Geology topics

Brian L. Sherrod

Publications and source records attributed to Brian L. Sherrod.

At least 19 recordsLinked to original sources

USGS2026

Colored shaded relief bathymetry of Lake Chelan, Washington

In 2023, the U.S. Geological Survey (USGS) completed a 3-week field operation in Lake Chelan, Washington, the third deepest lake in the conterminous United States, to acquire sediment cores, high-resolution bathymetry data, and chirp sub-bottom profiles. The high-resolution bathymetry displayed in this map publication was collected using a Norbit Winghead i67-curved array multibeam system; raw data were cleaned and gridded (3 by 3 meters [m]) in the Teledyne CARIS software. In some locations where the slopes of Lake Chelan are near-vertical, multibeam returns were sparse, resulting in artifacts resembling holes that appear as grey dots within the color-shaded bathymetric relief. Although the 2023 bathymetric mapping of Lake Chelan resulted in similar morphometric characteristics to previous hydrographic campaigns (table 1; Kendra and Singleton, 1987; Pelletier and others, 1989), these new data provide a much more detailed understanding of the two physiographic basins that comprise Lake Chelan: the deeper (as deep as 445.8 m [fig. 1]; previously reported as 453 m) and narrower (1–2 kilometers [km] wide) Lucerne Basin to the north, and the shallower (as deep as 127.8 m [fig. 1]), and wider (2–3 km wide) Wapato Basin to the south. With this detailed imaging of Lake Chelan, we find pervasive morphological evidence of slope failure and mass-transport deposits (MTDs) in at least 89 locations within the lake and 63 subaqueous deltas (fig. 2). The deeper Lucerne Basin is dominated by steep slopes and numerous subaqueous deltas fed by steep fluvial catchments, whereas the Wapato Basin contains numerous landslide scarps, scars, and debris aprons along the more gently sloping lake margins, and abundant relict kettle structures from the Pleistocene deglaciation. Lake Chelan’s geomorphic, geophysical, and sediment data contain important records of glacial retreat and advance, regional volcanic eruptions, and earthquakes that generate ground motions severe enough to induce landslides recorded in the sediment as MTDs. The epicenter of the 1872 Washington State Earthquake (M 6.5–7.2; Bakun and others, 2002; Brocher and others, 2017; Sherrod and others, 2021) is along the Spencer Canyon Fault, near the southern end of Lake Chelan. Shaking from this earthquake was strong enough to affect local aquifers, induce liquefaction of unconsolidated sediments, produce large seiches (Brocher and others, 2018), and likely cause landslides within Lake Chelan.

Washington↗

USGS2026

Latest Pleistocene to 19th-century earthquakes on bending-moment reverse faults of the Seattle fault zone, Washington

Fault-related folds and their associated secondary faults play a critical yet often underrecognized role in accommodating strain and generating earthquakes in active fold-and-thrust belts. In the Seattle fault zone (SFZ), Washington, USA, we present new paleoseismic, geomorphic, and geophysical evidence for late Pleistocene and Holocene earthquakes on shallow, south-dipping secondary faults—the Lytle Beach and Vasa Park faults—that lie within the hanging wall of the greater SFZ and are on trend with the primary, blind Blakely Harbor fault. Our data show that these structures have ruptured independently, producing localized uplift and deformation at the surface, with the most recent event (RH2) likely occurring in the early nineteenth century. While a temporal overlap between the late Pleistocene RH1 and VP1 earthquakes raises the possibility of a ≥35 km rupture along the Blakely Harbor fault, structural and temporal evidence instead supports independent rupture on individual faults related to folding. We interpret these faults as bending-moment reverse faults that formed within a synclinal hinge zone of the main fault, reflecting mechanical and kinematic influences of the broader fault system. Combined with prior studies, our findings indicate that faulting related to folding dominates the mode of strain release within the SFZ since the late Pleistocene with more frequent earthquake recurrence (∼350 yr) over the past ∼2500 yr.

Washington↗

USGS2025

Bathymetric and acoustic-backscatter mapping of Lake Sammamish, Washington, during USGS field activity 2021-656-FA

In 2021, scientists from the U.S. Geological Survey (USGS) acquired bathymetric and acoustic-backscatter data within Lake Sammamish, Washington. Mapping was completed to find evidence of past earthquakes such as underwater landslides and is part of a larger USGS project to understand the overall geologic hazards history of the Cascadia Margin region. The survey was conducted using the USGS research vessel Parke Snavely outfitted with an interferometric sidescan-sonar system for swath mapping. Three estimates of bathymetric uncertainty were calculated from the post-processed bathymetric data. This report provides a summary of the mapping mission.

