Microearthquake data from a permanent, telemetered array have been used to elucidate the structure and tectonics of the subduction zone in the Shumagin Islands, Alaska. The shallow microseismicity is characterized by active interplate thrusting in the 25--45 km depth range. Arcward of this interplate activity, microearthquakes in the overlying plate have a strike slip mechanism. The near-horizontal P axis of this mechanism is oriented in the direction of plate convergence, suggesting that the plate interface at shallow depths is currently efficiently coupled. Below 45 km depth, the dipping seismic zone appears to be double-planed. The upper plane dips at 32¿ from 45 to 100 km depth, where it exhibts a kneelike bend below the volcanic front. The lower plane begins at about 65 km depth, where it is separated from the upper plane by some 25 km, and appears to converge with the upper plane at about 120 km depth. The inferred geometry of the plate interface suggests that the double-planed portion of the dipping seismic zone is a product of elastic unbending of the subducted plate. A composite fault plane solution for events in the upper plane indicates, however, that downdip tension is currently present rather than the downdip compression that would be expected from unbending. It is proposed that because of locking of the plate interface at shallower depths, slab pull is currenlty overprinting the unbending stresses which predominate at times when the plates are unlocked. This model is consistent with previous interpretations that there is a high probability of a great earthquake in the Shumagin Islands seismic gap within the next decade or two.