Shallow Seismicity of the Anchorage, Alaska, Region (1964-1999)

Abstract

We have relocated over 4200 shallow (≤40 km) earthquakes occurring in the Anchorage region for ∼35 years following the 1964 great ( M w 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 σ 1 and near-vertical σ 3, 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. σ 1 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 M w 7.0 Susitna lowlands earthquake may have been associated with this region of complex deformation.