M 7.1 - 47 km ESE of Pedro Bay, Alaska

Tectonic Summary

The January 24, 2016 M 7.1 earthquake southwest of Anchorage, Alaska, occurred as the result of strike-slip faulting at intermediate depths, within the subducted lithosphere of the Pacific plate. In the region of the earthquake, the Pacific plate moves northwestward with respect to North America at a rate of 60 mm/yr, and begins its descent into the mantle at the Alaska-Aleutian Trench almost 400 km to the southeast of this earthquake. The mechanism and depth of the earthquake are consistent with its occurrence within the interior of the subducted Pacific plate, rather than on the shallower plate boundary thrust between Pacific and North America. The focal mechanism indicates faulting occurred on either a northwest striking right-lateral structure, or on a northeast striking, left-lateral fault.

Earthquakes like this event, with focal depths between 70 and 300 km, are commonly termed "intermediate-depth" earthquakes. Intermediate-depth earthquakes represent deformation within subducted slabs rather than at the shallow plate interfaces between subducting and overriding tectonic plates. They typically cause less damage on the ground surface above their foci than is the case with similar magnitude shallow-focus earthquakes, but large intermediate-depth earthquakes may be felt at great distance from their epicenters. Earthquakes have been reliably located to depths of about 220 km in this region.

The Pacific:North American plate-boundary zone of southern Alaska produces frequent earthquakes. The shallow interface between these plates to the southeast of the January 24, 2016 earthquake was part of the extended source-region of the second largest global earthquake ever instrumentally recorded, the M 9.3 Great Alaska earthquake of March 27, 1964. Twelve other earthquakes of M 6 or larger have occurred within 250 km of the January 24, 2016 earthquake since 1960, a time-period for which our earthquake catalog should be essentially complete for events of that size. The largest of these was an M 7.0 aftershock of the Great Alaska earthquake in July 1965. The largest intermediate-depth earthquake in the region during the same time-period was a M 6.8 event in July 2001, 120 km to the southwest of today's earthquake.