1 Earthquake induced landslides in the Sikkim-Darjeeling Himalayas - An aftermath of the 18 th September 2011 Sikkim earthquake Indranil Chakraborty, Dr. Saibal Ghosh, Debasish Bhattacharya & Anjan Bora Engineering Geology Division Geological Survey of India Eastern Region Kolkata Introduction According to the Indian Meteorological Department (IMD) 1 , a strong earthquake of Magnitude M: 6.8 shook Sikkim and Darjeeling areas of India and adjoining Nepal at 18.11 hours IST on 18 th September 2011 with its epicenter (27.7 o N, 88.2 o E) lying near the Sikkim-Nepal border, about 68 km northwest of Gangtok, Sikkim with the depth of its hypocenter at about 10 km. As per the USGS estimates 2 , the epicenter of this strong earthquake (6.9 M w ) lies slightly west of the location shown by IMD. This earthquake was widely felt in Sikkim, West Bengal to Rajasthan in the west and the northeastern states of India, and also in Nepal, Bhutan and Bangladesh. This major earthquake triggered several landslides in the region and a large number of which were studied during a recent post-earthquake fieldtrip (22-29 September 2011) to the Darjeeling-Sikkim Himalayas. The fieldwork was planned within four days’ of the event to the worst-affected areas in Sikkim-Darjeeling Himalayas that are accessible by roads. The aim of this study was to rapidly assess the damage caused by these earthquake induced landslides along with collection of the relevant landslide-related attributes for inventory and further study. It is also pertinent to mention here that moderate to heavy monsoon rainfall prior to the occurrence of 18 th September 2011 earthquake, during and post earthquake event is reported from some areas, which perhaps also contributed to some extent in lowering the shearing strengths of already-loosened (due to earthquake shaking) slope forming mass and ultimately triggered some landslides at selected locations just immediately after the earthquake shock. Due to this reason, it was considered wise to name these landslides as earthquake induced landslides instead of the co-seismic landslides in truest sense. Although, present field observation took due care not to mix up with landslides that are triggered by rainfall prior to 18 th September earthquake. Study area & Seismicity The study area falls within the Lesser and Higher Himalayan regions with an elevation ranging from 65 m (around Dudhia, Darjeeling District) through 800-1200 m around Ranipul, Mangan, 1400 m around Darjeeling, 1800-2200 m around Gangtok, Pelling, Ravangla to 2800–3200 m around Kyangsla (near Nathula) with elevations increasing gradually from south to north. In the Lesser Himalayas towards south, the slope is gentler and in the undulating stretches, numerous streams and rivulets flow along the depression and finally join with the trunk streams. In the Higher Himalayan terrain, the topography is highly rugged, characterized by steep slopes with prominent gully erosion. The Tista-Rangit water divide is the main north-south water divide within the Sikkim Himalayas. Another north-south water divide is between Lachen Chu & Lachung Chu which starts at about 3000 m and slowly increases up to 6700 m. North of Dikchu, steep ridges on the eastern side are there but of smaller extent as the Tista valley is sub-divided by the water divide between Lachen