The Queen Charlotte plate boundary (QCPB), a transform separating the Pacific and North American plates, accommodates ~55 millimeters per year of motion, is a source of large earthquakes in the northeast Pacific, and may be a modern site of subduction initiation. The southern QCPB experiences oblique convergence, showcased by the 1949 magnitude (M) 8.1 strike-slip earthquake and the 2012 M7.8 tsunamigenic thrust earthquake, both offshore Haida Gwaii, British Columbia. We present seismic reflection images of the southern QCPB, which constrain the crustal structure in unprecedented detail. The Queen Charlotte Terrace is underthrust by oceanic crust topped by a throughgoing, low-angle plate-boundary thrust, which ruptured in the 2012 earthquake. The Queen Charlotte Terrace is analogous to strain-partitioned, thin-skinned forearc slivers seen at oblique subduction zones, captured between a localized plate-boundary thrust and a mature strike-slip fault. Our imaging suggests that the system rapidly evolved from distributed to partitioned strain and is currently an incipient subduction zone.

Substantial morphological changes are rapidly occurring along the Canadian Arctic shelf edge (Paull et al., 2022, PNAS). During a 2022 IBRV Araon cruise, autonomous underwater vehicle mapping surveys identified several new craters that formed between 2019 and 2022. Five multibeam bathymetric mapping surveys, each partially covering a 15 km2 study area between 120 and 200 mwd have now been conducted over a 12-year time period. These repeat surveys reveal 65 new depressions developed averaging 6.5 m deep and reaching up to 30 m deep. Remotely operated vehicle investigations also discovered outcrops of massive ice exposed on the flanks of the newest craters. This ice is not believed to be relic permafrost formed during Pleistocene sea-level low-stands because the host sediments were deposited in a submarine setting. The low porewater salinity and light isotopic compositions in the meltwater of ice samples from sediment cores indicate brackish waters reflecting a meteoric source are discharging and freezing in this area. The ascending brackish groundwater is likely derived from melting relict permafrost under the shelf. The ~ -1.4°C bottom water temperatures provide conditions appropriate for freezing brackish porewaters within the near seafloor sediments. Conditions appropriate for the melting of ice also exist nearby where ice is in contact with seawater or warmed by ascending groundwater. Small variations in either temperature or salinity, over time, can shift equilibrium conditions of ice formation and degradation, which leads to repetitive freezing and thawing of ascending brackish groundwater and the development of wide-spread ice layers in the near seafloor sediments. These conditions have produced a dramatic submarine thermokarst morphology riddled with multi-aged depressions captured in the repeat mapping surveys. These findings suggest that the distribution of submarine permafrost ice should be reassessed as it may include extensive areas where ice formed during the Holocene where groundwaters discharge at sub-zero temperatures, in addition to relict Pleistocene permafrost.

Plate motion obliquity along the dominantly transform Queen Charlotte plate boundary (QCPB) peaks offshore Haida Gwaii. To investigate the effects of obliquity on plate boundary deformation, we analyze continuous seismic waveforms from temporary and permanent stations from 1998 to 2020 to generate a catalog of similar to 50,000 earthquakes across Haida Gwaii. We use an automated technique based on auto-regressive phase detection and onset estimation to obtain the initial seismic catalog, integrate existing catalogs, invert for 3D velocity structure using data from the best constrained period, and relocate the entire catalog using the new 3D velocity model. We investigate the seismically active sections of the transcurrent Queen Charlotte fault (QCF), noting that little seismicity locates directly along its bathymetrically defined trace. Instead, seismicity illuminates a complex system of segmented structures with variable geometries along strike. Other clusters highlight active shallow faults within the highly deformed Queen Charlotte terrace. Few aftershocks appear on the thrust plane of the 2012 Mw 7.8 Haida Gwaii earthquake except near its inferred intersection with the QCF at 15-20 km depths, suggesting elevated residual stress at the juncture of slip-partitioning. Deep crustal seismicity (up to similar to 20 km depths) beneath central Haida Gwaii aligned parallel to the strike of the thrust plane may represent landward underthrusting of the Pacific plate. Our results suggest possible coseismic strike-slip rupture on the QCF during the 2012 earthquake and add support to the thesis that highly oblique transform boundaries are viable settings for subduction initiation. We investigated the complex tectonics offshore Haida Gwaii, western Canada, where the Pacific and North American plates slide past one another obliquely. By compiling and analyzing the most comprehensive earthquake catalog in the area, spanning 1998-2020, we present the most detailed report to date of the earthquake-producing structures in the region, including previously unidentified and highly segmented faults. Clusters of seismicity illuminate (a) a highly deformed terrace of elevated seafloor west of Haida Gwaii, (b) a complex and segmented fault system adjacent to the previously-mapped surface trace of the main Queen Charlotte strike-slip fault, and (c) the inferred fault intersection at depth between the subvertical Queen Charlotte fault (which hosted the 1949 magnitude 8.1 earthquake) and the shallowly dipping Haida Gwaii thrust (which hosted the 2012 magnitude 7.8 earthquake). We also speculate that the 2012 earthquake may also have involved some motion on the Queen Charlotte fault. These results contribute to better constraints on regional tectonics and hazards, and provide insights into the mechanisms of subduction initiation. Seismicity off Moresby Island is distributed along multiple segments slightly off of the Queen Charlotte fault trace Aftershocks at intersection of Queen Charlotte Fault with the 2012 Mw 7.8 thrust plane reflect residual stress at slip partitioning juncture Previously undocumented deep seismicity beneath Haida Gwaii is consistent with an underthrusting Pacific Plate

