2014 M7.9 Rat Islands Earthquake

Mainshock and Aftershocks: 

A magnitude 7.9 earthquake occurred on Monday, June 23 at 12:53 pm AKDT (20:53 UTC) in the Rat Islands region (large red star on the map). It occurred at a depth of 118 km (73 miles) and was located 25 km (15 miles) WNW of Amchitka, 325 km (203 miles) W of Adak and 334 km (209 miles) ESE of Shemya. It was the largest earthquake to occur in Alaska since the Novermber 3, 2002 Denali Fault earthquake. The Alaska Earthquake Center recorded over 2,500 aftershocks through end of the year (1st month aftershocks are shown by open circles on the map). About 60 aftershocks had magnitudes of 4.0 or greater. The largest aftershock, magnitude 6.4 (smaller red star on the map), occurred about six hours after the mainshock at 7:15 pm (June 24, 3:15 UTC). This aftershock was located at a much shallower depth (19 km or 12 miles) and outside of the main rupture zone of the 7.9 earthquake.

Felt Reports: 

This earthquake was felt widely in western and central Aleutian communities. No reports of damage have been received. Maximum intensity of shaking, VI - strong, was reported on Adak.

Tectonic Setting: 

This is the largest event to occur in the region since the magnitude 7.7 earthquake on November 17, 2003, which occurred on the convergent boundary between the subducting Pacific and overriding North American crustal plates. Another major earthquake, M7.9, occurred farther east in 1996. This region, where the two plates are being forced directly into one another, is one of the world's most active seismic zones. Two great earthquakes occurred in this region, M8.7 Rat Islands earthquake in 1965 and M8.6 Andreanof Islands earthquake in 1957. These two earthquakes combined ruptured over half of the Aleutian megathrust length. The magnitude 7.9 earthquake on June 23 is different from these mentioned ruptures on the Aleutian megathrust. This earthquake was located at 118 km (73 miles) depth and was contained within the subducting Pacific plate (see cross-section below). This is the largest intra-slab earthquake ever recorded in Alaska. The next largest intra-slab earthquake, magnitude 7.3, was recorded on June 24, 2011 in Fox Islands at a depth of 74 km (46 miles).

Source Mechanism: 

The elastic-wave radiation pattern of the M7.9 event is consistent with the earthquake occurring as the result of down-dip pull of the subducting Pacific slab.