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Friday, October 7, 2011

Sikkim quake unique...Land Slide in Darjeeling Hill Areas

Sikkim quake unique
Y. MALLIKARJUN, TH : The recent 6.9-magnitude earthquake in Sikkim, which claimed more than 100 lives, was different from most temblors that had rocked the Himalayan region.
It did not occur on the well-known thrust faults formed due to under-thrusting of the Indian plate below the Eurasian plate as in 95 per cent of the major earthquakes in the Himalayan region. Instead, it occurred between two transverse faults represented by Tista and Gangtok lineaments where two segments of Himalayas have moved in a horizontal direction.
Although the transverse faults were also the result of under-thrusting of the Indian plate, this was the first time that a major earthquake had occurred along them in Sikkim region, according to R.K. Chadha, Chief Scientist, CSIR-National Geophysical Research Institute (NGRI) which deputed a team of five scientists to the affected region following the earthquake to carry out field investigations. The team has set up a network of four broadband seismic stations. The earthquake with its epicentre located 68 km of northwest of Gangtok occurred at a depth of 20 km beneath the Himalayas.
Seismically very active
With the Himalayan region being one of the most seismically active places in the world, the recent earthquake, however did not come as a surprise. Most other major earthquakes above magnitude (M) 8.0 in Shillong (1987), Kangra (1905), Bihar-Nepal border (1934), Arunachal Pradesh (1950) and many others between M 6 and 7 were caused by faults formed by the under-thrusting of Indian plate below the Eurasian plate. The Sikkim region experienced relatively moderate seismicity over the past 35 years with temblors of M 5 or greater occurring within 100 km of the epicentre of the September 18 event.
Large area impact
Dr. Chadha said another interesting feature of the recent earthquake was that it was felt over a large area, including Delhi and other major cities such as Lucknow, Patna and Ranchi along the Indo-Gangetic plains. With most of the region lying in the soft sediments in the Indo-Gangetic plains and the Brahmaputra Valley, the earthquake was felt strongly as the seismic waves get amplified in soft sediments when compared to hard rock regions. As experienced earlier, such temblors would cause damage in faraway regions too due the amplification process. The amplification phenomenon again showed that major cities located in the Indo-Gangetic plains face greater earthquake hazard than other regions.
Land Slide in Darjeeling Hill Areas
(compiled & shared by Abhinay Chhetri, Bangalore) The Blockwise landslide affected areas explain itself the comparative intensity of landslides in the blocks. It appears that the Kalimpong I, Kalimpong II and Rangli – Rangliot blocks are comparatively vulnerable or severly vulnerable regarding landslides. The rate of vulnerability is also high in Kurseong and some parts of Bijanbari and Gorubathan blocks. But as a whole the condition is critical in Kalimpong sub-division, where the land under agriculture exceeds that of the area under plantation or forests. Moreover, these areas are cultivated with root crops like potato, ginger, cardamom and onions. These root crops are harvested just after monsoon in the months of September – October. This particular practice changes the cohesiveness of the soil and makes it vulnerable to erosion.
The slope instability factors along the main thoroughfares due to heavy vehicular movements is another big problem that causes frequent land slides along the roads, especially during rainy season.
The problem of quarry operation and their effect on environment is also another threatening problem and was studied thoroughly by Prof. S.R Basu. The illegally operated quarries along the Lish and Gish basin and Tindharia region not only disturbs the slope stability but also overburdens the rivers and their tributaries with excessive amount of load that ultimately leads to massive siltation along the river beds and the adjoining plains. Thus they destroy the ecological equilibrium of the area. Other than coal mining, stone quarrying from the slope especially under the road is another way of human intervention that causes occasional slope failure. The recent landslide near Giddapahar may be attributed to the effect of stone quarrying under the main thoroughfare.
The Critical Area Zonation map has been prepared by examining the geology, soil, and climatic factors along with land use pattern. It appears from the map that the Grid no. E4 under Rangli-Rangliot block, G4 and H5 under Kalimpong II Block are the most landslide prone areas, where human intervention is maximum.
Possible causes of Landslide hazards in the Darjeeling Hill Areas
1.The trends of evolution or rising of young mountains is the basic reasons for frequent landslide hazards in the Himalayan region. This includes unstable geological structure, tectonic disturbances, parallel subsidence of Himalayan fore deep of slopes.
