Read at National Seminar on
GEODYNAMICS AND ENVIRONMENT
MANAGEMENT OF HIMALAYA
Department of Geology
EXTENSION OF ARAVALLI BASEMENT BELOW GARHWAL HIMALAYA AND ITS GEOLOGICAL CONTROL OVER THE OCCURRENCES OF NATURAL HAZARDS IN UTTARANCHAL STATE
Wadia Institute of Himalayan Geology, Dehradun
Mother nature has bestowed a plenty of natural resources and scenic beauty all around us on this globe, which have been exploited by human beings for their use. The utilisation of these resources has increased many folds in recent times and is being done many times unsystematically/unscientifically. This over exploitation in the greed of money is creating many problems and is disturbing the environmental set up. A need is being felt very rightly to minimise this exploitation and measures should be taken to conserve these resources, which are otherwise so vital for our survival. In fact, these resources are not only for the purpose these are being used or grown, but are source for other related resource e.g. the forests/vegetal cover is needed not only for the wood/timber, for source of energy, fuel wood, buildings, furnitures and decoration, but these forests are much needed for replenishment of depleting water resources, recharging of natural springs for drinking water, to check soil erosion to protect hill slopes from slope failures, to get fresh air, the greenery for scenic interest and improve the climate that has shown signs of change recently and too many other related benefits for preserving the forest cover. Public awareness for their conservation and protection is a must. Natural disasters in the form of earthquakes, landslides, creep, cloudbursts, flash floods, cyclones etc. have become quite common and their occurrences have increased substantially in the last 10 years, though these disasters are being influenced by human activity also. Therefore, public awareness and peoples participation in the endeavour of public safety is a must. This can minimise the loss of property and mankind. Proper study of causative factors can save considerable loss. It is vital to dispel popular misconceptions about the occurrences. Their effects can be combated effectively by proper understanding and having a sound network for alarm, land use regulations, quake proof structures, change in cropping pattern, erection of walls, gulley plugging, widespread plantation, fast growing soil binding grasses and bushes etc. It has been now realised that these natural disasters are being controlled by one very significant feature which is passing through this area and that is: Aravalli Basement below Garhwal in the form of Delhi-Hardwar-Harsil Ridge (DHH).
DELHI-HARDWAR-HARSIL RIDGE (DHHR)
This particular ridge (DHHR) is an underground ridge, which is passing through and below Dehradun. Its eastern limit is Hardwar-Lachchman Jhula and western limit is Paonta Sahib-Kalsi. It is 70-75 km in width and is expected at a depth of 2 to 2.5 km below Dehradun city, which is situated on its crest. The water divide of Ganga (Song River) and Yamuna (Tons-Asan) (Plate 1a) is fair reflection of this underground ridge passing through Asaraodi, Majra, Kaonli Ballupur, Anarwala and Shahanshahi Ashram (Rajpur). Its strike of extension is NNE-SSW. Its eastern and western flanks are fault controlled which are being followed by Ganga and Yamuna rivers respectively that have cut across the Siwalik ranges to flow down to the Indian plains.
This ridge is the direct extension of Aravalli ranges which originate in Gujarat (Amba Mata) and pass through Mount Abu, Ajmer, Alwar and Jaipur in Rajasthan and continue further north via Rewari, Sohna, Faridabad-Gurgaon and in Delhi in the form of Delhi Ridge upto Wazirabad bridge on Yamuna where the last exposure of this ridge is seen on the surface. From this point. this ridge gradually descends below surface and passes through Sonepat-Bagpat-Meerut. Karnal-Saharanpur-Hardwar, Paonta-Dehradun-Lachchman Jhula, Kalsi-Mussoorie-Nagni (Chamba). Tiuni-Uttarkashi-Bhatwari, Kalpa (Kinnaur)-Baspa-Harsil and continues further north via Rutog-Gartok and Lanzou in China to meet the Pacific boundary at Okhotsk.
Delhi-Hardwar-Harsil Ridge is a major water divide of Asian continent. All major rivers on its eastern and western sides flow to the east or west respectively and do not cross it (Fig. 1) e.g. all rivers in Indian peninsula namely Godavari, Krishna and Cauvery originate in the Western Ghats and flow to the SE to drain their water into the Bay of Bengal. Chambal, Yamuna, Ganga and Brahmputra flow into the Bay of Bengal. All rivers originating in the Tibetan plateau e.g. Irawadi, Salween, Mekong, Yangtse and Hwang Ho flow all the way to the east/south east to drain their water into the Bay of Martban, Bay of Thailand, South China Sea, East China Sea, Yellow Sea, and none goes to the Arabian Sea as in case of rivers originating on its western side e.g. Sabarmati. Luni: Indus and its tributaries - Satluj, Beas. Ravee, Chenab and Jhelum. Further north, rivers in Central Asia have either inland outlets in lakes/deserts or flow into the Arctic Sea e.g. Obe. Yensee. Leena.
It is worth mentioning that on western side of this ridge are deserts e.g. Rann of Kutch. Saline Lake system of Sambhar-Jaipur, Thar Desert, cold desert of Ladakh, Takla Makan, Tarim Basin of Sinkiang, and world famous Gobi desert of Mongolia. Similarly, eastern side of this divide is occupied by world famous plateau e.g. Deccan Plateau. Malwa Plateau, Tibetan Plateau and Ordos-Sanxi Plateau. Highest points of the earth are to the east of it, e.g. Everest, Nanda Devi, Trishul and lowest point on surface is on the western side. i.e. Turfan near Lop Nor which is 145 m below mean sea level. This divide is a submarine ridge in Arabian Sea -known as Chagos-Lakshdweep ridge (CLR), Aravalli hills in Gujarat, Rajasthan and Haryana, Delhi ridge in Delhi, Delhi-Hardwar-Harsil ridge below Gangetic plains in western U.P., Western Uttaranchal and eastern Himachal Pradesh. crosses below the Himalaya to reappear on surface in the form of Astin Tagh and Qilian mountain to mark the limits of western and northern Tibetan plateau.. Further north, it continues in the form of Khingan mountain in Mongolia, China and Russia upto Okhotsk. It takes a sharp turn in Okhotsk sea and meets the Pacific boundary on eastern side of Hokaido via Sakhalin.
