ALEUTIAN SUBDUCTION ZONE SEISMICITY, VOLCANO-TRENCH SEPARATION, AND THEIR RELATION TO GREAT THRUST-TYPE EARTHQUAKES

A seismic network has been operated in the Shumagin Islands and adjacent Alaska Peninsula, Alaska, since July 1973. Hypocentral cross sections based on the first 4yr of data from this network show a well-defined Benioff zone that is about 10km thick and extends to abotu 180km depth. Focal mechanism and strong-motion accelerograph data for an mb = 6.0 earthquakes at 40km depth indicate a slip plane subparallel to the Benioff zone. This event is interpreted as being directly caused by the underthrusting of the Pacific lithosphere beneath the Shumagin Islands. Comparison of the Shumagin hypocentral cros section with cross sections from other regions of the Aleutian arc shows that at depths greater than 40km 1) the Benioff zones are nearly congruent and 2) their positions relative to the volcano line are nearly the same along the entire length of the arc. This comparison also reveals that the eastward widening of the volcano-trench separation occurs predominantly in the shallowly dipping extension of the Benioff zone from its position at 40km depth to where it outcrops near the trench; this extension is herein termed the main thrust zone. Rupture zones of the 1964, the 1965, and possibly the 1938 great earthquakes along the Aleutian arc are confined to the main thrust zone.-Authors