2. Soil erosion and its conservation play an important role in the hill areas. Because of the presence of very thin soil cover plays an important role in the socio economic development of the hills and its people. All India Soil and Land use Survey under the Ministry of Agriculture, Govt. of India had carried out studies in some specified watershed areas. Otherwise, no systematic soil mapping has been carried out in the region. As such, there is no database, of how much soil cover has been destroyed.
3. The soils of Darjeeling hill areas have developed mainly Darjeeling gneiss, schists and Phyllites. Due to heavy deforestation and excessive cultivation of root crops like ginger, potatoes, onions, cardamoms etc. the extent of soil erosion has increased considerably in the recent times. It is a fact that the entire Darjeeling hill areas do not get any soil deposition. Deposition of soil is only found in the river valleys. Thus, the prevention of soil erosion and conservation of soil is very necessary in the hills.
4. By examining the land use pattern and changing characteristics since the last 150 years, it may be commented that the forest cover is in a precarious condition due to the rapid increase in cultivated land (with the exception of tea gardens), expansion of settlements, construction of roads. The rapid depletion of forest cover is noticeable in the tea plantation area. In most of the tea gardens in the hills, any type of shade tree or trees along the fringe line of the garden for the protection of the soil is more or less insignificant.
5. Rapid expansion of settlements and towns especially along the roads is one of the important causes of frequent landslide hazards in the hills. Multi storied buildings without proper planning along the roads and on the steeper slope increase the load on the already deteriorated slopes.
6. In the rural and inaccessible high hills. Demand for fuel is another important factor, which may be treated as an important cause for slope failure. Unscientific mining of low energetic coal seams and illegal felling of trees to meet the demand of firewood is practically unavoidable in the hills.
7. During the last 2 decades there has been an unprecedented growth of population in the hill areas, especially in the towns. The explosion has been followed by the rapid increase in vehicular movements. The continuous horizontal vibration along the roads gradually destabilizes the already unstable slopes and geological formations.
8. Lastly the demand of water for domestic and commercial purposes has also increased. The forest clearance, dissection of the upper portions of the slopes are reflected in the decrease in ground water level and consequent drying up of the streams during most part of the year.
9. Examining the above mentioned analysis, the future of the Darjeeling hill areas does not look very bright. Systematic and scientific utilization/management of the natural resources is required.
Earthquake-prone Himalayas under stress : India continues to sink within itself, creating a mega self-pull in the process that has put the mighty Himalayas under severe duress, triggering earthquakes..... A new theory on the formation of the world’s youngest mountain range suggests that the ongoing movement of India towards central Asia could be driven by a downward pull from the Indian continental plate.
This work provides a different interpretation of the forces that governed the collision of India and Asia between about 55 and 30 million years ago. It could also suggest why the Himalayas are so fragile and earthquake-prone. The research may eventually help understand why so many earthquakes occur in the Himalayas.
Almost 50 million years ago, the Greater Indian plate collided with the Eurasian plate leading to the formation of the mighty range. Usually, when two continental plates collide, the movement of the two plates stop. That did not happen with the Indian-Eurasian convergence.
The two plates continue to rub against each other generating stress, which is later released as earthquakes. An international team has now offered an explanation. In Sunday’s edition of “Nature Geoscience,” they suggest an internal mechanism that creates the drag which pulls the Indian plate towards its boundary with Eurasia.
“The India-Asia convergence cannot be explained by plate tectonics theory, which is deeply challenged,” said team leader Fabio Antonio Capitanio, a geoscientist at the University of Monash, Australia.
The scientists used a cutting edge software, aptly named “UNDERWORLD code,” to simulate the collision of the Indian plate and Eurasia before and during the collision. They found that the density of the uppermost layer of the Greater Indian plate is more than the underlying mantle. This would have continued to drag India towards Asia.
When the upper crust is scraped off at the Himalayan front during continental collision, the remaining dense continental lithosphere sinks into the mantle dragging the Indian plate with it. The bulk of the Indian plate was sufficiently dense to sink into the underlying mantle.
It is this sinking pull that has probably led to Indian plate’s continuous push along the plate boundary, which leads to stress generation and quakes later. “How the stress transforms into deformation, which is not always accompanied by earthquakes is another subject. But the boundary conditions are now a bit more clear,” Capitanio told Deccan Herald.
Darjeeling hill areas is unique from environmental Eco-perception.
