Alaska’s Subduction Zone and Potential for Large Tsunamis

Researchers: Roland von Huene, U.S. Geological Survey; Emeritus, John J. Miller, and Peter Dartnell, U.S. Geological Survey

Written by Linda Rowan
7 June 2016

Summary

Combined analysis of the seismic and bathymetric data collected along the Aleutian arch of Alaska near Chirikof Island shows a splay fault beneath a subducted ridge producing a steep and rough seafloor. These structural features suggest the possibility of a earthquake causing a large tsunami that could be directed toward the west coast of the contiguous United States. Significantly, the GPS data from Chirikof Island shows that the fault is 90% locked, strengthening the elastic strain to a critical level where failure in a large earthquake is possible.

Observations

The Pacific Plate subducts beneath the North American Plate along the Aleutian trench of Alaska. The collision generates an arc of active volcanoes, small to large earthquakes and on occasion small to large tsunamis. In 1946 and 1964, large tsunamis were generated from magnitude > 8 earthquakes near Unimak Island. The Semidi geologic segment lies between these great earthquakes and the last time an event on this segment generated a large tsunami was in 1788. The Semidi segment has a geometry that is capable of generating a transoceanic tsunami that could be directed toward the west coast of the contiguous United States.

Legacy seismic and bathymetric data have been improved and analyzed to determine the structure and state of stress along the Semidi segment. The legacy seismic data from the 1970s and 1980s was reprocessed to obtain a more detailed structural image of the segment. Bathymetric data has been collected in multiple patches for several decades and is combined here for a sharper image of the seafloor morphology.