Potential Geodynamic Drivers of Shallow Crustal Normal-Faulting Earthquakes in the Sichuan-Yunnan Block: Insights from the 2025 Mw 5.5 Xinlong County Earthquake

The Sichuan-Yunnan Block in the southeastern Tibetan Plateau is bounded by major strike-slip fault systems and undergoes clockwise rotation and southeastward extrusion. A series of shallow moderate-sized normal-faulting earthquakes including the 9 October 2025 5.5 Xinlong County earthquake occurred within the interior of the Sichuan-Yunnan Block, but the driving mechanisms of these events remain unclear. Using Sentinel-1 D-InSAR observations, we constrain the seismogenic fault geometry (strike 277°, dip 47°, rake -104°) and invert for the coseismic slip distribution with a maximum of 0.28 m at ∼4.5 km depth, implying a normal-faulting rupture. We then derive the present-day strain-stress rate fields from a dense multi-source GNSS velocity dataset, revealing pronounced boundary-fault shear, clockwise block rotation, and distributed intrablock extension. Under the present-day regional stress field, the 2025 rupture exhibits effective slip tendency values of 0.6-0.8, indicating that the earthquake fault is favorably oriented for normal-faulting reactivation. With joint analysis of active blocks, crustal thickness, and elevation, we suggest that the observed intrablock extension is spatially consistent with possible contributions from gravitational potential energy gradients and lower crustal flow, rather than elastic loading from boundary fault locking alone. The 2025 Xinlong County earthquake represents the latest manifestation of this persistent intrablock extensional regime.