The Seismogenic Structure of the 2025 Mw 7.1 Dingri Tibet Earthquake and the Shallow Stratigraphic Velocity Structure of the Dingmucuo Basin

On January 7, 2025, a M_w 7.1 earthquake struck Dingri County, Shigatse City, Tibet, causing significant casualties and infrastructure damage. The epicenter was located in the southern segment of the Shenza-Dingjie Rift, within the Dingmucuo Basin. Although the Dengmocuo Fault is recognized as the main seismogenic structure, its shallow and deep geometries remain poorly constrained, hindering a full understanding of the earthquake's mechanism and regional tectonic deformation. To address this, we used high-resolution seismic refraction imaging across the Dengmocuo Fault, along with focal mechanism and aftershock data, to investigate its seismogenic structure. The P-wave velocity (Vp) profile reveals a “bowl-shaped” sedimentary structure in the Dingmucuo Basin, with the depocenter offset eastward, indicating that long-term activity of the Dengmocuo Fault has influenced the basin’s formation and evolution. The stratigraphy is divided into four layers: modern lacustrine deposits, late to middle Pleistocene lacustrine and fluvial-lacustrine deposits, and underlying Jurassic bedrock. The main fault, F1 (Dengmocuo Fault), dips westward at ～67° and cuts through three sedimentary layers. Together with several eastward-dipping low-angle secondary faults, it forms a "Y"-shaped fault system. Analysis of the Vp profile and focal mechanism indicates that the Dengmocuo Fault defines a continuous shear plane with a "steep upper, gentle lower" geometry, characterized by steep slip at shallow depths and gentler slip at depth. Independent InSAR-based source models for this event are consistent with this listric geometry. We also investigate the wider surface rupture zone observed in the southern segment. Our results suggest that the thicker low-velocity Quaternary sediments and complex shallow fault system in this area facilitated the coordinated activity of the main and secondary faults, leading to a broader rupture zone. Ultimately, the "steep upper, gentle lower" geometry of the Dengmocuo Fault offers a plausible explanation for the seismogenic mechanism of the Dingri earthquake. This geometry aligns with existing models of the eastern boundary fault of the Shenza-Dingjie Rift, indicating that the southern Tibetan Rift is undergoing simple shear extension, primarily controlled by low-angle detachment faults. These findings offer valuable insights into the region's tectonic deformation and seismic behavior.