M6.8 - FOX ISLANDS, ALEUTIAN ISLANDS, ALASKA

Tectonic Summary

The September 2, 2011 Fox Islands earthquake occurred as a result of oblique-thrust faulting near the boundary between the Pacific and North American plates south of the Central Aleutian Islands of Alaska. The focal mechanism of the earthquake shows slip occurred on a fault almost perpendicular to expected northwest-southeast plate boundary deformation, and broadband depth modeling indicates a shallower rupture (32-35 km) than would be expected for slab interface events (45-50 km). This leads us to infer that the earthquake likely occurred within the overriding North American Plate, potentially related to block rotations within the arc as the upper plate responds to stresses generated from increasing obliquity of Pacific Plate subduction and relief on the downgoing slab. At the location of this event, the Pacific plate converges with North America at a rate of approximately 71 mm/yr in a northwesterly direction.

The September 2 earthquake is located approximately 25 km to the northeast of (and 30 km shallower than) an M 7.2 earthquake of June 24, 2011, which is thought to have ruptured a steeply dipping fault within the subducting Pacific Plate. Historically, the section of the Aleutians arc in the vicinity of these events is less active than other parts of this subduction zone, though it has hosted 12 events of magnitude 6 or greater since the early 1970's. The shallow subduction zone interface to the south of the September 2 event ruptured in the M 8.6 Central Aleutians earthquake of 1957, which spawned a large and damaging tsunami locally, that also impacted the shores of Hawaii and California.

Earthquake Information for Alaska