The seismic wavefield in Nenana basin and Cook Inlet basin of Alaska

Abstract

Sedimentary basins amplify ground motion from earthquakes which can have severe consequences for major cities on basins like Tokyo and Los Angeles. Ground motion on sedimentary basins is complex and it depends on the geometry and elastic properties. We study the seismic wavefield from ambient noise and earthquake sources in Nenana Basin and Cook Inlet Basin with seismic stations from the Fault Locations and Alaska Tectonics from Seismicity (FLATS) and Southern Alaska Lithosphere and Mantle Observation Network (SALMON) projects. The FLATS project consists of 13 seismic stations placed over the Nenana Basin region in Central Alaska from 2014 to 2019. In South-Central Alaska, 28 seismic stations were placed around the Cook Inlet Region from 2015-2017 for the SALMON project. In this thesis, we have established two valuable data sets of events that can be used to better understand the complex influence of sedimentary basins on the seismic wavefield. Our analyses help quantify the frequency-dependent amount of amplification that occurs within these sedimentary basins. A greater understanding of ambient noise can improve site selection as well as understanding other forces of nature. Many FLATS stations are near trees and a river, so we quantify the influence of river, wind and basin on ambient noise. We analyze ambient noise in the frequency domain with power spectral densities on annual, daily and hourly time scales. Rivers affect ground motion from shearing and turbulent forces. For FLATS stations within several meters of the Tanana River, we found consistent seasonal perturbations around 10 Hz. A comparison of the 10 Hz signal with river stage height, measured 14 km upriver, shows strong correlations exist during the summer and ambient noise increases by 40 dB. In the town of Nenana, a weather station shows wind occurs less during the winter than summer. Wind can directly interact with the ground through shearing forces or indirectly with the transfer of energy through trees. In this study, we found that wind can affect the ground motion by at least 10 dB for < 0.05 Hz. Basins amplify ground motion because it is easier to shear the materials. When we analyzed basin amplification from ambient noise we found that we had to correct for other noise sources like wind and river conditions. We also found that there were three classes of basin influence: Basin stations, marginal basin stations, and non-basin stations.