Adak local-network and teleseismically recorded data are used to describe shallow seismicity and tectonics in the Adak Canyon region of the central Aleutians. An examination of the locations and focal mechanisms of earthquakes up to 50 km in depth, occurring between July 1974 and December 1976, permits the subdivision of this activity into a northward dipping zone of thrusting and an overlying wedge-shaped zone of normal faulting. The thrust zone is characterized by an upper part which has been the source for all large thrust earthquakes occurring in the Adak region located with network data, and by a lower part representing a continuation of thrusting with smaller magnitude events. The wedge-shaped zone of low-level activity, in which east-west extension predominates, is found above and to the north of the thrust zone. The location, dimensions, and focal mechanisms of this region of normal faulting strongly suggest that Adak Canyon and similar features found to the west across the arc are caused by lateral extension of the overriding plate.
A Ms 7.1 shallow thrust event and four major aftershocks that occurred in the Adak Canyon region in May 1971 have been relocated by the master-event technique. The relocated sequence forms a tight cluster, about 20 km in diameter, and appears to be associated with the trenchward part of the thrust zone. This series of events lies in the center of a gap in recent large-magnitude seismicity.