Multi-frequency P-wave polarization angle inversion for shallow crustal structure

P-wave polarization analysis provides valuable constraints on near-surface S-wave velocity by measuring the polarization angles of P-wave arrivals at seismic stations. However, the depth sensitivity of this method has not been well quantified, limiting its broader application to velocity structure estimation. In this study, we investigate the depth sensitivity of P-wave polarization angles across multiple frequency bands using numerical approaches based on the reflectivity method. Using the resulting frequency-dependent sensitivity kernels, we perform multi-frequency inversion of P-wave polarization angle measurements to estimate the shallow crustal S-wave velocity structure from the near-surface to the mid-crust. We apply this method to three-component seismic data recorded at hundreds of stations across the southern Korean Peninsula. The resulting velocity structures from 1 to 20 km depth show good agreement with receiver function results, while shallow structures within the upper 1 km are consistent with local site survey measurements. These results demonstrate that multi-frequency P-wave polarization angle inversion provides a complementary constraint on near-surface to mid-crustal S-wave velocity structure and can enhance the characterization of near-surface seismic properties.