The relief varies from 100 Mts. above sea level to the mighty Kanchanjungha. There are different climatic zones with distinctive attributes and there are endangered animals like red panda etc along with memory orchids and medicinal plants are available in this hilly region.
The Darjeeling hill area is formed of comparatively recent rock structure that has a direct bearing on landslides. The causes of the landslides vary from one locality to another. Heavy monsoon precipitation is however a very common cause of these disasters. More over soils of Darjeeling hill areas are extremely varied, depending on elevation , degree of slope, negetative cover and obviously geolithology . The natural system of erosion in the hill gets more complicated when man interferes.
As the mountains serve as the source of resources for the population residing in the hills as well as in the plains, the form of environmental degradation is quite extensive other particularly is applied to the extraction of timber and other forest produces, mining and agriculture are taken into account. As human population expands in the hills, forests are being depleted for the extension of agricultural lands, introduction of new settlements, roadways etc. The growing changes coming in the wake of urbanization and industrialization leave deep impression on the hill ecosystem; disrupting normal functioning.
Due to unprecedented growth of population during the last few decades in the Darjeeling hill areas, nature has started reacting sharply to the accumulated human guilt. Landslide hazards, especially during rainy season have become a common factor to the people of the hill.
The Hill areas of Darjeeling District are located within the Lesser and Sub - Himalayan belts of the Eastern Himalayas. The area is bounded by the Sikkim Himalaya in the north, the Bhutan Himalaya in the east and Nepal Himalaya in the west. The southern foothill belt is demarcated by a highly dissipated platform of terrace deposits extending along the east west axis. The inner belt is defined by a ridgeline stretching from the Darjeeling Hill to the west and Kalimpong Hill to the east, overlooking the southerly flowing Tista valley in between. Prominent rivulets contributing to the Rammam - Rangit basin, dissipate the northern slope of Darjeeling Hills.
The Kalimpong Hill is rather rugged in topography and is dissipated by radically descending gullies and streams that contribute to the Tista and Jaldhaka River system.
The soils of Darjeeling Hill area have developed depending upon the underlying geological structure. But, in general the soils have been developed by both fluvial action and lithological disintegration. The soils that have developed in the Kalimpong area are predominantly reddish in color. Occasional dark soils are found due to extensive existence of phyllitic and schists. Soils in the highlands stretching from the west to the east of the district along most of the interfluvial areas are mainly mixed sandy loam and loamy, while those on the southern slopes of Mirik and Kurseong are mainly clayey loam and reddish in color. Sandy soils are mainly found in the east of the river Tista.
All the soils are definitely acidic in nature with the tendency to increase slightly in depth in most cases indicating the lacking of bases from surface and accumulation in the lower horizons. The weathering of lateritic type is the substantial mechanism in the transformation of the substratum. The variable thickness of the regolith and soils depend on the rate of weathering and gradient of the longitudinal slope profiles and intensity / gravity of mass movements. The basic soil types are yellow soils, red brown soils and brown forest soils. Red and yellow soils have developed on gneiss while brown on schists and shales. Coarse pale yellow to red brown soils are found on the Siwaliks while clayey dark soils are developed on Daling series.
The character of the bedrock is reflected only in the grain size composition of the soil. On the Darjeeling gneiss, very coarse-grained (50% -80%) particles are found. In Damuda and Daling series percentage of sandy and coarse particles in the soils are high. On the Siwaliks, silty – clay fraction is higher. The chemical content of the soil over Darjeeling gneiss is characterized by a high proportion of potassium derived from feldspar and muscovite mica. This soil is poor in lime, magnesium, iron oxides, phosphorous and nitrogen. Therefore lime is used in the tea plantation areas.
The loss of trees, which anchor the soil with their roots, causes widespread erosion throughout the tropics. Only a minority of areas have good soils, which after clearing are quickly washed away by the heavy rains. Thus crops yields decline and the people must spend income to import foreign fertilizers or clear additional forest. Costa Rica loses about 860 million tons of valuable topsoil every year, while the Great Red Island, Madagascar, loses so much soil to erosion (400 tons/ha) that its rivers run blood red, staining the surrounding Indian Ocean. Astronauts have remarked that it looks like Madagascar is bleeding to death, an apt description of a country with grave environmental degradation and an ever-declining agricultural economy which depends on its soils. The rate of increase for soil loss after forest clearing is astonishing; a study in Ivory Coast found that forested slope areas lost 0.03 tons of soil per year per hectare, cultivated slopes annually lost 90 tons per hectare, while bare slopes lost 138 tons per hectare.
