High-resolution seismicity and fault imaging of the Queen Charlotte Triple Junction region from the PACSAFE Leg1 ocean-bottom seismometer network

The Queen Charlotte Triple Junction (QCTJ) is a complex plate boundary offshore British Columbia, Canada, linking the Cascadia subduction zone, the Queen Charlotte transform fault, and the Juan de Fuca Ridge through the Revere–Dellwood fault (RDF) system. This region accommodates marked changes in plate motion and deformation style over short spatial scales, yet its offshore structure and seismicity remain poorly constrained due to distant land-based seismic coverage. The Pacific Coast Seismic Assessment for Faults and Earthquakes (PACSAFE) is a multi-year Canadian ocean-bottom seismometer (OBS) program designed to resolve this plate-boundary transition through dense, multi-deployment offshore monitoring.

During Leg 1 (October 2023-July 2024), 26 broadband OBS from the National Facility for Seismological Investigations (Dalhousie University) were deployed across the QCTJ and adjacent segments of the RDF and Explorer Ridge, and continental shelf break. During the deployment, two of these instruments located along the continental shelf break as well as one on the RDF prematurely released from the sea floor.  All remaining 23 instruments were successfully recovered. Clock drift corrections were applied by the NFSI technical team, then orientations of all instruments were computed using local and teleseismic waveforms. We then applied a deep-learned based approach to phase picking, utilizing the OBSTransformer on the 3-component data, resulting in ~1.23 million P phases and ~3.11 million S phases. These arrivals were associated and located using the maximum likelihood approach, to generate a catalog of ~11,000 events. These events were then reduced using additional quality control parameters, and double-difference relocated using both pick times and waveform cross correlations. The resulting relocated catalog contains more than 5,000 of the most robustly relocated events.

The catalog reveals dense, previously unobserved microseismicity that delineates near-vertical fault strands, fault-perpendicular seismicity lineations associated with the Revere-Dellwood transform and multiple seismicity strands associated with the northern Explorer ridge and transform system. Seismicity extends from near the seafloor to ~20 km depth, with most activity concentrated between 5 and 10 km. We provide new observations with unprecedented constraints on deformation and plate-boundary partitioning within the QCTJ. Ongoing analyses of focal mechanisms, seismicity, and tectonic context will further refine models of seismic and tsunami hazard for Canada’s Pacific margin.