THREAT OF LANDSLIDES AND SEISMICITY IN SIKKIM HIMALAYAS
T. Tashi, Department of Mines, Minerals & Geology
Government of Sikkim, Gangtok-737101, Sikkim
Abstract
With a total geographical area of 7096 sq. kms., Sikkim is the smallest amongst the North-eastern Council States. It lies between 27000’46” – 28007’48” North Latitude and 88000’58” – 88055’25” East Longitude and has elevation range of 250m to 8598m above main sea level. The State’s increasing population and developmental activities and depleting land and natural resources are in a collision course. Seismic and landslide vulnerability hazard compounds the evolving scenario. Government initiatives are in place to counter and mitigate the existing and perceived negative side effects. Their efficacy is yet to be proved.
INTRODUCTION
The common sight of high relief, swathes of verdant forest, raging fast flowing rivers and rivulets, steep slopes that are under failing and failed conditions, high seismicity and so on characterize the mountain ecosystem of the Sikkim Himalaya. Considering the fragility, diversity and complexity of the existing geoenvironmental settings and the ecosystem, manipulation of natural constants either by nature or man in an unsustainable manner can lead to irrepairable short as well as long term negative side effects and devastation.
The Sikkim Himalaya has never been and will never be free from ubiquity of weak geology, slope instability, frequent seismicity, soil erosion etc. mainly due to natural causes and partly as a result of accelerated degradation. These adverse conditions in tandem can exacerbate the existing fragile, vulnerable and multi-functional mountain ecosystem. So far disasters caused by landslides, earthquakes, floods etc. have not lead to large scale human tragedy in Sikkim in recent memory. However, there is ever increasing human demand of natural resources, especially land for urban development and mega dams in an apparently unsustainable manner, making some of the denizens to adapt and survive at dangerous margins. The emerging crisis can perhaps be minimized by indigenous knowledge based and modern technological interventions. To safeguard against accelerated degradation and improve the living standards of the hill people, the Governments (Centre and State) need to address hill specific issues through systematic and effective integration of the ecosystem services and development, highland and lowland linkages etc. Without a replicable
and hill specific developmental policy, the ever present threat from devastating landslides, earthquake, floods etc. remains and the options and the opportunities of the progeny in jeopardy. A beginning has been made by the State of Sikkim by asking the Department of Science & Technology (DST), Government of India to undertake systematic study of landslide problems in the State. The DST accordingly prepared a status report on landslides in Sikkim. Based on the report, the DST, Government of India, is considering establishment of a multi-disciplinary cell dedicated to landslide studies, not only in Sikkim but the entire Northeastern states. Such a initiative has become imperative because past experience shows that different agencies carry out landslide studies at will and without coordination with each other. Results of such exercises never actually got disseminated and proved futile. Whereas the seismicity monitoring is concerned, site response studies using digital accelrograms located at seven-station-strong motion network in Sikkim by year 2000 and twelve-station network by year 2006 has been done by IIT, Kharagpur and study results are widely published.
GEOLOGY AND TECTONICS
Mallet (1875) & Bose (1891) were the first explorers who gave an account of geology and mineral resources of Sikkim Himalaya. Subsequent explorations by Auden (1935), Heim and Gansser (1939), Ray (1947) Ghosh (1952), Raina & Bhattacharya (1975), Raina and Srivastava (1980), Thakur (1986), Ravikant (1993), Neogi et al (1989), Ray (2000) and others gave important and valuable contributions towards understanding of geology of Sikkim.
The current literature describes geology of Sikkim similar to that of the Eastern Himalaya where four distinct geomorphology based transverse zones or tectonostratigraphic domains, separated by major tectonic dislocations, are identifiable. Out of these four tectonostratigraphic units, namely; Sub-Himalaya, Lesser Himalaya, Higher Himalaya and the Tethys Himalaya, the Phanerozoic rocks of the Sub-Himalaya are not exposed in Sikkim. The Lesser, Higher and Tethys Himalaya of Sikkim are typically arranged in a domal shape or arch of thrust surfaces in the form of culmination across Teesta river, popularly known as Teesta culmination (Mcclay, 1992, Ray, 2000). The core of the Teesta culmination is occupied by Proterozoic Lesser Himalaya low grade metapelites of Daling Group of rocks. The distal part is made up of medium to high-grade Proterozoic Higher Himalaya crystalline complex, the Main Central Thrust (MCT) separates the Lesser and Higher Himalaya. Gondovana (Carboniferous to Permian) and Buxa Group of rocks are exposed in the Rangit window zone, small window near Rorathang, East Sikkim and as thrust/fault slices in South Sikkim. The Tethys Himalaya is represented by Cambrian to Eocene fossiliferous sediments of the North Sikkim Tethyan zone which tectonically overlie the Higher Crystalline Complex (see map).
