Chapter- 4

GEOLOGY AND LANDSLIDES

66

Geology and landslides are mutually related to each other. To assess the degree of

their interrelationship, one has to have an acquaintance with the geological framework of

the concerned area. This following is an endeavour to this effect.

The Darjeeling hills constitute a part of the Alpine Himalayan Orgenic Belt (Gansser,

1964159). It consists of the rocks of all four genetic groups, belonging to various ages, from

the Pre-Cambrian to the late Tertiary times (Wadia, 1938376) (Fig. 4.1) (Table 4.1 ). It repre­

sents one of the most complicated and varied geological structures of the world with intri­

cate folds, faults and thrusts (Auden, 1935", Heim and Gansser 1939193, Krishnan, 1956234

).

The study area, moreover, portrays a classic example of inverted metamorphism. From the

bottom of the Tista valley to the heights of the Darjeeling hills, one can find a gradual

change of rocks from clay slates to granite gneiss (Ray 194 7302). The region has been warped

and disturbed by geo-tectonic movements, bringing about complications in the geological

set up, form of fluvial adjustment and slope stabilisation processes. An analysis to this ef­

fect is given as follows :

4.1. Lithological Characteristics and Landslides

Lithological characteristics directly influence the stability of rocks and the occurrence

of landslides. Rocks belonging to various geological formations in the Darjeeling Hills have

been studied as follows, to determine their degree of relationship with landslides (Fig.4.1 ).

Pre-Cambrian : Darjeeling Gneiss & Dalings

The Pre-Cambrians occur in the northern part of the study area (Table 4.1 ). These are

represented by the Darjeeling gneisses and unfossiliferous Dalings which were thrown to-

66 A

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67

wards the south in a gigantic recumbent fold in the Pliocene- Pleistocene times during the

4th phase of the Himalayan orogenic movement of the post Miocene periods (De, 1980 108).

The Darjeeling gneiss, sensu stricto, is migmatic in character. It contains isolated len­

ticular and twisted, fine grained, concentric calc-silicate eye rocks. It occurs in the western

part of the area under study. The pre-Tertiary augen gneiss, belonging to the north-eastern

part of the study area, is intensely deformed, sheared, and lineated synchronously, with north­

erly directed metasedimentary sequence. There are also high grade gneisses and granites

intervened by the Rangli schist formation (Varma, 1972363, Sinha et al., 1975327

, Chatterjee,

1981 79). The Darjeeling gneiss is highly foliated. The foliation, in general, ranges from 30°-

500, while within the confines of Darjeeling town, it is oriented due east, at an amount of

20°-40° (Dutta, 1951 141).

The Darjeeling gneiss has been found to be exposed along the Ghum-Maneybhanjang

Road, Takvar Tea Garden as well as in the neighbourhood of Sidrapong and Singtham.

The Dalings are schistose in character. These are comprised of phyllites including

sericite and chlorite-phyllites, slates, flagstones, quartzites, greywackes, conglomerates, silt­

stone, and schists, such as biotite, garnet and muscovite-schists. These pass upwards into

silvery mica-schist at the contact with the overlying Darjeeling gneiss (Ghosh 1950164, Gansser

1964159, Sinha 1972330

, Sinha et al. 1975328). The Dalings, by their disposition, superimpose

the Gondwanas. Wadia (1938376) suggested that this overlying character of the Dalings may

represent an intensely folded isoclinal structure or a nappe.

In terms of its geographical spread, the Dalings have a convex, horse-shoe layout

and delimit the western and southern frontier of the Kanchenjunga gneiss, Paro graphite

schist, and Darjeeling gneiss. The Dalings broadly make up the left and the right bank

68

Table 4.1. Stratigraphic Sequence of Darjeeling Himalaya

Geographical Region

The Southern Plains and The Terai (65-300 m)

The Sub­Himalayans

The Lesser Himalayas (1000-4000 m)

Geological Time Scale

Recent & Sub-Recent

Quaternary

Pleistocene­Mid-Miocene

Stratigraphic Units

Subaerial deposits

Alluvial deposits

Unconformity

Siwalik Group

Geological Formation

Debris Cone, river sand and gravels

River Terraces

Neogene Siwaliks; Fine to medium grained arkosic and micaceous sandstones containing thin lignitous bands; coloured siltstones and shales; unconsoli­dated conglomerates and lime­stones.

