Fault motion and spatially bounded character of earthquakes in Amchitka Pass and the Delarof Islands

During the period from May 1969 to March 1970, seven moderate earthquakes occurred in the Delarof-Andreanof Island region. The focal mechanisms of these earthquakes correspond to the motion that would be expected on the basis of plate tectonics. Of more particular significance, the motion in one of these shocks, located at intermediate depth in the Benioff zone, indicates horizontal tension parallel to the plate, corresponding to lateral extension as the plate descends under an arcuate structure convex to the plate motion. The spatial and temporal relation of these earthquakes and of their aftershock sequences to the over-all activity of the arc and particularly to the seismicity of the Rat Islands during this period supports the hypothesis that the Aleutian Islands are active by independent blocks and that the boundaries of these blocks are permanent features.