By : 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 18th September 2011 with its epicenter (27.7oN, 88.2oE) 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 Mw) 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 18th 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 18th 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 Chu and Lachung Chu. On the western part long ridges are present – these are Talang Chu and Zemu Chu water divide. Geologically, the mountainous regions of the Darjeeling and Sikkim Himalayas are part of the active Himalayan Fold-Thrust Belt (FTB), which is geologically and structurally complex exposing a number of overturned (towards south) and thrusted sequence of variably metamorphosed pelitic and psammitic rocks over Mesozoic (Gondwanas) and foreland rocks composed of Tertiary (Siwaliks) sediments in the south³.
As per the Seismic Zonation Map of India⁴ the state of Sikkim comes under Seismic Zone IV – a zone of considerable vulnerability. The nearby region in the further north comes under Zone V and is highly susceptible to moderate to major earthquakes. In this area, most of the previous earthquakes were of shallow focus (< 40 km) and are commonly of 4.5 to 5.5 Magnitudes on the Richter scale. The regional distribution of earthquakes and lineaments/faults in Sikkim and adjoining Darjeeling areas indicates that a number of past moderate (M 5.0 to 5.9) and slight (M 3 to 5) earthquakes⁵ are clustered around the Tista Lineament trending NW-SE direction. Northern extension of the Tista lineament cuts across the NE-SW trending Kanchanjungha lineament. Another NW-SE trending lineament marked as a fault along the Tista River between Lachen and Chungthang (and further southeast) appears to have been the cause of some high Magnitude earthquakes in the region⁶.

Earthquake induced landslides

Due to the 18th September 2011 Sikkim earthquake (M: 6.8), several “new” and a few “reactivated” landslides have occurred right from the Himalayan foot-hill region (e.g., Dudhia in Kurseong Sub-division, Darjeeling District, West Bengal) up to the higher Himalayan range in the higher reaches of Sikkim-Darjeeling Himalayas. The new landslides that occurred in the lower elevations are mostly concentrated within the terrace deposits of trunk streams such as Tista, Rangit and Balason etc. and within the old colluvial deposits on the lower reaches of slope adjacent to trunk streams (e.g., Jorethang-Rishi-Legship section along the right bank of Rangit river). In the slopes having steep relief, the frequencies of rock/debris fall are more than to slides, whereas field observation clearly indicated that proximal to epicentral region, frequencies of both rock fall and rock slides are much higher than the debris/soil slides and the same are more concentrated towards the crest of steep ridges. Recent field observation of the landslide-related damages further indicated that lithology and geomorphology also played a significant role in causing these earthquake induced landslides. The debris-laden slope which is made of loose unconsolidated material and the slope covered by thin unconsolidated scree deposits have been more prone to failure by this earthquake. Frequency of rock fall and rockslides are more in areas which are generally vulnerable due to steep slope, weathered and fractured lithology and unfavourably jointed and kinematically-unstable slopes.

for more, go to:    http://earthquake-report.com/2011/11/25/earthquake-induced-landslides-in-the-sikkim-darjeeling-himalayas/