The Queen Charlotte Triple Junction (QCTJ) is a complex plate boundary offshore British Columbia, Canada, linking the Cascadia subduction zone, the Queen Charlotte transform fault, and the Juan de Fuca Ridge through the Revere–Dellwood fault (RDF) system. This region accommodates marked changes in plate motion and deformation style over short spatial scales, yet its offshore structure and seismicity remain poorly constrained due to distant land-based seismic coverage. The Pacific Coast Seismic Assessment for Faults and Earthquakes (PACSAFE) is a multi-year Canadian ocean-bottom seismometer (OBS) program designed to resolve this plate-boundary transition through dense, multi-deployment offshore monitoring.During Leg 1 (October 2023-July 2024), 26 broadband OBS from the National Facility for Seismological Investigations (Dalhousie University) were deployed across the QCTJ and adjacent segments of the RDF and Explorer Ridge, and continental shelf break. During the deployment, two of these instruments located along the continental shelf break as well as one on the RDF prematurely released from the sea floor. All remaining 23 instruments were successfully recovered. Clock drift corrections were applied by the NFSI technical team, then orientations of all instruments were computed using local and teleseismic waveforms. We then applied a deep-learned based approach to phase picking, utilizing the OBSTransformer on the 3-component data, resulting in ~1.23 million P phases and ~3.11 million S phases. These arrivals were associated and located using the maximum likelihood approach, to generate a catalog of ~11,000 events. These events were then reduced using additional quality control parameters, and double-difference relocated using both pick times and waveform cross correlations. The resulting relocated catalog contains more than 5,000 of the most robustly relocated events.The catalog reveals dense, previously unobserved microseismicity that delineates near-vertical fault strands, fault-perpendicular seismicity lineations associated with the Revere-Dellwood transform and multiple seismicity strands associated with the northern Explorer ridge and transform system. Seismicity extends from near the seafloor to ~20 km depth, with most activity concentrated between 5 and 10 km. We provide new observations with unprecedented constraints on deformation and plate-boundary partitioning within the QCTJ. Ongoing analyses of focal mechanisms, seismicity, and tectonic context will further refine models of seismic and tsunami hazard for Canada’s Pacific margin.

The seismic moment tensor (MT) delivers valuable information about the physical source process of a seismic event. It allows to distinguish between earthquakes, explosions and volcanic events, constraints the orientation of an earthquake rupture, and represents the most accurate estimate of the released seismic energy.The computation of absolute MTs by waveform inversion is a data intensive task that is oftentimes feasible only for the largest events in a data set. Relative MTs rely on less subsurface information and may be computed for a large number of closely spaced weaker seismic events that are connected to an absolute MT through relative amplitude measurements. The relative MT method assumes that the Green’s functions between events is similar and that relative amplitudes are measured below the corner frequency of the largest event. Under these assumptions, the relative amplitude between seismograms can be attributed to the difference in moment tensor between events. Compared to absolute methods, path and site effects cancel out and do not need to be considered.We here present relMT, an accessible, research-grade, open-source software package that facilitates computation of relative moment tensors for a large variety of data sets. The software takes as inputs seismic waveform, event locations, ray take-off angles, and a reference MT, as well as waveform headers and a configuration file. In synopsis, the similar waveforms are aligned to sub-sample accuracy under consideration of possible polarity reversals for P-waves and planar polarization of S-waves. Amplitude ratios between the aligned seismograms are measured on single seismic stations in a principal component framework. The relative amplitudes are combined mathematically with ray take-off angles, relative event distances and one absolute reference moment tensor in a linear system of equations. The solution of the equation system with algebraic methods yields all relative moment tensors at once. The uncertainty of the solutions is quantified using the bootstrap method. The software is under active development on GitHub (https://github.com/wasjabloch/relmt).We illustrate the application of relMT using data sets of induced seismicity and tectonic aftershock seismicity. For the induced seismicity of the enhanced geothermal system in Helsinki, Finland, we are able to lower the magnitude threshold for which MTs can be computed from 0.5 to -0.5. For aftershocks in the Pamir highlands of Central Asia from 4.0 to 2.0. For the data sets, this represents a 3- to 30-fold increase in the number of recovered MTs.

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.

