Southcentral Alaska is affected by a variety of tectonic events. The most severe earthquake in Southcentral Alaska is caused by a megathrust fault, which marks the point of contact between a subducting Pacific plate and an overriding North American plate. This fault originates under Prince William Sound and is still the largest earthquake ever recorded in the United States.
In Southcentral Alaska, intermediate-depth seismicity occurs at a depth of less than 20 miles/32 kilometers. This is due to the subduction of the Pacific Plate down-dip. The most recent intermediate-depth events in Southcentral Alaska have been the 2016 M7,1 Iniskin, and 2018 M7,1 Anchorage. Both of these events produced large-scale ground shaking in Southcentral Alaska and caused structural damage to structures and infrastructure.
Crustal Seismicity in Southcentral Alaska comes from three main sources: The Cook Inlet basin’s faults and folds, the Castle Mountain fault, and the broad band of diffuse seismic activity extending from the northern Cook Inlet out to the Denali fault are capable of producing large earthquakes. The mapped geological structures within upper Cook Inlet can generate large earthquakes, such as the April 1933 (M6.9) earthquake that caused significant damage in Anchorage. The Castle Mountain fault, about 25 miles north of Anchorage, shows geological evidence for Holocene offsets and produced the 1984 (M5.6) Sutton Earthquake. The wide band of seismic activity between Cook Inlet Out and the Denali fault may indicate a deformation zone that lies between the western Bering microplate on the one hand and the south Alaska block on the other. This broad band of seismic activity includes a series of mostly thrust faults, with a 1943 (M7.0) earthquake originating in this band.