Washington↗

USGS2025

Colored shaded-relief bathymetry and acoustic backscatter of Lake Sammamish, Washington

Evidence of strong earthquakes (such as underwater landslides and associated deposits) may be recorded within the lacustrine sediments of Pacific Northwest lakes. The floor of Lake Sammamish, Wash., an approximately 11 kilometer (6.8 mile) long, 2 kilometer (1.2 mile) wide, and 35 meter (114.8 feet) deep lake located in a populated region just east of Seattle, was mapped by the U.S. Geological Survey in November of 2021 to search for evidence of past earthquakes. Mapping was conducted using a SWATHplus-M 234-kHz interferometric side-scan sonar system was pole-mounted on the U.S. Geological Survey research vessel Parke Snavely, and the system collected full-coverage bathymetric and acoustic backscatter data, which were processed to 2-meter spatial resolution. Two maps were created, a colored shaded-relief bathymetric map showing lake floor morphology (sheet 1), and an acoustic-backscatter map showing backscatter intensities (sheet 2). The results may then be utilized together to investigate past earthquake activity.

Washington↗

USGS2025

Earthquake probabilities and hazards in the U.S. Pacific Northwest

Earthquakes and their cascading consequences pose a significant threat to the people, environment, infrastructure, and economy of the U.S. Pacific Northwest. The Pacific Northwest is susceptible to three types of earthquakes: deep (intraslab) earthquakes, subduction zone (megathrust) earthquakes, and shallow crustal earthquakes. For each of these earthquake types, earth scientists can use a variety of methods to estimate the probability of occurrence for future events, which constrains seismic hazard and informs building codes. The timing of past earthquakes indicates that there is an 85-percent chance of a magnitude 6.5 or greater deep earthquake in the Puget Sound region; a 10-15-percent chance of an approximately magnitude 9 earthquake on the Cascadia Subduction Zone; and a 17-percent chance of a magnitude 6.5 or greater crustal fault earthquake in the Puget Sound region in the next 50 years. Individuals and communities can take simple steps to prepare for and reduce the impact of future earthquakes.

California, Oregon, Washington↗

USGS2024

Post-glacial stratigraphy and late Holocene record of great Cascadia earthquakes in Ozette Lake, Washington, USA

Ozette Lake is an ~100-m-deep coastal lake located along the outer coast of the Olympic Peninsula (Washington, USA); it is situated above the locked portion of the northern Cascadia megathrust but also relatively isolated from active crustal faults and intraslab earthquakes. Here we present a suite of geophysical and geological evidence for earthquake-triggered mass transport deposits (MTDs) and related turbidite deposition in Ozette Lake since ca. 14 ka. Comprehensive high-resolution bathymetry data, seismic reflection profiles, and sediment cores are used to characterize the post-glacial stratigraphic framework and examine paleoseismic evidence in the lacustrine sediments. Stacked sequences of MTDs along the steep eastern flanks of the lake appear to grade basin-ward from thick, chaotic, blocky masses to thin, parallel-bedded turbidite beds. The discrete turbidite event layers are separated by fine-grained (silt and clay) lake sedimentation. The event layers are observed throughout the lake, but the physical characteristics of the deposits vary considerably depending on proximity to primary depocenters, steep slopes, and subaqueous deltas. A total of 30–34 event deposits are observed in the post-glacial record. Radiometric dating was used to reconstruct a detailed sedimentation history over the last ~5.5 k.y., develop an age model, and estimate the recurrence (365–405 yr) for the most recent 12 event layers. Based on sedimentological characteristics, temporal overlap with other regional paleoseismic chronologies, and recurrence estimates, at least 10 of the dated event layers appear to be sourced from slope failures triggered by intense shaking during megathrust ruptures; the recurrence interval for these 10 events is 440–560 yr. Thus, Ozette Lake contains one of the longest and most robust geological records of repeated shaking along the northern Cascadia subduction zone.

