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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

Description: 

In 2012, a M7.8 earthquake ruptured offshore the Haida Gwaii region of British Columbia and generated a small tsunami. The earthquake occurred in a region of the Pacific-North American (Pac-NA) plate boundary where >5 cm/year of transform plate 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. This convergence has been hypothesized to be accommodated by either crustal shortening, particularly along the Queen Charlotte Terrace (QCT), a prism-like structure just offshore Haida Gwaii, or through underthrusting, more similar to a subduction zone. In the summer of 2021, the Transform Obliquity on the Queen Charlotte Fault and Earthquake Study (TOQUES) collected wide-angle seismic refraction data along the QCF, including a ~75 km-long transect that crosses the plate boundary near the epicenter of the 2012 earthquake. Here we present P-wave tomography from this transect, where 5-km-spaced OBS recorded airgun shots of the R/V Marcus G. Langseth. Records of high-quality crustal and mantle refractions exist along the full offset of the line, as well as reflections from the Pac Moho and NA Moho or lower crust. Refractions were also recorded on a station on Moresby Island. Arrival-time tomography shows variation in seismic velocities transitioning from the eastern-dipping Pac lithosphere to the seismically fast and heterogeneous QCT offshore Haida Gwaii. Velocities support underthrusting of the Pacific slab beneath the QCT but do not show evidence of continued underthrusting east of the where the QCF has been mapped as a through-going, high-angle fault. The shallow structure does show a clear increase in seismic velocities east of the fault at the seafloor, consistent with Miocene uplift of Haida Gwaii, but does not show fault damage. Evaluating the relative contributions of underthrusting vs. crustal shortening is not straightforward, as both the shallow dip of the apparent underthrusting Pac and fast nappe-like structures in the wedge seem to require some of both deformation modes to accommodate convergence.

Session: Structure, Seismicity and Dynamics of the Queen Charlotte-Fairweather Fault System
[Poster Session]

Type: Poster

Date: 5/1/2024

Presentation Time: 08:00 AM (local time)

Presenting Author: Emily

Student Presenter: No

Invited Presentation: 

Authors