Revealing strong earthquake risks in seismic gaps of the Sichuan-Yunnan region based on crustal-scale magnetic inversion

The physical property structure of the crustal medium exerts a significant control on the formation of seismic gaps and the associated strong earthquake risk. Using EMAG2-v3 magnetic anomaly grids and 5561 field outcrop rock magnetic susceptibility data, we inverted the 0.1 × 0.1⁰ 3D magnetic structure of the Sichuan-Yunnan region. Based on this structure, we analyzed its influence on regional tectonic deformation and assessed the seismic risks in major seismic gaps. The strong magnetic bodies primarily originate from the NE-trending Neoproterozoic basement of the Sichuan Basin and the finger-like mafic-ultramafic channels in the central Yunnan block formed by the Permian Emeishan plumelets. These mafic components intensify the crust and exert varying degrees of resistance to the extrusion and escape of the Tibetan Plateau, indicating that the early strong magnetic basement plays a more significant role in craton strengthening and stabilization compared to the later mantle plume. Based on magnetic boundaries and magnetic intensity, combined with stress accumulation and fault coupling, the following seismic risks are identified: The southernmost segment of the Longmenshan Fault Zone and the Mianning-Xichang section, especially its southern half near Xichang, are at high risk of strong earthquakes (M_S 7+) and moderate-strong earthquakes (M_S 6-M_S 7), respectively. The Bamei-Kangding section has a moderate earthquake (<M_S 6) risk. The Dayi seismic gap shows no short-term strong earthquake risk but may shift to the Pujiang-Xinjin Fault Zone ∼60 km southeast.