REGIONAL SEISMICITY
The tectonic frame work and the seismicity of the northern eastern states including Sikkim are considered as a result of collision tectonics in the Himalayan arc and subduction tectonics below the Myanmarese arc. Studies have indicated a very complex tectonic setting of the region due to constant movement of the Indian plate from South to North & Myanmarese from East to West.
The two major structural elements in the Eastern Himalaya are the Main Central Thrust (MCT) and the Main Boundary Thrust (MBT). The Foot Hill Thrust (FHT)/Main Frontal Thrust (MFT) along the Southern edge of the Himalayan bring the Siwaliks in Juxta-position with the thick recent sediments of the Indo-gangetic plain. There are also a large number of prominent lineaments in this region, some of which are reported to extend for several kilometers beneath the Himalayan Foredeep. The Teesta lineaments which pass through Parbatipur area of Bangladesh to Bhadrapur area of Nepal, is considered to demarcate the Western limit of Eastern Himalayan seismicity.
The North Eastern India was subjected to severe shaking by a number of damaging earthquakes. The systematic account of which is available from the middle of nineteenth century only. Besides the Cachar earthquake of 10th January 1869 (Magnitude 7.5), the twenty other severe earthquakes exceeding magnitude of seven and affecting the region during the past hundred years have been tabulated (Ramchandran et al, 1981). Among the recorded earthquakes, the most damaging ones and macroseismically studied are the great Assam Earthquake of 12th June 1897 (M=8.7), Srimangal Earthquake of July 8, 1918 (M=7.1), Dhurbi Earthquake of July 3, 1930 (M=7.1), Assam Earthquake of August 15, 1950 (M=8.6), Assam Earthquake of July 8, 1975 (M=6.7), Cachar Earthquake of December 3, 1984 (M=5.4), Manipur-Burma Earthquake of August 6, 1988 (M=7.3), Assam-Tripura Earthquake of April 13, 1989 (M=5.7) and Manipur Earthquake of January 10, 1990 (M=5.4).
In the Eastern Himalayas, the seismicity is considered as a result of collision tectonics & correlated with the MBF and MCT (Gupta S.K. 2003). The MCT is shown passing through Gangtok to Mangan and then to lower Tolung to north of Sada from where it cuts through North
of Labdang-Tashiding to Gyalshing and then to Kaluk to Soreng before coming out of Sikkim border at an area where it meets the MBF (India-Nepal border).
The IIT, Kharagpur has carried out site response studies using strong motion network of accelerographs in Sikkim during the past ten years. The findings are available in the form of a paper with site response contour maps (Nath, et al 2000) and a paper with microzonation maps (Nath, et al 2006).
The existing record shows (Table 1) that the state has been subjected to local earthquakes of M=5.0 to M=6.5 and non-local high magnitude earthquakes of above M=8.0 (1897 and 1950 of Assam), (1934 of Bihar). The whole of the State is therefore, has been slotted in Seismic Zone IV of IS:1893-1984 and categorized as High Damage Risk Zone of MSK VIII.
Considering the seismically vulnerability of the State all new constructions are subjected to strict observance of BIS codes or seismic codes. The existing unsafe and non-engineered building stock still remains and is practically impossible to address the entire such building stock. The alternative left is to retrofit only the life line buildings such as hospitals, schools, cinema halls, multi-storied hostel/apartments etc.
LANDSLIDES
Mass wasting processes are a universal phenomina in mountain, hill and hilly areas of the world. Their magnitude, frequency and type differ from place to place and on the material that is undergoing displacement either through chemical or mechanical means. In the Himalayas, mass wasting process is dominated by landslides. The magnitude, intensity and frequency of Himalayan
landslides vary from East to West and from South to North. The variation is controlled, mainly by climate, neo-tectonism and seismicity. The eastern Himalaya including Sikkim is a hot-spot for natural hazards, particularly landslides and earthquakes.
