M 7.5 - 110 km SW of Edna Bay, Alaska

Tectonic Summary

The January 5, 2013, M 7.5 earthquake off the west coast of southeastern Alaska occurred as a result of shallow strike-slip faulting on or near the plate boundary between the Pacific and North America plates. At the location of this earthquake, the Pacific plate is moving north-northwestward with respect to the North America plate at a velocity of about 51 mm/yr. This earthquake is likely associated with right-lateral relative motion across the Queen Charlotte fault system offshore of Alaska and British Columbia, Canada, which forms the major expression of the Pacific-North America plate boundary in this region.

While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Strike-slip events of the size of the January 5, 2013, earthquake are typically about 120x20 km (length x width); modeling of this earthquake implies dimensions of about 80x20 km, predominantly north of the hypocenter.

The surrounding area of the plate boundary has hosted eight earthquakes of M 6+ over the past 40 years. In 1949, a M 8.1 strike-slip earthquake nucleated in the Pacific-North America plate boundary region approximately 200 km to the southeast of the January 5th event, and the causative fault ruptured close to, or into, the January 5th source region. In October 2012, the M 7.8 Haida Gwaii earthquake occurred in the plate boundary region approximately 330 km to the southeast of the January 5th earthquake, and was associated with oblique thrust faulting. The January 5, 2013, earthquake is related to the earthquake three months previously, and is an expression of deformation along the same plate boundary system. The January 5th event ruptured a fault approximately 50 km in length with 7–8 m of slip.

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)