Receiver functions are a powerful tool to image lithospheric stratigraphy. For flat lying structures, receiver functions can be stacked azimuthally to achieve high signal-to-noise ratios and h-κ-stacks allow to estimate the depth of interfaces (h) and P-to-S wave velocity ratio of the hanging layers (κ). For dipping layers, characteristic for the slab structure in a subduction zone forearc, these methods fail, because the moveout of phases arriving from different azimuths violates the basic assumptions of these methods. We here present a simple routine to simultaneously search for the depth of the top of slab and of the oceanic Moho, for strike and dip of the downgoing slab, as well as for the S-wave velocities and the P‑to-S wave velocity ratios of multiple layers of the overriding and downgoing plates in subduction zone forearcs. Our approach is based on the recent Python port PyRaysum of Frederiksen and Bostock's classic (2000) code for modeling ray-theoretical plane body-wave propagation in dipping anisotropic media, and on SciPy's simulated annealing global parameter search. We applied the routine to hundreds of azimuthally-dependent receiver function sections from the subduction zones of Cascadia (North America) and the central Andes (South America) and retrieved laterally coherent station measurements of the depth and orientation of the top of the subducting slab and the subducting Moho, with only weakly constrained seismic velocities. In Cascadia, we interpolated a regional slab model through fitting of regularized spline surfaces. Small scale structures that are not present in previous slab models can be resolved, e.g. under Olympic Peninsula (Cascadia) and Mejillones Peninsula (northern Chile). Where the receiver functions are more complex than can be accounted for by our model, the labeling of the modeled receiver function phases and comparison to the observed receiver functions allows us to confidently interpret the additional subsurface complexities and reconcile them with our interpretations.

Tremor is a weak seismic signal accompanying slow fault slip at plate boundaries. The relationship between tremor and slow slip and the tremor source mechanism have been widely debated, owing largely to the challenge of accurately locating tremor in depth. We assemble a tremor hypocenter catalog of 4,851 events in a 10 x 20 km2 area beneath Vancouver Island during three slow slip episodes between 2003 and 2005 using a cross-station detection method adapted from previous studies to recover accurate depths. Improved tremor locations provide key constraints on i) thickness of the tremorgenic zone, ii) the relative location of tremor to key structural features in the subduction complex, and iii) the geologic context and mechanism of tremor. Tremor occurs in quasi-planar clusters < 500 m thick at a depth near 39 km, beneath a high reflectivity layer and within a zone of elevated Poisson's ratio with P-wave velocities of ~7 km/s. We interpret tremor as originating in the fragmentation of the upper few hundred meters of basaltic oceanic crust. Comminuted and overpressured basalt with increasingly anisotropic fabric is underplated onto overriding lithosphere to generate high reflectivity. Tremor thus manifests areas of material transfer across the plate boundary during slow slip.

Tremor is a low-amplitude seismic signal that usually temporally coincides with episodic slow fault slip at plate boundaries worldwide. Since the discovery of tremor in Cascadia, significant effort has been devoted to understanding its relationship to slow slip. However, its source mechanism has been widely debated, owing in large part to the challenge of locating sources accurately in depth. We assemble a tremor catalog of 4,851 events for a ~ 10 X 20 km 2 area on southern Vancouver Island from slow slip episodes in 2003–2005 using a cross-station detection method adapted from previous studies, which we extend to use both P- and S- waves, thereby recovering accurate depths. Tremor occurs in distinct, quasi-planar clusters in the plate boundary region at a depth near 39 km, just beneath a layer of high reflectivity and within a zone of elevated Poisson’s ratio. We interpret this tremor to represent mafic underplating, wherein shearing generates tremor and continuously erodes basaltic material of the upper few hundred meters of the oceanic crust. Comminuted basalt with an increasingly anisotropic fabric is gradually plated onto the overriding lithosphere to form the highly reflective layer. Localized areas of material transfer within the subduction zone may manifest the distinct tremor clusters.

