Retrieval of Monsoon Landslide Timings With Sentinel-1 Reveals the Effects of Earthquakes and Extreme Rainfall

Monsoon rainfall triggers hundreds of landslides across Nepal every year, causing significant hazard and mass wasting. Annual inventories of these landslides have been mapped using multi-spectral satellite images, but these images are obscured by cloud cover during the monsoon, making it impossible to use them to constrain landslide timing. We employ recently developed techniques to derive individual timings from Sentinel-1 for 579 landslides in 2015, 2017, 2018, and 2019 in Nepal. We use this new timing information alongside satellite rainfall data to identify spatio-temporal clusters of landslides and associate these with periods of particularly intense rainfall. We also observed that during the 2015 monsoon, many landslides failed earlier and in dryer conditions than in 2017-2019. We use physical models to demonstrate how this requires a temporary loss of hillslope strength following the M-w 7.8 Gorkha Earthquake sequence and suggest a modeled cohesion loss in the range 1-3 kPa.