Read at National Seminar on
GEODYNAMICS AND ENVIRONMENT
MANAGEMENT OF HIMALAYA
Editors
H.C. Nainwal
C. Prasad
Department of Geology
H.N.B. University
Srinagar Garhwal
Uttranchal
India
2002
EXTENSION OF ARAVALLI BASEMENT BELOW GARHWAL
HIMALAYA AND ITS GEOLOGICAL CONTROL OVER THE OCCURRENCES OF NATURAL HAZARDS IN
UTTARANCHAL STATE
DEVENDRA PAL
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.).
Geological setting
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).
Central
Crystallines
_____________Main
Central Thrust (F)____________
Calc zone of
Tejam/Pithoragarh
Almora-Dudatoli
Crystalline Nappe
Krol Belt
____________Main
Boundary Fault (T)____________
Siwalik Belt
______________Main
Frontal Fault (T)______________
Gangetic Alluvium
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).
Central
Crystallines
___________________MCT
(F)____________________
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.
ACKNOWLEDGEMENTS
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