14 February 1983 earthquake sequence in the Shumagin Islands, Alaska

Abstract

Abstract Two moderate events (MS = 6.3 and 5.6) occurred just outside a local network in the Shumagin Islands, Alaska, on 14 February 1983. These were the first events of magnitude greater than 5.5 in the region since 1979. They occurred within the subduction zone at a depth of 26 km. Of the more than 50 aftershocks that were recorded at one or more stations, 12 events ranging from mb = 2.5 to 4.0 were recorded digitally and located. Focal mechanisms were calculated for the two main events from first-motion polarities and teleseismic body-wave modeling. The mechanisms for both events show similar nodal planes, with P axes in the approximate direction of plate convergence and nearly vertical T axes. The plane of motion is not constrained by the aftershocks but the shallowly dipping plane parallel to the subduction zone interface is the preferred solution. Dynamic stress drops of 80 and 70 bars are calculated from local strong-motion records. These events occurred in a mature seismic gap. They nucleated in a shallow portion of the main thrust zone that apparently has a relatively low state of stress and thus were not capable of rupturing into the higher strength region at the base of the main thrust zone.