abstract

Reconnaissance studies of Holocene coastal geomorphic features of the Aleutian Islands and the western part of the Alaska Peninsula indicate that, despite the high seismicity of the Aleutian Arc, most of the Aleutian Ridge has been vertically stable since at least the time sea level reached about its present position 2,000 to 4,000 years ago. Lack of youthful raised marine terraces, the presence of prograded beach ridges at the same general level as the modern ridge, and the widespread occurrence of a horizontal intertidal platform as much as 400 m wide are the primary lines of evidence for vertical stability. Detection of vertical movement is restricted by the natural vertical range inherent in the various coastal features, but it is believed that uplift of more than the local tidal fluctuation (≈ 2m) would be recognizable. Clear-cut evidence of uplift of coastal features that are thought to date subsequent to the time when sea level reached about its present position has so far been recognized only in a few areas in the Aleutians, and the amount of uplift in those localities is generally only several meters.

The evidence of vertical stability of the Aleutian Ridge in areas near the epicenters of earthquakes associated with destructive tsunamis indicates that large vertical displacements thought to produce tsunamis do not extend to the subaerial part of the ridge. A dislocation model for an underthrusting Pacific plate suggests to us that vertical displacements at the Aleutian Ridge may be only a small fraction of those in the vicinity of the Aleutian Terrace and Trench.

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