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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 ([M.sub.L] ~ 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. Key words: Mount Meager, deep long-period earthquake, volcano seismicity, Garibaldi Volcanic Belt, Bridge River Cones Les seismes profonds de longue periode (SPLP) designent une classe de seismes documentee dans des champs volcaniques partout sur terre, qui serait reliee au mouvement de fluides (magmas, volatiles) associes a des processus volcaniques. Nous avons applique un algorithme de detection automatique d'episodes aux donnees de stations du Reseau sismographique canadien dans le sud-ouest de la Colombie-Britannique et documente les premiers SPLP observes dans la ceinture volcanique de Garibaldi. Un total de 42 episodes satisfaisant les criteres spectraux des SPLP ont ete detectes a la station PMB entre 1993 et 1998, dont 26 ont ete localises par analyse de polarisation a trois composantes et lancer de rayon unidimensionnel. Six autres episodes ont ete cernes et localises a partir de la station MGMB entre 2016 et 2019. II s'agit de petits episodes (ML ~ 0), et la majorite des epicentres sont regroupes a ~45 km a l'ENE du mont Meager, lieu de la plus recente eruption d'importance au Canada, il y a 2400 ans. Ils sont situes plus pres du volcanisme quaternaire associe aux cones de Bridge River et pourraient y etre relies. [Traduit par la Redaction] Mots-cles : mont Meager, seisme profond de longue periode, sismicite volcanique, ceinture volcanique de Garibaldi, cones de Bridge River, Introduction Deep long-period earthquakes (DLPs) represent a type of volcano seismicity thought to be related to magma migration within the mid-to-lower crust atdepths from 10to 45 km (Power et al. [...]

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 the slab morphology. Where the backstop recedes landward beneath the 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 the 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. Plain Language Summary: The tectonic Juan de Fuca plate, that underlies the easternmost North Pacific Ocean off‐shore Vancouver Island, Washington, Oregon, and northern California, is being pushed beneath the North American continent by plate tectonics. On its way deep into the Earth, the plate deforms. In this study, we analyze seismograms of distant earthquakes which were recorded within the study area. Through signal and data processing, we decipher information about the location, orientation, and properties of the down‐going oceanic plate beneath the continent. The data show that the plate protrudes shallowly dipping under the continent beneath the Olympic Peninsula (Washington) and Cape Mendocino (California), while it dips down more steeply under central Oregon and Vancouver Island (British Columbia). This configuration suggests that Siletzia, an old and rigid basalt plateau that forms the central part of the study area, controls the shape of the down‐going plate. Furthermore, the oceanic plate appears to significantly thicken at depth, which may indicate that parts of it accumulate at the base of the continent. These results are important to better understand the subduction process and may help to infer the location of the deeper extent of rupture in a future potential strong earthquake. Key Points: We model Cascadia subduction stratigraphy as three dipping horizonsSlab morphology is controlled by crystalline terrane backstopsA near‐ubiquitous ∼2–10 km thick ultralow velocity zone in the tremor zone correlates with the E‐layer [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We present a detailed study of two M w 7.1 intraslab earthquakes that occurred in southern Alaska: the Iniskin earthquake of 24 January 2016, and the Anchorage earthquake of 30 November 2018. We have relocated and recovered moment tensors for hundreds of aftershocks following both events, and inverted for stress histories. The aftershock distribution of the Iniskin earthquake suggests that the rupture propagated updip along a fault dipping steeply into the Pacific Plate and terminated at a stratigraphic horizon, inferred to be either the interface or Moho of the subducting slab. In addition, four earthquakes ruptured the main fault in the preceding two years and had similar moment tensors to the mainshock. This evidence suggests that the mainshock likely reactivated a pre-existing, outer-rise fault. The Anchorage earthquake sequence is complex due to its location near the boundary of the subducting Yakutat and Pacific plates, as evidenced by the aftershock distribution. Aftershock hypocentres form two main clusters that appear to correspond to orthogonal, conjugate faults, consistent with the two nodal planes of the dominant focal mechanisms. Both geographic groups display many focal mechanisms similar to the mainshock, which could indicate simultaneous rupture on conjugate planes. The time dependence in stress ratio for the Iniskin sequence can be interpreted in terms of pore-pressure evolution within the mainshock fault zone. In particular, our observations are consistent with a dehydration-assisted transfer mechanism where fluids are produced during rupture through antigorite dehydration and raised to high pore pressures through matrix collapse and/or thermal pressurization. The Anchorage sequence exhibits a more complex stress ratio evolution that may be associated with stress adjustments within a distributed fault network, or reflect a strongly heterogeneous stress field. [ABSTRACT FROM AUTHOR]

