Microseismic records based on machine-learning reveal the crustal anisotropy beneath the southern Sichuan-Yunnan block in the SE Tibetan Plateau

Complex tectonics and significant crustal anisotropy are observed at the intersection of the Red River Fault (RRF) and the Xiaojiang Fault (XJF) in the southern Sichuan-Yunnan block, in the SE Tibetan Plateau. The fast S polarization of upper crustal anisotropy varies from NW-SE in the west to NE-SW in the east near the Yimen region. However, small-scale anisotropic structures remain challenging due to limited measurements. Using two years of seismic data from the temporary linear HX Array and permanent stations, this study employed machine learning to construct a high-precision earthquake catalog for S-wave splitting, revealing the upper crustal anisotropy. The new catalog has nearly twice as many earthquakes as the China Earthquake Networks Center. The seismicity is concentrated in the Yimen region with various strike-slip faults, which has a strong correlation with high- and low-velocity boundaries, especially near the edge of the low-velocity zone. Spatial variations in upper crustal anisotropy along the HX Array correspond to geological structures and regional stress. Despite a dominant NE-SW PFS (i.e., fast S-wave polarization) in the Yimen region, stations show dual dominant directions with high values of DTS (i.e., delay times between split S-waves), indicating intricate tectonic and stress interactions. The middle segment of the RRF shows significantly lower DTS values than either side, along with a vertically distributed earthquake swarm, possibly indicating locked structures with high seismic hazard. A comparison between the upper and whole crustal anisotropy reveals consistent deformation within blocks and nearly orthogonal deformation near the RRF and the XJF. The boundary faults likely play a crucial role in influencing the crustal anisotropy both horizontally and vertically. The faults like the Shiping-Jianshui and the Puduhe, running parallel to the RRF and the XJF, are believed to affect the crustal structure. This study highlights that microseismic detection enhances earthquake catalog completeness, providing insights into detailed structures [supported by NSFC Projects 42074065 & 41730212].