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

The entire editorial board of the Journal of Geophysical Research-Solid Earth would like to sincerely thank all our colleagues who reviewed manuscripts for us in 2023. The hours they spent reading in order to provide insightful comments on manuscripts not only help improve the quality of these manuscripts but also ensure the scientific rigor of our reviewing process and eventually, of the research published in the field of Solid Earth Geophysics by our journal. With the advent of open science and AGU's data policy, the reviewing process now also encompasses checking the accessibility and availability of data and developed software. This is a key objective of AGU's FAIR (Findable, Accessible, Interoperable and Reusable) policy, for which many reviewers have provided suggestions that helped to improve the data presentation and availability, and which also fed the editorial board's reflection on the matter. Of course, we particularly appreciate timely reviews, particularly in light of the growing demands imposed by the increase of manuscripts submitted to Journal of Geophysical Research-Solid Earth. We received 1,869 submissions in 2023, and 1,472 reviewers contributed to their evaluation by providing 2,237 reviews in total. We are deeply thankful for all of their contributions. The editorial board of Journal of Geophysical Research-Solid Earth: Rachel Abercrombie, Yves Bernab & eacute;, Michael Bostock (former editor), Mark Dekkers, Anke Friedrich, Shin-Chan Han, Satoshi Ide, Isabelle Manighetti (former EIC), Fenglin Niu, Douglas R. Schmitt, Alexandre Schubnel (EIC), Jun Tsuchiya, and all the associate editors of JGR-SE.

In this paper, we focus on the serpentinization of the forearc mantle wedge, just one of Roy Hyndman's many contributions to our understanding of subduction zones. Over the past 25 years, numerous advances in geophysics, petrology, and geology clearly document that H2O-rich fluids, derived from the subducting plate, hydrate portions of the overlying mantle to form serpentinite. The extent of mantle-wedge serpentinization depends, to first approximation, on the thermal evolution of the subducting plate. Dehydration reactions in warm subducting slabs occur at shallow (<100 km) depth making relatively large amounts of H2O available for forearc mantle-wedge hydration; in cool subduction zones, dehydration reactions occur at greater depth and less H2O is available to directly hydrate the shallow mantle wedge. High-resolution seismological studies, complemented by numerical modelling, reveal that serpentinization in a subduction zone varies spatially, with strong evidence for a serpentinite layer at the base of the mantle wedge. Serpentinite mineralogy plays an important role in controlling the rheologic behaviour of the subduction plate interface, but appears not to control the downdip extent of large thrust earthquakes as originally proposed. Weak serpentinite along the base of the mantle wedge acts to mechanically isolate the forearc mantle wedge from induced corner flow, and serpentinized regions of the forearc mantle wedge may localize deformation. Rare mantle-wedge earthquakes may reflect the subvertical flow of fluids along fractures. Future work in wellinstrumented subduction zones is expected to clarify the spatial distribution and extent of serpentinization in the forearc mantle wedge.

The relative abundance of small earthquakes affords significant opportunities for improved understanding of regional seismotectonics; however, determining moment tensors for such events recorded on regional networks is complicated by low signal-to-noise ratios, sparse station sampling and complex wave propagation at short periods. We build upon previous work in designing a multiple-event, simultaneous moment tensor inversion scheme for small earthquakes that employs constraints from P-wave polarities, relative amplitudes of P- and S-waves recorded at common stations, and local magnitude estimates. Our method does not require a priori knowledge of a reference moment tensor. High-fidelity polarity and relative amplitude data are recovered using principal component decomposition of clustered-event waveforms. These data are employed within a multi-stage iterative framework to invert for moment tensors and incorporate local magnitude information. Synthetic examples employing as few as four high-quality and spatially-distributed stations yield accurate moment tensor estimates. We demonstrate our approach on a cluster of seismicity near San Juan Island, Washington, USA, within the Cascadia forearc. Our results are consistent with previous characterization of the local stress regime, and support an interpretation of swarm behaviour resulting from migration of fluids originating from dehydration of the subducting Juan de Fuca plate.

