Seismic Response of Cook Inlet Sedimentary Basin, Southern Alaska

Abstract

Cook Inlet fore‐arc basin in south‐central Alaska is a large, deep (7.6 km) sedimentary basin with the Anchorage metropolitan region on its margins. From 2015 to 2017, a set of 28 broadband seismic stations was deployed in the region as part of the Southern Alaska Lithosphere and Mantle Observation Network (SALMON) project. The SALMON stations, which also cover the remote western portion of Cook Inlet basin and the back‐arc 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 18 basin stations relative to a nonbasin reference station, (2) earthquake ground‐motion metrics for 34 crustal and intraslab earthquakes, and (3) spectral ratios (SRs) between basin stations and nonbasin 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 (0.1–0.5 Hz), ambient‐noise ratios and earthquake SRs are in a general agreement with power amplification of 6–14 dB, corresponding to amplitude amplification factors of 2.0–5.0. At high frequencies (0.5–4.0 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 3D wavefield simulations and 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.

Teleseismic Attenuation, Temperature, and Melt of the Upper Mantle in the Alaska Subduction Zone

“…Slow arrivals in this region are partly due to the thick sedimentary section; on average this is the slowest region within the study area with Δ & Δ of 1.67 ± 0.03 s and 0.58 ± 0.02 s respectively, and shows little dependence on back azimuth. The Cook Inlet Basin, Region 2, contains a sedimentary cover sequence reaching depths approximately 8 km at its deepest parts (Plafker & Berg, 1994; Shellenbaum et al., 2010; Smith & Tape, 2020), and substantially delayed arrivals relative to surrounding regions, by 0.81 and 0.27 s for Δ and Δ, respectively.…”

“…The influence of sediments from the Cook Inlet Basin and Yakutat Terrane (Plafker & Berg, 1994; Smith & Tape, 2020) on travel times can be easily explained. In the Cook Inlet Basin, a 5 km sequence with mean S‐wave velocity ( V S ) of 2.2 km/s surrounded by basement of 3.5 km/s, as estimated in recent high‐resolution velocity models (Figure S8 of Nayak et al., 2020), would explain the observed 1.0 s difference in ΔT S between regions 3 and 2.…”

“…Perhaps, basin amplification effects are altering the spectra in ways not accounted for by the attenuation model (Equations 2-5). Frequency-dependent amplification has been well documented in the Cook Inlet Basin and to lesser extents in other Alaska basins (Moschetti et al, 2020), showing 6-14 db S-wave amplification in the 0.1-0.5 Hz frequency band (Smith & Tape, 2020). To test the effects of possible amplification, we re-ran the spectral fits varying the weighting factor A between fits to amplitude and phase (Equation 6).…”

Geotechnical lessons from the Mw 7.1 2018 Anchorage Alaska earthquake

“…Previous site response studies in the region (e.g. Boore, 2004; Dutta et al, 2003; Martirosyan et al, 2002; Moschetti et al, 2020; Smith and Tape, 2019) have found significant ground motion amplification, particularly on the western side of the basin. For instance, Boore (2004) used ground motions from the 2002 M w 7.9 Denali earthquake and found that the spatial distribution of site amplification in Anchorage for low-frequency motions roughly corresponded to increasing depth to bedrock in the basin.…”

“…Moreover, recent studies of the Cook Inlet sedimentary basin (e.g. Moschetti et al, 2020; Smith and Cape, 2019) have shown that it amplifies ground motions in the region. However, those studies have not focused on the Anchorage metropolitan area.…”