We examine upper-mantle seismic anisotropy beneath the cordillera and craton of north-western Canada using the multi-event station averages of shear-wave splitting of SKS, SKKS, and sSKS phases recorded during the Canadian Northwest Experiment (CANOE). Splitting times derived from multi-event averaging at each station range from 0 to 1.4 s, with an array average of ~0.65 s. Over broad portions of the array, fast directions are coherent and roughly consistent with the direction of absolute plate motion in a hotspot reference frame, suggesting that coherent asthenospheric fabric underlies the plate in much of the region. Within this broad framework, fast directions and splitting times show marked variability over length scales of 50-200 km, and are generally correlated with surface and/or crustal tectonics. Anomalous splitting is observed across an ancient suture within the cratonic lithosphere, apparently associated with complex dipping fabric produced during continental assembly. The deformation front of the Canadian Rockies correlates with a significant change in splitting behavior, consistent with the front range demarking the craton-cordillera transition within the mantle. Splitting times are small across much of the cordillera, indicating that lithospheric and/or asthenospheric fabric is weaker or less coherent than beneath the craton. In the western cordillera, fast directions rotate abruptly to parallel the plate boundary, implying that fabric associated with plate-boundary deformation extends ~200 km into the North American continent. doi: 10.1130/G31120.1

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. [ABSTRACT FROM AUTHOR]

We investigate an isolated cluster of temporally persistent, intraslab earthquakes (⁠ML<3.2⁠) at >60 km depth below the Georgia Strait in southern British Columbia that is unique in Cascadia and meets the criteria for identification as an earthquake nest. A total of 129 relocated hypocenters define two northwest-dipping structures in the subducting Juan de Fuca mantle within an ∼30×10×10 km³ volume. Focal mechanisms for 15 events represent a mix of strike-slip and reverse faulting, and a stress regime of down-dip tension and plate-normal compression, consistent with a previous regional study. Converted seismic phases inferred to originate at the boundaries of subducted oceanic crust are observed at several receivers and are consistent with a local slab depth of ∼45 km⁠, shallower than some JdF plate models. The geographical isolation of the nest within the confines of an extrapolated propagator wake suggests that its location is controlled by this pre-existing and presumably hydrated structure. [ABSTRACT FROM AUTHOR]

The Queen Charlotte plate boundary, near Haida Gwaii, B.C., includes the dextral, strike-slip, Queen Charlotte Fault (QCF) and the subduction interface between the downgoing Pacific and overriding North American plates. In this study, we present a comprehensive repeating earthquake catalog that represents an effective slip meter for both structures. The catalog comprises 712 individual earthquakes (0.3≤ M W≤3.5) arranged into 224 repeating earthquake families on the basis of waveform similarity and source separation estimates from coda wave interferometry. We employ and extend existing relationships for repeating earthquake magnitudes and slips to provide cumulative slip histories for the QCF and subduction interface in six adjacent zones within the study area between 52.3°N and 53.8°N. We find evidence for creep on both faults; however, creep rates are significantly less than plate motion rates, which suggests partial locking of both faults. The QCF exhibits the highest degrees of locking south of 52.8°N, which indicates that the seismic hazard for a major strike-slip earthquake is highest in the southern part of the study area. The 28 October 2012, M W 7.8 Haida Gwaii thrust earthquake occurred in our study area and altered the slip dynamics of the plate boundary. The QCF is observed to undergo accelerated, right-lateral slip for 1-2 months following the earthquake. The subduction interface exhibits afterslip thrust motion that persists for the duration of the study period (i.e., 3 years and 2 months after the Haida Gwaii earthquake). Afterslip is greatest (5.7-8.4 cm/yr) on the periphery of the main rupture zone of the Haida Gwaii event. [ABSTRACT FROM AUTHOR]