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

• Published in: Geophysical research letters 2023-08, Vol.50 (16), p.n/a
• Main Authors: Burrows, K., Marc, O., Andermann, C.
• Format: Article
• Language: English
• Subjects: Cloud cover; Earth Sciences; Earthquakes; erosion; Extreme weather; Geomorphology; Hydrologic data; Landslides; Landslides & mudslides; Mass wasting; Monsoon rainfall; Monsoons; natural hazards; Precipitation; Rain; Rainfall; Rainfall data; Rainfall measurement; remote sensing; Satellite imagery; Satellites; Sciences of the Universe; Seismic activity; Storms; Wind
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2023GL104720

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 Mw 7.8 Gorkha Earthquake sequence and suggest a modeled cohesion loss in the range 1–3 kPa. Plain Language Summary Earthquakes, storms and long periods of rain during the monsoon all contribute to landsliding in Nepal. Understanding these different triggers requires information on where and when landslides happen every year. We applied a new landslide timing method based on Sentinel‐1 satellite images alongside satellite measurements of rainfall to model the amount of rain needed to trigger individual landslides during the monsoon. This allowed us to connect landslides with particular storms and to observe the influence of a large earthquake in 2015 on landslide timing. Landslides occurred in unusually dry conditions following the earthquake, which can be interpreted as a temporary weakening of the landscape. Key Points We use Sentinel‐1 time series to constrain the timings of hundreds of monsoon‐triggered landslides across Nepal in 2015, 2017, 2018, and 2019 We identify spatio‐temporal clusters of landslides and associate them with particular periods of intense rainfall We use landslide timing information to calibrate numerical models of soil saturation and infer post‐seismic hillslope weakening in 2015