After heavy tropical rains fall on cleared forest lands, the run-off carries soil into local creeks and rivers. The rivers carry the eroded soils downstream, causing significant problems. Hydroelectric projects and irrigation infrastructure lose productivity from siltation, while industrial installations suspend operations due to lack of water. Siltation also raises river beds, increasing the severity of floods, and creates shoals and sandbars that make river navigation far more troublesome. The increased sediment load of rivers smothers fish eggs causing lower hatch rates. As the suspended particles reach the ocean, the water clouds, causing regional declines in coral reefs, and affecting coastal fisheries. The loss of coral reefs worldwide, often labeled the rainforests of the sea, is especially distressing to scientists because of their tremendous diversity and the important services they provide. Coastal fisheries are affected not just by the loss of coral reefs and their communities, but by the damage inflicted on mangrove forests by heavy siltation.
Besides damaging the fisheries industry, deforestation-induced erosion destroys infrastructure of roads and highways that cross through the forest. The government does not shut down the roads, but must rebuild them repeatedly, using money that could be used more productively elsewhere.
Erosion is extremely costly for developing countries. Besides the damage to infrastructure, fisheries, and property, erosion of precious topsoils costs tens of billions of dollars worldwide each year. For example in the late 1980's, the Indonesian island of Java was losing 770 million metric tons of topsoil every year at an estimated cost of 1.5 million tons of rice, enough to fulfill the needs of 11.5-15 million people.
Land- Use Pattern in the Hill Areas: The land use practices play the most important role in determining the stability factors in respect of landslide hazards. The land use map of Darjeeling Hill Areas explains that there are agricultural activities, tea and medicinal plant plantations, construction works along with forests, rivers, jhoras etc.
The main problem in respect of land use in the Darjeeling Hill Areas is related to high density of population. There is very limited scope for extension of agricultural land to cope up with increasing pressure of population. As a result pressure on forested and other restricted areas is gradually increasing.
Another problem related to land use and consequent landslide is that in Darjeeling Hill Areas, roads have never been examined with its carrying capacity respect with geology etc. Along with new road construction the vehicular movements have increased to a great extent with the rapid growth of trade and commerce. Heavy traffic movements along with heavy rainfall are responsible for most of the landslide occurrences especially on the roads. In recent years, it has been observed that there is a constant increase in the vehicular traffic, especially heavy vehicles like trucks and buses. The record reflects that at present, the number of registered vehicles in the hill subdivisions are more than 6500.
According to the District Gazetteer of Darjeeling district (1980) the road lengths (in km) in the district are as follows;
i National Highway------------ 100km
ii State Highway--------------- 80km
iii Major district road --------- 37km
iv Ordinary district road------ 516km
(the figure includes Siliguri subdivision) During the last 50 years, the length or pattern of the roads in the hill subdivision did not change significantly, though there has been at least 5 to 7 times increase in the number of vehicles, especially goods vehicles like trucks etc. It has been observed that even during night times, the traffic movement on the roads continues. As a result, due to constant lateral vibrations, the weaker geological structure has become unstable. Beside the above-mentioned roads, there are many roads, which are maintained by the Forest Department. Mention should be made about some of them like: (1) the Cart Road from Sukhiapokhri to Maneybhanjan and Batasia, (2) Cart Road from Simkona to Lalkuthi in Darjeeling Forest Division (3) Sukna-Sevok Road in Kurseong Forest Division (4) the Rassium – Labha Cart Road, South Boundary Cart Road, Central Cart Road and Dalgaon Tar Cart Road in Kalimpong Forest Division.
Earthquakes pose the greatest threat to the Himalayan region. 

Almost the entire Himalayas are prone to high seismic activity or earthquakes. Earthquakes have hit the region several times in the past and similar threats remain inevitable in the future too.
The Himalayas, as we know, were formed by the head-on collision of Indian and Eurasian plates. The mountain building process is still going on because the Indian plate is still moving towards the Eurasian plate. The Indian plate is pushing the Asian plate northward at the rate of about 2 cm per year. This means that in every 100 years India moves 200 cm north against the Asian plate. This colliding force builds up pressure continually for several years and this pressure is released in the form of earthquakes from time to time.
