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The Queen Charlotte plate boundary (QCPB), a transform separating the Pacific and North American plates, accommodates ~55 millimeters per year of motion, is a source of large earthquakes in the northeast Pacific, and may be a modern site of subduction initiation. The southern QCPB experiences oblique convergence, showcased by the 1949 magnitude (M) 8.1 strike-slip earthquake and the 2012M7.8 tsunamigenic thrust earthquake, both offshore Haida Gwaii, British Columbia. We present seismic reflection images of the southern QCPB, which constrain the crustal structure in unprecedented detail. The Queen Charlotte Terrace is underthrust by oceanic crust topped by a throughgoing, low-angle plate-boundary thrust, which ruptured in the 2012 earthquake. The Queen Charlotte Terrace is analogous to strain-partitioned, thin-skinned forearc slivers seen at oblique subduction zones, captured between a localized plate-boundary thrust and a mature strike-slip fault. Our imaging suggests that the system rapidly evolved from distributed to partitioned strain and is currently an incipient subduction zone.
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Incipient Subduction and Slip Partitioning at High Obliquity: The Haida Gwaii Plate Boundary
Abstract 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 2012Mw7.8 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.
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- PAR ID:
- 10539411
- Publisher / Repository:
- AGU
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Solid Earth
- Volume:
- 129
- Issue:
- 5
- ISSN:
- 2169-9313
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1029/2024JB028752