Washington↗

USGS2024

Plan to coordinate post-earthquake investigations supported by the National Earthquake Hazards Reduction Program (NEHRP)

Introduction This report presents a plan supported by the National Earthquake Hazards Reduction Program (NEHRP) to coordinate domestic and international post-earthquake investigations (herein called “the Plan”). Post-earthquake scientific and engineering investigations are undertaken to capture critical information to understand the causes and impacts of the event, lessons from which can substantially improve the Nation’s resilience after future earthquakes. NEHRP is the Federal Government’s coordinated nationwide program to reduce risks to life and property from earthquakes. The Plan describes the activation and coordination of the four designated NEHRP Agencies in the Federal Government: Federal Emergency Management Agency, National Institute of Standards and Technology, U.S. National Science Foundation, and U.S. Geological Survey (USGS). The Plan also describes coordination between NEHRP Agencies and other organizations that may participate in pre-event and post-earthquake investigations, including non-NEHRP Federal agencies; State, regional, local, Tribal, and territorial agencies; domestic nongovernmental organizations; academic institutions and affiliated organizations; private companies; foreign governmental agencies and nongovernmental organizations; and international organizations. The Plan delineates the coordination of NEHRP post-earthquake scientific and engineering investigations to document the direct, indirect, and cascading physical and societal impacts from fault rupture and ground shaking hazards and from secondary hazards such as landslides, liquefaction, and tsunamis. In addition, the Plan identifies pre-event activities necessary to ensure that post-earthquake investigations are executed effectively. The USGS is the lead NEHRP Agency for activating and coordinating NEHRP post-earthquake investigations and for implementing this Plan. The USGS also leads coordination of the NEHRP Agencies in completing the pre-event activities identified in the Plan. The Plan has new information and supersedes USGS Circular 1242, “The Plan to Coordinate NEHRP Post-Earthquake Investigations,” which was published in 2003. This second-generation Plan was developed with the assistance of the Applied Technology Council (ATC) of Redwood City, California, under USGS contract 140G0121P0309, ATC-155. A 17-member Project Review Panel provided guidance on plan development, and input was solicited from subject matter experts representing key stakeholder groups and from participants at a public workshop.

Circular↗

USGS2024

Debris avalanches in the northern California Coast Range triggered by plate boundary earthquakes

Determining the timing and cause for ancient hillslope failures proves difficult in the western United States, yet critical as it ties directly into groundmotion estimates for hazardous events. This knowledge gap is important to confront as hillslope failures are candidates to be triggered by earthquakes along active plate boundaries. We identify two prehistoric, i.e., preinstrumental history, debris avalanches (3 10^6 and 6 10^6 m^3) in the Coast Ranges of northern California. These debris avalanches are well suited for studying the timing (to the exact year) and trigger as the densely forested landscape enables effective use of dendrochronology and high-resolution radiocarbon to pinpoint time of failure, and the legacy of landslide studies in the region provides context for evaluating a seismic trigger as the most likely triggering mechanism. Neither debris avalanche was triggered by the largest precipitation event of the instrumental record in 1964 CE, and the two debris avalanche sites are physiographically suited to accommodate topographic amplification of seismic shaking. Through a suite of geologic and dendrochronologic evidence, we establish the likely time of failure of the two debris avalanches as 1906 CE and 933 CE. The failure of the younger landslide coincides with the 1906 CE San Francisco earthquake on the Pacific/North American plate boundary, while the older landslide dates to 933 CE falling within the broad age range (850-966 CE) of a likely megathrust earthquake on the Cascadia subduction zone as recorded in coastal marshes in Humboldt Bay, California. The precise age on the 933 CE debris avalanche could trim the uncertainty on a Cascadia subduction zone event from 116 years to one year. Utilizing debris avalanche records from sites suitable to record seismic shaking improves understanding of plate-boundary earthquake timing and extent of shaking.

Bulletin of the Seismological Society of America↗

USGS2024

Colored shaded relief bathymetry and acoustic backscatter of Ozette Lake, Washington

