Fault slip behavior during episodic tremor and slow slip (ETS) events, which occur at the deep extension of subduction zone megathrust faults, is believed to be related to cyclic fluid processes that necessitate fluctuations in pore-fluid pressures. In most subduction zones, a layer of anomalously low seismic wave velocities [low-velocity layer (LVL)] is observed in the vicinity of ETS and suggests high pore-fluid pressures that weaken the megathrust. Using repeated seismic scattering observations in the Cascadia subduction zone, we observe a change in the seismic velocity associated with the LVL after ETS events, which we interpret as a response to fluctuations in pore-fluid pressure. These results provide direct evidence of megathrust fault-valve processes during ETS.

Northwestern Canada has experienced >2.5 Gy of tectonic evolution including the formation of the cratonic core of North America and the development of the Phanerozoic Canadian Cordillera, which started with the break-up of supercontinent Rodinia around ∼750 Ma (Nelson & Colpron, 2007). The rifting of a continental fragment (Laurentia) from Rodinia led to the formation of a continental passive margin. The continent-ocean passive margin persisted until the middle Devonian, when a convergent plate boundary

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The thrust mechanism of the 2012 M w 7.8 Haida Gwaii earthquake suggests convergence across the transpressive Pacific-North America plate boundary in the region is accommodated by underthrusting, with important consequences for seismic-and tsunami-hazard analysis. This article investigates the crustal structure and extent of subduction beneath Haida Gwaii by nonlinear inversion of receiver function data processed from teleseismic recordings at five land-based seismograph stations. Three of these stations were deployed since the 2012 earthquake to extend coverage to the southeast and have not been analyzed previously. The inversions provide estimates of the shear-wave velocity structure beneath much of Moresby Island. Results indicate a positive velocity contrast at approximately 18-26 km depth, interpreted as a shallow continental Moho. A 12-17 km thick low shear-wave velocity zone is also identified, which increases in depth from ∼25 to 42 km along the direction of plate convergence, which is interpreted as subducting oceanic material. These results provide the first evidence that the subducting oceanic plate extends beneath the entirety of Moresby Island.

Summary Surface wave tomography is a valuable tool for constraining azimuthal anisotropy at regional scales. However, sparse and uneven coverage of dispersion measurements make meaningful uncertainty estimation challenging, especially when applying subjective model regularization. This paper considers azimuthal anisotropy constrained by measurements of surface wave dispersion data within a Bayesian trans-dimensional (trans-d) tomographic inversion. A recently-proposed alternative model parameterization for trans-d inversion is implemented in order to produce more realistic models than previous studies considering trans-d surface wave tomography. The reversible-jump Markov-chain Monte Carlo sampling technique is used to numerically estimate the posterior probability density of the model parameters. Isotropic and azimuthally-anisotropic components of surface wave group velocity maps (and their associated uncertainties) are estimated while avoiding model regularization and allowing model complexity to be determined by the data information content. Furthermore, data errors are treated as unknown, and solved for within the inversion. The inversion method is applied to measurements of surface wave dispersion from regional earthquakes recorded over northern Cascadia and Haida Gwaii, a region of complex active tectonics but highly heterogeneous station coverage. Results for isotropic group velocity are consistent with previous studies that considered the southern part of the study region over Cascadia. Azimuthal anisotropic fast-axis directions are generally margin-parallel between Vancouver Island and Haida Gwaii, with a small change in direction and magnitude along the margin which may be attributed to the changing tectonic regime (from subduction to transform tectonics). Estimated errors on the dispersion data (solved for within the inversion) reveal a correlation between surface wave period and the dependence of data errors on travel path length. This paper demonstrates the value of considering azimuthal anisotropy within Bayesian tomographic inversions. Furthermore, this work provides structural context for future studies of tectonic structure and dynamics of northern Cascadia and Haida Gwaii, with the aim of improving our understanding of seismic and tsunami hazards.