Abstract It is critical to characterize submarine landslide hazards near dense coastal populations, especially in areas with active faults, which can trigger slope failure, subsequent tsunamis, and damage seabed infrastructure during earthquake shaking. Offshore southern California, numerous marine geophysical surveys have been conducted over the past decade, and high‐resolution bathymetric and subsurface data now cover about 60 percent of the total region between Point Conception and the United States‐Mexico border from the California coast out to the base of Patton Escarpment ∼200 km offshore. In a comprehensive compilation and interpretive mapping effort, we find evidence of seafloor failure throughout offshore southern California with nearly 1,500 submarine landslide‐related features, including 63 discrete slide deposits with debris and >1,400 slide‐related scarps. In our analysis, we highlight new mapping of submarine landslides in Catalina Basin, the Del Mar slide, the San Gabriel slide complex, and the 232 km2 San Nicolas slide, the largest area of any known submarine landslide mass offshore southern California. Analysis of the spatial distribution of submarine landslide features suggests that most mapped slide features are located relatively near coastal sediment sources, particularly during sea‐level lowstand conditions, which underscores the importance of sediment supply and sediment accumulation on low‐gradient slopes as failure preconditioning processes. Tectonically driven uplift at shelf edges and along basin flanks is another key preconditioning factor, and our results also suggest that earthquakes along active faults trigger mass wasting, especially for repeated, small‐scale failures on tectonically steepened slopes.

Substantial seafloor morphological changes are rapidly occurring along the Canadian Arctic shelf edge. Five multibeam bathymetric mapping surveys, each partially covering a 15 km2 study area between 120- and 200-m water depth, were conducted over a 12-year time period. These surveys reveal that 65 new craters have developed between 2010 and 2022, averaging 6.5 m and reaching up to 30 m deep. Remotely operated vehicle investigations revealed massive ice outcrops exposed on two newly formed crater flanks. This ice is not relict subaerially formed Pleistocene permafrost because it is hosted in sediments which were deposited in a submarine setting post-deglaciation. Low salinity porewater and sediment core ice samples with depleted oxygen isotopic compositions indicate waters with a meteoric signature are discharging and freezing in this area. These ascending brackish groundwaters are likely derived in part from thawed relict permafrost hundreds of meters under the continental shelf. They refreeze as they approach the -1.4 degrees C seafloor, leading to the development of widespread, near seafloor, sub-bottom ice layers. Conditions appropriate for ice melting also exist nearby where ice is exposed to seawater or warmed by ascending groundwater. Small variations in temperature and salinity lead to shifts between freezing of ascending brackish groundwater or melting of near seafloor ice layers. These conditions have produced a dramatic submarine thermokarst morphology riddled with multi-aged depressions. Thermokarst geohazards may exist, unmapped, on other Arctic margins with groundwater channeled toward the shelf edge by a relict permafrost cap, and sufficiently cold shelf edge bottom water temperatures. Significant seafloor changes are rapidly happening along the Canadian Arctic shelf edge, where numerous craters and mounds occur. Five seafloor mapping surveys collected over 12 years covering the same area reveal 65 new craters, reaching up to 30 m but averaging 6.5 m deep, were formed between 2010 and 2022. Observations from a remotely operated vehicle showed massive ice outcrops along the flanks of two newly formed craters. Chemical analyses of the ice show that the source of the frozen water is ascending brackish groundwaters that refreeze near the -1.4 degrees C seafloor, forming widespread sub-bottom ice layers that blister the seafloor producing ice-cored mounds. The source of the groundwaters is likely melted relict permafrost from beneath the continental shelf. Where ice is exposed to seawater salinity or warmer groundwater, ice melting causes seafloor collapses. Minor temperature and salinity variations cause shifts between freezing of ascending brackish groundwater and melting of near-seafloor ice layers. These ongoing processes create a dramatic submarine landscape composed of numerous depression and ice-filled mounds of varying ages. This discovery of sub-seafloor ice with a groundwater origin significantly expands our understanding of submarine permafrost within the Arctic continental shelves. Massive outcrops of submarine ice layers were found within recently formed seafloor craters along the edge of the Arctic continental shelf On-going ice growth and decomposition is occurring around seafloor seepages to produce a distinctive submarine thermokarst topography Thermokarst geohazards may exist on other Arctic margins where sub-zero water temperatures and submarine groundwater seepage occur

