Aleutian subduction zone seismicity, volcano‐trench separation, and their relation to great thrust‐type earthquakes

Abstract

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 4 years of data from this network show a well‐defined Benioff zone that is about 10 km thick and extends to about 180‐km depth. Focal mechanism and strong‐motion accelerograph data for an mb = 6.0 earthquake at 40‐km 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 cross section with cross sections from other regions of the Aleutian arc shows that at depths greater than 40 km (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 40‐km 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. This observation and the very low level of seismic activity at the present time in the main thrust zone compared to that in the Benioff zone below 40‐km depth suggest that seismic activity in the main thrust zone is dominated by great earthquakes and their aftershocks, followed by long intervals of quiescence. The Benioff zone below 40‐km depth shows more continual seismic activity. For the Aleutian arc there exists a weak correlation between the width of the main thrust zone and the rupture length and magnitude (Mw) of great earthquakes.