Crustal structure crossing the Queen Charlotte Fault and Trough in the region of the Haida Gwaii 2012 M7.9 Thrust Earthquake using P-wave tomography

In 2012 a M7.8 earthquake ruptured offshore the Haida Gwaii region of British Columbia. This earthquake generated a small tsunami and caught the attention of the scientific community as, based on its low-angle thrust slip mechanism, outer-rise-like normal faulting aftershocks and patterns of up-lift and subsidence, it looked very similar to a subduction zone thrust event. The earthquake occurred in a region of the Pacific-North American (Pac-NA) plate boundary where >5 cm/year of transform motion is accommodated on the Queen Charlotte Fault (QCF). Along the southern portion of the QCF, a bend in the plate boundary leads to ~15° of convergence. Convergence may be accommodated by either crustal shortening, particularly along the Queen Charlotte trough and terrace, a prism-like structure just offshore Haida Gwaii, or through underthrusting, more similar to a subduction zone. How strain is partitioned between compressional and transform structures, the latter which are easily mapped to the north, has not been resolved. In the summer of 2021, the Transform Obliquity on the Queen Charlotte Fault and Earthquake Study (TOQUES) collected several thousand km of wide-angle seismic refraction and reflection data along the QCF, including profiles specifically designed to characterize crustal structure through the highly-oblique southern segment. This dataset includes a ~75 km-long transect that crosses the plate boundary near the epicenter of the 2012 earthquake. Here we present initial P-wave tomography results from this transect, where 5-km-spaced OBS recorded the shots of the R/V Langseth. Crust and mantle refractions are recorded along the full offset of the line, as well as reflections from the Pac and NA Moho. Initial modeling shows variation in seismic velocities as the line transitions from the Pac lithosphere, to the complex region within the terrace, and up to the continental shelf offshore Haida Gwaii, where some studies have mapped a through-going, high-angle Queen Charlotte transform fault that is distinct from the thrust surface that slipped in 2012. This data provides some of the first constraints on the complex plate boundary structure as well as the prism offshore Haida Gwaii, which are key features associated with the tectonic evolution of the oblique transform plate boundary and seismic and tsunami hazards.