The Queen Charlotte plate boundary marks a transpressional system between the Pacific and North American plates, extending from offshore Haida Gwaii in Canada into southeastern Alaska. Using continuous seismic waveforms from temporary and permanent seismic networks from 1998–2020, we produced a comprehensive catalog of ~50,000 earthquakes across the region near Haida Gwaii. We used an automated processing technique of auto-regressive phase detection and onset estimation to obtain the initial seismic catalog, integrated existing catalogs, inverted for 3D velocity structure using data from the most well constrained period, and relocated the entire catalog using the new 3D velocity model. We investigate the seismically active sections of the transcurrent Queen Charlotte fault (QCF), noting that little seismicity locates directly along the bathymetrically defined QCF trace. Instead, the seismicity illuminates a complex system of multiple segmented structures, featuring variable geometries along strike. Clustered shallow seismicity could indicate active shallow faults within the highly deformed Queen Charlotte terrace. Few aftershocks appear on the thrust plane of the 2012 Mw 7.8 Haida Gwaii earthquake except near its inferred intersection with the QCF between 15 and 20 km depths, suggesting elevated residual stress. Deep (up to ~20 km) crustal seismicity below central Haida Gwaii aligned parallel to the strike of the thrust plane may manifest the landward underthrusting of the Pacific plate. We also explore the possibility of coseismic strike-slip rupture on the QCF during the 2012 earthquake. Our results provide insights into postseismic strain accommodation and partitioning across this complex oblique transpressive system.

A high-resolution multibeam survey from a portion of the San Simeon Channel (offshore Morro Bay, California) captured a zone of recurring troughs and ridges adjacent to prominent submarine meander bends. Through an integrated study using surveying data, sediment core analysis, radiocarbon dating, and stable isotope measurements, we hypothesize that turbidity current event frequency was higher during the late Pleistocene than at present conditions. We speculate that the rise in sea-level following the Last Glacial Maximum sequestered sedimentation largely to the shelf during the Holocene. This work suggests that the occurrence of sediment gravity flows in this region, particularly away from any submarine channels, is appreciably lower than at times of continental shelf subaerial exposure.

Oceanic crust formed at mid-ocean ridges may be later modified by off-ridge magmatism forming seamounts, guyots, and islands. We investigate processes associated with seamount formation in the Gulf of Alaska Seamount Province using two coincident seismic reflection/wide-angle profiles. A north-south profile crosses the Kodiak-Bowie Seamount Chain and Aja fracture zone (FZ), and an orthogonal east-west profile is located about 90 km south of the seamount chain over Pacific plate oceanic crust. Structure along the profile away from the seamount chain is consistent with typical oceanic crust. Crust in our study region is thinnest (about 5.6 km) at the Aja FZ. Unlike observations from active transform faults, no low-velocity anomaly is observed at the Aja FZ suggesting that the crustal velocities have recovered to normal values through crack closure and crack healing. Higher lower crustal velocities (-7.3 and > 7.5 km/s) and thicker crust (-8.5 and-7.0 km) are observed near the Pratt and Durgin Seamounts and at the intersection of the Kodiak-Bowie Seamount Chain linear trend, respectively. These observations are attributed to magmatic underplating associated with seamount province magmatism. Lithospheric thickness variations across the Aja FZ may form a barrier or impediment to magmatic flow. The thickest crust (8.5 km) along our two profiles is located on the younger side of the FZ, and we suggest that the majority of magmatism jumped south of the Aja FZ when thinner lithosphere was encountered by the Bowie hot spot. The crustal structure near the Kodiak-Bowie Seamount Chain is most similar to that of other seamounts and guyots that formed on similarly young lithosphere (8-12 Ma). Our results suggest that litho-spheric thickness at the time of hot spot interaction has a large control on magmatic underplating at seamounts and seamount provinces.