Thrust (Main Boundary Fault) ----------------------

Permo­Carboniferous

Gondwana Group

------------------------------ Thrust

Carboniferous Buxa Group

------------------------------- Thrust

Pre- Daling Cambrian Group

Pre- Darjeeling Cambrian Gneiss

Palaeogene and variably meta­morphosed belts ofthrust-sheets; coarse grained quartzitic sand stone; thin slate bands; carbona­ceous sandstones and shales semi-anthracitic thin coal seams with basic intrusions

Pink and purple limestones, grey limestones interbedded with green calcareous slates; stromato­litic dolomite with purple quart­zites and phyllites

-----------------------------------------

Central crystallines; Phyllites, quartzites, quartz-sericite schist, chlorite-sericite schist, Golden silvery mica schist, biotite staurolite, garnet, kyanite and sillimanite bearing schists and gnetsses

(Source Acharyya, 19792; Bhattacharya et al., 196437

; Dutt, 1954139; Raina, 1976292)

69

terrain of the R. Tista (Fig.4.2). These are exposed, therefore, in the Tista-Rangit Valley

area, especially between Sevoke and Kalijhora. In the Tindharia - Gayabari - Chunabhati

sector of the Hill Cart Road, within the Paglajhora- Shiv khola basins, the Dalings have

again been exposed. It is to be noted in this context that the Dalings and the DarJeelings

represent argillaceous sedimentary facies in and around the township of Darjeeling

The Dalings occurring between Kalijhora and Rangpo in the Tista valley sector, form

the core of a domal uplift running north-south for more than 50 km. The structure has been

complicated by local disturbances. There is also evidence of increasing metamorphosis up­

wards. From the bottom of the Tista river, the clay-slates are found to pass through mica­

schist to gneiss, or the Darjeeling gneiss. This phenomenon is known as inverted metamor­

phism and it suggests large scale warping.

Slates and greywacke-schists form an elbow-bend outcrop along the Tindharia­

Kalijhora front as well as in the Tista valley. Other beds run in sinuous belts of outcrop,

trending roughly north-north east to south-south west through the settlements, like, Gayabari,

Mana, Mangpu, Rangli, and Pashok. The Dalings have been found to dip always towards

the hills but at varying directions. It is easterly in the western slopes of the Kalimpong hills,

while it is between the westerly and north-westerly directions in the Tista valley as well as

along the eastern slope of the Takdah and Mangpu hills. Near Kalijhora, the Dalings are

almost vertical in disposition. The stretch between Kurseong and Darjeeling experiences

variable dips, like, towards the north near Kurseong and to the south near Darjeeling.

In so far as landslides are concerned, the geolithological characteristics of Darjeeling

gneiss, and its two allied formations, viz. the Kanchenjunga augen gneiss and the Paro

graphite schist gneiss, have been found to be poorly resistant. The same holds good for the

Dalings, too. The structural disturbances within the two Pre-Cambrian formations of the

70

Darjeeling gneiss and the Dalings, as noted above, minimise their quantum of resistance

against slope failures. As a result, these group of rocks are vulnerable to landslides, for

example,

(i) The Tindharia- Gayabari - Chunabhati sector along the Hill Cart Road (26°52'N -

26°52'40"N and 88°17'40"E - 88°19'1 O"E), formed on the Dalings; Single rotational

debris slides, and compound block slides are common in this region.

(ii) The Yangmakun highland, lying to the east of the Tista, formed on the Dalings

(26°56'05"N and 88°29'57"E); Rotational slides are common place occurrence here.

(iii) The hill slopes, opposite Melli Bazar (27°5'20"N and 88°27'25"E), being composed

of Kanchenjunga augen gneiss and Paro schist; Rockfall and rotational slides occur

frequently.