This ridge is quite active and is responsible for several natural hazards that have become so common. It is the movement of magma that is surging upward to produce tremendous amount of energy by way of baking, fracturing, breaking and even falling/shifting/rupturing the lower most strata/blocks of the ridge (Fig. 2). This energy is being generated regularly and in a continuous process. This energy is temporarily stored in the gap/cavity/open space below the Himalaya and tries to get released through the planes of weaknesses. In the Himalayan region, there are two primary fractures (Fig. 3) that are planes of weakness for the energy release; these are Main Boundary Thrust (MBT) and Main Central Thrust (MCT). Besides these thrusts, there are a number of faults, cracks, joints etc. that can facilitate the movement. The MBT and MCT are nearly parallel to Himalayan trend where MCT brings the Central Crystallines of Himalaya over the Lesser Himalaya and passes through Tiuni, Mori, Jamlola, Barkot, Nald, Sainj, Gorsali, Bhatwari, Mallan, Medh, Budhakedar, Ghansyali,) Chirbatia, Mayali, Chandrapuri, Ukhimath, Chopta, Mandal, Ghingran, Urgam, Helang, Nijmula, Tharali, Dewal and goes to Loharkhet, Girgaon and Dharchula in Kumaun.
The MBT occupies a place in lower altitudes and is responsible for bringing the Lesser Himalaya over the Siwalik ranges and passes through Kalsi, Langha, Dhoonga, Kiarkuli, Rajpur (Shahanshahi Ashram), Gujrada, Raipur (Kesarwala-Dwara), Bhopalpani, Thano, Bhogpur, Barkot (Chandra-Bhaga), Rishikesh, Mundal, Kimsar, Dugadda, Rathwadhab, Kumariya, Danda, Kota Dun, Baijun, Baldiyakhan, Jeolikot Chanda Devi, Amritpur, Heda Khan, Durgapipal-Danda (Nandhaur valley), Sukhi Dhang, Chalthi (Ladiya valley).
The author has very closely studied the occurrences of natural hazards in this part of the globe, especially the earthquakes, landslides and the cyclones. Their frequency and devastating power has increased many fold in recent time, and this last decade of the century in particular. The author is convinced that the Aravalli basement activity in the form of Delhi-Hardwar-Harsil ridge is primarily responsible for such hazards (Fig. 2). This is in sharp contrast to the common belief that the earthquakes are being generated due to collision of the Indo-Australian Plate and Eurasian Plate along the Himalaya. It is believed that the Indo-Australian Plate is moving in NNE direction and is underthrusting below the Eurasian Plate. The movement is to the tune of 4-5 cm per year. Several eminent workers have carried out detail geological work in Kumaun Himalaya and adjacent regions, a few to mention are Auden (1933 & 1934), Heim and Gansser (1939), Gansser (1964), Wadia (1928 & 1966), Bhattacharya and Niyogi (1971), Qureshy (1971 , Bhargava ( 1972), Saklani ( 1972), Rupke (1974), Karunakaran & Rao (1979), Raiverman (1992) etc. However, the trend of the Delhi-Hardwar-Harsil ridge is almost perpendicular to the Himalayan mountain belt. The Aravallis and the Astin Tagh (Altyn Tagh) mountains on southern side and the northern side of the Himalaya show same orographic axis and are in perfect NNE-SSW trending alignment.
This underground ridge is the prime cause of earthquakes in this area (Pal, 1993,1995, 1997; and Pal et al., 1998). This ridge is northward extension of Aravalli basement, which is extending from Amba Mata in Gujarat to Deri, Ajmer and Alwar in Rajasthan. Faridabad-Sohna in Haryana and finally at Delhi upto northern part of Delhi, this ridge is exposed on surface and becomes underground beyond Wazirabad bridge on Yamuna. It continues below the Gangetic alluvium in Sonepat, Bagpat-Meerut, Panipat-Kandhla-Muzaffar Nagar, Yamuna Nagar-Saharanpur-Roorkee and finally below the Himalaya at Dehradun, Tehri, Uttarkashi and Kinnaur and beyond into Tibet (China). This ridge is gradually sloping towards north. Therefore, its estimated depth at Meerut is hardly 1/2 km, 2 km at Hardwar and 5-6 km at Uttarkashi-Ukhimath (Fig. 2). This ridge remained positive across the Himalayan basin. Therefore, the geological continuity of various formations is not seen across this ridge (Pal et al., 2000) except the Middle and Upper Siwalik sequence when this ridge got submerged. The DHHR coincides with the trans Himalaya condutor of Arora et al. (1993), trans-asiatic lineament of Raiverman (1992) along Yamuna and Tin-Tungstan line of Misra ( 1996).
