A multi-inventory study of regional- and local-scale controls on earthquake- and rainfall-induced landslides in the Himalaya

Landslide spatial distribution and dependency on geo-environmental controls vary significantly with triggering mechanisms, with important implications for landslide hazard and risk assessment. This paper analyzes three different landslide inventories covering an area of ∼25,000 km2 affected by the 2015 Gorkha earthquake. These include 21,151 earthquake-induced landslides (EQLs); an original collection of 14,943 pre-event shallow landslides, most of which are attributed to monsoon rainfall; and a new inventory of 5648 large landslides. Regional swath profiles, frequency analysis, and principal component analysis were used to investigate regional- and local-scale controls on landslide distribution, considering geological, geomorphological, land use, and meteo-climatic variables. Furthermore, the role of antecedent landslides in reactivating newer landslides was assessed. In general, the distribution of all three landslide inventories is mainly controlled by the litho-morphological landscape imprint associated with the regional-scale transition from the Lesser Himalaya (LH) to the Greater Himalaya (GH), locally modulated by land use, slope aspect, and slope gradient. This control was strongest for the 2015 Gorkha EQLs, influencing both their density and size, and extended beyond the expected, inherently localized effect of seismic variables (peak ground acceleration and net vertical coseismic displacement). At the local scale, EQLs appear evenly distributed along slopes, with no clear topographic clustering along ridges or cliffs. Rainfall induced landslides (RLs) tend to concentrate in the LH zone, characterized by smoother topography, generally weaker rock strength, and greater anthropogenic disturbance, with local-scale control from land use and slope aspect. Large landslides exhibit a more even distribution, though their size clearly increases with the regional-scale transition from LH to GH. Remarkably, the presence of pre-existing large landslides also controls the regional-scale distribution of RLs and EQLs. It is shown that the proportion of reactivated landslides increases in areas characterized by regional-scale conditions less favorable for landsliding, where instability appears to be reached largely on slopes already weakened by pre-existing landslides.