Mapping Near-Surface Rigidity Structure using Co-located Pressure and Seismic Stations from the EarthScope Transportable Array

We propose a single-station approach to retrieve shallow elasticity structure using co-located pressure and seismic instruments and apply it to stations in the EarthScope Transportable Array. This approach can retrieve rigidity in the top 50-100m and provide complementary information to other techniques that retrieve S-wave velocity. Co-located data show that the coupling between atmosphere and solid Earth is strong for frequencies between 0.01 Hz and 0.05 Hz. Using a homogeneous half-space model for the solid Earth, for which analytical solutions exist, we estimate near-surface rigidity structure at 716 TA stations. The method consists of measuring the ratios between the horizontal seismic power spectral density (PSD) and the co-located pressure PSD at each station, from which we derive the near-surface rigidity. This approach differs from our previous work in which we used ratios between the vertical PSD and the co-located pressure PSD. The rigidity map shows good spatial agreement with large-scale surface geological features, such as the Appalachian Mountains and the Mississippi Alluvial Plain. Our results correlate with various Vs30 models, although there are some stations that show clear differences. While some variation may be due to methodological differences, our approach can serve as an independent parameter to resolve potentially problematic locations in proxy-based models. We believe close examinations on these differences will lead to improvements of Vs30 models. We also extend the half-space model to a layered model using an inversion scheme. Density, P-wave and S-wave velocities vary with depth in the layered model by incorporating data at different frequencies. We derive depth sensitivity kernels by numerical differentiation and invert for near-surface elasticity to match surface observables η(f) = Sz/Sp, where f is frequency and Sz and Sp are the PSDs of vertical seismic data and of surface pressure data. We present near-surface rigidities at many TA stations by using both the half-space model and the layered model.