The author has observed that none of the geological formations continue across this ridge as it remained positive and therefore, the eastern side oft!te basins got limited by this divide e.g. Vindhyan system, Gondwana system and Deccan traps of peninsular and central India are confined to the east of it. In the Himalaya, two prominent sedimentary belts of Krol and Calc Zone of Tejam/Pithoragarh are confined to the eastern side while the western side of the Himalaya does not have the Lesser Himalaya, which are otherwise so broad and prominent in Uttaranchal and Nepal (Fig. 4). On the contrary, the sub-Himalaya/outer Himalaya are too broad and well developed in Jammu and Kashmir and Himachal in the form of Murrees/ Dharamsalas, Subathus, Dagshais and Kasaulis. These get limited to a very narrow strip in south eastern Himachal and finally pinch out to the east of Nahan near Paonta-Kalsi (Tons valley). As such, Lower Siwaliks are also missing completely in the Uttaranchal. The crystalline nappes/thrusts e.g. Almora Nappe, Baijnath Nappe, Askot Nappe. Lansdowne Klippe, Almora Thrust etc. are so well defined in eastern side and these are totally absent in
western side. Further, the Tethyan sequences of Dhauli-Girthi Ganga get closed to the west of Gangotri and do not continue in Sangla-Chitkul-Baspa valley. Instead of it, much younger granite which is transverse to the Himalayan trend in Khab-Leopargji-Shipki La-Gartok restricts the Tethyan basins of Zanskar, Spiti and Indus to the west.
It is quite interesting to note that Main Boundary Fault/Thrust and Main Central Thrust/ Fault are also not continuing across this ridge. The MBT of Himachal becomes Krol Thrust in Uttaranchal, as there are no older Tertiaries on eastern side. Similarly the MCT of Uttaranchal is not present in J&K/westem Himachal as the Lesser Himalaya is totally absent and instead the Panjal volcanics/Panjal traps are thrusted along with the Tethyan sequence.
Similarly geomorphic and geophysical contrasts are quite obvious on eastern and western side of this feature i.e. difference in the geology of Garhwal and Himachal (Pande, 1991, p.95). He firmly negates the westward physical extension of North Almora Thrust (Pande. 1991. p. 94 ). The author opined that the earthquakes in this area are due to presence of magma at a depth of 10-11 km. and it is the magmatic surge upward which is producing tremendous energy and its periodic release is causing frequent tremors and occasional devastating earthquakes. Total thickness of Aravalli Basement and the Himalaya comes out to be 11 km only. That means the magmatic surge is precisely below the Aravalli Basement where the Main Central Thrust is passing (Fig. 1). The stored energy tries to get released through these fractures of prime importance viz. MCT and MBT and the waves get ideal medium to travel through the DHH to Delhi. Therefore maximum damage or impact of shocks is felt in the vicinity of MCT (Uttarkashi-Ukhimath-Gopeshwar-Chamoli) and MBT (Dehradun-Rishikesh-Nilkanth-Dugadda) and the areas that are located directly above the DHH-Meerut, Bagpat, Sonepat upto Delhi. It is from the experience of Uttarkashi quake of 20th October '91 that maximum impact was felt on the DHH upto Delhi and even in north eastem part of Rajasthan (Pal et al.. 1998. p. 296). Similarly in the Chamoli quake of 1999, Shanker and Narula ( 1998) have shown isoseismal map where isoseist show VI intensity zone of MSK scale at Delhi i.e. same as in Garhwal area. This southerly bulging of isoseist is following definite trend of DHH i.e. from Faridabad (Haryana) to Shipki La (Kinnaur border area) in NNE-SSW direction. Higher intensity zone at Delhi is attributed to the fact that the underground ridge appears on the surface when the waves travel from NNE (11-12 km depth) to SSW (ground level).
Mandal et al. (1999) and Venna & Suresh (1999) reported that a number of aftershocks were located at a depth of 4 to 6 km. As per autl.'or's view, main shocks come from a depth of 11-12 km. This is quite natural and can be easily explained due to the downward movement of Aravallis (DHH) leading to creation of open vacant space below the Himalayan sequence. The energy is partially released during tremors and it quite often gets trapped below the Himalaya and above the Aravallis, which gets released in the form of shocks from a much less depth i.e. 4-6 km. Its release is at frequent interval and the magnitudes of these shocks are also less compared to the energy release of quakes from 11-12 km depth. The DHH passes through Hardwar-Rishikesh-Lachchman Jhula-Kunjapuri, Nagni, Chamba, Sirain (Tehri), Lambgaon, Mallan. Bhatwari, Gangnani and Harsil (eastern flank of the DHH) while the western flank passes through Paonta, Kalsi, Kuwa. Tiuni, Kalpa, and Khab in H.P.
EARTHQUAKES AND ENHANCED FREQUENCY OF TREMORS
The Main Central Thrust/Fault and Main Boundary Fault/Thrust have become quite active. Their reactivation is linked to underground seismic activity that has increased now. The epicentres of the Uttarkashi earthquake (6.4 magnitude) of 20th October '9l and Chamoli-Rudraprayag earthquake (6.6 Magnitude) of 29th March '99, precisely fall in the DHH and MCT intersection zone, which is responsible for these earthquakes. Both the earthquakes have a plenty of things in common. Their causative factors and affected areas are almost overlapping. Both the quakes got generated at a depth of 12 km, which is quite shallow, and both were felt southward in the areas falling on the DHH as far as Delhi and Alwar quite severely. The quake was felt more severely in Delhi and quite a few buildings developed cracks. Its destructive effects are noticeable in Dehradun, Hardwar, Meerut, Noida and Delhi. These areas lie above the DHH. The rumblings of the quake waves were clearly heard by people in these areas. Even the aftershocks of these quakes were felt southward more, as compared to eastern and western sides of the epicentres. All these observations are quite pertinent and very well support the hypothesis of DHH as a controlling factor. These quakes show quite similar features and can be linked to one single factor of origin i.e. DHH instead of prevailing idea of plate collision in this area (Indo-Australian plate underthrusting below the Eurasian plate along the Indus Tsangpo Suture Zone).
Chamoli earthquake struck the Garhwal Himalaya in the wee hours of 29th March '99, inflicting injuries to hundreds of persons, killing more than one hundred persons and devastating a large area in Garhwal and its shocks were severely felt southward upto Faridabad and Alwar beyond Delhi. It did considerably damage a large number of structures in and around Delhi besides western Uttar Pradesh and Haryana.
