We found that repeated slow slip events observed on the deeper interface of the northern Cascadia subduction zone, which were at first thought to be silent, have unique nonearthquake seismic signatures. Tremorlike seismic signals were found to correlate temporally and spatially with slip events identified from crustal motion data spanning the past 6 years. During the period between slips, tremor activity is minor or nonexistent. We call this associated tremor and slip phenomenon episodic tremor and slip (ETS) and propose that ETS activity can be used as a real-time indicator of stress loading of the Cascadia megathrust earthquake zone.

Continuous Global Positioning System sites in southwestern British Columbia, Canada, and northwestern Washington state, USA, have been moving landward as a result of the locked state of the Cascadia subduction fault offshore. In the summer of 1999, a cluster of seven sites briefly reversed their direction of motion. No seismicity was associated with this event. The sudden displacements are best explained by approximately 2 centimeters of aseismic slip over a 50-kilometer-by-300-kilometer area on the subduction interface downdip from the seismogenic zone, a rupture equivalent to an earthquake of moment magnitude 6.7. This provides evidence that slip of the hotter, plastic part of the subduction interface, and hence stress loading of the megathrust earthquake zone, can occur in discrete pulses.

The active tectonic setting of the southwest coast of Canada and the Pacific northwest coast of the United states is dominated by the Cascadia subduction zone. The zone can be divided into four segments where oceanic lithosphere is converging independently with the North American plate: the Winona and the Explorer segments in the north, the larger Juan de Fuca segment that extends into both Canada and the United States, and the Gorda segment in the south. The oceanic lithosphere entering the Cascadia subduction zone in all segments is extremely young, less than 10 Ma. Of the other six zones around the Pacific where young (< 20 Ma) lithosphere is being subducted, five have had major thrust earthquakes (megathrust events) on the subduction interface in historic time. An estimation based on potential area of rupture gives maximum possible earthquake magnitudes along the Cascadia subducting margin of 8.2 for the Winona segment, 8.5 for the Explorer segment, 9.1 for the Juan de Fuca segment, and 8.3 for the South Gorda segment. Repeat times for maximum earthquakes, based on the ratios of seismic slip to total slip observed in other subduction zones, are predicted to be up to several hundred years for each segment, well beyond recorded history of the west coast, which began about 1800. Thus the lack of historical seismicity information provides a few constraints on the assessment of the seismic potential of the subduction zone.

ABSTRACT We evaluate the potential performance of the ShakeAlert earthquake early warning system for M 9 megathrust earthquakes in the Pacific Northwest (PNW) using synthetic seismograms from 30 simulated M 9 earthquake scenarios on the Cascadia subduction zone. The timeliness and accuracy of source estimates and effectiveness of ShakeAlert alert contours are evaluated with a station-based alert classification scheme using an alert threshold equal to the target threshold. We develop a population-based alert classification method by aligning a population grid with Voronoi diagrams computed from the station locations for each scenario. Using raster statistics, we estimate the PNW population that would receive timely accurate alerts during an offshore M 9 earthquake. We also examine the range of expected warning times with respect to the spatial distribution of the population. Results show that most of the population in our evaluation region could receive alerts with positive warning times for an alert threshold of modified Mercalli intensity (MMI) III, but that late and missed alerts increase because the alert threshold is increased. An average of just under 60% of the population would be alerted for MMI V prior to the arrival of threshold level shaking. Large regions of late and missed alerts for thresholds MMI IV and V are caused by delays in alert updates, inaccurate FinDer source estimates, and undersized alert contours due to magnitude underestimation. We also investigate an alerting strategy where ShakeAlert sends out an alert to the entire evaluation region when the system detects at least an M 8 earthquake along the coast. Because large magnitude offshore earthquakes are rare in Cascadia, overalerting is most likely to occur from an overestimated M 7+ on the Gorda plate. With appropriate criteria to minimize overalerting, this strategy may eliminate all missed and late alerts except at sites close to the epicenter.

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.

SUMMARY The Cascadia subduction zone in the Pacific Northwest of the United States of America capable of producing magnitude ∼9 earthquakes, likely often accompanied by tsunamis. An outstanding question in this region is the degree and spatial extent of interseismic strain accumulation on the subduction megathrust. Seafloor geodetic methods combining GNSS and underwater acoustic ranging (GNSS-A) are capable of imaging this strain accumulation on the offshore portion of the subduction zone and therefore anticipating the potential size and rupture pattern of a future earthquake. However, the high cost of seafloor geodesy means that only a limited number of stations may be deployed and monitored. To facilitate expansion of current geodetic networks offshore, we develop a quantitative recommendation of optimal locations for future seafloor geodetic observations, based on the amount of new information provided by that observation. The optimal network depends on the problem that one is trying to solve with those observations (mapping subduction locking rates, coupling rates, constraining total moment rate, etc.), and on a number of modelling and data uncertainty assumptions. In particular, data uncertainty assumptions will change over time, as more position observations reduce velocity uncertainties. We find that near-trench observations on the megathrust hangingwall, distributed along-strike, consistently provide significant reduction in differential entropy over a large suite of

Increasing public awareness that the Cascadia subduction zone in the Pacific Northwest is capable of great earthquakes (magnitude 9 and greater) motivates the Cascadia Initiative, an ambitious onshore/offshore seismic and geodetic experiment that takes advantage of an amphibious array to study questions ranging from megathrust earthquakes, to volcanic arc structure, to the formation, deformation and hydration of the Juan De Fuca and Gorda Plates. Here, we provide an overview of the Cascadia Initiative, including its primary science objectives, its experimental design and implementation, and a preview of how the resulting data are being used by a diverse and growing scientific community. The Cascadia Initiative also exemplifies how new technology and community-based experiments are opening up frontiers for marine science. The new technology—shielded ocean bottom seismometers—is allowing more routine investigation of the source zone of megathrust earthquakes, which almost exclusively lies offshore and in shallow water. The Cascadia Initiative offers opportunities and accompanying challenges to a rapidly expanding community of those who use ocean bottom seismic data.