Offshore of the Pacific Northwest of the United States is the Cascadia Subduction Zone, a 1,000-kilometer-long tectonic boundary defined by a large fault, called a megathrust, that extends from the Mendocino Junction off northern California to the Nootka Fracture Zone off Vancouver Island, Canada (U.S. Geological Survey, 2023). The Juan de Fuca and Gorda oceanic plates to the west of this boundary subduct under the North America continental plate to the east. Several other smaller faults that cut through the North America plate crust also affect the region. Although their effects upon Ozette Lake are uncertain, geological evidence for past earthquakes, such as underwater landslides, may be found in Pacific Northwest lakes. Underwater landslides caused by past earthquakes should be well preserved in these relatively undisturbed lake environments. The floor of Ozette Lake, Washington, located along the Pacific coast of the United States, west of the Puget Sound region and about 140 kilometers east of the megathrust was mapped by the U.S. Geological Survey in July of 2019 to search for evidence of past earthquakes. Mapping was completed using a SWATHplus-M 234-kHz interferometric side scan sonar system pole-mounted on the U.S. Geological Survey research vessel San Lorenzo. The system collected full-coverage bathymetric and acoustic backscatter data that were processed to 2-meter spatial resolution (Dartnell and others, 2024). This two-map series displays the results of this mapping. A colored shaded relief bathymetry map (sheet 1) and an acoustic backscatter map (sheet 2) show the lake floor morphology and backscatter intensities, respectively, that can be analyzed for evidence of past earthquakes.

Washington↗

USGS2024

Field observations and logs from the Rose Hip trench exposure across a north-facing scarp within the Seattle Fault Zone, southern Bainbridge Island, Washington

The Seattle Fault Zone is an approximately 70-km-long, east-west-trending zone of south-dipping blind reverse faults within the Puget lowland region in Washington. Because of the proximity, the Seattle Fault Zone poses a significant earthquake hazard to the Puget sound and Seattle metropolitan regions. We present preliminary mapping and trench-site information from a paleoseismic investigation across a newly identified active fault scarp located within the hanging wall of the Seattle Fault Zone on southern Bainbridge Island, Washington. The trench exposed monoclinally folded Miocene bedrock, fractured and faulted glacial-related deposits, and laminated lacustrine deposits capped by slope-derived colluvium. The observations from this investigation record late Pleistocene to Holocene north-vergent folding and faulting along this new fault scarp.

Washington↗

USGS2023

Logs and data from the Starthistle trench across a scarp within the Wallula Fault Zone, southeastern Washington

Introduction The Wallula Fault Zone is composed of a series of northwest-trending faults and folds that coincide with a prominent magnetic anomaly that extends uninterrupted for approximately 120 kilometers within the Cascadia back arc of southeastern Washington and northeastern Oregon. It is part of the geologic structures associated with the topographic lineament known as the Olympic-Wallowa lineament and represents a relatively narrow zone of active faulting and seismicity that trend along the northern flank of the Horse Heaven Hills. Prior paleoseismic study of the Wallula Fault at an exposure at Finley Quarry indicates multiple Quaternary ruptures, including a Holocene liquefaction event, demonstrating that this fault zone poses a seismic hazard to the Tri-Cities region (Richland, Pasco, Kennewick) in southeastern Washington. Recent airborne light detection and ranging (lidar) data coverage east of the Columbia River reveals an approximately 0.5-meter-high south-facing scarp east of the Columbia River that extends, almost continuously, for about 25 km to the east along the base of the Horse Heaven Hills. As part of an effort to assess and characterize the seismic hazard posed by the Wallula Fault Zone, we excavated and studied a trench exposure across the scarp to understand its origin and the potential history of rupture along the fault zone. We present preliminary mapping and trench site information from a paleoseismic investigation. These field and laboratory data may support development of a history of the latest Pleistocene and Holocene surface rupture within the Wallula Fault Zone.

Washington↗

USGS2023

Witnessing history: Comparison of a century of sedimentary and written records in a California protected area

We use a combination of proxy records from a high-resolution analysis of sediments from Searsville Lake and adjacent Upper Lake Marsh and historical records to document over one and a half centuries of vegetation and socio-ecological change—relating to logging, agricultural land use change, dam construction, chemical applications, recreation, and other drivers—on the San Francisco Peninsula. A relatively open vegetation with minimal oak ( Quercus ) and coast redwood ( Sequoia sempervirens ) in the late 1850s reflects widespread logging and grazing during the nineteenth century. Forest and woodland expansion occurred in the early twentieth century, with forests composed of coast redwood and oak, among other taxa, as both logging and grazing declined. Invasive species include those associated with pasturage ( Rume x , Plantago ), landscape disturbance ( Urtica , Amaranthaceae), planting for wood production and wind barriers ( Eucalyptus ), and agriculture. Agricultural species, including wheat, rye, and corn, were more common in the early twentieth century than subsequently. Wetland and aquatic pollen and fungal spores document a complex hydrological history, often associated with fluctuating water levels, application of algaecides, raising of Searsville Dam, and construction of a levee. By pairing the paleoecological and historical records of both lakes, we have been able to reconstruct the previously undocumented impacts of socio-ecological influences on this drainage, all of which overprinted known climate changes. Recognizing the ecological manifestations of these impacts puts into perspective the extent to which people have interacted with and transformed the environment in the transition into the Anthropocene.

