Estimation of Vs30 at the EarthScope Transportable Array Stations by
Inversion of Low-Frequency Seismic Noise
Abstract
One of the main sources of seismic noise below 0.05 Hz is the
atmospheric pressure variation, especially when surface pressure
variations are large. The amount of ground deformation under surface
pressure variations reflects the characteristics of shallow elastic
structure. When a surface broadband seismic stations is equipped with a
pressure sensor, we can estimate near-surface shear-modulus structure
using an inversion method. In the inversion method, we have the surface
observable η(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
η(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 meters. 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-meters 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.