Investigating Rainfall-Induced Instability in Wayanad through Sentinel-1 SAR Interferometry and Geotechnical Context

The 30 July 2024 Wayanad landslide in the Western Ghats represents one of the most destructive rainfall-induced mass movements in India, characterised by an ~8 km runout and catastrophic downstream impacts. To advance process understanding for hazard assessment, we integrate hydrometeorological forcing, geotechnical constraints from published field investigations, and Sentinel-1 SAR interferometry. Extreme monsoonal precipitation (~586 mm in 48 h) combined with sustained 15-day antecedent rainfall repeatedly exceeded global intensity-duration thresholds, indicating prolonged saturation and pore‐pressure accumulation. The landslide source area comprises 2-8 m thick lateritic soil mantles over weathered and fractured gneiss, where laboratory evidence from recent studies shows high saturated hydraulic conductivity and marked reductions in unsaturated shear strength under 30-40 kPa suction.

We processed pre- and post-event Sentinel-1 (IW mode) interferograms, applying coherence-based masking and zero-reference correction to quantify line-of-sight deformation. The InSAR signal exhibits distinct displacement concentration at the crown zone coincident with a documented pre‐existing fracture system, and spatially continuous deformation aligned with the observed debris-flow channel. These patterns corroborate a failure mechanism involving rainfall‐induced saturation of lateritic covers, mobilisation along structurally weakened bedrock interfaces, and rapid transformation into a fluidised debris flow.

The results demonstrate the utility of spaceborne InSAR for characterising pre‐ and post-failure kinematics in inaccessible terrain and highlight the need to couple rainfall-soil moisture thresholds with routine SAR-based monitoring for early warning in the monsoon-dominated Western Ghats.