This electronic supplement contains one table and two figures. Table S1 contains details of the aftershock moment tensors used in the calculations in the main article. The table is not strictly necessary to the average reader, because the focal mechanisms for these events are already displayed in Figure 3, but they are worth archiving nonetheless. Figure S1 shows the remaining optimally oriented failure plane results for all model runs. These are excluded from the main article for brevity and because the same general patterns are observed in all cases. Figure S2 shows premainshock moment tensors as beachballs. These are interesting in that they illustrate why we did not normalize nodal plane results against a control group, as well as showing that there is a location near shore at about 52.75° north that seems to have been seismically active even before the 28 October 2012 Haida Gwaii event.
Table S1. Aftershock moment tensors used in this study. The table was compiled from the Canadian National Seismic Network (CNSN) database of moment tensor solutions from immediately following the October 2012 mainshock until May 2013. There are 50 in total, most of which have normal mechanisms. For more information see Kao et al., 2015.
Figure S1. Maximum stress change (bar) over 1 km increments for optimally oriented fault planes, overlain by all Mw 2+ aftershocks. The finite-fault model, friction value, and sense of slip are listed in the bottom left corner (a)–(f), and the aftershock and depth interval are displayed in the bottom left of each panel within (a)–(f).
Figure S2. All CNSN moment tensor solutions (compressive quadrants scaled by depth) and focal mechanism solutions (compressive quadrants in black) (Bird, 1997; Ristau et al., 2007) for the pre-mainshock period (January 1982–October 2012). There are fewer control events than aftershocks (see Fig. 3 in main article). Interestingly, there is a persistent cluster of aftershocks near shore at approximately 52°45′ north (52.75° N), both before and after the 2012 mainshock.
Bird, A. L. (1997). Earthquakes in the Queen Charlotte Islands Region: 1982–1996. Master’s Thesis, University of Victoria, British Columbia, 123 pp.
Kao, H., S.-J. Shan, and A. Farahbod (2015). Source characteristics of the 2012 Haida Gwaii earthquake sequence, Bull. Seismol. Soc. Am. 105, no. 2B, doi: 10.1785/0120140165.
Ristau, J., G. C. Rogers, and J. F. Cassidy (2007). Stress in western Canada from regional moment tensor analysis, Can. J. Earth Sci. 44, 127–148.
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