Crustal Seismic Anisotropy Along the Continental Margin in Western Canada From Receiver Function Analysis

Coastal British Columbia (BC), Canada, has the highest seismic hazard in the country due to convergent and transpressive deformation at offshore plate boundaries between the Pacific, Juan de Fuca (JdF) and North American (NA) plates. Further landward, the crust of the NA plate is made up of several geologically unique exotic terranes and is unusually thin. Investigating the geophysical features in this area can help us better constrain its tectonic history and the geophysical processes that are currently underway. Here, we conduct an analysis of teleseismic body-wave scattering data (i.e., receiver functions [RFs]) recorded at stations across western coastal BC including northern Vancouver Island and southeastern Alaska. Using these RFs, we perform a harmonic decomposition with respect to earthquake back-azimuths to determine the orientation of seismic anisotropy over a series of depth ranges, attributable to either mineral alignment or dipping structures. We find a coherent pattern of margin-parallel orientations at upper crustal depths that persist onto the mainland at distances 420 km from the margin. Furthermore, dominant receiver function orientations at depth are attributed to dipping faults and interfaces, and fabrics due to lower crustal shearing or inherited from tectonic assembly along the margin. This work supports models for the tectonic assembly of this region that involve a combination of plate subduction and transpressive motion along crustal scale faults that pervade a wide portion of the margin. This work also helps to constrain the current geometries of the subducting Pacific and JdF slabs. Plain Language Summary Coastal British Columbia (BC) is a geologically complex region, but could it also be the epicenter of large tsunami causing earthquakes that put the lives of residents all around the Pacific at risk? Better understanding how this region formed and the current geometry of the tectonic plates can help us address this question. We study seismic waves from distant earthquakes recorded across western BC to see if wave speeds depend on direction. This directional dependence could be caused by a dipping interface between two different media within the Earth, or by the effect of past tectonic deformation on minerals and rocks. We detect directional dependency in seismic wave speeds at various depths beneath all seismometers in our study area. From this data set, we determine the strike of dipping boundaries and/or the trend of material properties, coming from various sources such as changes in the thickness of the crust, faults, and material fabrics caused by deformation. Results of this study add to the body of knowledge of the formation history of western BC and the assessment of earthquake and tsunami hazards.