Identification of landslide-prone slopes at Paglajhora area, Darjeeling Himalaya, India

Paglajhora area, located in the southeastern part of the Kurseong town in the Darjeeling Himalaya region, India, experiences frequent slope failure especially in monsoon season. Due to the intricate disposition of major thrust zones (MCT, MBT, etc.), heavy rainfall, neotectonic and anthropogenic activities, etc., the cause of slope failure is not similar all over the area, which makes difficult to categorize the landslides based on their mode of failure. The present work is aimed to demarcate the landslide-prone areas from the surface displacement (horizontal) map derived by optical image correlation (OIC) technique applying normalized cross-correlation (NCC) algorithm of two temporal Landsat 8 OLI (level 1TP) images, supported by the ground field survey. Results indicate that the overall range of surface displacement (horizontal) rate in the study area varies from 0.4 to 8 m/year out of which the range 0.5-1.5 m/year being dominant classifying the slide type as slow-to-very slow earth flow with a velocity class varying from 3 to 2. Places showing relatively higher displacement values (3 to 8 m/year) match accurately with the already reported slide zones and the large exposed scarp surfaces noticed during the field surveys or distinguished from the satellite imagery. Areas with lower displacement rates (< 1.5 m/year) experience very slow earth flow, which are confirmed by the presence of cracks and steps on the major roads. Depths of shallow subsurface slip planes are established from dipole-dipole geo-electric surveys in few locations showing moderate surface velocity.