The Aleutian arc is one of the most seismically active regions of the world. It consists of a number of segments or blocks that are rotating clockwise and translating westward along the North American-Pacific plate boundary. Two of the largest recorded earthquakes have occurred along the interplate boundary south of the Andreanof Islands in 1957 and the Rat Islands in 1965. The large magnitudes of these earthquakes implied that the probability of the recurrence of a large-magnitude event at these same locations was low. However, the occurrence of a great earthquake south of the Andreanof Islands in 1986 less than 30 years after the 1957 event has prompted a reevaluation of the seismic potential of the arc. We present new marine geophysical observations and synthesize previous geologic interpretations of the Aleutian arc to show that the epicenters of these great thrust-type earthquakes coincide with upper plate segments of the arc characterized by a coherent forearc structural fabric. These segments, which are associated with large plutonic masses, are mechanically stronger and are related to the location of asperities positioned at depth along the main thrust zone; they appear to be more strongly attached to the North American plate than adjacent blocks. The tectonically controlled locations of these asperities thus appear to influence the seismicity along the arc.

The Andreanof and Rat segments of the arc are separated by the Delarof segment, which is considered to have a high probability of a large-magnitude earthquake in the near future based on probabilistic estimates of seismic potential (Nishenko, 1991). The Delarof segment of the arc, however, is highly disrupted by arc-transverse faults. This segment is structurally weak and does not appear to be capable of accumulating the interplate strain necessary to generate a great, subduction zone earthquake. We propose that variations in upper plate structural strength and mobility affect the mechanical properties of the interplate thrust zone and need to be considered in localizing interplate asperities. Forearc tectonic segmentation associated with the partitioning of strike-slip and thrust motions may exert long-term controls on the rates of seismic moment release.