Interpreting the Queen Charlotte plate boundary from 3D seismic tomography and relocated earthquakes

The Queen Charlotte plate boundary, offshore Western Canada, is predominantly a right-lateral strike-slip fault with up to ~20º oblique convergence in the southern section. One of its important characteristics is the juxtaposition of an oceanic (Pacific) plate against a continental (North American) plate along the majority of its length. The 2013 Mw 7.5 Craig earthquake in the northern part of the plate boundary was primarily strike-slip, whereas the 2012 Mw 7.8 Haida Gwaii earthquake on the southern end was nearly pure thrust. The latter underscored the complicated tectonics and highlighted the tsunami hazards in the region. The Queen Charlotte plate boundary provides an opportunity to examine the effects of transpression, continent-ocean juxtaposition, and variations in plate age and compositional structure on the evolution of transform plate boundaries, and lithospheric thickening or potential incipient subduction. In preparation for an upcoming active/passive marine seismic experiment in summer 2021, we perform a reconnaissance study of the structure of the Queen Charlotte plate boundary by using a hybrid method of auto-regressive phase detection and onset estimation on various existing seismic waveform archives from temporary and permanent seismic arrays from 1998-2020. The new automated earthquake catalog includes ~59000 events with at least 4 P and 2 S picks, compared to ~19000 events in the Canadian National Seismic Network (CNSN) catalog, in the region between 50.5-57ºN and 128-137ºW. We use a combined (automated + CNSN) seismic catalog to invert for a 3D velocity model using double-difference seismic tomography. Initial locations corroborate the existence of seismicity concentrations along the Pacific plate with evidence for along-strike segmentation along Haida Gwaii. We interpret fault structure from tomography and relocated seismicity, noting along-strike and across-strike variations. We discuss the implications of our results for regional geodynamics in light of local hazards and the upcoming seismic experiment.