The great Mw8.7 Rat Islands event was an underthrusting earthquake that ruptured a 600-km segment of the western end of the Aleutian Islands along the Aleutian Megathrust. The waveform analysis indicates unilateral rupture from east to west and three large pulses of moment release located along the fault (Beck and Christensen, 1991).

The mainshock was closely followed by a large tensional outer-rise event on March 30, 1965 (Ms7.5), which was located oceanward of the largest moment release associated with the Rat Islands mainshock rupture. This event may have been triggerred by the large displacement near the mainshock epicenter. The overriding plate along the western Aleutian subduction zone is laterally segmented into a series of rigid tectonic blocks separated by fault controlled canyons and extensional basins.

This tectonic setting controlled the rupture behavior of the M8.7 event.