Site Response Analysis of Anchorage, Alaska Using Generalized Inversions of Strong-Motion Data (2004–2019)

Anchorage, Alaska, is located in one of the most active tectonic settings in the world. The city and region were significantly impacted by the M W 9.2 Great Alaska Earthquake in 1964, and they were recently shaken by a M W 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 M W 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.