Prelude of Chamoli quake
There were two noticeable events before the quake in this area, which signalled the forecoming hazards. They are subsidence and large scale erosion in Mandakini valley and substantial number of tremors in Sonepat-Bagpat-Meerut region within 40 days.
Subsidences in Mandakini Valley
The problem of landslide, subsidence and erosion are quite common in the hilly region due to combination of several factors like geological movements, structure, lithology, water seepage, soil cover, vegetal cover; weather and climatic changes. In this decade, cases of large scale subsidences on hilltops, hill slopes, river banks and settlements have increased alarmingly. In the 2nd week of August '98, precisely between 11 and 19 August, problem of subsidences, slides and severe erosion played havoc along the Madhyamaheshwar valley, engulfing 34 villages and converting 25 sq. km fertile land into waste land, bringing boulders and cobbles over the fields and blocking the Madhyamaheshwar river. This was supposed to be due to cloudburst and torrential rains that lashed this valley from 11 to 19 August '98. This led to over saturation of topsoil and superficial unconsolidated material. This area has been experiencing frequent tremors in the near past and the epicentres of Uttarkashi quake of 1991 and Chamoli quake '99 are located in adjacent areas. Many epicentres are located within the Mandakini valley and the M.C. T. is passing through this area. These tremors have made this area very fragile due to shearing/shattering. The slopes have become steeper due to later modifications. It is derived here that fissuring and shearing has given rise to severe erosion problem in rainy season. This erosion is so severe along the right bank of Madhyamaheshwar that it has threatened the existence of several villages on hill slopes The hill slopes are quite steep and given to rise gullying and deep rills, that changes their courses quite frequently and cut deeply on the slopes (Plate 2).
The left bank of Mandakini is very densely covered by a very thick forest. Still during the heavy downpour, the hill subsided along already existing fissures, cracks along with the forest (Plate 3). This was a case of huge subsidence and slided finally of a 600 m x 500 m hill mass that buried 3 villages viz Koti, Bheti and Paundar. This slided mass came down in a few seconds with a thud and blocked the Madhyamaheshwar, which was already overflowing its banks, flooding the area. This blockade contined for more than a day and a huge lake was created along Madhyamaheshwar river upstream of the buried village Paundar. The discharge downstream of Paundar decreased drastically due to obstruction/damming in the course of flow. Since this blockade was created by the slided debris or loose rock mass, on saturation it gave way to seepage and finally flows of water. The river valley was filled up to its capacity, 250 to 400 m deep, and finally its over flow carved out a channel by eroding the debris (Plate 40). This had threatened the population downstream as the burst of this lake would have led to flash floods and rising of river water all along its course from Ukhimath to Deoprayag.
The problem of Mandakini valley is directly linked to frequent occurrences of earthquakes in this area and neotectonic activity in the form of downward gradual movement of Delhi-Hardwar-Harsil ridge (DHH) -Aravalli Basement. This made the slopes vulnerable to failure and severe erosion. Secondly the downward movement of DHH gives rise to creation of open space between the Aravalli and Himalaya. The sinking and adjustment of base finally leads to cave in subsidences and slumping of hills. This happens even if the hills are clad with thick forest. Madhyamaheswar valley is one of the best preserved forest ( Kedarnath Musk Deer Sanctuary) and it still experienced large scale erosion and slumping, The subsidences are noticeable in first phase by cracks in houses. fissures in ground and wide cracks in open areas. Tremors in Sonepat-Bagpat Meerut region: Prior to Chamoli earthquake of 29th March '99. several foreshocks of this quake were recorded and felt in the areas falling on the Delhi-Hardwar-Harsil ridge (DHH) within 40 days preceding the main event (Fig. 5). Out of these 7 were in the Gangetic plains in Bagpat-Meerut of western Uttar Pradesh and adjacent Sonepat (Haryana) and Delhi. These are shown in the Table I.
It is worth mentioning that no tremors were felt in Garhwal before the main event and the activity was confined to western U.P. and Haryana. In fact, the epicentres were changed from one flank of the DHH (Sonepat) to the other flank of DHH (Meerut) (fig. I ). This is believed that the stored energy might have migrated below DHH to the Garhwal Himalaya that finally gave rise to the main shock of Chamoli on 29th March '99 at 0.35 hrs. (I.S.T.).
There are three distinct domains separated by two major thrusts/faults known as Main Central Thrust/Fault and Main Boundary Fault/Thrust. Two sedimental belts are well developed between these two faults, these are Krol belt in south (to the north of MBF) and the calc zone of Tejam and Pithoragarh in the north (to the south of MCT (F).
_____________Main Central Thrust (F)____________
Calc zone of Tejam/Pithoragarh
Almora-Dudatoli Crystalline Nappe
____________Main Boundary Fault (T)____________
______________Main Frontal Fault (T)______________
The Garhwal Group of rocks are parautochthonous, representing a thick, broad and oldest sedimentary belt of quartzites and limestones between the MCT(F) and the North Almora Thrust (NAT).
Patroli Formation Quartzites
Pipalkoti Formation Dolomites and Phyllites
Garhwal Group Chamoli Formation Metavolcanics, Quartzites
Lameri Formation Metavolcanic, Quartzites
Rudraprayag Formation Metavolcanics, Slates
and Schistose Quartzites
_______________North Almora Thrust ________________
Almora Dudatoli Crystallines
North Almora and South Almora Thrusts constitute the two flanks or base of the Almora- Dudatoli Thrust sheet close in a semicircular pattern near Khirsu and therefore, the exposures of Almora Crystallines are restricted to the east of Khirsu (Pauri) only. The Inner Sedimentary Belt comprising Garhwal Group of rocks lies to the north of North Almora Thrust in the form of the Calc zone of Pithoragarh and Calc zone of Tejam. These two calc zones are also separated by the Askot-Baijnath Crystallines in-between. The Askot-Baijnath-Nandprayag Crystallines are much less in comparison to Almora Crystallines in their extent.
