The Wadati-Benioff zone beneath Unalaska Island, in the eastern Aleutian arc, is shown to dip at approximately 48° between the depths of 50 and 150 km. Above this depth, the seismic zone appears to dip at a more shallow angle (∼12°). The depth of the Wadati-Benioff zone beneath Makushin Volcano on Unalaska Island is about 100 km. Events located within the upper plate near the Wadati-Benioff zone indicate that the maximum depth of seismic coupling between the subducting and overriding plates is about 30 km. This suggests that the maximum width of seismic coupling between the downgoing and overriding plates is about 75 km. Earthquake locations based on the local network data and 20 yr of teleseismic data (PDE), define similar seismically active and quiescent regions. A major seismicity gap exists for events of magnitudes greater than at least 4.6 in the forearc region near Unalaska Island. This seismicity gap correlates well with the eastern edge of the tsunami source area of the great (Mw = 8.6) 1957 earthquake, and a large forearc basin 75 km south of Unalaska Island suggesting a similar process may be controlling the location of all three phenomena. Only one intermediate-sized (mb = 6.1) earthquake has occurred within the seismicity gap during the past 20 yr. This event is shown to be a high stress drop (690 bars) intraplate event within the subducting slab at a depth of 35 km that may be related to lateral compressive stress induced by a change in slab curvature with depth. A review of the historic record from about 1770 reveals that the Unalaska region may have ruptured in a great earthquake in 1878 and a large earthquake in 1902. If this region is currently locked, an earthquake of magnitude 8.2 could rupture this section of the Alaska-Aleutian arc. Estimates of the recurrence interval for events of this size vary from between 40 and 70 yr.

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