south. [ABSTRACT FROM AUTHOR]

Presents a study which used tree-ring dating of living trees and trees killed by an earthquake, to place the time of the event along a stretch of coast of Washington, Oregon and northern California between the growing seasons of A.D. 1699 and 1700. Methodology of experiments conducted; Results obtained; What the results suggest.