STRESS PATTERN IN THE DOUBLE SEISMIC ZONE BENEATH COOK INLET, SOUTH-CENTRAL ALASKA

Relocation of south-central Alaskan earthquakes recorded by the region al seismograph network, using the joint hypocenter determination metho d, revealed a double seismic zone in the underthrusting Pacific plate beneath the Cook Inlet area. The lower zone is separated from the uppe r one by a decrease in seismic activity within a 5-10 km wide band, an d extends from a depth of 50 km to about 90 km. Stress tensor inversio n for earthquakes in the upper and lower zones shows different stress regimes. In the upper zone minimum principal stress is oriented in the down-dip direction and maximum principal stress is normal to the Wada ti-Benioff zone (WBZ). In the lower zone both maximum and minimum prin cipal stresses are horizontal and rotated by 40-60 degrees from the do wn-dip direction of the WBZ. Below 80-90 km depth, stress tensor inver sion results show down-dip orientation of the minimum principal stress and the maximum stress aligned in the direction of convergence of Pac ific plate.