Out of these two Calc zones, the southern one (Fig. 4) stretches from Jhulaghat to Pithoragarh-Gangolihat-Someshwar-Chaukhutja-Lameri and Deoban in Chakrata area. This belt closes in Tons valley and does not continue further west in Himachal Pradesh. However, the Calc zone of Tejam occupying northern part upto the Main Central Thrust (Fault) continues from Tejam to Thal-Kapkot-Dhar-Birehi valley and Pipalkoti to finally get closed to the west of Pipalkoti (east of Gopeshwar) due to fault controlled or final basinal periphery (Fig. 6) and does not continue further west in contrast to the southern belt of Calc zone of Pithoragarh.
The oldest Formation of Garhwal Group is Rudraprayag Formation, which comprises alternate sequences of quartzites and metavolcanics. The quartzites are generally fine to medium grained with ripple marks and slaty horizons in between. The quartzites are quite often thickly bedded and become gritty in upper parts. Lameri Formation forms a part of Calc zone of Pithoragarh, and is represented by stromatolitic limestones, pyritic phyllites and slates in between. It has restricted exposures in Alaknanda valley near Lameri.
Most dominant sequence of this area is alternate bedding of quartzites and penecontemporaneous lava flows around Nagnath. Pokhri and Karanprayag. The quartzites are quite thickly bedded, fine to very coarse grained, sometimes pebbly. These are dirty white to purple pinkish in colour. These quartzites show plenty of sedimentary structure like cross bedding, current bedding, graded bedding, ripple marks etc. The basic flows are prominent. encountered often. These are dark grey to green in colour, show vesicles and amygdaloids. Due to metamorphism, these are altered to chlorite phyllites. Quite often, dark bands in these rocks are prominently seen due to presence of iron minerals or tourmaline, ilmenite etc. In upper parts, the volcanic bands are metamorphosed to the extent that these are easily mistaken to be metamorphic actinolite-biotite schists. In the upper most horizons, two bands of calcareous slates and marbles are observed at Deothan near Nagnath. Another basic flow is seen near Dungri in Kyunja gad where ophiolite tourmaline veins of varying sizes are seen.
Another important formation of Garhwal Group is Pipalkoti Formation. which is represented by marbles, limestones and calc silicate rocks, which are quite extensively developed in Birehi catchment and between Pipalkoti and Helang in Alaknanda valley. These are quite rich in magnesite, which appear to be synsedimentary deposits. The limestones are algal and stromatolitic in nature. The limestones are overlain by quartzites and their exposures are seen to the south of MCT(F), over the limestones near Helang in a linear NNW-SSE trending belt. The quartzite is generally quite hard. Since it is also metamorphosed. the excess of chlorite and sericite makes it less resistant.
The Garhwal Group rocks are also intruded by two types of granites viz. biotite-granite and tourmaline granite. Dolerite dykes are also seen. The epidiorites are greenish grey. medium grained. weekly foliatet~d show spheroidal weathering.
Kumar and Agarwal ( 1975) divided the Central Crystallines into Tungnath and Pandukeshwar Formations. Tungnath Fonnation shows lower grade of metamorphism and is directly in contact with the rocks of Garhwal Group along the MCT(F). Ragsi schists and gneisses form the base of Tungnath Formation near K'!fsir. It is profusely intruded by tourmaline gral1ite '4round Ragsi. These are generally sericite schists with silvery appearance and easily eroded. These are followed by a quartzitic horizon and finally overlain by Chandersila schist. These are followed by Pandukeshwar Formation comprising augen gneisses and high grade metamorphics.
Main Central Thrust (Fault)
Main Central Thrust (MCT) was defined by Heim and Gansser (1939) in Kumaun Himalaya as a thrust plane, which has brought older Central Crystallines of Great Himalaya over the younger metasedimentary belt of Lesser Himalaya, which has become the boundary between the Lesser Himalaya and the Higher Himalaya (Great Himalaya). It is believed that the Central Crystallines and the crystalline nappes of Askot-Baijnath and Almora-Dudatoli were transported southward along this gently dipping thrust plane. The author believes that the MCT is a high angled deep seated fault, which dips quite steeply (65°-85°) north. This fault plane is the original basinal limit of the sea (Fig. 7) and the Central Crystallines formed the boundary of the basement in the northern side. whereas and in the south existed the Birehi-Tejam sea which was closed on the western side by the Aravalli ridge (Delhi-Hadwar-Harsil ridge) and in the east continued to Nepal at least upto Kali Gandaki. It implies that the Crystallines and the DHH formed positive landforms and gave rise to a closed basin of Birehi-Tejam. Further, the MCT is a deep fracture along which repeated intrusions of granites and dolerites have come.
Geographically, the MCT(F) is not a straight line but is zig-zag and runs from Tiuni (Tons valley) to Jauljibi in Kali valley via Wazri (Yamuna valley). Uttron-Nald (Assi Ganga valley), Gorsali-Sainj-Mallan (Bhagirathi valley), Chhatera-Budhakedar (Balganga valley), Kanda-Mahar-Thela-Chirpatiya Khal (along Nailchami). Thap1a (Helaun-Lastar gad), Chandrapuri-Parkandi- Ukhimath (Mandakini valley) (Plate 5). Makku (Kakra gad), Kalsir (Nagol gad), Mandal-Siroli (Amrit ganga), Ghingran-Helang (Alaknanda valley), Karamnasa gad and Tharali in Pindarvalley. This fault takes almost a N-S trend along the left bank of Mandakini from Tilwara to Ukhimath and turns eastward to pass through Kalsir and Mandal and again changes its trend towards NE upto Helang and then swiftly turns to its general trend ofNW-SE along Karamnasa gad to Pindar valley.
