Building the 3D seismic fault models for the 2021 Ms 6.4 Yunnan Yangbi earthquake: the potential role of pre-existing faults in generating unexpected moderate-strong earthquakes in southeast Tibet

The three-dimensional (3D) geometry of a fault is a critical control on earthquake nucleation, dynamic rupture, stress triggering, and related seismic hazards. Therefore, a 3D model of an active fault can significantly improve our understanding of seismogenesis and our ability to evaluate seismic hazards. Utilising the SKUA GoCAD software, we constructed detailed seismic fault models for the 2021 Ms 6.4 Yangbi earthquake in Yunnan, China, using two sets of relocated earthquake catalogs and focal mechanism solutions following a convenient 3D fault modeling workflow. Our analysis revealed a NW-striking main fault with a high-angle SW dip, accompanied by two branch faults. Interpretation of one dataset revealed a single NNW-striking branch fault SW of the main fault, whereas the other dataset indicated four steep NNE-striking segments with a left-echelon pattern. Additionally, a third NEE-striking short fault was identified NE of the main fault. In combination with the spatial distribution of pre-existing faults, our 3D fault models indicate that the Yangbi earthquake reactivated pre-existing NW- and NE-striking fault directions rather than the surface-exposed Weixi-Qiaohou-Weishan Fault zone. The occurrence of the Yangbi earthquake demonstrates that the reactivation of pre-existing faults away from active fault zones, through either cascade or conjugate rupture modes, can cause unexpected moderate-large earthquakes and severe disasters, necessitating attention in regions like southeast Tibet, which have complex fault systems.