Atka M2.3 | Alaska Earthquake Center

Tectonic framework of the Aleutian Islands

The Aleutian Arc marks the region where the Pacific Plate is thrusting into the mantle beneath the North American Plate. It is a seismically active region, as evidenced by the thousands of earthquakes that occur each year. Since 1900, this region has been the site of several major megathrust earthquakes, including the 1957 magnitude 8.6 Andreanof Islands earthquake, the 1965 magnitude 8.7 Rat Islands earthquake, the 1986 and 1996 magnitude 7.9 Andreanof Islands earthquakes, and the 2003 magnitude 7.8 Rat Islands earthquake. Together, they have ruptured the entire length of the megathrust boundary in this region. Another notable source of seismicity in the arc is the 10- to 15-km-wide zone of intermediate-depth earthquakes in the subducting Pacific plate, known as the Wadati-Benioff zone. The largest intermediate-depth earthquake recorded in this region was the magnitude 7.9 Little Sitkin event of 2014. A third source of seismicity in this region is shallow earthquakes associated with volcanic processes and crustal faulting within the North American plate. These occur regularly and can produce vigorous aftershocks or swarm-like sequences.