Earthquake source parameters and stress distribution in the Adak Island region of the central Aleuti

Source parameters have been systematically determined for all earthquakes with Mw≥5 that occurred between 172°W and 179°W longitude in the Adak Island region of the central Aleutian Islands during 1977-March 1987. We relocate the events using a plate velocity model developed for the region and use two alternative methods of depth determination. The first method uses arrival times of direct and prominent reflected phases, primarily pwP. The second method uses broadband P wave displacement seismograms in an inversion for source depth. The analysis of these events provides a well-constrained data set for the study of stress release along an active subduction zone. Three earthquakes, which occurred seaward of the trench axis, are located just below the crust-mantle interface and show extension nearly perpendicular to the trench axis. Seven events occurred in the Wadati-Benioff zone. Sixty-four events are located in the main thrust zone, and, except for five unusual events, are characterized by thrust mechanisms with one nodal plane dipping north at a shallow angle. In cross section the thrust zone appears as a thin (10–15 km thickness) interplate region which extends from 15 to 50 km depth. A 10°–15° average discrepancy between observed slip vector azimuths and the predicted relative motion direction between the Pacific and North American plates exists for these events. The observed slip vectors are oriented more normal to the trench than is predicted by plate motions. Five earthquakes, which occurred as aftershocks to the May 7, 1986, earthquake (Mw = 8.0), in the crust of the overriding plate have strike-slip mechanisms consistent with right-lateral motion on arc-parallel fault planes. Observations of slip vectors along the whole Aleutian arc show a similar trend to that observed in the Adak Island region. The largest differences (∼30°) between observed and predicted slip azimuths occur around 175°E. We propose a model of plate interaction in which a portion of the along-arc motion occurs along a weak strike-slip shear zone in the upper plate, near the volcanic line. The slip azimuths in the main thrust zone fit this model well, if the amount of transcurrent slip occurring in the upper plate is ∼60% of the arc-parallel relative plate motion. A consequence of the model is along-arc extension of the overriding plate between the accretionary wedge and the volcanic line, especially in the western part of the Aleutian arc. Calculations based on a tectonic model by Geist et al. (1988) for the formation of arc summit basins through block rotation and translation, suggest that along-arc extension has been significant since late Miocene or early Pliocene.