Chatwapipal-Sankri-Amrit Ganga Fault: One of the most important transverse cross fault is Chatwapipal-Sankri-Amrit Ganga Fault. This fault is NNE-SSW trending and is controlled by the basement. In the southern part of this area it has offset/displaced the course of Alaknanda at Chatwapipal between Karanprayag and Gauchar. Alaknanda flows to North for 2-3 km from Chatwapipal. Banigoan gad follows this fault and similarly Patroli gad takes a straight course to have confluence with Nagol gad at Sankri. Amrit ganga flows along this fault to have its confluence with Balasuti near Mandal. Thus at Ansuya Devi in the north to Chatwapipal, this fault is followed by straight courses of channels and it has displaced all the geological formations along with the Alaknanda Fault. It is a very prominent feature of this area.
Alaknanda Fault: This fault was identified by Kumar and Agarwal (1975), running in E-W direction on the right bank of Alaknanda between Tilwara and Langasu via Satera, Nagrasu, Gauchar, Ranu, Kulara. The trend of Lastar gad-Bhandari gad, Mandakini Fault are N-S trending faults while Chamethi Fault and Nandakini Fault are WNW ESE trending faults to follow the strike of the beds.
The tear faults (N-S or NNE-SSW trending faults) are directly linked to the Aravalli Basement (DHH) and are potential escape routes of energy release while the E-W or ESE-WNW faults/cracks were generated by the quake activity in the past. The Garhwal Himalaya has a long history of earthquakes. Most severe one was in the month of May 1803 when its three-fourth population vanished due to landslides and extensive damage to human settlements.
Instability of hill slopes
The epicenter of the Chamoli quake (main event 6.8 magnitude on open ended Richter scale) and the epicentres of the aftershocks precisely lie in the vicinity of intersection of the MCT(F) and Chatwapipal-Sankri-Amrit Ganga Fault (basinal limit of Tejam-Birehi sea) i.e. Rudranath hill, NW of Gopeshwar or eastern water divide of Madhyamaheshwar (Mandakini) river. This implies that the origin of the quake is closely linked to the seismically active DHH and the MCT. The quake is quite similar to Uttarkashi quake 6.6 magnitude on Richter scale ( of 20th October 1991) in genesis, magnitude, geological and geomorphological setting, along with the control of basement ridge (magmatic surge below the ridge). Even the affected areas of these quakes are by and large overlapping. Isoseismals show maximum
VIII zone on MSK scale in the vicinity of Chamoli-Pipalkoti, Kyunjha gad while VII zone are isolated pockets representing damage in Hafla-Pokhri, Parkandi-Makku, Mahar- Chirpatiya Khal-Mayali. Zone VI engulfs a much larger tract of hilly region encompassing entire Garhwal and quite large areas of NE Delhi. It is worth mentioning that some areas in between have partially escaped the fury of the quake. The damage is confined to cracks in walls and houses in zone i.e. Gangetic plains and adjacent areas of Haryana, Rajasthan and Himachal Pradesh. Ground damages were more severe on boulders, terrace deposits or loosely dumped material compared to slopes on hard rocks in hilly region.
Two most prominent crack/fissure patterns have appeared on the ground, (a) NNW-SSE trending as seen near Makku-Hafla and Gairpoonga (Plate 6). In this case, fissures are seen upto 400-500 m length, 30-40 cm width and more than 10 m depth. South western block has moved 20-30 cm downward compared to the north eastern block as seen near Garpoonga; (b) Other prominent fissures are almost E-W trending. One such fissure is in SW of Mayali alongthe left bank of Helaun Gad near Indira Nagar (Makhait village). Here also the southern block has moved down. A fissure of 60 m length. north of Sari has developed parallel to the MCF.
Besides fissures, this quake also triggered a large number of new landslides and reactivated many earlier slides. The new slides are (a) 1.5 km NW of Gopeshwar, near to the epicentral part and the MCF: (b) Gairpoonga-Kyunjha gad near the N-S trending fault (Plate 7). It has highly sheared and shattered the hillock-quartzites and opened their joints making it more fragile: (c) new slides at 1.5 km SW of Karanprayag in the vicinity of Alaknanda Fault. It is continuing nonstop. Another nonstop slide which is continuing without stoppage developed and got accelerated near Banswara i.e. along the left bank of Mandakini and has adversely affected the clearance of road to Kedarnath shrine and Ukhimath. The slides will bring miseries to travellers, pilgrims and local villagers during rains, when the percolation of water will make the planes mobile and will act as lubricant on building up the pore pressure of the unconsolidated material dumped on slopes.
It is also interesting to note that a well marked low pressure area (atmospheric) was observed over Chamoli and Latur on 17th and 25th March - the days of foreshocks - and just before the main event of Chamoli quake and persisted for next 2-3 days too. Quite heavy rains followed the main event within next 3 days over the entire DHH (Chamoli-Dehradun-Meerut). The winter rains and snow had eluded this region for the last 6 months (precisely since 2nd week of October '98), which are otherwise lashed by frequent winter rains in lower areas and heavy snowfall in higher reaches. The development of low pressure area can help in forecasting of the hazards and mitigation of after effects.
The Chamoli quake has made a huge area. quite vulnerable to slope failures due to shearing and fracturing, which is already prone to subsidence and slides. A few pockets that were very badly affected by the quake are quite close to MCT (under thrust sheet) and the active faults. These pockets are Mahar-Chirpatiya, Khal-Mayali, Kyunjha gad. Parkandi-Makku, Hafla-Pokhri. Chamoli-Pipalkoti etc. Some areas have partly escaped the fury of the quake.
