M 7.8 - Rat Islands, Aleutian Islands, Alaska

Tectonic Summary

The November 17, 2003, M 7.8 Rat Islands earthquake occurred as the result of shallow thrust faulting on or near the plate interface between the overriding North America plate and the subducting Pacific plate, along the subduction zone defining the Aleutian Arc of Alaska. The Pacific plate subducts northwest relative to the North America plate at the Aleutian Trench south of the November 17th earthquake, at a velocity of about 66 mm/yr in the arc’s eastern region where convergence is nearly perpendicular to the plate boundary and up to 86 mm/yr near its western extent. However, because of the boundary’s curvature, as one travels westward along the arc, the subduction becomes more and more oblique to the boundary until the relative plate motion becomes almost parallel to the boundary at its western edge. The Aleutian Arc extends about 3,000 km from the Gulf of Alaska to Kamchatka.

While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Thrust-faulting events of the size of the November 17, 2003, earthquake are typically about 120x50 km (length x width); modeling of this earthquake implies dimensions of about 120x100 km, mostly to the west and up-dip of the hypocenter.

Subduction zones such as the Aleutian Arc are geologically complex and produce numerous earthquakes from multiple sources. Deformation of the overriding North America plate generates shallow crustal earthquakes, whereas slip at the interface of the plates generates interplate earthquakes that extend from near the base of the trench to depths of 40 to 60 km. At greater depths, Aleutian Arc earthquakes occur within the subducting Pacific plate and can reach depths of 300 km.

The depth and thrust mechanism of the November 17th event are consistent with those of an interplate earthquake. The Aleutian subduction zone has produced four great interplate earthquakes in the last century: the 1938 M 8.2 earthquake that occurred off the Alaska Peninsula, the 1957 M 8.6 Andreanof earthquake, the 1964 M 9.2 Gulf of Alaska earthquake that caused $311 million in property damage and 125 fatalities, and the 1965 M 8.7 Rat Island earthquake.

Hayes et al. (2016) Tectonic summaries of magnitude 7 and greater earthquakes from 2000 to 2015, USGS Open-File Report 2016-1192. (5.2 MB PDF)