P-wave particle motions in anisotropic and dipping structures: theoretical simulation and practical application

P-wave particle motions have been widely utilized to study the S-wave velocity structure of the shallow crust beneath seismic stations. However, existing studies mostly assume horizontally layered isotropic media, overlooking the influence of dipping and anisotropic structures on P-wave particle motion. In this study, we present synthetic modeling of P-wave particle motions in dipping and anisotropic structures. The results show that dipping and anisotropic structures lead to periodic variations in P-wave particle motions with back-azimuth. The periodic variations are also dependent on frequency, reflecting the depth-dependent variation in structures. The Sichuan Basin is a key area to study the growth and outward escape of the Tibetan Plateau. We apply the P-wave particle motion method to one station (S124) within the basin to evaluate its potential to investigate the dipping and anisotropic structures of the shallow crust. The results show that the shallow crust, to a depth of approximately 4 to 5 km beneath the station, exhibits approximately 20% anisotropy, with the fast axis direction aligning with the strike of Central Sichuan strike-slip fault belts. These findings indicate that faulting may control the deformation at shallow crustal depths in the study area. Both synthetic tests and real data analyses demonstrate the effectiveness of using the back-azimuth dependence of multi-frequency P-wave particle motions to constrain the anisotropic and dipping structures of the shallow crust.