# 1979 Saint Elias earthquake

The 1979 Saint Elias earthquake affected southeastern Alaska on 28 February at 12:27 (AKST). The thrust-faulting M$_w$ 7.5 earthquake had an epicenter in the Chugach Range. Though the maximum recorded Modified Mercalli intensity was VII (Very strong), damage was minimal and there were no casualties due to the remoteness of the faulting. Damage also extended across the border in parts of Yukon, Canada.

## Tectonic setting

In the Saint Elias region, the North American plate and Yakutat Terrane converges with the Pacific plate. The Pacific plate undergoes subduction beneath North America along the Aleutian Trench, west of that area, while strike-slip faulting between the two plates occur 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. The Saint Elias area represents a transition zone between the convergent and strike-slip plate motions. This area was also the location of two 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 magnitudes 8.0 or larger have a recurrence interval of 80 years. Only a section of this gap ruptured during the 1979 earthquake.

## Earthquake

On the surface-wave magnitude (M$_s$ ) scale, the earthquake measured between 7.1 and 7.7, while the moment magnitude (M$_w$ ) was 7.5. The shock was the largest to strike the continental United States since 1964. It occurred at a shallow depth of 15.0 km (9.3 mi), beneath the Chugach and Saint Elias ranges. The earthquake was recorded by 50 United States Geological Survey seismic stations in the southeastern part of the state.

### Geology

The earthquake occurred in the Yakataga seismic gap, a section of a subduction zone between the rupture zones of a 1964 earthquake and the Fairweather Fault. This major transform fault, east of the gap, last ruptured during a 1958 earthquake. The Yakataga seismic gap last ruptured during a major earthquake sequence in 1899. Beneath the Saint Elias range, subduction of the Pacific plate and possibly including the Yakutat Block occurs at a rate of 5.3 cm (2.1 in) annually. The earthquake accommodated around 30 percent of the accumulated stress in the area since the 1899 events. A possible seismic gap exists between the western extent of the 1979 and eastern end of the 1964 earthquake ruptures.
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. It propagated further east-southeast on the fault before transitioning onto a separate northeast-dipping dextral fault. 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 into Canada's Yukon Territory. There was no foreshock activity, and the largest aftershock measured M$_w$  5.2. These aftershocks displayed focal mechanisms involving thrust, strike-slip, and normal faulting.
Three accelerographs located within 250 km (160 mi) of the epicenter recorded strong ground motion data from the earthquake. A peak horizontal acceleration of 0.16 g recorded in Icy Bay, 74 km (46 mi) away, was the highest recorded ground motion. The Alyeska Pipeline Service Company also recorded digital ground motion data in Valdez using their accelerographs.

### Impact

The earthquake occurred in the sparsely populated and glaciated Chugach Range. It was felt the strongest, VII (Very strong) on the Mercalli intensity scale, at Icy Bay Lumber Camp, 73 km (45 mi) away, where a logging truck bounced violently. Damage to drywall and plaster, and displaced furniture was observed in Mendenhall, Haines, Border City, Juneau and Cape Yakataga. At Border City, a wooden building situated on permafrost was displaced on its base. Two fractures appeared in the concrete floor of a restaurant in Yakutat. Wallboards cracking occurred at Valdez Airport though the building did not sustain structural damage. Many people in the airport reported difficulties in standing up. Likewise, at Juneau Airport, passengers disembarking a plane struggled to stand and a heavy desk in the terminal was shaken.
In Yukon Territory, fractures appeared in the local school wall at Beaver Creek. A building in Burwash Landing sustained cracks while cracked plasters appeared in a lodge at Destruction Bay. The shaking was widespread, with people reportedly feeling the earthquake 1,200 km (750 mi) away, and shaking being felt over a distance of 500,000 km$^2$ (190,000 sq mi). In Iowa, some wells reported water level fluctuations of up to 1.42 ft (0.43 m).
At Sitka, a 6 in (150 mm) tsunami was recorded while a 2 in (51 mm) tsunami occurred in Yakutat. An erroneous entry that was included in a 1979 report stated that a 30 cm (12 in) tsunami was recorded. Many snow avalanches were aerially observed to the south and southeast of the epicentral region. On the Saint Elias range, between Mount Huxley and Mount Augusta, severe avalanches travelled more than 1 mi (1.6 km) away from the north face of the range. Multiple large (6-10×10$^6$ m$^3$) rock avalanches fell from slopes near the Seward Glacier. Many large (3–5 km$^2$ (1.2–1.9 sq mi)) landslides also occurred. The mainshock and aftershock distribution on land suggest the tsunami was triggered by a small and local landslide.