California↗

USGS2023

The Searsville Lake Site (California, USA) as a candidate Global boundary Stratotype Section and Point for the Anthropocene series

Cores from Searsville Lake within Stanford University’s Jasper Ridge Biological Preserve, California, USA, are examined to identify a potential GSSP for the Anthropocene: core JRBP2018-VC01B (944.5 cm-long) and tightly correlated JRBP2018-VC01A (852.5 cm-long). Spanning from 1900 CE ± 3 years to 2018 CE, a secure chronology resolved to the sub-annual level allows detailed exploration of the Holocene-Anthropocene transition. We identify the primary GSSP marker as first appearance of 239,240 Pu (372–374 cm) in JRBP2018-VC01B and designate the GSSP depth as the distinct boundary between wet and dry season at 366 cm (6 cm above the first sample containing 239,240 Pu) and corresponding to October-December 1948 CE. This is consistent with a lag of 1–2 years between ejection of 239,240 Pu into the atmosphere and deposition. Auxiliary markers include: first appearance of 137 Cs in 1958; late 20th-century decreases in δ 15 N; late 20th-century elevation in SCPs, Hg, Pb, and other heavy metals; and changes in abundance and presence of ostracod, algae, rotifer and protozoan microfossils. Fossil pollen document anthropogenic landscape changes related to logging and agriculture. As part of a major university, the Searsville site has long been used for research and education, serves users locally to internationally, and is protected yet accessible for future studies and communication about the Anthropocene.

California↗

USGS2023

A multifault earthquake threat for the Seattle metropolitan region revealed by mass tree mortality

Compound earthquakes involving simultaneous ruptures along multiple faults often define a region’s upper threshold of maximum magnitude. Yet, the potential for linked faulting remains poorly understood given the infrequency of these events in the historic era. Geological records provide longer perspectives, although temporal uncertainties are too broad to clearly pinpoint single multifault events. Here, we use dendrochronological dating and a cosmogenic radiation pulse to constrain the death dates of earthquake-killed trees along two adjacent fault zones near Seattle, Washington to within a 6-month period between the 923 and 924 CE growing seasons. Our narrow constraints conclusively show linked rupturing that occurred either as a single composite earthquake of estimated magnitude 7.8 or as a closely spaced double earthquake sequence with estimated magnitudes of 7.5 and 7.3. These scenarios, which are not recognized in current hazard models, increase the maximum earthquake size needed for seismic preparedness and engineering design within the Puget Sound region of >4 million residents.

Washington↗

USGS2021

LiDAR and paleoseismology solve earthquake mystery in the Pacific Northwest, USA

One of the largest historical earthquakes in the U.S. Pacific Northwest occurred on December 15, 1872 near the south end of Lake Chelan. Lack of recognized surface deformation suggested that the earthquake occurred on a blind, perhaps deep, fault. New LiDAR data revealed a NW-side-up scarp along the north side of Spencer Canyon near Entiat, Washington. Landslides triggered during the earthquake impounded small ponds in Spencer Canyon; the larger of the two landslides obliterated a portion of the scarp. Tree-ring counts show that the oldest trees on each landslide are 130 and 128 years old, and lend credence to the idea that the earthquake triggered the landslides. Trenches across the scarp exposed a NW-dipping thrust fault offsetting young soils and Mesozoic bedrock. Radiocarbon and tree ring data shows that the last fault movement was between 1856 and 1873 CE, and was most likely during the 1872 CE earthquake.

Idaho, Montana, Oregon, Washington↗

USGS2021

Toward an integrative geological and geophysical view of Cascadia subduction zone earthquakes

The Cascadia subduction zone (CSZ) is an exceptional geologic environment for recording evidence of land level changes, tsunamis, and ground motion that reveals at least 19 great megathrust earthquakes over the past 10 kyr. Such earthquakes are among the most impactful natural hazards on Earth, transcend national boundaries, and can have global impact. Reducing the societal impacts of future events in the U.S. Pacific Northwest and coastal British Columbia, Canada requires improved scientific understanding of megathrust earthquake rupture, recurrence, and corresponding hazards. Despite substantial knowledge gained from decades of research, large uncertainties remain about the characteristics and frequencies of past CSZ earthquakes. In this review, we summarize geological, geophysical, and instrumental evidence relevant to understanding megathrust earthquakes along the CSZ and associated uncertainties. We discuss how the evidence constrains various models of great megathrust earthquake recurrence in Cascadia and identify potential paths forward for the earthquake science community.