First posted April 5, 2022 For additional information, contact: Pacific Coastal and Marine Science CenterU.S. Geological Survey2885 Mission St.Santa Cruz, CA 95060 Coastal and Marine Ecological Classification Standard (CMECS) geoform, substrate, and biotic component geographic information system (GIS) products were developed for the U.S. Exclusive Economic Zone (U.S. EEZ) of south-central California in the region of Santa Lucia Bank motivated by interest in development of offshore wind-energy capacity and infrastructure. The Bureau of Ocean Energy Management (BOEM), in coordination with the State of California and many other members of the California Task Force, issued calls for information in 2018 for the study area offshore of Morro Bay, California. The study area is in depths of 500 to 1,200 meters (m) and adjacent to a decommissioned nuclear power plant with a developed electric grid connection, and in an area of high wind resource. BOEM is the lead agency responsible for planning and leasing in the U.S. EEZ and funded this project to assess baseline conditions of, and the potential effects on, the seafloor environment. This project, carried out by the U.S. Geological Survey (USGS), resulted in three reports: one on biological analysis of seafloor video data, one on analysis of the geologic framework and hazards, and this report on seafloor habitat. The study area consists of 8,424 square kilometers (km2) of multibeam echo sounder (MBES) data acquired during five surveys from 2016 to 2019. Remotely operated vehicle (ROV) video was acquired in 2019 to supervise the classification of the MBES data into habitats. Derivatives of the MBES data were classified into 16 unique biotopes, 6 substrate types, 28 modifier groups, and 22 geoforms. The study area substrate is predominantly soft sediment (mud and fine sand) covering 7,804 km2 (92.7 percent) of the area. Mixed substrate areas on rocky banks, channel scarps, and the shelf break comprise 404 km2 (4.8 percent) of the study area. Hard substrate areas are found predominantly on the tops and flanks of banks and on bank ridges that separate canyons incising the banks. Hard substrates comprise 211 km2 of the study area (2.5 percent). After the bathymetry and backscatter raster images (rasters) were classified, manual editing was also done to remove noise artifacts. This effort was not completely successful and there are numerous erroneous small areas in the rasters that have been passed on to the CMECS polygon product. Nearly 120,000 annotations of organisms and their habitat were made from 25 video transects selected from 185 hours of ROV video. In total, 2,714 km2 of seafloor were successfully assigned to biotopes. Some biotopes were assigned to separate areas spatially distant from the transects that define the biotope. Expected relations between physical habitat and biota such as the number of species and the substrate induration and rugosity were verified. Slope is typically a predictive variable and was used in the classification of habitat, but the ground truth used for biotic component analysis included very little steeply sloping area. Ground-truth ROV operations were reduced by the sea state; additional ground truth could improve the biotic results and increase confidence in the spatial distribution of classifications reported here.

A series of large earthquakes in 1899 affected southeastern Alaska near Yakutat and Disenchantment Bays. The largest of the series, a MW 8.2 event on 10 September 1899, generated an ~12-m-high tsunami and as much as 14.4 m of coseismic uplift in Yakutat Bay, the largest coseismic uplift ever measured. Several complex fault systems in the area are associated with the Yakutat terrane collision with North America and the termination of the Fairweather strike-slip system, but because faults local to Yakutat Bay have been incompletely or poorly mapped, it is unclear which fault system(s) ruptured during the 10 September 1899 event. Using marine geophysical data collected in August 2012, we provide an improved tectonic framework for the Yakutat area, which advances our understanding of earthquake hazards. We combined 153 line km of 2012 high-resolution multichannel seismic (MCS) reflection data with compressed high-intensity radar pulse (Chirp) profiles, basin-scale MCS data, 2018 seafloor bathymetry, published geodetic models and thermochronology data, and previous measurements of coseismic uplift to better constrain fault geometry and subsurface structure in the Yakutat Bay area. We did not observe any active or concealed faults crossing Yakutat Bay in our high-resolution data, requiring faults to be located entirely onshore or nearshore. We interpreted onshore faults east of Yakutat Bay to be associated with the transpressional termination of the Fairweather fault system, forming a series of splay faults that exhibit a horsetail geometry. Thrust and reverse faults on the west side of the bay are related to Yakutat terrane underthrusting and collision with North America. Our results include an updated fault map, structural model of Yakutat Bay, and quantitative assessment of uncertainties for legacy geologic coseismic uplift measurements. Additionally, our results indicate the 10 September 1899 rupture was possibly related to stress loading from the earlier Yakutat terrane underthrusting event of 4 September 1899, with the majority of 10 September coseismic slip occurring on the Esker Creek system on the northwest side of Yakutat Bay. Limited (~2 m) coseismic or postseismic slip associated with the 1899 events occurred on faults located east of Yakutat Bay.

