Historic Earthquakes: Tectonic Summary

2003 March 17 16:36:16 UTC

The Aleutian arc extends about 3,000 km from the Gulf of Alaska to Kamchatka. It marks the region where the Pacific plate subducts into the mantle beneath the North American plate. This subduction is responsible for the generation of the Aleutian Islands and the deep offshore Aleutian Trench. Relative to a fixed North American plate, the Pacific plate is moving north-west at a rate that increases from 6.6 cm per year in the arc's eastern region to 8.6 cm per year near its western edge. In the east, the convergence of the plates is nearly perpendicular to the plate boundary. 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.

Subduction zones such as the Aleutian Arc are geologically complex and produce numerous earthquakes from multiple sources. Deformation of the overriding North American 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 or 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 March 17th event are consistent with those of an interplate earthquake. The Aleutian subduction zone produced four great interplate earthquakes in the last century: the 1965 magnitude 8.7 Rat Island earthquake, the 1957 magnitude 9.1 Andreanof earthquake, the 1938 magnitude 8.2 earthquake that occurred off the Alaska Peninsula, and the 1964 magnitude 9.2 Gulf of Alaska earthquake that caused $311 million in property damage and took 125 lives.