Seismicity of the Wrangell and Aleutian Wadati‐Benioff Zones and the North American Plate along the Trans‐Alaska Crustal Transect, Chugach Mountains and Copper River Basin, southern Alaska

Abstract

The configuration of the Aleutian and Wrangell Wadati‐Benioff zones in southern Alaska between longitudes 142°W and 148°W is determined from regionally recorded earthquakes from 1971 to 1986. The earthquakes are located with a simple velocity model calibrated with seismic refraction measurements conducted for the Trans‐Alaska Crustal Transect (TACT). West of the transect meridian (approximately 145°20′W) and beneath the Chugach Mountains, shocks in the depth range 25–60 km compose the shallow part of the northwest to north dipping Aleutian zone. This zone can be traced updip and seaward beneath Prince William Sound. East of the transect meridian and north of 61°N latitude, the weakly active, north‐northeast dipping Wrangell zone extends to a depth of at least 100 km beneath the Wrangell volcanoes and along strike for at least 150 km. The two zones appear to be continuous, at least in the depth range 20–45 km, and may define adjacent limbs of a buckle in the subducted oceanic lithosphere. North of 61°N, focal mechanisms in the shallow Aleutian zone are compatible with a uniform stress field characterized by in‐plane least compressive stress directed toward the steeply dipping, deeper part of the Aleutian zone north of Cook Inlet. Stress orientation in the Wrangell zone is unresolved because of the low level of seismicity. No focal mechanisms corresponding to slip on a gently northward dipping megathrust have been observed. Along the transect meridian, a change of focal mechanism with depth near the top of the dipping seismic zone is inferred to reflect the interplate boundary beneath the Chugach Mountains. This boundary coincides with a band of strong reflectors imaged by TACT vertical seismic profiling. The overriding plate appears to be aseismic beneath the northern Chugach Mountains, but minor shallow seismicity occurs beneath the Copper River Basin. First‐motion patterns for these shallow shocks suggest the most compressive stress within the overriding plate is oriented subhorizontally and approximately north‐south.