Willow M6.0 | Alaska Earthquake Center

Tectonic setting of southern Alaska

Earthquakes in Southcentral Alaska are caused by various tectonic features. (1) The most powerful earthquakes in southcentral Alaska originate from the megathrust fault that marks the contact zone between the subducting Pacific and overriding North American plates. The 1964 M9.2 Great Alaska earthquake, the second largest earthquake ever recorded worldwide, originated under Prince William Sound. (2) Intermediate-depth seismicity (below 20 mi/32 km) occurs in the Wadati–Benioff Zone, where the subducting Pacific Plate descends toward the mantle beneath the North American Plate. This region extends across the Aleutian Arc, the Alaska Peninsula, and Cook Inlet and ends below the northern foothills of the Alaska Range. In southern and central Alaska, this seismicity diminishes at a depth of about 140 mi (225 km), reflecting down-dip extension of the Pacific Plate. The 2016 M7.1 Inniskeen and 2018 M7.1 Anchorage earthquakes are the most recent notable intermediate-depth events. Both caused significant ground shaking in the south-central 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 seismicity extending from northern Cook Inlet to the Denali Fault. Geological structures mapped in upper Cook Inlet are capable of generating strong earthquakes. The M6.9 earthquake of April 1933, which caused significant damage in Anchorage, appears to have occurred on such a structure. The Castle Mountain Fault, which passes 25 miles (40 km) north of Anchorage, displays geological evidence of Holocene offset and produced the 1984 M5.6 Sutton earthquake. The extended zone of seismicity 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 zone of seismicity consists primarily of a series of thrust faults, and the 1943 M7.0 earthquake may have originated in this band.