Usually the barren cold desert regions have experienced less devastation from earthquakes than other parts of the Himalayan mountain chain, probably due to the low population.
Four major earthquakes have occurred in the Himalayan region in the past 100 years. The famous earthquake that hit Nepal in 1933 A.D. killed thousands of people in Nepal and northern India. Several earthquakes have occurred since that time.
The Indian Himalayas have experienced some significantly strong earthquakes in the last few decades.
Kinnaur Earthquake (1975)
This earthquake struck in the early afternoon of January 19, 1975. It caused havoc in parts of the Kinnaur, Lahaul and Spiti regions of India. It is believed to have been caused by movements along a fault known as the Kaurik fault. This quake killed hundreds of people and caused severe damage to property.
A massive landslide was triggered off by this earthquake near Maling in the Spiti Valley. Another giant landslide blocked the Paro chu River near Sumdo. Many smaller occurrences of slope failure were caused by this earthquake. As a result, communications remained disrupted for several days and helicopter services had to be pressed into operation to bring relief to the worst-affected areas.
Dharchula Earthquake (1980)
This was another devastating earthquake that struck Dharchula and surrounding areas of the Pithoragarh District in the Kumaon Himalayas. It occurred in December 1980. Displacement along a deep-seated fault is believed to have been the cause of this quake that affected parts of the inner dry valleys of Pithoragarh District.
The 2011 Sikkim earthquake , was a magnitude 6.9 (Mw) earthquake centered within the Kanchenjunga Conservation Area, near the border of Nepal and the Indian state of Sikkim, at 18:10 IST (12:40 UTC) on Sunday, 18 September 2011. The earthquake was felt across northeastern India, Nepal, Bhutan, Bangladesh and southern Tibet.
Earthquake Resistant Structures of Himalayas
DP Agrawal and Manikant ShahEarthquakes don't kill; badly built houses do. Though this is a time-tested lesson, it is often forgotten soon after an earthquake. Already, lessons of the massive earthquakes are being forgotten. But, quakes are not new to India as 55% of the country is prone to seismic shocks. Several earthquake prone regions in the country have traditionally built houses that minimize the damage to life and property and stand up well when the quake hits. These techniques are based on traditional material that have been used to build such houses�timber and bamboo, for example: The traditional wisdom and attention to detail can be applied to modern materials as well.
No building can remain entirely free of damage during a quake; still, all houses, big or small, can be made safer. Structures can be made to withstand earthquakes of a particular magnitude by taking certain precautions. Buildings collapse as a result of inertial forces. During an earthquake, the lower part of a building tends to vibrate as it is in direct contact with the ground. The forces of inertia, however, keep the upper portions static. This conflict of forces leads to collapse.
The magnitude of these forces is directly proportional to the weight of the building the heavier the structure, the greater is the damage. If the structure is light, lesser number of people die in case of a collapse. Avoiding compression structures like domes, vaults and arches is another option, according to architect Das Gupta. The structural system needs to be tensile and the material should be flexible, as is the case with timber, steel and bamboo. It also helps if the structure is constructed in a way that it vibrates as one unit and sways together. Traditional constructions in India�s Northeast follow this principle. So also the tall sky scrapers of Tokyo. One can even notice their swaying if there is a strong wind.
At 2.53 am October 25, 1991, a moderate earthquake measuring 6.6 on the Richter scale affected more than 425,000 people of about 2,100 villages in the districts of Uttarkashi, Tehri and Chamoli in Garhwal Himalayas in the hill state of Uttaranchal. Traditionally built timber houses proved to be the most effective at keeping damage at bay. It would, however, be incorrect to say that the old-fashioned houses survived just because of materials used. The construction techniques, too, determined the amount of damage.
Most new constructions with heavy roofs (slate tiles or RCC) supported by weak walls (random rubble in mud mortar) proved deadly. Older houses at higher elevations have timber roofs held together by timber tie-bands- horizontal timber beams spanning across the entire building, connecting the entire structure and giving it the character of a cage. Such houses suffered little damage despite the mud-and-stone masonry.
Quake resistant houses should have tie-bands just above the level of the floor, the level of the doors and windows, and another at the roof level. Corners are the most vulnerable and ought to be strengthened. Elasticity of the structure can be enhanced with flexible steel rods or wood batons at corners. Doors and windows should be few, small and symmetrically placed away from the corners. The house should be as light as possible.

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