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JOURNAL GEOLOGICAL SOCIETY OF INDIAVol.75, February 2010, pp.383-392

Macroseismic Study of 20th May 2007 Sikkim Earthquake –Its Seismotectonic Implications for the Region

K. C. JOSHI1, S. SENGUPTA2 and G. C. KANDPAL1

Geological Survey of India, 1Northern Region, Lucknow -226 024; 2Eastern Region, Gangtok – 737 101

Abstract: An earthquake of magnitude 5.0 with its epicenter in the West Sikkim district occurred on the evening of 20th

May 2007. The macroseismic study revealed an ENE-WSW isoseismal pattern indicating a trend conforming to that ofthe major tectonic discontinuities in this part of the Himalaya.

The relatively moderate event, felt even at far away places like, Siliguri, Darjeeling, etc induced a maximum intensityof VI (MSK) in the form of development of cracks in the walls, fall of thick chunks of plaster in Type A or even Type B-C structures at localities like Tashiding, Ralang, Rabang, Keozing, Sakyong, etc.

The study also revealed that the NW-SE trending Tista lineament is playing an important role in the release of thetectonics strain in this region and that the rupture initiated under the influence of the northward directed tectonic stressesat an asperity formed at one of the decollements within the crust by the intersection of this tectonic discontinuity wasinterpreted to be responsible for this seismic event.

Keywords: Seismology, Earthquake, Sikkim.

of the Sub-Himalaya constituting the northern parts of WestBengal, which expose a variety of rocks belonging to a widerange of geological age and tectonic status. Broadly theseinclude the comparatively thin sedimentary cover rocks ofTeritary age, affected by the fold thrust movement duringthe terminal phase of Himalayan orogeny in the south,followed northward by the pre-Tertiary metasedimentaries,dominantly of Proterozoic age, and the intrusives. Theboundary between the Tertiaries and pre-Tertiaries is markedby the Main Boundary Thrust (MBT) (Fig.2). The pre-Tertiary metasedimentaries to the north of MBT, alsoclassified as Lesser Himalaya, are represented here bythe Daling Group of rocks. These rocks comprise quartz-chlorite-sericite phyllite/schist, slaty phyllite, dark greyslates, quartzites and their intercalations of the Gorubathanand Reyang Formations, and dolomite, limestone andphyllites of the Buxa Formation (Acharyya, 1989; Ray,1989). In tectonic perspective, these Lesser Himalayanformations are described as the older folded cover sequenceoverprinted by Himalayan fold thrust movement (GSI,2000). Another regional tectonic discontinuity of theHimalaya, the Main Central Thrust (MCT), is exposedparallel and north of MBT. The MCT separates the highlymetamorphosed central crystallines in the north from thecomparatively low-grade rocks of the Lesser Himalayatowards south. The central crystallines, which like the LesserHimalayan rocks have been reworked by the Himalayan fold

INTRODUCTION

A moderate earthquake of magnitude M 5.0 (IMD,USGS) jolted the western and southwestern parts of Sikkimat 19Hr: 48Min: 16.5Sec (IST). The epicentral coordinates27.5°N: 88.3°E given at IMD site vary from those - 27.30°N:88.19°E - given at USGS site (Fig.3). The depth of the eventas given by USGS and IMD is 17 and 15 km, respectively.The tremors with the maximum effects around few localitiesof West Sikkim district, were not severe enough to causeany serious damage to life and property - albeit the felt areaencompassed areas of the adjoining localities like Siliguri,Darjeeling, etc of the neighbouring state of West Bengal.Interestingly, though the area is considered to be tectonicallyhighly active, there are no records of a large earthquake inSikkim region. However, recent study by Bilham andWallace (2005) puts this part of the Himalaya in seismicallyhighly vulnerable regions. The authors undertook thedamage survey with a view to assess the seismic intensity atvarious places, and to constrain the isoseismals of the seismicevent, thereby delineating the source fault.

During the survey, the authors visited thirty five localitiesin West, South, East and North districts of Sikkim to makean on-the-spot assessment of the intensity.

GEOTECTONIC SETUP

The earthquake has affected Sikkim Himalaya and parts

sonal

Highlight

JOUR.GEOL.SOC.INDIA, VOL.75, FEB. 2010

384 K. C. JOSHI AND OTHERS

thrust movement, northwards are followed by the poorlymetamorphosed (Tethyan) Paleozoic-Mesozoic sequence,which got folded during Himalayan orogeny. The Tertiaries,comprising the Siwalik Group of rocks, are followed byGangetic alluvum further south.

Interestingly, the ENE-WSW trending Siwaliks abutagainst a NNE-SSW trending fault, beyond which theSiwaliks are not exposed and the Gangetic Alluvium isjuxtaposed with the pre-Tertiaries along the MBT.

Besides these, another important tectonostratigraphicfeature here is the presence of the Gondwana Group of rocksexposed in a tectonic window, known as Rangit Window.Similar lithopacket is also exposed towards south in the formof a lenticular tectonic wedge along the MBT (Narula et al.2000).

One significant feature in the tectonic scenario of thisregion is the reentrant formed by one of the most importanttectonic surfaces of the Himalaya, the MCT, which thoughdisposed parallel to the MBT at a more or less constantaerial distance of about 5 km in this part of the EasternHimalaya, suddenly swerves northward and makes areentrant of considerable spread, forming a narrow neck atits base. Seismotectonically, most of the thrusts, which areeither parallel or sub parallel to the Himalayan trend arebelieved to merge with a gently north dipping tectonicdiscontinuity formed by the upper surface of theunderthrusting Indian Plate, known as decollement (Seeberand Armbruster, 1981, Ni and Barazangi, 1984). Nakata etal. (1990) have reported neotectonic activity along theHimalayan Frontal Fault (HFF).