The Queen Charlotte triple junction/Explorer microplate region offshore British Columbia, Canada, is marked by poorly understood and rapidly evolving microplate tectonics. Although the region hosts abundant seismicity, it has received relatively scant attention in recent years due to its remote, offshore location. We use the Regressive ESTimator (REST) algorithm to generate a new catalog of automatically detected earthquakes from 1995 to 2021, which, when merged with the existing Geological Survey of Canada catalog, yields the most extensive seismicity data set offshore British Columbia to date. We apply double‐difference relocation to these events and perform stress inversions using moment tensors for subregions within the study area. Our results confirm and extend previous models of microplate deformation processes. We suggest the Revere‐Dellwood‐Queen Charlotte fault system has evolved as a NW‐migrating, pull‐apart system between Haida Gwaii and the Explorer ridge that obeys global length/width scaling and whose bathymetric expression is influenced by volcanism plausibly induced by interaction with the Kodiak‐Bowie hotspot. Seismicity within the Explorer microplate is dominated by prominent, northeast‐trending lineations that emanate from the Sovanco fracture zone and parallel the Nootka fault zone. Alignment of these features with spreading structures that bound the microplate suggests that its breakup is controlled primarily by a strength fabric inherited at spreading ridges. Stress inversions are dominated by near‐vertical intermediate compressive stress reflecting the dominance of strike‐slip faulting. Stress varies systematically between transpression to the north along southern Haida Gwaii and seafloor spreading to the south along the Juan de Fuca ridge.

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.

In the Coast Mountains of western British Columbia, an anomalous seismicity concentration exists near the intersection of the Coast Shear Zone, a major northwest–southeast trending Eocene-age shear zone that accommodated deformation between the Pacific and North America plates, with the Anahim Volcanic Belt, an east-northeast–west-northwest trending zone of volcanic features that decrease in age to the east. To better characterize seismicity in the Coast Mountains, we augment the existing Natural Resources Canada seismicity catalogue by applying an automatic detection and location algorithm to both permanent Canadian National Seismic Network stations and temporary stations from the 2005–2006 BATHOLITHS deployment, resulting in 837 relocated events with at least three paired P- and S-phase picks. Double-difference relocation reveals several small-scale linear strands subparallel to the Coast Shear Zone and within the Anahim Volcanic Belt and three clusters of events striking at a high angle to the Coast Shear Zone that occurred as swarms in 2015 and 2017. First-motion focal mechanisms exhibit extensional and strike-slip faulting. Our observations indicate that most of these events are not associated with surficial processes such as landslides, but rather, we hypothesize that the interaction of the Anahim Volcanic Belt and Coast Shear Zone has weakened the lithosphere in this region, leading to current-day strain localization and high heat flow that manifest seismicity, including swarm-like activity.

ABSTRACT We propose new methods for assessing temporal changes in seismic velocity using the S-wave coda for repeating earthquakes and cross-correlation functions of ambient noise. For a pair of seismic waveforms representing a common source–receiver path, the relative change in path-averaged velocity over the corresponding time interval is directly proportional to the factor by which one waveform needs to be stretched or compressed with respect to the other to achieve maximum coherence. For an arbitrary number of waveforms, initial pair-wise stretch factors determined through standard approaches can be improved through solution of an overdetermined system and further refined through an iterative approach exploiting the singular value decomposition to minimize rank of the stretched waveform section. We apply this combined approach to both repeating earthquakes and ambient noise correlations for Haida Gwaii in western Canada, the site of a Mw 7.8 thrust earthquake in 2012. Optimal stretch factors for repeating earthquake families indicate that path-averaged S velocities dropped by up to 0.16% after the earthquake. Ambient noise correlations indicate that velocities dropped by between 0.26% and 0.39%, which we interpret to be more pronounced in the uppermost levels of the crust. We explore these results in terms of changes in crustal porosity and hydrogeologic conditions by considering the observation that hot spring activity on Haida Gwaii ceased following the 2012 mainshock and recovered over the next several years.

Editors of JGR‐Solid Earth express their appreciation to those who served as peer reviewers for the journal in 2022.

We map the characteristic signature of the subducting Juan de Fuca and Gorda plates along the entire Cascadia forearc from northern Vancouver Island, Canada to Cape Mendocino in northern California, USA, using teleseismic receiver functions. The subducting oceanic crustal complex, possibly including subcreted material, is parameterized by three horizons capable of generating mode-converted waves: a negative velocity contrast at the top of a low velocity zone underlain by two horizons representing positive contrasts. The amplitude of the conversions varies likely due to differences in composition and/or fluid content. We analyzed the slab signature for 298 long-running land seismic stations, estimated the depth of the three interfaces through inverse modeling and fitted regularized spline surfaces through the station control points to construct a margin-wide, double-layered slab model. Crystalline terranes that act as the static backstop appear to form the major structural barrier that controls slab morphology. Where the backstop recedes landward beneath Olympic Peninsula and Cape Mendocino, the slab subducts sub-horizontally, while the seaward-protruding and thickened Siletz terrane beneath central Oregon causes steepening of the slab. A tight bend in slab morphology south of Olympic Peninsula coincides with the location of recurring large intermediate depth earthquakes. The top-to-Moho thickness of the slab generally exceeds the thickness of the oceanic crust by 2-12 km, suggesting thickening of the slab or underplating of slab material to the overriding North American plate.

