Tectonic position of southern Alaska

Earthquakes in southcentral Alaska are caused by a number of 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 Plate and the overlying North American Plate. The 1964 Great Alaska Earthquake of magnitude 9.2, which remains the second largest earthquake ever recorded worldwide, originated beneath Prince William Sound. (2) Intermediate depth seismicity (less than 20 miles/32 km) occurs in the Wadati-Benioff Zone, where the subducting Pacific Plate descends toward the mantle beneath the North American Plate. This zone extends along the Aleutian Arc, the Alaska Peninsula, and Cook Inlet, and ends beneath the northern reaches of the Alaska Range. In southern and central Alaska, this seismicity subsides at a depth of about 140 miles (225 km), reflecting the downward extension of the Pacific Plate. The magnitude 7.1 Iniskin quake in 2016 and the magnitude 7.1 Anchorage quake in 2018 are the most recent notable intermediate-depth events. Both caused 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 main 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 the upper Cook Inlet can trigger strong earthquakes. The magnitude 6.9 earthquake of April 1933 that caused significant damage in Anchorage appears to have occurred on such a structure. The Castle Mountain Fault, which runs 40 kilometers north of Anchorage, shows geological evidence of Holocene slip and triggered the magnitude 5.6 Sutton earthquake of 1984. The diffuse seismic zone between Cook Inlet and the Denali Fault may mark a deformation zone between the Bering microplate to the west and the southern Alaska block to the east. This broad seismic zone includes a series of predominantly thrust faults, and a magnitude 7.0 earthquake in 1943 may have originated in this band.