Seismic Response of Cook Inlet Sedimentary Basin, Southern Alaska

Cook Inlet forearc basin in south-central Alaska is a large (250 km X 100 km), deep (7.6 km) sedimentary basin that has the Anchorage metropolitan region on its margins. From 2015-2017 a set of 28 stations was deployed in the region as part of the SALMON (Southern Alaska Lithosphere and Mantle Observation Network) project. The SALMON stations, which also cover the remote western portion of Cook Inlet basin and the backarc region, form the basis for our observational study of the seismic response of Cook Inlet basin. We quantify the influence of Cook Inlet basin on the seismic wavefield using three data sets: (1) ambient noise amplitudes of basin stations relative to a non-basin reference station, (2) earthquake ground motion metrics for 34 crustal and intraslab earthquakes, and (3) spectral ratios between 40 basin stations and non-basin stations for the same earthquakes. For all analyses we examine how quantities vary with the frequency content of the seismic signal and with the basin depth at each station. Seismic waves from earthquakes and from ambient noise are amplified within Cook Inlet basin. At low frequencies (0.1-0.7 Hz), ambient noise ratios and earthquake spectral ratios are in general agreement, with power amplification of 10-15 dB, corresponding to amplitude amplification factors of 3.2-5.6. At high frequencies, at least up to 4 Hz, the basin amplifies the earthquake wavefield by similar factors. Our results indicate stronger amplification for the deeper basin stations, such as near Nikiski on the Kenai peninsula, and weaker amplification near the margins of the basin. Future work devoted to three-dimensional wavefield simulations and to treatment of source and propagation effects should improve the characterization of the frequency-dependent response of Cook Inlet basin to recorded and scenario earthquakes in the region.