abstract

Analysis of leveling data from Middleton Island in the Gulf of Alaska confirms our earlier observation of high rates of vertical deformation. Although the island is only 8 km long, the deformation pattern is quite complex. The north end of the island is falling relative to the remainder at a rate of 3 mm/yr. This rate of movement is decreasing exponentially with a time constant of about 8 yr. In addition, a short level line across the island indicates some tilting down in the direction N50°W. This deformation also exhibits a declining rate.

The direction of all deformation appears to be controlled by the strike of the Aleutian Trench or, equivalently, by the axis of the 1964 Alaska earthquake deformation field. The deformation at Middleton Island is inconsistent with motion in the direction of the Minster-Jordan relative plate motion vector.

The deformation is inferred to be a long-lived aftereffect of the 1964 earthquake.

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