Many studies are ongoing within Alaska's most populous city to understand better its unique seismogenic setting as well as its seismic hazard and risk. With its relative proximity to the Aleutian megathrust subduction zone and other earthquake sources, Anchorage has been subjected to regular earthquakes, including the 1964 Great Alaska earthquake. In 2004, a downhole array was installed near downtown Anchorage within the Bootlegger Cove Formation, which was responsible for much of the ground failure during the 1964 earthquake. This study provides new information regarding the downhole array and the dynamic soil properties found at the array site. Shearand compression-wave velocities were measured at the site. Evaluation of the transfer function of the new velocity model is compared with the measured response at the site. In addition, several comparisons are performed utilizing nearby historic cone penetration test (CPT) and standard penetration test (SPT) data measured during installation of the deepest accelerometer at the site. A significant improvement in the theoretical modeling of the site is achieved utilizing the new shear-wave velocity profile.

The Mw 7.1 47 km deep earthquake that occurred on 30 November 2018 had deep societal impacts across southcentral Alaska and exhibited phenomena of broad scientific interest. We document observations that point to future directions of research and hazard mitigation. The rupture mechanism, aftershocks, and deformation of the mainshock are consistent with extension inside the Pacific plate near the down‐dip limit of flat‐slab subduction. Peak ground motions >25%g were observed across more than 8000  km2, though the most violent near‐fault shaking was avoided because the hypocenter was nearly 50 km below the surface. The ground motions show substantial variation, highlighting the influence of regional geology and near‐surface soil conditions. Aftershock activity was vigorous with roughly 300 felt events in the first six months, including two dozen aftershocks exceeding M 4.5. Broad subsidence of up to 5 cm across the region is consistent with the rupture mechanism. The passage of seismic waves and possibly the coseismic subsidence mobilized ground waters, resulting in temporary increases in stream flow. Although there were many failures of natural slopes and soils, the shaking was insufficient to reactivate many of the failures observed during the 1964 M 9.2 earthquake. This is explained by the much shorter duration of shaking as well as the lower amplitude long‐period motions in 2018. The majority of observed soil failures were in anthropogenically placed fill soils. Structural damage is attributed to both the failure of these emplaced soils as well as to the ground motion, which shows some spatial correlation to damage. However, the paucity of instrumental ground‐motion recordings outside of downtown Anchorage makes these comparisons challenging. The earthquake demonstrated the challenge of issuing tsunami warnings in complex coastal geographies and highlights the need for a targeted tsunami hazard evaluation of the region. The event also demonstrates the challenge of estimating the probabilistic hazard posed by intraslab earthquakes.

Anchorage, Alaska, is located in one of the most active tectonic settings in the world. The city and region were significantly impacted by the MW 9.2 Great Alaska Earthquake in 1964, and they were recently shaken by a MW 7.1 event in 2018. The city was developed in an area underlain by complex soil deposits of varied geological origins and stiffnesses, with the deposits’ thicknesses increasing east to west. Situated at the edge of the North American Plate, with the actively subducting Pacific Plate below, Anchorage is susceptible to both intraslab and interface earthquakes, along with crustal earthquakes. Strong-motion stations were installed across the city in an attempt to capture the variability in site response. Several previous studies have been performed to evaluate that variability but have not included larger magnitude events and have not benefited from the current density of instrumentation. The work presented here provides background information on the geology and tectonic setting of Anchorage and presents details related to the dataset and methods used to perform the site-response analysis. This study has collected strong-motion recordings from 35 surface stations across Anchorage for 95 events spanning from 2004 to 2019, including the MW 7.1 Anchorage Earthquake in 2018. The more than 1700 three-component recordings from those 95 events with moment magnitudes ranging from 4.5 to 7.1 were used to evaluate site response variability across the city. Using the Generalized Inversion Technique and a reference rock site, spectral amplifications were calculated and analyzed for frequencies between 0.25 and 10 Hz for each strong-motion station. The study results were used to develop contour maps at 1 Hz and 5 Hz, using logarithmic-band averages, to describe the variability of spectral amplifications at these two frequencies of interest. The results were also compared to geologic conditions across Anchorage, and the overlaying of different soil deposits can be seen to have an impact on the spectral amplification at the sites. The results of this study provide improvements on past microzonation studies and, using sensitivity analyses, offer support for the use of small and moderate earthquakes to evaluate spectral amplifications.