Spatial-temporal building stability assessment of Himalaya town Joshimath from long-term multi-sensor InSAR analysis

Post-event topography of ancient landslides often provides favorable terrain for settlement in mountainous regions. However, such landslides can undergo long-term creep or even reactivation, posing significant risks to the buildings constructed on them. Joshimath, a town situated on an ancient landslide in the northwest Himalaya, has experienced persistent land subsidence since the early 20th century, with clear indications of accelerated deformation in recent years. Although building damage is widely observed, a regional-scale stability assessment for the roughly 10,000 buildings is still absent. Interferometric Synthetic Aperture Radar (InSAR) enables long-term monitoring of creeping landslides, and its time-series deformation measurements provide a powerful means to evaluate building stability at regional scales due to their millimeter-level sensitivity and extensive spatial coverage. In this study, we integrate the long-term temporal record of Sentinel-1 with the high spatial resolution of TerraSAR-X to assess building stability from both temporal and spatial perspectives. Our results show that Joshimath has undergone continuous subsidence since 2017, with a marked acceleration beginning in 2021. Buildings across the town exhibit widespread settlement, while those located in zones with strong spatial variations in deformation rates are subject to pronounced differential settlement. Based on deformation characteristics, we classify the stability of individual buildings, providing a framework for prioritizing maintenance, reinforcement, and future land-use planning. Given the ongoing deformation in Joshimath, continued monitoring is essential for evaluating both slope stability and structural safety. Overall, our findings highlight the effectiveness of multi-sensor InSAR for assessing building stability in remote, landslide-prone mountain communities.