(iv) Mahakal-Badamtam spur (27°5'15"N and 88°18'45"E), formed of the Darjeeling

gneiss; Rotational slides are common.

Carboniferous: Buxa Formation

The rocks belonging to the Carboniferous or the Purana age are represented in the

Buxa formation, within the confines of the area under study (Table 4.1 ). It is to be noted in

this context that the geological age of the Buxa remains a matter of controversy. Powde et

a/. (1976290) and Nautiyal eta/. (1966269

•270

) consider the formation to be of older origin,

between the Pre-Cambrian and the Triassic. Acharyya ( 1971 4), and Raina ( 197 6292 ), on the

other hand, have strongly argued that the Buxa belong to the Carboniferous age.

The Buxa formation include rocks, like shale, quartzite and dolomite, the last being

the most frequent, predominant, and representative ofthe geological sequence under study.

It is hard, compact, massive and thinly bedded (Raina, 1976292 ).

The Buxa group of rocks occur between the Permo-Carboniferous Gondwana and the

69A

GEOLOGICAL MAP OF

PEOONG - YANGMAKUN - GORUBATHAN EASTERN DAR)EEL lNG HILLS

,... ,.. ,... ""'

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71

Pre-Cambrian Dating. It is separated from these two formations by thrusts. It forms a part of

the Rang it Tectonic Window (Mallet 1875249) (Fig.4.3 ).

The Buxa set in approximately at the north-eastern boundary of the study area between

the Tista and the Great Rang it (Fig.4.4 ). It is tectonically very disturbed and, as such, it

constitutes a less competent geological formation against landslides (Chatterjee and

Choudhuri 198978). The rotational failures of slope between 27°4'N - 27°4'30"N and

88°26'30"E - 88°27'15"E are examples of the geolithological incompetence of the Buxa

formation, in so far as landslides are concerned.

Permo-Carboniferous: Gondwana Formation

The Permo-Carboniferous of the study area is represented by the Gondwana forma­

tion (Gansser 1964159, Ray 1976301 ). It is composed of sandstone, shales and quartzites with

a few limestone beds interbedded with lesser horizons (Singh and Bhattacharyya 1968325).

The sandstones are coal bearing and are, at places, micaceous and felspathic by nature. The

coal seams are crushed, dislocated, crumpled and somewhat anthracitic. The coal bearing

strata are inverted and mixed with lamprophyre intrusives or mica peridotite at many a places.

The coal measures are enriched with fossil impressions like Glossopteris, Vertebraria,

Schizoneura etc. The shales are often splintery while carbonaceous shales are metamor­

phosed at places to graphite schist. The Gondwanas have a generalised strike ENE-WSW

with variable dip between 40°-90° in NWN direction (Sinha eta/. 1975328).

In the Sub-Himalayan region, the Gondwanas have formed recumbent folds and

overthrusts. The thrust zone is exposed and generally coincides with the Main Boundary

Fault. Acharyya and Sastry ( 19792) have observed that the Gondwana group is affected by

co-axial deformation and sub-parallel lineation. They overlie the younger Tertiary beds and,

in turn, are overlain by the older rocks of the Dalings and Darjeelings.

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The Damuda series consitute an integral part of the Lower Gondwana. It is a narrow

belt between the Dating series in the north and the Siwaliks in the south. These extend in a

roughly east-west direction from the Jaldhaka in the east to the Balasan in the west There

is however a break in this latitudinal extension as observed between the Chel and the Murti ' '

river (Gansser 1964159, Gangopadhyay 1979158

). In the Tista Valley, region, the Damudas

attain a maximum width and thickness of 2.5 km and I 00 m respectively.

The geographical disposition of the Gondwana conforms to an invered 'T'. It is

meridionally oriented almost between 88°22'E- 88°23'E and 26°52'N - 27°5'N. It changes

its alignment, however, at 26°52'N and bifurcates diagonally, reaching almost upto 26°45'30"N

in the west and 27°0'45"N in the east.

