1979 Saint Elias earthquake

In the Saint Elias region, the North American plate and Yakutat Terrane converge with the Pacific plate at rates of 45–50 mm (1.8–2.0 in)/yr. The Pacific plate undergoes subduction beneath North America along the Aleutian Trench west of the Saint Elias area, while strike-slip faulting between the two plates occurs to the east on the Queen Charlotte Fault. Faults in the Saint Elias region exhibit thrust and reverse slip while there is localised strike-slip deformation evident in some earthquake fault plane solutions.^[2] In the area of the epicenter, several surface faults exist that may have slipped during the earthquake.^[3] The Saint Elias area represents a transition zone between the convergent and strike-slip plate motions. This area was also the location of a series of large earthquakes in 1899. Aside from the earthquake of 1979, no other major earthquakes have occurred in the Saint Elias region since 1899, and the 1964 and 1958 earthquakes ruptured adjacent segments of the plate boundary. This part of the plate boundary, dubbed the Yakataga Gap, extends 250 km (160 mi) across, where great earthquakes of magnitude 8.0 or larger have a recurrence interval of 80 years. Only a section of this gap ruptured during the 1979 earthquake.^[4]

The earthquake, which occurred at a shallow depth of 15.0 km (9.3 mi), was the largest to strike the continental United States since 1964.^[5][1] It measured 7.5 on the moment magnitude scale (M_w ),^[6] while on the surface-wave magnitude scale (M_s ), measured between 7.1 and 7.7.^[1][7] The earthquake occurred beneath the Chugach and Saint Elias ranges and was observed by 50 seismic stations in Southeast Alaska.^[7]

Beneath the Saint Elias range, the Pacific plate and possibly the Yakutat Block subduct at a rate of 5.3 cm (2.1 in) annually. The earthquake occurred in this region, known as Yakataga, where a seismic gap was previously identified. This gap represented a section of the subduction zone where no large earthquakes have occurred since the 1899 Yakutat Bay earthquakes. Adjacent sections of the subduction zone ruptured during the 1964 and the 1958 earthquakes.^[8][9] While the 1979 earthquake occurred on this seismic gap, it did not fully rupture the entirety of it. Therefore, a possible seismic gap remains west of the 1979 and east of the 1964 ruptures.^[3]

A focal mechanism analysis suggest shallow thrust and strike-slip faulting was responsible for the earthquake. The rupture initiated on a thrust fault that dipped to the north and propagated east-southeast on the fault before transitioning onto a separate northeast-dipping dextral fault.^[8] The distribution of aftershocks in the area suggest a maximum rupture dimension of approximately 65 km (40 mi) by 80 km (50 mi) and extending southeast of the epicenter. Part of the rupture extended beneath Canada's Yukon Territory.^[7][2] There was no foreshock activity,^[7] and the largest aftershock measured M_w  5.2.^[10] The largest aftershocks displayed a variety of focal mechanisms involving thrust, strike-slip, and normal faulting.^[2] The earthquake caused 0.4–1.0 m (1 ft 4 in – 3 ft 3 in) of movement on the fault; only a portion (10–30%) of the total slip accumulated since the 1899 events.^[8]