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.

The Northern Canadian Cordillera (NCC) is an actively deforming orogenic belt in northwestern Canada. Geochemical and geophysical data show that the NCC is underlain by a thin and hot lithosphere, in contrast with the adjacent cold and thick cratonic lithosphere to the east. This juxtaposition of cold/hot and thick/thin lithosphere across a narrow transition zone has important implications for regional geodynamics. The recent deployment of USArray Transportable Array and other seismic stations across Alaska, USA, and northwestern Canada allows us to image lithosphere and upper mantle three‐dimensional seismic velocity structure at significantly improved resolution. Our model reveals a broad high‐velocity anomaly across northern Yukon and Northwest Territories, which is interpreted as buried cratonic lithosphere and which we refer to as the Mackenzie craton. Another prominent high‐velocity anomaly is imaged beneath northeastern British Columbia and is interpreted to indicate cratonic lithosphere beneath the Northern Rocky Mountains. These two mechanically strong lithospheric blocks, also suggested by regional magnetic data, are interpreted to buttress the ends of the Mackenzie Mountains fold and thrust belt, guiding intervening cordilleran mantle flow toward the Canadian Shield and controlling the arcuate geometry of the Mackenzie Mountains fold and thrust belt. Both P and S wave models also reveal the signature of a northward dipping, subducting Wrangell slab across the southern region of the Alaska/Yukon border. Strong P and S wave velocity contrasts across the Tintina Fault suggest that it is a lithosphere‐scale shear zone that extends into the upper mantle beneath the NCC and demarcates distinct regions of lithospheric mantle.

Moho morphology in orogens provides important constraints on the rheology and density structure of the crust and underlying mantle. Previous studies of Moho geometry in the northern Canadian Cordillera (NCC) using very sparse seismic data have indicated a flat and shallow (∼30–35  km) Moho, despite an average elevation of >1000  m above sea level attributable to increased thermal buoyancy and lower crustal flow due to elevated temperatures. We estimate Moho depth using receiver functions from an expanded dataset incorporating 173 past and recently deployed broadband seismic stations, including the EarthScope Transportable Array, Mackenzie Mountains transect, and other recent deployments. We determine Moho depths in the range 27–43 km, with mean and standard deviations of 33.0 and 3.0 km, respectively, and note thickened crust beneath high-elevation seismogenic regions. In the Mackenzie Mountains, thicker crust is interpreted as due to crustal stacking from thrust sheet emplacement. The edge of this region of thickened crust is interpreted to delineate the extent of the former Laurentian margin beneath the NCC and is associated with a transition from thrust to strike-slip faulting observed in regional seismicity. More geographically extensive seismograph deployments at EarthScope Transportable Array density and scale will be required to further extend crustal-scale and lithosphere-scale imaging in western Canada.

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

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.