The occurrence of the overthrusts and recumbent folds and similar other structural

deformations make the Gondwanas, a structurally weak formation. It is found to have been

incompetent in arresting landslides. Thus, it is susceptible to slope failures, as observed by

the author, in the neighbourhood of the following places :

(i) 27°3'50"N and 88°22'45"E in the headwater region of the Handi or Lopchu Khola;

slumps and rotational failures are common in the area.

(ii) Along the left bank of the MangwaJhora from 27°2'30"N and 88°22'45"E to 27°3'50"N

and 88°24'1 O"E; A valley-side free-fall on the Gondwanas, formed by incision of the

Mangwa Jhora has been found to be responsible for a series of single rotational slope

failures.

(iii) Around 27°0'1 O"N lat. and 88°21'20"E long. on both the valley side inward facing

slopes of the Rambi khola; slumps and rotational failures are frequent in this locality,

etc.

73

Pleistocene -Mid Miocene : Siwalik

These are represented in the study area by the Siwalik system or the Nahans. The chief

rocks include sandstone, grit, conglomerate, siltstone with occasional patches of lignite,

limestone and iron ore. These are found to have been arranged in a homoclinal sequence

dipping gently due north with alternating sandstone, siltstone and pebble bed (Acharyya

1971 4). In some parts of Siliguri-Kalimpong and Siliguri-Darjeeling roads, they dip at 20°-

800 towards NWN, Nor NEN. The Siwaliks differ from the Gondwanas by their sedimento­

logical characteristics, although there is no structural discontinuity between them. These are

tectonically disturbed by the forethrust older rocks.

The Siwaliks are moderately competent against landslides, in so far as their

geolithological properties are concerned. The weakness of rocks has been further aggra­

vated by intense fluvial action, as observed by the author in the Lish-Gish catchments of the

Eastern Darjeeling Hill region. The following locations on the Siwalik are frequented by

landslides, for example,

(i) 26°57'30"N- 26°58'15"N and 88°0'35"E- 88°1'25"E;

(ii) 26°57'N- 26°57'20"N and 88°1'25"E- 88°1'35"E;

(iii) 26°55'50"N- 26°57'1 O"N and 88°36'15"E- 88°37'20"E etc.

4.2. Tectonic Set Up and Landslides

The study area covers two tectonically diverse and structurally complex belts of the

Darjeeling Himalaya, viz. (i) The Sub-Himalayan Belt, and (ii) The Lesser Himalayan Belt.

The Sub-Himalayan region is separated from the Lesser Himalaya by the Main Boundary

Fault (MBF), while a number of thrust planes occur within the Lesser Himalayan Belt.

The Sub-Himalayan Belt

The Sub-Himalayan belt composes the southernmost range of the Himalayas. It ranges

74

between 65-300 min altitude, having a variable width of 2-10 km and is oriented in an east­

west direction. It emerges from the Quaternary-Recent alluvial deposits ofthe 'Duar' plains

with consistently warped terrace deposits (Mallet 1875 249, Acharyya and Tech 1972', Powde

and Saha 1976290, Ray 1976301

) The Sub-Himalayan zone belongs to the Siwalik group of

rocks of the Tertiary or the Pleistocene-Mid Miocene age. Near its contact with the Lesser

Himalayas, the Sub-Himalayan belt shows a number of discrete shear planes. The Siwaliks

gradually become overlain by a 'belt of schuppen' constituted of the Gondwana group of

rocks. The Main Boundary Fault separates the Gondwana ensembles lying atop the Siwalik

(Gansser 1964 159, Raina 1979292

).

The Sub-Himalayan Siwalik range is remarkably continuous both in strike extension

and facies along the Himalayan foot-hills except a few gaps of post tectonic erosion or

downfaulting (Acharyya and Sastry 19792). It is broadly divided into the lower, middle and

upper lithological units. The marly northern deformed belt of Darjeeling-West Bhutan foot­

hills, comprising the Chunabhati formation, is broadly similar to the basal Gish formation

of homoclinal sequence (Acharyya and Sastry 19792). The Siwalik group represents coars­

ening upward and increasing immature sequence.

