Geologic records of fault slip in subduction forearcs provide critical data on stress and strain in the upper plate and the seismogenic potential of hazardous faults. However, few active upper-plate faults have been identified in the northern Cascadia forearc. Here we investigate the slip history of the Beaufort Range fault (BRF) on Vancouver Island, BC, Canada, a proposed source of the 1946 M 7.3 Vancouver Island earthquake, the largest recorded in Cascadia. We use recently-collected lidar data, field mapping, and surveying of offset landforms to map the extent of previously unidentified post-glacial (<14 ka) tectonic scarps and reconstruct 3D fault slip vectors. Post-glacial landforms show increasing displacement with age, suggesting at least three Mw~6.5-7.5 earthquakes since ~14 ka, the most recent <4 ka. These displacements suggest the BRF is one of the fastest-slipping faults in the northern Cascadia forearc (0.5-2 mm/yr). Kinematic slip inversions of offset geomorphic piercing lines are consistent with right-lateral transtension along a steeply NE-dipping fault. Because BRF fault geometry and kinematics are similar to the 1946 earthquake, it is a plausible source. The kinematic similarity of millennial and decadal slip data suggests the BRF has accommodated transtension over multiple earthquake cycles.
Lynch et al. (Wed,) studied this question.