A complex Queen Charlotte Plate Boundary offshore Haida Gwaii

Abstract

The Queen Charlotte plate boundary marks a transpressional system between the Pacific and North American plates, extending from offshore Haida Gwaii in Canada into southeastern Alaska. Using continuous seismic waveforms from temporary and permanent seismic networks from 1998–2020, we produced a comprehensive catalog of ~50,000 earthquakes across the region near Haida Gwaii. We used an automated processing technique of auto-regressive phase detection and onset estimation to obtain the initial seismic catalog, integrated existing catalogs, inverted for 3D velocity structure using data from the most well constrained period, and relocated the entire catalog using the new 3D velocity model. We investigate the seismically active sections of the transcurrent Queen Charlotte fault (QCF), noting that little seismicity locates directly along the bathymetrically defined QCF trace. Instead, the seismicity illuminates a complex system of multiple segmented structures, featuring variable geometries along strike. Clustered shallow seismicity could indicate active shallow faults within the highly deformed Queen Charlotte terrace. Few aftershocks appear on the thrust plane of the 2012 Mw 7.8 Haida Gwaii earthquake except near its inferred intersection with the QCF between 15 and 20 km depths, suggesting elevated residual stress. Deep (up to ~20 km) crustal seismicity below central Haida Gwaii aligned parallel to the strike of the thrust plane may manifest the landward underthrusting of the Pacific plate. We also explore the possibility of coseismic strike-slip rupture on the QCF during the 2012 earthquake. Our results provide insights into postseismic strain accommodation and partitioning across this complex oblique transpressive system.