Implications and inclusion of size-dependent scenario earthquakes on landslide hazard zonation: A case study of the Indian Himalayas

Seismically induced landslide hazard zonation (LHZ) studies typically examine the correlation between the magnitude of a single earthquake event with the spatial patterns of coseismic landslides. However, studies on the impact of different scenario earthquakes (in terms of their return periods) on landslide hazards are found to be rare, especially in case of the Himalayas. The study presents a novel approach to integrate scenario earthquakes generated probabilistically for different return periods with the persistent landslide hazards in the Himalayas. The Peak Ground Acceleration (PGA) is estimated for scenario earthquakes with 10, 50, 100, 225, and 475 years return periods. A dynamic weightage system is developed to rank the scenario earthquakes. Subsequently five LHZ maps under different seismic conditions have been prepared. The results indicate that the maximum static Landslide Potential Index (LPI) for the study area increased by 25% and 35% respectively when scenario earthquakes with return periods of 225 and 475 years were considered. When the scenario earthquake with 475 years return period was considered, 51% of the total area fell in the very high landslide hazard zone. Even with a moderate earthquake size viz. 100 years’ return period, the seismic LHZ map demarcated 28% of the study area as very high landslide hazard zone. This was an approximately 211% spatial increment for the same zone corresponding to the static landslide susceptibility map that excluded the seismic factor. The trend (significant areal increments for the high and very high landslide hazard zones) remains the same in all scenario earthquakes. Based on the results, the study concludes that seismicity becomes the main landslide causative factor for moderate to great earthquakes. Thus, it is recommended to include earthquake parameters for a more pragmatic evaluation of the landslide hazard in seismically active mountain belts.