The Sub-Himalayan belt is congenetically weak for the reasons noted above, and is,

therefore, frequented by landslides. There are innumerable landslide scars of diverse form

and orientation. The Lish-Gish Catchment, referred to in 4.1 and under the sub-section

Pleistocene - Mid Miocene, is one of the fine examples to this effect.

The Lesser Himalayan Belt

This is a zone of complicated stratigraphy and structure. It comprises the Palaeogene

as well as the poorly fossiliferous and variably metamorphosed belts of thrust sheets, win­

dows and semi-windows. It rises upto 4000 m and has a variable width of about 75 km. It is

75

the Inner Belt of the Himalayas lying between the Great Himalayan Belt in the north and the

Sub-Himalayan zone in the south. Geologically, it extends to the north upto the Main Cen­

tral Thrust, thereby roughly corresponding to the Lesser Himalayan Orogenic Belt ( Gansser

1964 159) Within the confines of the study area, the Lesser Himalayas are composed of the

Gondwanas, the Buxa group as well as the Pre-Cambrian Dalings and Darjeeling gneiss

(Fig.4.3).

The Gondwanas represent the youngest and well dated formations. These formations

are broadly subdivided into a basal diamicite bearing Rangit Pebble-Slate and a younger

carbonaceous and arenaceous Damuda Subgroup (Acharyya and Sastry 19792). The Lower

Gondwanas represent the para-autochthonous unit and is tectonically overlain in the Inner

Tectonic Belt by the allocthonous Daling and Buxa formations (Sinha Roy 1977331).

Foliated granites are embedded within the Buxa formation. Geologists have observed

that such granites have formed the basement for Daling sedimentation (Sinha Roy 1977331 ,

Acharyya and Sastry 19792). The basal Dalings represent eugeosynclinal facies or the shelf

facies of the deeper water geosynclinal massive rocks (Mukhopadhyay, 1982265).

The Lesser and the Sub-Himalayan Belts are spatially diverse with regard to their

competence against landslides. The author confirms a six-tier classification of rocks to this

effect, as indicated by Chatterjee 198476, Chatterjee and Bhattacharya 198577, Chatterjee

and Chaudhuri 198978. The competence scale of rocks for the Darjeeling hilly region may be

as follows (Fig.4.5) :

I) Competent rocks made up of Gneisses of the Darjeeling group.

2) Moderately competent sandstone ofthe Siwalik group.

3) Less competent rocks belonging to the high grade schists of the Darjeeling group.

ROCK COMPETENCY AGAINST LANDSLIDES & SLIDE PRONE AREAS

IN DARJEELING HILLS

N

· ~ . . . . . . . . . . .

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i ( ___ r-.

-1 (

Explanations -

KALIMPONG

. 0 • 0 • 0 . """"'

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1. competent rocks made up ot gneisses of the Darieel ing group

2. Moderatly eompelent ; sandstone of the Siwalik 9roup

3. Less competent; h igh grade schists of the Oarjeeling group

.. ,~

Index-­- -1

-- 2

--3

- -4

- -5

-- 6

Fault line

Landslide prone areas

4. Poorly competent ; low grade schists , phyllites & banded Quartzites

of Dating group

s . Incompetent : mytonit ic & Carbonaceous Phy I lite

6. Tectonically d isturbed rocks of the Gondwana & Buxa group

( Rock competency as per G. 5·1· Report )

75A

76

4) Poorly competent rocks made up of low grade schists, phyllites and banded quartz­

Ites of the Daling group.

5) Incompetent rocks like mylonitic gneiss and carbonaceous phyllite.

6) Tectonically disturbed rocks of the Gondwana and the Buxa formations.

Landslides are, therefore, rampant in the Lesser Himalayan Belt, due to its compli­

cated stratigraphy, complex structure and variegated rock competence, for example,

(i) the area lying to the east of the Singalila range, particularly the river basins of the

Lodhama, Rato, Gurdum and the Rammam;

(ii) the northern slope of the Gok spur and the Laring Khola basin in the Lingtin-Mane­

Gok region;

(iii) the middle Tista valley region, etc.

