Incipient Subduction and Slip Partitioning at High Obliquity: The Haida Gwaii Plate Boundary

• Published in: Journal of geophysical research. Solid earth Vol. 129; no. 5
• Main Authors: Oliva, S. J., Bostock, M. G., Schaeffer, A. J., Nissen, E., Merrill, R., Hughes, A., Roecker, S. W., Nedimovic, M. R., Roland, E., Worthington, L. L., Walton, M. A. L., Gase, A.
• Format: Journal Article
• Language: English
• Published: 01.05.2024
• Subjects: double‐difference; earthquake location; oblique tectonics; sliver tectonics; subduction initiation; transpression
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1029/2024JB028752

Plate motion obliquity along the dominantly transform Queen Charlotte plate boundary (QCPB) peaks offshore Haida Gwaii. To investigate the effects of obliquity on plate boundary deformation, we analyze continuous seismic waveforms from temporary and permanent stations from 1998 to 2020 to generate a catalog of ∼50,000 earthquakes across Haida Gwaii. We use an automated technique based on auto‐regressive phase detection and onset estimation to obtain the initial seismic catalog, integrate existing catalogs, invert for 3D velocity structure using data from the best constrained period, and relocate 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 its bathymetrically defined trace. Instead, seismicity illuminates a complex system of segmented structures with variable geometries along strike. Other clusters highlight 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 at 15–20 km depths, suggesting elevated residual stress at the juncture of slip‐partitioning. Deep crustal seismicity (up to ∼20 km depths) beneath central Haida Gwaii aligned parallel to the strike of the thrust plane may represent landward underthrusting of the Pacific plate. Our results suggest possible coseismic strike‐slip rupture on the QCF during the 2012 earthquake and add support to the thesis that highly oblique transform boundaries are viable settings for subduction initiation. Plain Language Summary We investigated the complex tectonics offshore Haida Gwaii, western Canada, where the Pacific and North American plates slide past one another obliquely. By compiling and analyzing the most comprehensive earthquake catalog in the area, spanning 1998–2020, we present the most detailed report to date of the earthquake‐producing structures in the region, including previously unidentified and highly segmented faults. Clusters of seismicity illuminate (a) a highly deformed terrace of elevated seafloor west of Haida Gwaii, (b) a complex and segmented fault system adjacent to the previously‐mapped surface trace of the main Queen Charlotte strike‐slip fault, and (c) the inferred fault intersection at depth between the subvertical Queen Charlotte fault (which hosted the 1949 magnitude 8.1 earthquake) and the shallowly dipping Haida Gwaii thrust (which hosted the 2012 magnitude 7.8 earthquake). We also speculate that the 2012 earthquake may also have involved some motion on the Queen Charlotte fault. These results contribute to better constraints on regional tectonics and hazards, and provide insights into the mechanisms of subduction initiation. Key Points Seismicity off Moresby Island is distributed along multiple segments slightly off of the Queen Charlotte fault trace Aftershocks at intersection of Queen Charlotte Fault with the 2012 Mw 7.8 thrust plane reflect residual stress at slip partitioning juncture Previously undocumented deep seismicity beneath Haida Gwaii is consistent with an underthrusting Pacific Plate