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Today’s outer-rise earthquake in Alaska complicates hazard assessment

Media Contact

Daryl Lovell

A magnitude 7.9 earthquake was reported earlier this morning in the Gulf of Alaska, about 170 miles off the Alaskan coast, according to the United States Geological Survey. Officials issued a tsunami warning for many on the U.S. West Coast and Canada’s British Columbia, which has since been canceled.

Geoff Abers

Geoff Abers

Geophysicist, Department of Earth and Atmospheric Sciences

Starting this spring, Cornell University geophysicist Geoffrey Abers will be leading the largest single deployment of seismometers along the Alaskan Peninsula, as part of the Alaska Amphibious Community Seismic Experiment. Today’s earthquake location is within the footprint of a major seismic experiment that will be deployed this May.

Abers says:

“Though somewhat indirectly, this morning’s magnitude 7.9 earthquake is related to subduction of the Pacific Plate beneath North America. The largest earthquakes we record are directly on the giant fault where one plate slides under the other, often called the Megathrust. This includes the 1964 magnitude 9.2 Gulf of Alaska earthquake, the second-largest ever recorded, which ruptured a fault rupture area that includes the Kodiak area and region to the east, just north of today’s earthquake.

By contrast, today’s earthquake is south of the deep-sea trench that marks the start of the plate contact area. These are known as ‘outer rise’ earthquakes. Elsewhere we have seen ‘outer rise’ earthquakes in the low 8’s. Some examples: off Samoa in 2009, the Kuril Islands in 2007, and Indonesia in 1977. These earthquakes are in some ways similar to the Mexico earthquakes in September, in that they are within a subducting plate rather than on the plate boundary, although they are much more shallow and offshore. They similarly complicate hazard assessment, which is dominated by consideration of the great earthquakes on the plate boundary.

“Although they are far from land so the direct shaking is less of a concern, some outer rise earthquakes can produce tsunami. Today's did not, owing to the side-to-side ‘strike slip’ fault motion – a tsunami is produced by vertical motions of the sea floor.”

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