Several cracks, ground fissures and active landslides got developed due to fracturing and shaking viz. Garpoonga, Parkandi, Mayali (Indra Nagar), Karanprayag, Gopeshwar-Mandal, Damar, Hafla-Pokhri, Gohna-Nizmula, Pipalkoti, Sari-Deoriya Tal etc. Banswara slide on left bank of Mandakini, landslides 1.5 km to the NW of Gopeshwar, landslides near Garpoonga-Akhori and slides near Karnaprayag are almost continuing non stop.
LANDSLIDES AND RELATED HAZARDS
Another important natural hazard is landslide. It is very common in the hilly region and is the movement of slope down the hill. It is sometimes quite destructive as it moves the soil, its vegetal cover/crops, houses, settlements down and buries the fertile land below, threatens the existence of villages down slope, blockades of channels/rivers and creates chances of flash floods owing to bursting of temporary lakes that are formed upstream of these blockades. The artificial damming is facilitated by uprooted trees, trunks. branches and rock debris filling the gaps and provides hurdle for flow of water downstream. Excessive rainfall, building up of pore pressure, over saturation of moist rock and soil, weaker lithology, presence of thrust or fault planes, toe cutting by rivers, lack of vegetal cover and soil binding species, steep slopes, too much irrigated water through guls (unlined canals) on high slopes. excessive excavation or building structures, road cuttings and vibrations cause the landslides. These are quite common in a narrow belt along MBT, MCT and the DHH fringes. Along the thrust zones, the areas 1 km south and 4 km northwards are more vulnerable to slides due to crushing along the thrust planes (Fig. 8).
Along the MBT(F), majority of the slides were observed in the sedimentary rocks of Krol belt of Lesser Himalaya and are located in a linear belt on the southern aspect of the hill slopes. These slides are along the structurally controlled streams e.g. Dwara slides along the left bank of Song River in Dehradun district (Plate 1b). Along the MCT(F), the slides are generally observed in the crystalline rocks of Central Crystallines of the Higher Himalaya and are located in a linear belt on the southern aspect of the hill slopes. These slides are facilitated to a great extent by percolation of rain water, frost action or snow melt in highly crushed/sheared schistose rocks e.g. Karamnasa gad in Chamoli (Plate 1c) and Barasu-Chandrapuri in Rudraprayag along Mandakini.
In contrast to the slides along the MET and MCT, the slides along the DHH fringes are on the eastern/western side of hill slope as the DHH trends in NNE-SSW direction e.g. Mansa Devi slides in Hardwar facing eastward, Sirai slides in Tehri along Bhagirathi facing east and Bhatwari-Gangnani slides in Uttarkashi towards east/west along the banks of the Bhagirathi river.
These slides block the road, communication and pose danger to the settlements while slow movements or creep that are seen in soil, that are quite thin (hardly 20 cm thick) on the hill slope. Excess-water percolation, grazing or cattle movements lead to creep move faster than normal. These slides in the hills are linked to subsidence that is so prominent everywhere and it is not the uplift of the Himalaya, the cause as has been thought by so many learned workers. The subsidence is due to the descending movement of DHH in this part. Therefore, maximum number of slides/creeps is seen here as compared to the other parts of this young mountain belt. Slow and steady subsidence of hill slopes in the topsoil has been initiated by the perma-frost conditions during the Quaternary period. After the deglaciation. a plane of separation was formed between the rock and the topsoil horizons due to the seepage of melt water. while it escaped down hill. This plane became mobile due to rain, frost action and anthropogenic activity, especially digging. ploughing on dry hill slopes and sometimes due to over irrigation in agricultural fields. The creep is more than 1 cm in a large number of cases and the height of terracettes is getting increased giving rise to the degradation of the agricultural land.
The landslides in Uttaranchal State are along:
MCT: Arakot, Mori. Khanera, Wazri - Barkot -Uttron- Sangamchatti -Nald-Sainj - Bhat\vari - Medh- Budhakedar-Ghansyali -Thela -Chirbatiya -Mayali -A.gastyamuni -Chandrapuri -Ukhimath-Barasu -Chopta -Mandal- Gopeshwar -Ghingran- Urgam-Helang-Karamnasa -Gohna -Narayan Bagar- Tharali etc.
MBT: Kalsi -Langha -Dhunga -Kyarkuli -Nalota- Kalagarh- Rajpur -Raipur (Dwara) - Thano- Bhogpur (Bidhalna) -Muni Ki Reti- Binj -Bidasni -Mundal- Lal Dhang- Dugadda -Rathwadhab -Marchula -Kumariya -Kota Dun -Baijun -Khurpa Tal- Nihal -Jeolikot - Dogaon- Chanda Devi -Amritpur -Hedakhan -Jamrani -Durgapipal -Danda -Bastiya - Sukhi Dhang -Chalthi.
DHH: Mansa Devi. Nilkanth, Nir Gad. Narendra Nagar. Kunjapuri. Phakot. Nagni. Jakh. Sirai, Lambgaon, Saur, Chowrangi Khal. Malan. Bhatwari. Gangnani and DM slides upto Harsil.
There are several tear faults parallel to the trend of the DHH, which are causing problems of mass movement. Quite a few slides are controlled by lithology, structural aspects and quite a large number are due to unscientific development activity. The landslides/slope failures can be minimised by taking suitable remedial measures and the treatments have to be location specific. Each slide deserves proper attention and care. The slides can be checked by erecting check dams, retaining walls, breast walls, toe support, gully plugging, gabion check dams, catch pits, silt traps, lined channels. contouring. bunding. breaking of slopes, afforestation of soil binding plantss and grasses, properly suited land use and cropping pattern, proper drainage etc.
