Skwentna M1.0 | Alaska Earthquake Center

Tectonic setting of southern Alaska

Earthquakes in southcentral Alaska are caused by several different tectonic features. (1) The largest earthquakes in southcentral Alaska are caused by the megathrust fault that marks the contact zone between the subducting Pacific and overriding North American plates. The Great Alaska Earthquake of 1964, still the second largest earthquake ever recorded worldwide, originated beneath Prince William Sound. (2) Seismicity at intermediate depths (below 20 miles/32 km) occurs in the Wadati-Benioff zone, where the subducting Pacific plate descends into the mantle beneath the North American plate. This zone extends along the Aleutian Arc, the Alaska Peninsula, and Cook Inlet, and terminates beneath the northern reaches of the Alaska Range. In southern and central Alaska, seismicity decreases at depths of about 140 miles (225 km), reflecting the downward extension of the Pacific plate. The 2016 M7.1 Iniskin and 2018 M7.1 Anchorage earthquakes are the most recent notable intermediate-depth events. Both produced significant ground shaking in the Southcentral region and resulted in structural damage to buildings and infrastructure. (3) Crustal seismicity in this region can be attributed to three major sources: the faults and folds of the Cook Inlet basin, the Castle Mountain fault, and the broad band of diffuse seismicity extending from northern Cook Inlet to the Denali fault. Mapped geologic structures in upper Cook Inlet are capable of generating strong earthquakes. The April 1933 magnitude 6.9 earthquake, which caused significant damage in Anchorage, appears to have occurred on such a structure. The Castle Mountain Fault, which runs 25 miles (40 km) north of Anchorage, shows geologic evidence of Holocene shifting and produced the 1984 5.6 M Sutton earthquake. The diffuse zone of seismicity between Cook Inlet and the Denali Fault may mark a zone of deformation between the Bering microplate to the west and the Southern Alaska Block to the east. This broad zone of seismicity includes a series of predominantly thrust faults, and a magnitude 7.0 earthquake in 1943 may have originated in this belt.