Fault Reactivation Above a Fossil Magmatic Structure Revealed by Dense-Array Receiver Functions and High-Resolution Earthquake Catalogs

Abstract To investigate what structure may control the site of fault reactivation, we focus on the Weishan–Fengqing region in the southeastern Tibetan Plateau with unusual earthquake clusters. Using two-phase dense seismic array data, we conduct the 3D common conversion point imaging and H−κ−c analysis to retrieve key crustal parameters (crustal interfaces, VP/VS ratio, and anisotropy), and apply the machine-learning-based earthquake location scheme LOC-FLOW to build a high-resolution catalog. The results reveal a localized high-velocity body in the mid-lower crust that strongly correlates with high VP/VS (>1.80), anisotropy fast-axis, and concentrated seismicity. The structure is interpreted as a fossil magmatic intrusion, which may link to slab-tear-related mantle upwelling. Comparison between the relocated earthquake catalog and the pre-existing faults suggests that this mechanically stronger block concentrates stress in the overlying brittle crust, promoting reactivation of probably four subparallel pre-existing faults with clustered earthquakes.