Spatial Heterogeneity of Upper Crustal Anisotropy in the Southern Sichuan Basin (China) Revealed by Local Shear-Wave Splitting

The recent increase in seismic activity in the southern Sichuan Basin has attracted substantial public interest and simultaneously provides an important opportunity to investigate upper crustal anisotropy, which offers key constraints on the regional stress field and crustal deformation. In this study, we obtained 1,845 high-quality local shear-wave splitting measurements from 15 stations, along with 2,027 null measurements from 19 stations. The results reveal a single anisotropic layer characterized by a horizontal symmetry axis at depths of approximately 3–7 km. The fast polarization directions exhibit clear spatial variability, which is primarily controlled by the spatial distribution of earthquakes rather than temporal evolution. Near the Baimazhen Syncline, the fast polarization directions align with the strike of the strata and form a circular pattern around the synclinal core, indicating that the anisotropy in this region is dominantly structure-controlled. In contrast, stations located in the southern Weiyuan Anticline and the western Baimazhen Syncline display fast directions of N171.7°E and N45.9°E, respectively. These orientations are consistent with the P axes derived from earthquake focal mechanisms, suggesting that anisotropy in these areas is primarily governed by the regional stress field. Overall, this study enhances our understanding of the complex geological framework of the southern Sichuan Basin and underscores the need for caution when interpreting potential temporal variations in seismic anisotropy in future investigations. This work was supported by the National Natural Science Foundation of China (Grant 42374124).