Recent satellite-based radar and optical monitoring of the activity of a slow-moving landslide in Nepal during monsoon 

The steep Himalayan slopes are highly exposed to landslides, primarily triggered by earthquakes and monsoon intense precipitation. Along the Himalayan southern slopes, a specific landslide type involves old slided hillslope, characterized by intense internal fracturing, and prone to rapid retrogressive erosion through deep gullies incision, ultimately leading to catastrophic collapse of secondary landslides. Anticipating such events requires understanding if subsiding slices at the edge of the deeply incised talweg exhibit signs of acceleration preceding their rapid collapse and establishing a potential relationship between triggering factors (such as rain) and displacement amplitude. While optical images are commonly used for rapid landslides (with displacements superior to 20 cm/yr), their effectiveness is hindered by cloud cover during monsoon period, limiting sampling frequency and impeding the identification of transient deformation signals. In this study, we integrated satellite-based optical and radar remote sensing data with high spatial and temporal resolution to characterize the dynamics of a slow-moving landslide located in the Marsyandi valley (84.418° E ; 28.411° N ; 1900m a.s.l.) in Nepal, and to understand how it responds to monsoon rainfall. We developed a processing chain to apply sub-pixel image correlation to a data set comprising spotlight TerraSAR-X and PAZ radar images (1m spatial resolution), as well as medium resolution Sentinel-2 (10m), and high-resolution Pleiades (1m) amplitude optical images. We derived time series of ground displacements in range, azimuth, east-west, and north-south directions. Vertical displacements were additionally produced by comparing high-resolution Digital Surface Models (DSM) obtained from tri-stereo Pleiades images.  The displacement time series revealed metric transient ground displacements in the upper part of the landslide at the end of the monsoon, along with linear displacements in downstream gullies. Field observations validated our satellite measurements, indicating that during the monsoon, the south-eastern part of the landslide remained relatively stable and revegetated, while the north-western part experienced downward sliding. By comparing these displacements with precipitation data, we characterized the response of the slow-moving landslide to seasonal forcing and gained insights into the mechanisms of collapsed hillslopes.