Earthquakes, plate subduction, and stress reversals in the Eastern Aleutian Arc

Plate subduction beneath the 1500-km-long segment of the eastern Aleutian arc between Kodiak and Atka islands (154°W and 176°W longitude) is studied with observations from teleseismic data. The primary data base consists of hypocenters of earthquakes (for the period 1965–1975), carefully selected from the bulletins of the International Seismological Centre, and of 44 new focal mechanism solutions. The principal results of this study are that hypocenters of intermediate-depth earthquakes in the eastern Aleutians appear to define a weakly developed double seismic zone at depths between 70 and 170 km. Additional evidence for a double seismic zone comes from focal mechanisms which generally show downdip-directed P axes for earthquakes in the upper zone and downdip-directed T axes in the lower zone. Major features of the double zone can be explained by thermoelastic stresses in the downgoing plate. The observed predominant downdip stress polarity at intermediate depths in the descending plate reverses along strike of the arc. This stress reversal coincides in map view with a change from a continental to an oceanic arc. The coincidence may result from spatial differences either in the coupling between the plates at shallow depths or in the rheology of the surrounding (oceanic versus continental) mantle. Alternatively, the stress reversal may be related to the time since the last great earthquake. If the observed stress polarity relates to the elapsed time since the last great earthquake, it may be a significant observation for long- to intermediate-term prediction of great thrust earthquakes in subduction zones. Portions of the eastern Aleutian arc where downdip tension predominates contain one or more seismic gaps that appear to have a high probability for great earthquakes in the next few decades. A moderate-sized bathymetric feature in the Pacific plate, the Amlia Fracture Zone, modifies subduction and produces a bend or offset in the downgoing plate as well as an offset in the line of active volcanoes in the upper plate. Subduction of the fracture zone appears also to cause formation of summit basins on the arc platform and some aseismic behavior of a 100-km-long segment of the main thrust zone.

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