Source Characteristics of the 2012 Haida Gwaii Earthquake Sequence

Abstract All the significant ( M L ≥4) events in the 2012 Haida Gwaii earthquake sequence are systematically relocated, and their moment tensor solutions are determined from waveform inversion. The focal mechanism of the mainshock shows low‐angle thrust faulting along a shallowly dipping plane with a strike parallel to the Queen Charlotte fault (QCF), consistent with the inference of Pacific plate underthrusting beneath the overriding North American plate. The epicenter of the mainshock is located ∼5  km landward (northeast) of the surface trace of the QCF, suggesting the nucleation of the rupture was near the bottom of the seismogenic (locked) interface. Significant aftershocks appear to cluster on the periphery of the main rupture zone with most events located immediately seaward of the deformation front. The majority of these events show normal‐faulting mechanisms that are probably associated with the bending stress within the Pacific plate near the deformation front. Several normal and strike‐slip events at greater depths within the subducted Pacific slab show a consistent pattern of T ‐axis in the down‐dip direction, implying the subducted plate is under a stress regime of down‐dip extension. Only a few strike‐slip events were observed along or near the QCF. The limited size and distribution of these events suggest that most of the elastic strain accumulated along the QCF was not released during the 2012 Haida Gwaii sequence. Major strike‐slip earthquakes are likely to occur along the southernmost part of the QCF system in the future.