Geodetic Evidence for Cascading Landslide Motion Triggered by Extreme Rain Events at Joshimath, NW Himalaya.

Slope instability due to tectonic, hydrological and anthropogenic activities cause severe landslides in Himalaya. Joshimath, a densely populated Himalayan town witnessed a catastrophic landslide event during December 2022 and January 2023 causing damages to ∼700 buildings. We use Interferometric synthetic aperture radar, Global Positioning System and rainfall measurements to probe the kinematics of the Joshimath landslide. We separate the seasonal and episodic deformation components using singular spectrum analysis. While the low amplitude annual landslide motions are modulated by seasonal precipitation, acceleration phases are triggered by extreme rain events. Our analysis revealed episodes of cascading motions triggered by extreme rain events resulting an overall increase in landslide velocity from −22 mm/yr during 2004–2010 to −325 mm/yr during 2022–2023. We estimate the landslide depth (∼30 m) and hydraulic diffusivity (∼3 × 10−5 m2/s) using a 1‐D pore‐water pressure diffusion model. Our study reveals the importance of systematic monitoring of ground deformation and weather parameters for landslide hazard mitigation. Plain Language Summary: The Himalayas, world's largest and youngest mountain range formed as a result of the India‐Eurasia continental collision hosts thousands of landslides each year related to tectonic and anthropogenic activities. It is important to characterize the landslide kinematics, especially the slow moving landslides at the fast‐developing Himalayan urban centers for hazard assessment. A recent catastrophic landslide event in the Joshimath town in NW Himalaya occurred between December 2022 and January 2023 caused damages to ∼700 buildings leading to mass evacuation. We map the spatio‐temporal evolution of land deformation at Joshimath using about two decades of geodetic measurements and singular spectrum analysis technique. While the decadal‐scale landslide motions match well with the overall increase in rainfall intensity, the episodic accelerations are triggered by extreme rain events. The extreme rainfall event on 19 October 2021, triggered the latest episode of landslide acceleration, which eventually led to the tragic disaster. The present study, for the first time, provides constraints on the landslide depth and hydraulic diffusivity. Our study also emphasis on the systematic monitoring of ground deformation and weather parameters at the Himalayas for an effective landslide hazard preparedness and mitigation. Key Points: Joshimath landslide displays episodic and seasonal kinematic changes; episodic accelerations are triggered by extreme rain eventsThe 19 October 2021 extreme rain event triggered the latest episode of landslide acceleration culminating to the 2022 disasterDepth and hydraulic diffusivity of the landslide is constrained using 1‐D pore‐water pressure diffusion model [ABSTRACT FROM AUTHOR]