Southern Haida Gwaii is part of an island archipelago off the west coast of British Columbia, Canada adjacent to the Queen Charlotte transform fault that exhibits slip partitioning with strike-parallel motion taken up along the Queen Charlotte Fault and convergence motion along an incipient subduction thrust as emphatically manifest by the Mw 7.8 October 28, 2012 earthquake. In this study, seismicity located in the vicinity of the crustal sliver, the so-called Queen Charlotte Terrace, is analyzed to gain a better understanding of slip partitioning. The sparse regional seismic network poses challenges for studying microseismicity. To address this shortcoming, we employ a novel relative moment tensor inversion technique to recover mechanisms from 115 aftershocks of the 2012 M7.8 event, distributed over 4 groups. Three of these groups lie at depths of 12-18 km near but downdip of the intersection between the Queen Charlotte Fault and the thrust interface, and exhibit predominantly reverse faulting mechanisms. We thus interpret these events to occur on, or along fault strands adjacent to, the main thrust. The fourth group comprises some 130 events at depths of 15-24 km distributed along a nearly trench-parallel lineament farther landward below Moresby Island and which occurred between 9 and 13 months after the mainshock. These events exhibit a variety of focal mechanisms with no systematic spatio-temporal progression and are hypothesized to manifest swarm activity resulting from vertically migrating fluids sourced from former outer-rise faults in the underthrusting Pacific plate.
Drolet et al. (Thu,) studied this question.