Tensional character of earthquake foci beneath the Aleutian Trench with relation to sea-floor spreading

Abstract

The relation between the hypothesis of sea-floor spreading and features noted in previous studies of the Alaska earthquakes of 1964 and the Rat Islands earthquake of 1965 has prompted a further investigation of the larger earthquakes of the entire Aleutian chain. Sixty-six earthquakes which occurred in the decade 1957–1966 are examined. The hypocenters of these shocks are found to be separated spatially into two distinct groups. Twelve earthquakes occurred along a narrow line immediately below the axis of the Aleutian trench or under the seaward slope of the trench. The remaining hypocenters were distributed over a broader tabular region beginning midway between the trench and the islands proper and extending under the islands. Foci of the first group are uniformly extensional in character with axes of tension aligned normal to the local axis of the trench. Foci of the second group are conformable to an underthrust of the island arc, and the direction of underthrust at the individual foci is in agreement with a rotation of the oceanic plate about a pole located at 85°W, 50°N. Certain foci represent related effects such as an arc-arc transform fault, or a strike-slip shear at the eastern extremity of the arc. All features could be predicted by the hypothesis of sea-floor spreading and are presented as evidence in support of island arcs as the locale of a sink mechanism required by the hypothesis.