The areas mentioned above are tectonically characterized by shear faults across their

foliation planes, structural superimposition, as well as intense folding to the tune of over­

turned folds and plicated nappes.

Apart from these, the dip direction of the rocks have also influenced the incidence of

landslides. The dip slopes have been found to be more slide-prone than the obsequent or the

anti-dip inclination of the ground surface. This explains the occurrence of landslides on the

north-east facing slopes of the river Chel, Sel, Kali and the Jit Khola catchments in the

Kalimpong sector of the Darj eeling hills.

The occurrence of platy minerals within the rocks. has led to planar anisotrophy influ­

encing the direction of water movement along them. Phyllite, mica-schists, silicified felspathic

quartzites etc. are vulnerable to destabilisation due to their genetic association with such

minerals (De and Chattopadhyay 1990117).

77

It is to be noted in this context that the qistribution of bare rock outcrop hardly affects

the occurrence and frequency of landslides in the study area (Fig.4.6). Slope failures are

equally common on exposed rock as well as on rock waste, talus and scree.

4.3. Earthquakes vis-a-vis Landslides

The study area suffers from earthquakes nearly at regular intervals in the form of

shock waves of varying intensities. This is due to its site and situation in the dynamic belt of

the Himalayas which is the locus of frequent earthquakes (Table 4.2).

Assam Earthquake : 12 June, 1897

The earthquake (magnitude 8. 7) was caused due to a movement along thrust plane/s

and along secondary thrust and fault planes (Oldham, 1899274). At the Chedrang fault, the

movement took place over a distance of I 0.66 m. Large fractures were also developed,

having a throw of0.3 em, along the comparatively minor Samin fault. The earthquake had a

hinterland of over 3,885,000 sq.km. (Fig.4.7). The zone of influence included Darjeeling

and its adjacent hilly region. It has brought about widespread landslides in addition to the

damages caused in the socio-economic sector.

Srimangal Earthquake: 8 July, 1918

The magnitude of the Srimangal earthquake (Srimangal :a town in Bangladesh) was

to the tune of 7.6 in the Richter scale (Fig.4.8). Extensive subterranean adjustments as well

as violent shock waves, that followed, had inflicted large scale damages including landslips

in and around the Darjeeling hills (Dunn, 1939134).

Dhubri Earthquake : 3 July, 1930

The Dhubri earthquake was a seismic tremore of intermediate intensity. Its hinterland

extended almost upto Patna in the west, Imphal in the east, Sikkim in the west and Bay of

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Bengal in the south (Fig.4.9). It was responsible, like its predecessors, for numerous land-

slips in the Darjeeling hill region.

Table 4.2. Parameters of Earthquakes (magnitude above 7) around Darjeeling Hills : 1897-1954

Date Original Epicentre Magnitude Area over time Lat0 N Long0 E which damages GMT caused

I897, June I2 II 06 00 26.0 91.0 8.7 Severe damage over (Shillong) 4I4,000 km~, felt over

(Assam earthquake) 3,885,000 km2 area.

I906, Aug 3I I4 57 30 27.0 97.0 7.0

I9I8, July 8 IO 22 07 24.I 91.8 7.6 Severe damage near (Srimangal) epicentre at Srimangal;

felt over 2,072,00 km2

area.

I923, Sept 9 22 03 43 25.5 91.5 7.I Some damage at Dhubri and Garo Hills; felt over 905,000 km2

area.

I932, Aug I4 04 39 32 26.0 95.5 7.0

I934, Jan IS 08 43 I8 26.5 86.5 8.4 Severe damage over (Bihar- Nepal 93,240 km2

; felt over earthquake) 4, 92I, 000 km2 area.