CLOUDBURST, LIGHTNING AND FLASH FLOODS
Another major hazard is the cloudburst and the flash floods. This is mainly confined to narrow valleys, higher reaches in the catchment, where the configuration of the valley does not allow the clouds to move outside. Whatever cloud enters the valley gets staked, piled up one layer above the other. The density becomes higher, lightning and thundering get increased manifold, if the relative humidity reaches higher percentage, the entire cloud leaves its total water in the form of very heavy rainfall/down pour in a few minutes/hours, say 25 cm to 35 cm in 10 hours, the channel and valley do not have enough width or course to facilitate flow of gushing water. The water level reaches very high level. Thundering/lightning leads to slope failures along with uprooting of trees, tumbling of huge rock masses and boulders, slope/soil slips that give rise to formation of temporary lakes. Gushing down of muddy water pouring into the lakes fills it upto capacity and the artificial lake cannot retain it and the final burst of these lakes leads to flash floods in downstream areas. The flash floods play havoc, devastating fertile land, converting it into waste bouldery beds, severe bank erosion, threat to settlements and towns, bridges, roads etc. This total phenomenon is known as cloud bursts and flash floods. Each year. during monsoon one or two incidents of cloud burst and flash floods are occurring e.g. Belakuchi in Alaknanda valley (1970), Jalkur (1978). Pilldar (1986), Nilkanth (1990), Mansa Devi-Hardwar (1991 ). Gadni (1992), Mori-Arakot- Tiuni & Dewal (1993 ). Chirgaoll (1997), Budhakedar, Govilldghat-Alaknanda (1998). Khab- Kinnaur in Satluj valley (2000).
CYCLONES AND STORMS
Cyclones are another devastating natural disasters that are so common in the coastal areas of our country. These are more common on the east coast in Bay of Bengal than the west coast in Arabian sea. The cyclones bring very heavy rainfall and very high speed winds in coastal areas, submerge a large tract of agricultural land, disturb communication network, road, rail and uproot large number of trees and cause destruction of a large number of houses in that area. The cyclones are generated after the sea water temperature becomes 26°C and wind starts coiling in a spiral cylindrical form. Initially a low pressure area is created in the sea, which gets intensified into a deep depression cyclone and finally very severe cyclone. Air pressure becomes quite low in the centre i.e. 996 h pa. The wind speed may be 120-200 km anticlockwise, but the movement of cyclone on its track is not that fast. It is 50-60 km/hour or slightly more. That is how its advance can be tracked by satellite and the areas facing the fury can be partially saved by shifting people to safer places and bringing the traffic and rails to halt. The track of the cyclone e.g. 19.5.97, is normally along the 90° E Ridge in Bay of Bengal and it moves to Bangladesh, Tripura, Mizoram, Manipur, Nagaland and Arunanchal Pradesh before these go to China. Sometimes, these are deflected e.g. super cyclone of Orissa on 29.10.99, towards Andhra Pradesh and Orissa on 10° channel (South Andamans)- Machchlipatnam (Kakinada)-Latur- Cambay axis (well preferred alignment) and bring miseries to the people of Gopalpur, Visakhapatnam, Godavari, Prakasham. Nellore districts. The cyclones that move along west coast in Arabian Sea strike Gujarat-Saurastra coast e.g. 9.6.1998 cyclone with wind speed of 150-200 km. heavy rainfall and move north, north eastward along the DHH-Aravalli in south Rajasthan (Jallore, Sirohi, Bam1er, Pali, Jodhpur) and sometimes reach Delhi with high speed winds (Figs. 9 and 10: The Hindu, 1999). Their impact in the hills of Uttaranchal are felt in the form of low pressure areas and heavy rains accompanied by storms.
The cloud movement along DHH gives very heavy rainfall in Dehradun and Garhwal. This indirectly leads to drought conditions in Central-Peninsular India, parts of Vidarbha, Orissa. Marathwara and NE Kamataka.
It has been observed in all the cases of earthquakes that intense atmospheric low pressure area is created which brings immediate rainfall (Fig. 11), sometimes unseasonal rains, within 36-48 hours after the earthquake has struck. This adversely affects the rescue and relief works. Not only it disrupts the vehicular traffic, already badly affected due to bridges collapse, damage to roads and communication, it brings miseries to shelterless people and creates hindrances in the extrication of bodies, shifting of injured people and removal of debris by heavy machinery. In cold areas it is the snowfall in place of rainfall. It is to be understood what brings the rains-release of gases, after or during the earthquakes or some other cosmic phenomenon. Radon and Helium most commonly emitted gases are inert gases. If this phenomenon is clearly understood. it can help in prediction of the quakes - a much awaited answer to long due forecast.
It is true that natural disasters cannot be prevented from occurring but the losses these inflict on the human beings in the form of life and property can be minimised many fold, by way of educating people, people awareness, physical and mental encouragement, adjustment, adoption of new technology for resistant designs that can withstand the hazard or shifting from vulnerable or disaster prone areas. Proper recording of events, their tracking. chances of happenings should be scientifically manned and the latest technology of satellite and remote sensing should be extensively utilized. The information gathered should be disseminated to the concerned users at the earliest by way of establishing a network. This will certainly mitigate the suffering of the affected/likely affected people.
The author is thankful to the Director, Wadia Institute of Himalayan Geology, Dehradun for the permission of publication of this conceptual article and facilities extended during the course of studies. The author expresses his heartfelt thankfulness to his NRDMS colleagues: Dr. Bhagwat Sharma, Dr. A.K.L. Asthana, Dr. M.P.S. Bisht and Dr. D.S. Bagri (Tehri Campus. HNB Garhwal University) for company in the field and discussions held at the Wadia Institute, and to Smt. Suman Nanda for typing the manuscript.
N.B. List of references available if required