Seismic Activity in Southern Alaska
Southcentral Alaska experiences earthquakes due to various tectonic features. The most powerful earthquakes in the region arise from the megathrust fault, which is located at the boundary between the subducting Pacific Plate and the overriding North American Plate. Notably, the 1964 M9.2 Great Alaska Earthquake, the second largest recorded worldwide, occurred under Prince William Sound.
Intermediate-depth seismic activity, occurring at depths exceeding 20 miles (32 km), is found in the Wadati-Benioff Zone. In this area, the Pacific Plate descends into the mantle beneath the North American Plate. This zone stretches along the Aleutian Arc, through the Alaska Peninsula, and into Cook Inlet, terminating beneath the northern foothills of the Alaska Range. In southern and central Alaska, this seismic activity diminishes around 140 miles (225 km) deep, aligning with the deeper extension of the Pacific Plate. The most recent significant intermediate-depth earthquakes include the 2016 M7.1 Iniskin quake and the 2018 M7.1 Anchorage quake, both of which caused considerable shaking in the region and led to damage to buildings and infrastructure.
Crustal seismic activity in the area is primarily linked to three main sources: the faults and folds within the Cook Inlet basin, the Castle Mountain Fault, and a widespread zone of diffuse seismicity extending from northern Cook Inlet to the Denali Fault. The geological structures in upper Cook Inlet have the potential to produce strong earthquakes. For instance, the April 1933 M6.9 earthquake that caused significant damage in Anchorage likely resulted from such a geological feature. The Castle Mountain Fault, situated 25 miles (40 km) north of Anchorage, shows evidence of recent geological activity and was responsible for the 1984 M5.6 Sutton Earthquake. The diffuse seismic zone between Cook Inlet and the Denali Fault might represent a deformation area separating the Bering microplate to the west from the southern Alaska block to the east, encompassing numerous thrust faults. A notable earthquake in this zone was the M7.0 event recorded in 1943.