I94I, Jan 21 02 30 15 27.5 92.5 7.I Damage restricted to I943, Oct 23 17 23 I6 27.5 93.5 7.3 epicentral tract I947, July 29 13 43 22 28.5 94.0 7.9 J I950, Aug I5 I4 09 30 28.5 96.5 8.6 Severe damage over

(Rima) I94,250 km~; felt over 2,926,600 km2 area

I954, Mar 23 23 42 05 24.0 95.I 7.2 Damage confined within epicentral region

(Source Ramachandran, Pradhan, Dhanota, I98I 293)

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Bihar-Nepal Earthquake : 15 January, 1934

It was an earthquake of the magnitude 8.4 and an acceleration of0.3 g (Dunn, 1939 134)

(Fig.4.1 0). The epicentre was located about 50 km away from the Himalayan foot hills. The

most remarkable feature of the earthquake was "sanding" from the fissures. and vents as well

as lands lips along hill-slopes. Slopes were reported to have been destabilised near Kathmandu,

Udaipur, Garji in eastern Nepal and Darjeeling hill region in India (Chattopadhyay 1983 85).

Movement along the fracture zone between Motihari and Purnea is considered responsible

for the earthquake (Wadia and Auden, 1939377).

Assam Earthquake: 15 August, 1950

The epicentre of the earthquake was at Rima in Mayanmar. It ravaged nearly 27,200

sq.km. of the hill area in North-East India. The total earth removed due to landslides, which

followed the earthquake, amounted to 4.59 x 1010 cu.m. Ray (1953 304) concluded that the

seismic tremor was caused due to displacement along a major fault lying across the Assam

axial belt trending NE-SW direction with an acceleration of 0.5 g.

The major epicentral tracts are aligned in an east-west direction in the Himalayan

belt and in the NE-SW direction in the Naga-Patkoi area. This is in general conformity with

the major tectonic lineaments in the region (Ramachandran eta/. 1981 293 ).

According to the estimates of the Indian Standard Institute, the Darjeeling hills fall

within the zone V, with reference to earthquake intensity, measured in the Modified Mer­

calli Scale (seismic co-efficient 0.08 to 0.12 g). Ramachandran et a/. ( 198 J293 ), however,

differ from that of the I.S.I's observation, referred to the above, and suggested earthquake

intensity to the tune of IX-X and VI, respectively, for the said region (Fig.4.11 ).

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configuration are the byproducts of earthquakes. This has been corroborated by earth scien­

tists, like Oldham ( 1899274), Dunn ( 1939 134

), Wadia and Auden ( 1939377), Ramachandran e/

a!. (198 F 93), Chattopadhyay (1983 85

) etc. in India; Youd and Hoose ( 1978391) in northern

California; Harp, Wieczorek and Wilson ( 1981 189) in Guatemala, etc.

It is also felt that good vibrations, set in as a result of thunder producing supersonic

sound waves in the lightning, might be aiding and abeting landslides along the critical hill

slopes (Sinha eta!. 1975 327).

4.4. Hydrogeological Characteristics and Landslides

The hydrogeological status of the study area is varied over the geographical space.

The nature of aquifer, for example, changes directly with the change in geology and land­

form. The Darjeeling hills, including the belts of the Sub-Himalaya and the Lesser Himalaya,

are characterized by the aquifers with the secondary intergranular porosity and fractures. In

so far as the water table contour is concerned, it appears to rise gradually due north, towards

the Darjeeling Hill region, as the elevation rises to this direction. The Darjeeling hills, for

instance, belong to the 70-80 m water table contour above the mean sea level, as contrasted

with the 60 m contour below the Terai. The yield rate in the hilly parts of Darjeeling is 1-5

litre per second. The chances of large scale accumulation of water in such an area is very

remote. In the absence of ground water storage, a considerable portion of subterranean

water is circulated through joints, fissure and weathered mantle. Thus, a situation of sub­

surface lubrication is accomplished. As a result, the permeable top layer of the slope-form­

ing materials becomes unstable. Landslides at Pradhan Busti and those below the Mount

Hermon School in the Darjeeling town; the Chunabhati slide on the Hill Cart Road; the

Berrik slide on the Sevoke-Tista sector of the NH 31 A, etc. are examples to this effect. Thus

the geological characteristics, in general, have a direct bearing on the hydrological behaviour

inducing landslides.