Editors of JGR-Solid Earth express their appreciation to those who served as peer reviewers for the journal in 2021.

Abstract We employ an automatic earthquake detection algorithm to seismic waveforms recorded between the years 2000 and 2020 in southwest British Columbia. 32,121 events which possess at least three paired P‐ and S‐wave arrival times are located, compared to 21,538 seismic events in the existing Geologic Survey of Canada catalog. This augmented catalog is employed for double‐difference seismic tomography across Vancouver Island, with particular focus on the Nootka Fault zone (NFZ). The NFZ is a transform boundary that separates the Juan de Fuca and Explorer plates in a zone of distributed left‐lateral strike‐slip faulting. Tomographic results indicate that a double seismic zone exists within the NFZ that parallels Vp/Vs structure typically observed in subduction zones. Specifically, a dipping high Vp/Vs layer is underlain by reduced Vp/Vs material. Structural complexities are revealed by three Mw > 6 events and their aftershocks. The 2004 Mw 6.3 and 2011 Mw 6.4 events are interpreted to reside southeast of the NFZ within the Juan de Fuca plate at depths as shallow as 20 km, and the 2014 Mw 6.6 event is interpreted to reside within oceanic lithosphere of the NFZ at depths >35 km. Vp/Vs structure further indicates that underthrust oceanic lithosphere of the Explorer plate extends to 20 km depth beneath Brooks Peninsula. Our results support the hypothesis that the NFZ represents a structurally independent slice of oceanic lithosphere that exhibits (a) a more northerly trajectory than typically depicted, and (b) increased plate curvature when compared to either the Juan de Fuca or Explorer plates.

Deep long-period earthquakes (DLPs) are a class of seismicity that has been documented in volcanic fields worldwide and are thought to be related to the motion of fluids (magmas and volatiles) associated with volcanic processes. We applied an automatic event detection algorithm to stations of the Canadian National Seismograph Network in southwestern British Columbia and documented the first DLPs observed within the Garibaldi Volcanic Belt. A total of 42 events satisfying the spectral criteria of DLPs were detected at station PMB between 1993 and 1998, of which 26 events were located using three-component polarization analysis and 1-D ray tracing. An additional six events were identified and located from station MGMB between 2016 and 2019. The events are small ( ML ∼ 0) and the majority of the epicenters are clustered at ∼45 km ENE of Mount Meager, the site of Canada’s most recent large eruption at 2.4 ka. They lie in closer proximity and may be related to Quaternary volcanism associated with the Bridge River Cones.

ABSTRACT High-resolution earthquake locations and structural inversions using body waves rely on precise delay-time measurements. Subsample accuracy can be realized for P waves using multichannel cross correlation (MCCC), as developed by VanDecar and Crosson (1990), which exploits redundancy in pairwise cross correlations to determine delays between similar waveforms in studies of mantle structure using teleseismic sources (common source and multiple stations) and regional studies of structure and seismicity (multiple sources and common station). For regional S waves, alignment is complicated by the additional degree of freedom in waveform polarity that is expressed for sources with different moment tensors. Here, we recast MCCC within a principal component framework and demonstrate the equivalence between maximizing waveform correlation and minimization of various singular value–based objective functions for P waves. The singular-value framework is more general and leads naturally to an MCCC linear system for S waves that possesses an order of magnitude greater redundancy than that for P waves. Robust L1 solution of the system provides an effective means of mitigating outliers at the expense of subsample precision. Residual time shifts associated with higher-order singular vectors are employed in an iterative adaptive alignment that achieves subsample resolution. We demonstrate application of the approach on a seismicity cluster within the northern Cascadia crustal fore-arc.