2003 M7.7 Rat Islands Earthquake

Mainshock and Aftershocks: 

At 9:43 p.m. AST (06:43 11/17 UTC) on Sunday evening, November 16, 2003, a major earthquake occurred in the Rat Islands region of the Aleutian Islands. This earthquake was situated 95 kilometers (59 miles) south-southwest of Amchitka. The epicenter (red star) was located at 50.7487N and 178.4427E, the estimated moment magnitude is 7.7. This earthquake is the largest to occur in North America since the magnitude 7.9 Denali Fault, Alaska earthquake of November 3, 2002, and the largest in the Aleutian Islands since the June 1996 magnitude 7.9 Adak earthquake. The AEIC located 182 aftershocks (open circles) in the 17.5 hours after the main shock. The largest aftershock occurred at 10:50 p.m. AST on November 17 (7:50 UTC, 11/18) and had a moment magnitude of 5.7. 

The M 7.7 earthquake occurred on the convergent boundary between the subducting Pacific and overriding North American crustal plates. This region, 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.

Tectonic Setting: 

The 1965 Mw 8.7 Rat Islands earthquake ruptured an ~600 km-long portion of the plate boundary. In November 17, 2003 M7.7 earthquake, the easternmost part of the 1965 zone failed again.


The main shock generated a small tsunami of about 50 cm (1.64 ft) at Shemya and 12 cm (0.39 ft) at Adak (Alaska Tsunami Warning Center).