Seismic Noise in Central Alaska and Influences From Rivers, Wind, and Sedimentary Basins

Abstract

Ambient noise is useful for characterizing frequency-dependent noise levels and for assessing data quality for seismic stations. We use 4 years of ambient noise spectra from 16 stations in central Alaska to examine environmental and structural influences on seismic stations. The region contains a major river (Tanana River) that is ice covered for half the year and is underlain by a sedimentary basin (Nenana basin) that strongly influences the seismic wavefield. Nenana basin amplifies ambient seismic noise by 12–16 dB at 0.1–0.7 Hz and 17–30 dB at 0.7–3 Hz. A meteorological station and river gauge at Nenana provide environmental data for comparison with seismic stations. During the summer, the Tanana River produces noise levels elevated by 30–40 dB at frequencies near 10 Hz, as recorded by all five stations within 100 m of the main river channel. The Tanana River sediment is predominantly silt and sand in this region; therefore, we attribute the 10-Hz river signal to turbulence within the water and to unsteady shearing on the river bottom. The influence of wind is apparent on seismic noise at low (<0.05 Hz) frequencies, due to atmospheric-induced tilting, and at high (>2.0 Hz) frequencies, due to unsteady shearing and turbulence near the ground. Our empirical findings motivate future studies, such as how flow from air or water couples to the ground and how deep sedimentary basins influence the ambient noise wavefield. Our results have implications for seismic site selection, environmental monitoring, and detection and characterization of earthquakes.