The Cascadia subduction zone is a 620-mile (1,000 km) long convergent plate boundary, about 70–100 miles (110–160 kilometers) off the Pacific coast of North America, that stretches from northern Vancouver Island in Canada to Northern California in the United States. It is capable of producing 9.0+ magnitude earthquakes and tsunamis that could reach 100 feet (30 m) high. The Oregon Department of Em The… The…

Complex interactions between strain accumulation, fault slip, and fluid migration influence shallow subduction zone dynamics. Using a decade of continuous ambient seismic data from Cascadia's seafloor observatories, we identified distinct regional variations in subduction dynamics. Northern Cascadia exhibits a fully locked megathrust with persistent strain accumulation, while central Cascadia displays a slow slip event on protothrusts and rapid fluid migration along fault systems in the overriding plate. Effective fluid transport through the décollement and the Alvin Canyon Fault likely modulates the earthquake behavior but does not cause slow slip events on the megathrust and likely stabilizes large earthquakes, promoting rupture arrest.

The Pacific Northwest (PNW; French: Nord-Ouest Pacifique), also referred to as Cascadia, is a bi-national geographic and cultural region in Western North America defined by its coastal waters of the Pacific Ocean to the west and, loosely, by the Rocky Mountains to the east. Though no official boundary exists, the most common conception includes the U.S. states of Oregon, Washington, Idaho, and the From 1792 to 1794, George Vancouver charted the Pacific Northwest on behalf of Great Britain, including the Strait of Georgia, the bays and inlets of Puget Sound, and the Johnstone Strait–Queen Charlotte Strait and much of the rest of the British Columbia Coast and southeast Alaska shorelines. For him the city of Vancouver and Vancouver Island are named, as well as Vancouver, Washington. From Mexico, Malaspina dispatched the last Spanish exploration expedition in the Pacific Northwest, under Dionisio Alcalá Galiano and Cayentano Valdes aboard the schooners Sutil and Mexicana. They met Vancouver in the Strait of Georgia on June 21, 1792. Vancouver had explored Puget Sound just previously. The Spanish explorers knew of Admiralty Inlet and the unexplored region to the south, but they decided to sail north. They discovered and entered the Fraser River shortly before meeting Vancouver. After sharing maps and agreeing to cooperate, Galiano, Valdés, and Vancouver sailed north to Desolation Sound and the Discovery Islands, charting the coastline together. They passed through Johnstone Strait and Cordero Channel and returned to Nootka Sound. As a result, the Spanish explorers, who had set out from Nootka, became the first Europeans to circumnavigate Vancouver Island. Vancouver himself had entered the Strait of Juan de Fuca directly without going to Nootka first, so had not sailed completely around the island. In 1786, Jean-François de La Pérouse, representing France, sailed to Haida Gwaii after visiting Nootka Sound, but any possible French claims to this region were lost when La Pérouse and his men and journals were lost in a shipwreck near Australia. Upon encountering the Salish coastal tribes, either Pérouse or someone in his crew remarked, "What must astonish most is to see painting everywhere, everywhere sculpture, among a nation of hunters". Maritime fur trader Charles William Barkley also visited the area in Imperial Eagle, a British ship falsely flying the flag of the Austrian Empire. American merchant sea-captain Robert Gray traded along the coast, and discovered the mouth of the Columbia River. 1862 Pacific Northwest smallpox epidemic Atlantic Northeast,…

Tsunamis cause direct damage to property and destroy infrastructure. In addition, power outages can lead to death, especially for patients who rely on medical equipment requiring a power supply. Recently, Nankai Trough Earthquakes have been predicted, and much effort has been put into developing countermeasures in Japan. Kochi City on Shikoku Island is expected to suffer in the event of a large tsunami. The present study identifies individuals living in Kochi who need evacuation assistance and depend on electrical medical devices, simulates evacuation behavior and inundation during a tsunami u

To meet the challenges of hazards impacting coastal communities, demand is growing for more equitable coastal natural hazard adaptation and disaster mitigation approaches, supported by co-productive research partnerships. This review paper outlines contemporary advances in hazard adaptation and disaster mitigation with attention to how an equity and justice framework can address the uneven impacts of hazards on marginalized and underserved communities. Drawing upon the allied concepts of distributive, procedural, systemic, and recognitional equity and justice, we illustrate how these concepts form the basis for equitable coastal resilience. To demonstrate how equitable resilience can effectively advance contemporary adaptation and mitigation strategies, we present two vignettes where collaborative partnerships underscore how equitable coastal hazard planning and response practices complement these processes in coastal zones subject to large earthquakes and tsunamis. The first vignette focuses on disaster response and takes place in the Tohoku region of Japan, with diverse gender and sexual minority community members' experiences of, and responses to, the 2011 Tohoku disasters. The second vignette centers on hazard planning and takes place on the U.S. Pacific Northwest coast along the Cascadia Subduction Zone to demonstrate how principles of distributive, procedural, systemic, and recognitional equity can inform the co-production of alternative coastal futures that prioritize equitable resilience. From this discussion, we suggest applying an equity lens to research processes, including alternative futures modeling frameworks, to ensure that the benefits of hazard adaptation and disaster mitigation strategies are equitably applied and shared.