Anisotropy of middle-upper crust in the Anninghe-Zemuhe fault zone and its adjacent area and its implications

In this study, the polarization of fast shear-waves (PFS) and time delay of slow shear-wave beneath 42 stations (including fixed seismic stations and mobile seismic stations) in/around Anninghe-Zemuhe fault zone were calculated by shear-wave Splitting Analysis Method (SAM) based on the seismic data (Jan. 2013-Dec. 2019). Because of the impacts from both regional principal compressive stress and local geological structures, the parameters of PFS show obviously local characters. The Anninghe fault zone is bounded by Mianning. In the north of Anninghe fault Shimian-Mianning section, the PFS is in NW-SE direction, which agrees with the direction of principal compressive stress in Southern China, meaning that the anisotropy in the middle-upper crust is mainly dominant by the regional stress field. In the south of Anninghe fault Mianning-Xichang section, the dominant direction of PFS is NE-SW, which is different from that in other regions, showing the complicated characteristics of crustal structure and stress distribution under the interaction between sub-blocks. The dominant direction of PFS in the north section of Zemuhe fault zone is consistent with that in the north section of Anninghe fault zone, which is NW-SE, indicating the influence from the maximum principal compressive stress in the region. Before and after the Kangding M(s)6. 4 and Shimian M(s)4. 5 mainshocks, the direction of PFS in/around this area varies, suggesting that the anisotropy of the middle and upper crust is related to the accumulation and release of seismic stress. Xichang area (near XC27 station), at the junction of Anninghe fault and Zemuhe fault, is both a structural intersection and anisotropic inconsistence, with the largest time delay, meaning that this area has complex structure and the strongest anisotropy. In addition, Muli County (MLI station), Puge County (PGE station), the intersection area of Xianshuihe fault and Longmenshan fault (HCP station) and Meigu County (XC36 station), these stations show high time delay and various polarizations compared with the surrounding stations. Considering that they are located at the intersection or segmentation boundary of the fault zones, we should pay more attention on these areas in the future.