2009 M6.5 and M6.4 Fox Islands Earthquakes

Mainshock and Aftershocks: 

A magnitude 6.5 earthquake occurred on Monday, October 12, 2009 at 9:37 pm AKDT (October 13, 05:37 am UTC) in the Fox Islands region of Alaska (red star on the map). It was located 136 km (85 miles) ESE of Nikolski and 146 km (91 miles) S of Dutch Harbor. Another strong earthquake of magnitude 6.4 occurred on Tuesday, October 13 at 12:21 pm AKDT (20:21 UTC) in the same area (red star on the map). The AEIC staff located 440 aftershocks (open circles) through the end of October, 13 of which had magnitudes 4 or greater. The largest aftershock of magnitude 5.6 (small red star) occurred on Wednesday, October 15 at 4:13 pm AKDT (October 16, 00:13 UTC).

Felt Reports: 

M6.5 earthquake was reported felt in Nikolski and Unalaska, intensity III, weak.

Tectonic Setting: 

This is the largest event to occur in the region since the M6.4 earthquake on December 26, 2007. Another magnitude 6.4 earthquake occurred in the region on May 10, 2006 (yellow stars). October 2009 sequence of earthquakes occurred on convergent boundary between the subducting Pacific and overriding North American crustal plates. This region, where Pacific plate is being forced under the North American plate, is one of the world's most active seismic zones. In 1957, magnitude 8.6 Andreanof Islands earthquake ruptured a ~600 km-long portion of the plate boundary in the central Aleutian Islands. The current sequence of earthquakes is located near eastern end of the 1957 rupture zone.

Source Mechanism: 

Waveform modelling of the mainshocks and largest aftershocks indicate thrust type of faulting, consistent with the slip on plate interface.