2006 M6.6 Andreanof Islands Earthquake
On July 8, 2006, at 12:39 pm ADT (8:39 pm UTC) a strong earthquake, magnitude 6.6, occurred in the Andreanof Islands region of the Aleutian Islands (larger red star on the above map). The event was situated 138 kilometers (86 miles) east-southeast of Amchitka and 204 km (128 miles) west-southwest of Adak. The epicenter was located at 50.97N and 179.22W. It was felt at Adak. This earthquake is the largest to occur in this region since the magnitude 6.8 event on June 14, 2005 (smaller white star on the map). The July 8th event was preceded by a series of moderate foreshocks that started on July 1st. The strongest foreshocks (M5.5-5.6) occurred on July 1st at 11:34 am ADT, and on July 2nd at 8:57 am and 9:20 am ADT. The AEIC located over 250 aftershocks of the M6.6 event within the first week after the mainshock and over 500 events total between July 1st and 31st in the source region of the M6.6 earthquake (crosses). There were three aftershocks with magnitudes greater than 5.0 through the end of July. The largest aftershock (M5.5) occurred on July 8 at 8:16 pm ADT (July 9, 4:16 am UTC).
The July 2006 earthquake sequence occurred in the area separating the rupture zones of the 1965 M8.7 Rat Islands earthquake to the west and the 1957 M8.6 Andreanof Islands earthquake to the east. The westernmost area of the 1957 fault zone was reruptured repeatedly in the 1986 M8.0 and 1996 M7.9 earthquakes. The eastern portion of the 1965 fault zone was reruptured recently in the M7.7 earthquake on November 17, 2003. The June 2005 sequence of earthquakes was located east of the 2003 rupture zone, at the easternmost end of the 1965 rupture zone. All these events ruptured the boundary between the subducting Pacific and overriding North American plates (approximate extent of the rupture zones is shown in red for the 1957 and 1965 events, and in black for smaller events). The Aleutian megathrust, where the two plates are being forced directly into one another, is one of the world's most active seismic zones. Over one hundred earthquakes of magnitude seven or larger have occurred along this boundary in the past hundred years, or one magnitude 7+ earthquake every year or two.