Ground Failures Induced by Seismic Shaking During the 2018 Anchorage, Alaska M7 Earthquake

Date: 4/26/2019

Time: 11:15 AM

Room: Cascade II

Strong ground motions during the November 30, 2018 M7 Anchorage, Alaska earthquake triggered numerous ground failures in artificial fill and natural materials over a >5000 km² area in south-central Alaska. Shaking generated by the intraslab earthquake (40 km deep) produced peak ground accelerations of 0.3-0.8 g throughout much of the greater Anchorage Metropolitan area. Post-earthquake aerial and ground surveys, spanning December 1–10, focused on coseismic ground failures (liquefaction, lateral spreads, and landslides) in natural materials. The costliest failures occurred in artificial fill, which damaged engineered road and rail embankments and buildings, particularly residential homes. Ground failures in natural materials occurred where landslides were triggered by prior earthquakes, including the 1964 M9.2 Great Alaska earthquake and the 1954 M6.4 Kenai Peninsula earthquake. For example, landslides threatened but did not damage the Alaska Railroad at Potter Hill in 2018, adjacent to an area where landslides destroyed the railroad in 1954 and 1964. Liquefaction-related lateral spreading and sand boils occurred in tidal and deltaic environments along upper Cook Inlet and in areas of Anchorage underlain by sandy, wet soils. We also observed debris avalanches on steep slopes underlain by glacial outwash and rockfalls and snow avalanches in steeper terrain of the Chugach Mountains. Field surveys of translational landslides triggered by the 1964 earthquake identified minor cracking (≤0.01 m wide, ~32 m long) along the margins of landslide blocks and grabens, but no evidence for significant net displacement. We speculate that the 20–40 sec of shaking in the 2018 Anchorage earthquake stopped short of reactivating large translational landslides that failed during much longer (4 to 5 min) shaking in 1964. Our observations will be used to empirically check the USGS ground-failure products that depict areas prone to coseismic liquefaction and landsliding.

Presenting Author: Robert C. Witter

Authors