British Columbia, Oregon, Washington↗

USGS2020

Improving paleoseismic earthquake magnitude estimates with rupture length information: Application to the Puget Lowland, Washington State, U.S.A.

Both earthquake displacement and rupture length correlate with magnitude, and therefore observations of each from past earthquakes can be used to estimate the magnitude of those earthquakes in the absence of instrumental records. We extend the Bayesian inversion method of Biasi and Weldon (2006), which estimates paleoearthquake magnitude from displacement observations, to incorporate both rupture length and surface displacement measurements into the magnitude inversion. We then use this method on 27 late Pleistocene to Holocene paleoearthquakes in the Puget Lowland region of Washington. Observations of (typically vertical) fault separation per event range from 0.6 to 7 m, implying net displacement per event of up to 10 ± 4 m for the largest event. Rupture lengths are estimated to vary between the smallest contiguous mapped scarps to the full extent of the faults mapped from geology and geophysical observations. Although a few of the ruptures may be longer than 150 km, the ruptures have a median of 53 km, indicating that earthquakes in the Puget Lowland have relatively high displacement to length ratios. By considering both datasets, we find that all events were between M 6.3 and 7.5, generally consistent with the expected seismicity from the USGS National Seismic Hazard Map for the region. The simultaneous use of both length and displacement data in the magnitude inversion decreases both the estimated earthquake magnitudes and the uncertainty. The magnitude reduction in particular is due to the relatively short rupture lengths possible for Puget Lowland faults. This implies a decrease in the seismic hazard (relative to a displacement-only assessment) to a highly populated and rapidly urbanizing region.

Washington↗

USGS2020

Post-glacial Mw 7.0-7.5 earthquakes on the North Olympic fault zone, Washington

Holocene crustal faulting in the northern Olympic Peninsula of Washington State manifests in a zone of west‐northwest‐striking crustal faults herein named the North Olympic fault zone, which extends for ∼ 80    km "> ∼ 80 km ∼80 km along strike and includes the Lake Creek–Boundary Creek fault to the east and the Sadie Creek fault and newly discovered scarps to the west. This study focuses on the Sadie Creek fault, which extends for > 14    km "> > 14 km >14 km west‐northwest from Lake Crescent. Airborne light detection and ranging (lidar) imagery reveals the trace of the Sadie Creek fault and offset postglacial landforms showing a history of Holocene surface‐rupturing earthquakes dominated by dextral displacement along a steeply dipping fault zone. Paleoseismic trenches at two sites on the Sadie Creek fault reveal till and outwash overlain by progressively buried forest and wetland soils developed on scarp‐derived colluvial wedges. Trench exposures of complex faulting with subhorizontal slickenlines indicate dextral displacement with lesser dip slip. Correlation of broadly constrained time intervals for earthquakes at the Sadie Creek sites and those to the east along the Lake Creek–Boundary Creek fault is consistent with rupture of much of the length of the North Olympic fault zone three to four times: at about 11, 7, 3, and 1 ka, with a shorter rupture at about 8.5 ka. Dated ruptures from trenches only partially coincide with coseismic landslides and megaturbidites in Lake Crescent, indicating that some earthquakes did not trigger megaturbidites, and some turbidites were unrelated to local fault rupture. Landform mapping suggests single‐event dextral displacement of 4 ± 1    m "> 4 ± 1 m 4±1 m on the Sadie Creek fault. Inferred maximum rupture length and single‐event slip imply earthquake magnitudes M w "> M w Mw 7.0–7.5. Dextral slip rates of 1.3 – 2.3    mm / yr "> 1.3 – 2.3 mm / yr 1.3–2.3 mm/yr and the ∼ 11 , 000    yr "> ∼ 11 , 000 yr ∼11,000 yr slip history suggest that the North Olympic fault zone is a prominent contributor to permanent strain in the northern Cascadia fore‐arc.

Washington↗