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 US 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. ▪ Despite outstanding geologic records of past megathrust events, large uncertainty of the magnitude and frequency of CSZ earthquakes remains. ▪ This review outlines current knowledge and promising future directions to address outstanding questions on CSZ rupture characteristics and recurrence. ▪ Integration of diverse data sets with attention to the geologic processes that create different records has potential to lead to major progress.

First posted July 30, 2021 For additional information, contact: Pacific Coastal and Marine Science CenterU.S. Geological SurveyPacific Coastal and Marine Science Center2885 Mission St.Santa Cruz, CA 95060 The California Continental Borderland is the complex continental margin in southern California that extends from Point Conception southward into northern Baja California (Fisher and others, 2009). This colored shaded-relief bathymetry map of the northern continental borderland in southern California was generated primarily from multibeam-echosounder data collected by the University of Washington in 2016, the Ocean Exploration Trust-Nautilus Exploration Program in 2015–17, and the National Oceanic and Atmospheric Administration in 2017. These datasets were processed in part by the U.S. Geological Survey. Additional smaller amounts of publicly available multibeam-bathymetry data collected by other federal and local agencies, academic institutions, and private firms were also incorporated into this map. Since the production of this map, other multibeam-bathymetry data have been collected in this region.

The lack of historic seismicity along the Cascadia subduction zone (CSZ) fuels the uncertainty of the megathrust rupture process and resulting hazards in the US and Canadian Pacific Northwest. Diverse types of paleoseismic proxies uniquely record different aspects of the earthquake rupture process. Thus, margin-wide integration of paleoseismic proxy records leverages these unique transcripts to provide a richer understanding of past earthquake variability in time and space. In a series of virtual workshops, the USGS Powell Center research group on CSZ earthquake science and hazards have compiled paleoseismic records of coseismic coastal land-level change, tsunami inundation, and ground shaking proxies such as landslides, liquefaction, and turbidites. To consistently integrate different datasets, we constructed ranking schemes to quasi-quantitatively evaluate the quality of geochronologic age control and certainty of earthquake evidence at 49 coastal land level change sites, 81 tsunami sites, 76 terrestrial paleoseismic shaking sites, and 13 marine paleoseismic shaking sites. At each site, we evaluated how well geochronologic samples and results (14C dates, stratigraphy, geological context etc.) constrain the timing of candidate earthquake events. Similarly, we applied an evidence quality ranking scheme to assess how certain the observations of coastal land-level change, tsunami inundation, and strong ground motion were the result of a CSZ megathrust earthquake source. We also focused on historic records of non-local events, such as the 1964 Alaska earthquake and the 1992 Cape Mendocino earthquake, to identify the characteristics of distal subduction zone earthquakes and small local events, respectively, in the paleoseismic record. In the future we plan to identify the likely rupture characteristics of major CSZ earthquakes during the Holocene and to synthesize paleoseismic, geophysical, and instrumental datasets to identify the existence and persistence of rupture boundaries along the subduction zone.

Characterizing short‐term temporal variations of fault creep provide insight into the evolution, mechanics, and strength of fault systems. Using spirit leveling and an extensometer, we measured active slip of a surface fault southwest of the Needles District, Canyonlands National Park, Utah, where exteis driven by differential unloading of a subsurface salt layer due to incision of the Colorado River. Results show continuous creep at maximum rates of 0.7 ± 0.2 mm/yr without large temporal variations typical of episodic creep events. Occasional, minor transient events in fault slip velocity coincided with water infiltration; however, we found no significant relation between precipitation and transient events. Detailed mapping of widespread, fault‐parallel sinkholes provide evidence for dilation of faults at shallow depth, a process that lowers fault strength. We propose continuous slip is related to low fault strength and differential unloading, as opposed to other salt systems where dissolution has been linked to episodic slip.

Detailed methods, figures, and analysis.