Stress pattern in the double seismic zone beneath Cook Inlet, south-central Alaska

Abstract

Relocation of south-central Alaskan earthquakes recorded by the regional seismograph network, using the joint hypocenter determination method, revealed a double seismic zone in the underthrusting Pacific plate beneath the Cook Inlet area. The lower zone is separated from the upper one by a decrease in seismic activity within a 5–10 km wide band, and extends from a depth of 50 km to about 90 km. Stress tensor inversion 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 Wadati-Benioff zone (WBZ). In the lower zone both maximum and minimum principal stresses are horizontal and rotated by 40–60° from the down-dip direction of the WBZ. Below 80–90 km depth, stress tensor inversion results show down-dip orientation of the minimum principal stress and the maximum stress aligned in the direction of convergence of Pacific plate.