Landslides of all types and size occur in almost all types of rocks and quaternary formations of Sikkim. The Daling Group of rocks, especially, Gorubathan Formation appears more prone to landslides than the inhomogeneous quaternary deposits and gneisses and schists of Higher Himalaya. The high landslide susceptibility of the Daling Group of rocks has been attributed to their severe shear distortion due to loading and unloading during orogenesis, higher rate of weathering and mineral composition. A cursory survey of frequency of occurrence of landslides in Sikkim was done in 1991 and the result clearly showed that the East and South Districts where Daling rocks dominate are affected by maximum number of landslides (Table below)
| DISTRICT | VERY SEVERE | SEVERE | MODERATE | TOTAL |
| North | 32 | NA | NA | 32 |
| South | 90 | 148 | 130 | 368 |
| East | 5* | 9* | 3* | 153 |
| West | 40 | 16 | 33 | 89 |
|
| 167 | 173 | 166 | 642 |
Almost all the landslides in Sikkim occur after prolonged exposure to monsoon rains and occasionally during or just after cloudbursts or precipitation intensity exceeding 135-145 mm in 24
hours. The red letter month of October 1968 was considered the most disastrous month in recent years of Sikkim history. On 5th September 1995 heavy rain triggered off a mud avalanche near Gangtok killing 32 people. Similarly, non-stop heavy rain since early June 1997 damaged 300 houses completely, 1000 houses partially and caused death of 51 people in East and North districts of the State. Rain, therefore, has been one of the main triggers besides weak geology, steep slopes with thick overburden, frequent earthquakes etc., causing landslides.
There has never been a systematic study of landslide problems in Sikkim. Most of the existing literature on landslides of Sikkim are written by some fancy expert expatriate on a whirlwind visit to the State. Since no two landslides are identical in any given area, it is not always easy to know the ground realities of mechanism to trigger off landslides. The State Department of Mines, Minerals & Geology, took up a number of landslide prone and affected areas for systematic study and mitigation measures. Besides treating some chronic rotational type of landslides along State highways with slope dressing, benching, installation of effective drainage systems and so on, some planar type slides, controlled by joint, foliation and bedding planes were further subjected to grouting and rock bolting, both tensioned and untensioned (see pics). The notoriety of a landslide is known by its maximum capacity to cause damage to life and property. Some of the well known landslides falling in that category are as shown in Table 2. Sikkim being a land locked State, disruption of road communication by landslides is an annual affair. For every linear kilometer of Sikkimese road there are atleast 10 minor to major slips.
Tensioned Rock Bolting
Tensioned Rock Bolting
CONCLUSION
Natural hazardous events such as earthquakes, landslides, floods etc. in the Himalayas are a reality. Man and man-made structures stand no chance against the awesome power and fury of such events when they strike. Therefore, a mechanism is needed to safeguard against massive and unwarranted loss of life and property in the event of a calamity. In August 2004, the Government of India came out with a detailed status report on Disaster Management in India. The report specifies various programmes and strategies of the Nation to tackle and mitigate all forms of destructive natural events. Translation of some of the recommendations have already begun in Sikkim. The Department of Land Revenue & Disaster Management, Government of Sikkim and UNDP has undertaken various initiatives in this direction. The general public is sensitized through awareness training/talk shows, mock drills, banners and so on. The North eastern states including Sikkim being in high seismogenic domain, landslide and flood prone areas require special attention and constant vigilance. The on-going research by established institutions in various fields of adverse events needs up-gradation and monitoring by an apex authority for proper and effective coordination of long term research and dissemination of information to stake holders. The existing scenario of haphazard and secretive study of natural events by all sorts of agencies needs to be discouraged.
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About Mr Tshering Tashi :-
Mr Tshering Tashi is a 55 yr old geologist who is Director (Dept. Of Mines, Minerals & Geology) Govt. of
I know him personally and when he is not immersed in work he is a fun loving person who loves fishing or just relaxing with his family.
praful rao
