Differential Rock–Ice Deformation Before the 2026 Langtang Hazard Cascade, Nepal: NISAR Observations and Geological Interpretation

Deformation of glacier–bedrock systems provides evidence of slope adjustment before catastrophic failure. In a post-event assessment, we investigate the 26 August 2026 Langtang rock–ice failure in Nepal using NASA-ISRO Synthetic Aperture Radar (NISAR) observations to relate pre-event displacement to rock–ice interaction and valley confinement. Seven ascending line-of-sight (LOS) intervals characterize temporal change, while the final pre-event ascending–descending pair (P4) provides east–west and vertical displacement estimates. P4 spans 24-day windows ending on 19 and 16 August 2026. Relative to the surrounding terrain, median eastward and vertical displacements are +24.85 and −19.03 mm within the sampled possible detachment mapped by the United Nations Satellite Centre (UNOSAT), compared with +41.05 and +33.12 mm in the neighboring LOS deformation area. Opposing vertical tendencies and local east–west contrasts indicate differential movement across the rock–ice system. A positive LOS cluster lies 1.393 km from the possible detachment boundary and retains 78.8% of its connected area at coherence ≥ 0.70. The ascending record shows a positive pre-event contrast, a negative event-spanning contrast and a mixed post-event field. Together, these observations underpin a geological model in which oblique sliding increases loading at neighboring ice–bedrock contacts, promotes uplift and westward deflection, and transfers loading toward a valley constriction. We interpret this sequence as south to north, counter-clockwise adjustment accompanying differential sliding. The model connects the measured deformation pattern to structural weakness, changing glacial support and topographic confinement, providing a geological explanation for pre-event rock–ice adjustment within the source to valley hazard cascade.