M 7.8 - 206 km SW of Prince Rupert, Canada

Tectonic Summary

The October 28, 2012 (October 27 at the location of the epicenter), M 7.7 earthquake offshore of the island of Haida Gwaii, Canada, occurred as a result of shallow, oblique-thrust faulting near the plate boundary between the Pacific and North America plates. At the location of this earthquake, the Pacific plate moves approximately north-northwest with respect to the North America plate at a velocity of about 50 mm/yr. This earthquake is likely associated with relative motion across the Queen Charlotte fault system offshore of British Columbia, Canada. Studies of tectonics in this region suggest plate motions are taken up by strike-slip faulting parallel to the plate boundary, accompanied by lesser amounts of thrust motion to accommodate the oblique nature of the plate motion vector between the two plates relative to the orientation of the main plate boundary fault structure. This oblique component of plate motion may involve either underthrusting of the western edge of the Pacific plate beneath North America, or be accommodated by crustal faults within the North America plate. The October 28th earthquake is consistent with either scenario. Its rupture extended approximately 100–150 km along strike, along the shallow extent of the source fault. Slip amounts reached upwards of 5 m, in a patch predominantly to the south of the epicenter. Aftershocks clearly delineate the rupture.

While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Reverse-faulting events of the size of the October 28, 2012, earthquake are typically about 130x40 km (length x width); modeling of this earthquake implies dimensions of about 140x30 km, predominantly up-dip of and bilaterally from the hypocenter.

This region of the Pacific-North America plate boundary has hosted seven earthquakes of M 6+ over the past 40 years, the largest of which was a M 6.6 earthquake in November 2009, 80 km to the southeast of the 2012 earthquake that caused no documented damage. In 1949, a M 8.1 earthquake occurred closer to the Pacific-North America plate boundary, likely as a result of strike-slip faulting, approximately 100 km northwest of the October 28th earthquake, near the northern extent of Haida Gwaii region (formerly Queen Charlotte Islands).

Hayes et al. (2016) Tectonic summaries of magnitude 7 and greater earthquakes from 2000 to 2015, USGS Open-File Report 2016-1192. (5.2 MB PDF)