2012 M6.4 Andreanof Islands Earthquake

Mainshock and Aftershocks: 

A magnitude 6.4 earthquake occurred on Wednesday, September 26, 2012 at 3:39 pm AKDT (18:37 UTC) in the Andreanof Islands region of Alaska (red star on the map). It was located 114 km (71 miles) WSW of Adak and 197 km (127 miles) E of Amchitka. It was felt on Adak and Atka. AEIC located nearly 900 aftershocks within the following month with magnitudes as low as 1.0 (white circles). Seven aftershocks had magnitude 4 or greater, the largest aftershock of magnitude 4.4 occurred about 3 hours after the mainshock.

Tectonic Setting: 

This is the largest event to occur in the area since the magnitude 6.6 earthquakes on April 15 and May 2, 2008 (yellow stars). The faulting parameters of these earthquakes estimated from the waveform inversion indicate strike-slip type of faulting. Their locations, shallow and above the down-dip end of the locked plate interface, are consistent with the events occurring within the crust of overriding North American plate. Similar earthquakes have occurred in the past in western and central Aleutian arc. In western Aleutians, the crust is partitioned into rotating blocks. In addition, direction of convergence between the two tectonic plates is oblique to the plate interface in this region. The 2008 and 2012 strike-slip events manifest deformation along the Aleutian arc that accommodates relative motions of the crustal blocks and/or slip partitioning between the plate subduction and strike-slip motion.