Tectonic Setting of Southern Alaska
Earthquakes in Southern Alaska are produced by a number of different tectonic features. (1) The
strongest earthquakes in Southern Alaska are generated by the megathrust fault that marks the contact
zone between the subducting Pacific and overriding North American plates. The 1964 M9.2 Great Alaska
Earthquake, which is still the second largest earthquake ever recorded worldwide, began under Prince
William Sound. (2) Intermediate depth seismicity (below 20 miles) occurs in the so-called Benioff Zone,
where the subducting Pacific Plate descends towards the mantle beneath the North American Plate. This
zone extends along Aleutian Arc, Alaska Peninsula and Cook Inlet and terminates beneath the northern
foothills of the Alaska Range. In southern and central Alaska, this seismicity abates at a depth of
approximately 140 miles, reflecting the down-dip extension of the Pacific Plate. Historically,
magnitude 6+ earthquakes of this type have been recorded beneath Cook Inlet. (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 wide band of diffuse seismicity extending from northern Cook Inlet
to the Denali Fault. Mapped geological structures in upper Cook Inlet are capable of generating strong
earthquakes. The April 1933 M6.9 earthquake, which caused considerable damage in Anchorage, appears to
have occurred on such a structure. The Castle Mountain Fault, which passes 25 miles north of Anchorage,
exhibits geological evidence of Holocene offsets and generated the M5.6 1984 Sutton earthquake. The
diffuse 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 includes a series of predominantly thrust faults, and a 1943 M7.0 earthquake may have
originated in this band.
