Nucleation Mechanism of the 2022 Ms6.8 Luding Earthquake in Sichuan, China: Insights from Lithospheric Effective Elastic Thickness and Numerical Simulation

On September 5, 2022, Luding County in Sichuan Province, China experienced a Ms 6.8 earthquake. It is of significance to investigate the deep geodynamics and nucleation mechanism of the Luding earthquake for seismic hazard analysis and earthquake monitoring and forecasting in this region. In this study, we utilized EIGEN6C4 Bouguer gravity anomalies and SIO V15.1 topographic data to calculate the effective elastic thickness, Te, of the lithosphere in the southeastern margin of the Tibetan Plateau by finite difference method. The results indicate  that the Luding earthquake occurred in a gradient zone with a steep change in Te between the eastern edge of the Tibetan Plateau and the Sichuan Basin, and that Te changes significantly, increasing from approximately 0-10 km in the west to about 10-30 km in the east. Based on the rheological structure information provided by Te , we constructed a two-dimensional numerical model to study the distribution of strain energy and displacement vector fields in the region. The results reveal that lower crustal flow is the key factor of the Luding earthquake nucleation. The lower crustal flow in the eastern edge of the Tibetan Plateau, impeded along the Shimian-Mianning-Xichang segment , migrates upward along the strong and weak lithospheric interface between the Sichuan-Yunnan rhumb block and the Gongga Mountain blocks, resulting in the gradual accumulation of stress and strain energy in the transitional zone between the upper and middle crust. This, in turn, triggers fluid migration upwards, facilitating the rupture of shallow rocks and the nucleation of the Luding earthquake. Drawing insights from Te and numerical simulation results, we proposes a plausible deep geodynamic model for the nucleation of the Luding earthquake and analyzes the seismic hazard of the Shimian-Mianning-Xichang segment along the Juanshui River-Anning River fault.