Depth-dependent in-situ stress state of the Longmen Shan fault Belt: Evidence from Wenchuan earthquake fault scientific drilling project Hole-2 image logs and core

The Longmen Shan Fault Belt, as a critical tectonic boundary along the eastern margin of the Tibetan Plateau, exhibits stress field characteristics that are important for understanding the kinematic features of the fault zone and its seismic activity. Recent studies have demonstrated significant variations in the shallow stress field of the fault zone, particularly when seismic ruptures propagate to shallow depths where the kinematic characteristics undergo marked changes. To investigate the stress states at different depths within the fault zone, this study systematically analyzed the in-situ stress field of the Yingxiu-Beichuan Fault through borehole wall image logs and core samples from the Wenchuan Earthquake Fault Scientific Drilling Project Hole 2 (WFSD-2). Combined with previous research findings, six depth intervals (424–569 m, 958–1163 m, 1271–1357 m, 1523–1623 m, 1870–2081 m, 2112–2178 m) were identified, with mean maximum horizontal principal stress orientations of 318°, 301°, 291°, 306°, 328°, and 190°, respectively, showing ∼20° inter-segment variations. Regional fault geometry analysis revealed distinct kinematic behaviors: Interval 1 exhibited pure thrust motion; Intervals 2–4 showed thrust-dominated motion with right-lateral strike-slip components; and Intervals 5–6 showed thrust motion with left-lateral strike-slip components. Integrated with existing studies, the 750–950 m and 1300–1500 m depth segments correspond to pure thrust and right-lateral strike-slip thrust regimes, respectively, matching the coseismic slip patterns of the Yingxiu-Beichuan Fault and Shenxigou Fault during the Wenchuan earthquake. These depth-dependent differential stress states, manifested as coseismic response features along the fault zone, result from the vertically heterogeneous rupture processes during the Wenchuan earthquake. This postsismic stress configuration reflects the kinematic adjustments of different fault segments, which fundamentally explains the structural complexity of the Longmen Shan Fault Belt and the multi-stage rupture characteristics observed in the 2008 Wenchuan earthquake.