Seasonal and Anthropogenic Controls on Slow-moving Landslides: A Case Study of Uttarakhand Himalaya, India

The 2023 landslide event in Joshimath, Uttarakhand Himalaya, signifies the coupling between hydrological cycles and anthropogenic modifications in controlling long-term slope instability of slow moving landslides (SMLs). Using multi-temporal Interferometric Synthetic Aperture Radar (InSAR) data spanning from 2017 to 2023, combined with field observations, land use land cover analysis, and numerical modeling, we quantify the temporal evolution, driving mechanisms, and potential failure scenarios of the Joshimath landslide and adjacent Hailang and Kalpeshwar slopes. InSAR displacement time series reveal the onset of slow creep in 2018, followed by pronounced acceleration after extreme precipitation in October 2022 in the hillslope of Joshimath. Spectral and cross-correlation analysis between InSAR derived LOS displacement, rainfall, equivalent water height, and modeled vertical hydrological loading (LSDM) after long term trend removal demonstrate a dominant annual (~12-month) deformation cycle with a rainfall-deformation lag of 0 to 3 months, consistent with delayed pore-pressure propagation in the subsurface. Concurrently, land use change mapping indicates a >25% decline in forest canopy between 2000 and 2022, attributed to urban expansion and deforestation. Numerical slope stability modeling confirms that the factor of safety reduces through decreased root cohesion and increased surface saturation. While Hailang and Kalpeshwar exhibit hydrologically modulated creep, Joshimath displays an additional long-term acceleration trend, suggesting progressive failure behavior under compounded hydro-mechanical forcing. Runout simulations using the D-Claw framework highlight that a potential slope failure event could severely impact the downstream Tapovan Vishnugad Hydropower Project. Collectively, our results demonstrate that the interplay between seasonal cyclic hydrological loading and anthropogenic land-cover alteration exerts first-order control on deformation dynamics of SMLs, emphasizing the necessity of integrating hydro-geomechanical monitoring for anticipatory hazard assessment in rapidly urbanizing mountain terrains.