Estimation of Vs30 at the EarthScope Transportable Array Stations by Inversion of Low-Frequency Seismic Noise

One of the main sources of seismic noise below 0.05 Hz is the atmospheric pressure variation, especially when surface pressure variations are large. When surface broadband seismic stations are equipped with pressure sensors, there is high coherence between pressure and seismic signals at low frequencies. The amount of ground deformation under surface pressure variations reflects the characteristics of near-surface elastic structure and allows us to estimate near-surface shear-modulus structure using an inversion method. In the inversion method, we have the surface observable eta(f) = Sz/Sp, where f is a frequency between 0.01 and 0.05 Hz, and Sz and Sp are the power spectral densities of vertical seismic data and of surface pressure data. We derive depth sensitivity kernels for eta(f) with which we invert for elastic moduli of the shallow structure. Between 0.01 and 0.05 Hz, sensitivity kernels typically have peaks at depths within the uppermost 100 m. Based on vertically heterogeneous 1-D structures, we estimate Vs30 at 744 USArray Transportable Array stations. Vs30 is the time-averaged shear-wave velocity from the surface to the 30-m depth. We compare our results with various surficial geology maps. Although Vs30 has high horizontal variability over a short distance on the scale of hundreds of meters, we find correlations between Vs30 and large-scale geological structures, such as mapped units and surficial materials. We find good agreement between estimated Vs30 and mapped Quaternary sediment depths, where stations with thicker underlying sediment tend to have slower Vs30.