Abstract

We have relocated over 4200 shallow (≤40 km) earthquakes occurring in the Anchorage region for ∼35 years following the 1964 great (Mw 9.2) Alaska earthquake. The shallowest (<20 km) earthquakes delineate several faults within the crust, including one associated with mapped folds located north of Upper Cook Inlet. Inversion of first-motion data for the stress field orientation in Upper Cook Inlet indicates east–west oriented horizontal σ1 and near-vertical σ3, a condition favoring reverse faulting along north–south striking faults with trends similar to the orientation of mapped faults and fault cored anticlines within the inlet. σ1 is rotated 60° to 90° counterclockwise from the direction of plate convergence, in agreement with Global Positioning System gps/geodesy studies that indicate the western portion of the Kenai Peninsula and Upper Cook Inlet do not appear to be moving in the direction of plate motion due to a change in coupling across the plate interface. The stress regime north of the Castle Mountain fault is conducive to strike-slip or normal faulting along faults striking east-northeast or north-northwest. Similar to previous studies we observed a persistent aseismic zone in the upper crust that appears to be located above and immediately downdip of the portion of the plate interface that slipped 20–25 m in the 1964 mainshock. Deeper (20–40 km) earthquakes indicate intense deformation and a rapidly changing stress field near the boundary between the Kenai and McKinley segments of the subducted slab. The 1943 Mw 7.0 Susitna lowlands earthquake may have been associated with this region of complex deformation.

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