Detecting slow-moving landslides in parts of Darjeeling–Sikkim Himalaya, NE India: quantitative constraints from PSInSAR and its relation to the structural discontinuities

Abstract

The difficulty of monitoring slow-moving landslides is attributed to its highly dynamic spatial and temporal character, especially in a tectonic regime like the Himalayas, where the control of structural discontinuities to determine the risk at a local and regional scale is essential. Although many methods have been used for landslide monitoring in the past, the identification, measurement and categorization of landslide-induced relative surface displacements, its relevant relationship with relative slope and structural features, and forecasting remain elusive. The present work explores a holistic approach towards studying slow-moving landslides in and around the urban locales (Darjeeling, Kalimpong and Gangtok) of Darjeeling–Sikkim Himalaya. In order to establish an effective relationship, 109 successive C-band Sentinel-1A dataset acquired from February 2017 to October 2020 has been analysed using PSInSAR to identify the potential landslide risk zone in the selected blocks. Results indicate nearly 10.63% (~ 42km2) of the total Sikkim monitoring area (~ 395km2) is subjected to a 20-40 mm/yr of mean annual displacement rate and thus considered to be under the expression of potential risk. The contextual relationship between slope instabilities and regional thrusts/faults is also established. The derived time series displacement estimates are further integrated with corresponding slope estimates derived from the ALOS PALSAR DEM to determine the nature of Line of Sight (LOS) displacement followed by its classification into different categories and are further used in forecasting using ARIMA (1,1,1; 2,1,1) model. Results indicated that the area may experience cumulative displacement of 200-240 mm with a predicted 2.5–threefold increase between 2020 and 2023. © 2022, Springer-Verlag GmbH Germany, part of Springer Nature.