Crustal Anisotropy Characteristics in the North China Craton

    The North China Craton (NCC) exhibits a marked east-west contrast in its present-day tectonic framework. The eastern region  features a thinned lithosphere hosting extensional basins like the Zhangjiakou-Bohai seismic belt and the Shanxi Graben. This area experiences intense crustal deformation and frequent seismicity. Conversely, the western region possesses a thicker lithosphere, greater crustal stability, and weak seismic activity. The central orogenic belt acts as a transitional zone in crust-mantle structure, characterized by dramatic crustal thickness variations, evidence of lithospheric modification, remnants of ancient structures, and is key for studying crust-mantle coupling/decoupling. 
    Crustal anisotropy serves as a crucial indicator for revealing lithospheric deformation. Analysis of seismic wave velocity anisotropy quantitatively constrains crustal stress, assesses fault activity, and infers deep material flow. This provides  essential constraints for seismic hazard assessment and tectonic dynamics research.
    This study utilized local-earthquake data (M>1) from the National Fixed Seismic Network to investigate crustal anisotropy across the NCC using the SAM (Shear-wave splitting Analysis Method) method. The extensive dataset, covering multiple temporal windows of seismic activity, provides strong temporal continuity and spatial coverage for analyzing anisotropy characteristics. Results reveal complex fast-wave polarization directions (FPDs) across the study area. The average FPD (~73°) aligns well with the regional mean maximum horizontal compressive stress (SHmax) direction in the NCC. However, the FPDs also display distinct local features correlating with specific structures, indicating significant local variability in the regional stress field. This manifestation of localized crustal anisotropy characteristics is vital for understanding the region's geodynamic activity. The local stress field variations suggest a heterogeneous crustal stress distribution, likely influenced by multiple geological factors.