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Geometry of the subducted plate near Kodiak Island and Lower Cook Inlet, Alaska, determined from relocated earthquake hypocenters

GEOPHYSICAL INSTITUTE UNIVERSITY OF ALASKA, FAIRBANKS, ALASKA 99701
INSTITUTE FOR GEOPHYSICS UNIVERSITY OF TEXAS AT AUSTIN, P.O. BOX 7456, AUSTIN, TEXAS 78712

Abstract

Using observations from 28 seismograph stations in southern Alaska, we have used the JHD method to relocate 341 well-recorded events with focal depths beneath about 40 km which occurred between 56.4°N and 60.5°N, and between 151.6°W and 156.5°W. These hypocenters show that the Wadati-Benioff zone has a dip of about 45°. However, its strike changes by about 15° beneath Lower Cook Inlet at about 59°N near where the volcanic line also bends northward. Near this bend there is also a distinctly higher level of seismic activity than in the area to the southwest of 59°N. A few events are situated about 20 km beneath the main Wadati-Benioff zone in Cook Inlet, and a focal mechanism we have determined for one of these differs from other mechanisms for events in this area. Focal mechanisms available from previous work show that at intermediate focal depths the area between 59°N and 60.5°N is controlled by horizontal north-south compression, and as such differs from the region to the southwest. Relying primarily on the observed change in strike and apparent thickness of the Wadati-Benioff zone, we interpret these data to suggest that the subducted lithophere separates into two distinct segments as it subducts beneath Cook Inlet. Because the direction of subduction is not perpendicular to the volcanic arc, the events situated beneath the Wadati-Benioff zone in Cook Inlet may be part of the southwestern plate segment. Although the geometry and available focal mechanisms for events beneath Cook Inlet are similar to what would be expected in a "double Wadati-Benioff zone," we suggest instead that these observations result because we are at the boundary between two adjacent segments of the subducting plate.

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