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 lowangle 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.

Basement and Regional Structure Along Strike of the Queen Charlotte Fault in the Context of Modern and Historical Earthquake Ruptures

“…A tsunami with up to ∼13 m runup also indicates a significant component of underthrusting and slip (Leonard and Bednarski, 2014). Normal-faulting aftershocks in the Pacific plate suggest plate bending and downdip extension, consistent with young subduction (Lay et al, 2013;Farahbod and Kao, 2015;Kao et al, 2015). GPS data and new thermal models are consistent with these interpretations, supporting a shallow megathrust event (Nykolaishen et al, 2015;Wang et al, 2015).…”

“…The Haida Gwaii earthquake fits in well with this observation, as the mainshock ruptured a fault dipping 18.5°to the east at 14 km depth beneath the seafloor of the QCT (James et al, 2013;Lay et al, 2013;USGS, see Data and Resources). In addition, several recent studies based on seismicity, GPS observations of coseismic and postseismic motion, and thermal modeling Kao et al, 2015;Nykolaishen et al, 2015;Wang et al, 2015) support the Haida Gwaii event as a shallow megathrust on the Pacific-North American plate interface near the QCF.…”

“…The entirety of the Haida Gwaii aftershock zone is just south of 53.2°N, where the strike of the QCF changes from 328°to 338° Kao et al, 2015). Aftershocks of the Haida Gwaii event are primarily within the Pacific plate, and the majority of the aftershocks have normal fault mechanisms (Lay et al, 2013;Kao et al, 2015).…”

“…The area with the highest-density normal faults as mapped in this study is highlighted by a gray ellipse. The approximate area of Haida Gwaii normal fault aftershocks (derived from the Global Centroid Moment Tensor catalog; Ekström et al, 2012; see also Kao et al, 2015) is denoted by a white ellipse. The extent of the Queen Charlotte (QC) Trough at the seafloor is also plotted by a thick dashed line.…”

“…The Haida Gwaii event was exceptional because it was significantly larger in moment magnitude than other thrust events along the transpressional southern QCF system (Ristau et al, 2007;Rabinovich et al, 2008). It was followed by normal-faulting aftershocks within the Pacific plate, likely generated from bending of the Pacific plate in response to underthrusting (Lay et al, 2013;Kao et al, 2015).…”