Subduction or Crustal Faulting? Evaluating Ground-Motion Models to Characterize Haida Gwaii in the Canadian Seismic Hazard Model

ABSTRACT The tectonic classification of the Haida Gwaii thrust fault (HGT) as either a subduction interface or a crustal fault is important for seismic hazard assessment in northwest British Columbia. To inform future versions of the Canadian Seismic Hazard Model (CanadaSHM), we reviewed evidence from the literature for subduction beneath Haida Gwaii, and found that various independent geophysical datasets suggest localized incipient underthrusting along an ∼180 km segment, roughly corresponding to the 2012 rupture plane. We evaluate the performance of newly developed ground-motion prediction equations (GMPEs) for Haida Gwaii by analyzing instrumental ground motion and shaking intensity data from the 2012 Haida Gwaii earthquake, considering two magnitudes for the event (Mw 7.8 from teleseismic data and Mw 7.4 from a regional seismicity catalog). We also assess GMPE performance for another reverse-faulting earthquake that could plausibly have occurred on the HGT. Our results show that the GMPEs predict horizontal ground motions reasonably well, especially for the 2012 mainshock. On average, GMPEs overestimate shaking by a factor of 1.1 (Mw 7.4) or 1.6 (Mw 7.8), improving on earlier estimates of overprediction by a factor of two or more. Although residuals for roughly half the data overlap with the zero-residual line within uncertainty, residuals for all GMPEs tested overlap within the aleatory uncertainty of active crustal GMPEs. Therefore, ground motions are not diagnostic in terms of a crustal faulting or subduction interface designation for the HGT. We recommend that the HGT remain classified as a subduction interface in future versions of CanadaSHM, consistent with previous hazard assessments and recent studies that advocate for incipient subduction, while noting that reclassifying the HGT as an active crustal fault would have minimal impact on ground-motion simulations.