Additionally, there are a number of transverse lineaments/ faults having great relevance to the contemporary tectonics.The NW-SE trending Tista lineament, Purnia-Everestlineament and another lineament east of Tista Lineament,passing through west of Cooch Bihar, appear to have asignificant influence on the seismicity of the region.

SEISMICITY

The affected area falls in Zone IV of the Seismic ZonationMap of India, which implies that the area may be jolted byseismic activity equivalent to intensity VIII.

As per the Seismotectonic Atlas of India and its Environs(2000) and the USGS website, a total of one hundred andforty four seismic events have been recorded in the areabounded by latitude 25°-29° and longitude 87°-90° tillAugust 2007, which show following frequency distribution.

A perusal of the table shows that about half of the numberof total events are of magnitude between �4.0 and <5.0 andthat there is only one event in the range of �6.0 <7.0. As

a matter of fact, the magnitude of this solitary event in thisrange is Mb6.0. There is no record of a larger eventoriginating in this region. However, 1934 Bihar-Nepalearthquake induced ground motions equivalent to intensityVII in the Sikkim region, which means that the area suffereddamage comprising extensive cracks in many structures andpartial collapse in some of the stone masonry constructions(GSI, 1939). Bilham and Wallace (2005) have mentionedabout an earthquake in the year 1713 in Western Bhutan,which damaged several monasteries in the area. Also on thebasis of their study of the slip potential in the Himalaya,they have come up with the possibility of a future earthquakein this region with a maximum magnitude of Mw 8.2.

The records also reveal that most (82.6%) of the recordedseismic events had a shallow crustal origin with their depthbeing less than 40 km.

The fault plane solutions of the events belonging to theyears 1965, 1980 and 1982, which occurred in the Himalaya,indicate predominantly strike slip type mechanism withnodal planes striking in NW-SE and NE-SW directions(Table 2) (Fig.1). The other two solutions for the earth-quakes of 1979 and 1993, which occurred in the foredeepregion, however, indicate a predominantly normal faultmechanism with both the nodal planes, striking in almostN-S direction for the former and NE-SW and E-W directionsfor the latter.

De and Kayal (2003) presenting the seismotectonicmodel of Sikkim Himalaya concluded that the earthquakeactivity was concentrated mainly along the MBT andNW-SE trending lineament. A composite fault planesolution of a cluster of micro earthquakes recorded just northof MCT, during a survey conducted by GSI indicated apredominantly thrust type mechanism with nodal planesoriented in E-W directions (De et al. 1995).

Recently, another micro earthquake study was carriedout by the GSI in the southern part of Sikkim and northernpart of West Bengal, through a four-station network between19th Feb. and 19th April 2007 (Chakraborty, 2006-07). Thesurvey revealed that a majority of the seismic events, andthus the seismic activity, is concentrated on the eastern sideof the Teesta lineament (Fig.2).

Table 1. Frequency distribution of earthquakesbetween 25°-29° N and 87°-90° E

Mag. range No. of events Percent

�6.0 <7.0 1 0.69�5.0 <6.0 38 26.38��.0 <5.0 70 48.60<4 12 08.33Not determined 23 15.97


MACROSEISMIC STUDY OF 20 MAY 2007 SIKKIM EARTHQUAKE 385

Sankar Kumar Nath et al. (2000) has carried out siteresponse studies in Sikkim Himalaya by using strong motionsrecorded with the help of an array of 7 instruments for about4 months in 1999. A keener look on the Fig.3 of this paperindicates that there exists a somewhat predominance ofseismic events on the eastern side of the Tista lineament.Additionally, although their studies were of preliminarynature, they concluded that, in general, in Sikkim Himalaya,

the overall site response was higher at higher frequencies.Site response at lower frequencies (1 – 5 Hz) was, however,either low or moderate.

Interestingly, there was a spate of seismic activity justprior to and after the instant event of 20th May 2007. Twoevents of M 4.0 and 4.7 occurred on 16th and 18th Mayrespectively, and another event of M 3.5 took place on 23rd

May 2007 (Figs. 2 and 3). It was even more interesting that

Fig.1. Seismotectonic map of Himalaya and the foredeep region of North Bengal and Sikkim (modified after Seismotectonic Atlas ofIndia and its environs, GSI, 2000).

Lachung C

hu

2500

4000

3000

2500

(F)C Mz(F)F Pt

(F)FC

(F) Cz

(F)C Mz

Tista

Lin

eam

ent

Purn

iaE

vere

st L

ineam

ent

Kan

chen

jung

a

L

inea

men

t

20.05.07Earthquake

Gangtok

Darjeeling

Siliguri

Kishanganj

Jalpaiguri

Kalimpong

(F)C

(F)FC Pt23

(F)FPt1

(F)FPt1

(F) Cz

Kanchenjunga

-100

-120

-140

F

M C T

Thimpu

(F)F

(F)FC Pt

MC

T

Al

FF

F

F

F

-180

MC

T

8586 m

Kooch Bihar

Alipur Duar

-80

Forbesganj

Morang

Tista

R.

Rangit R.

Wangdi

Dogan

Cz1

MC

T

Earthquake epicentres (Deep)

Earthquake epicentres (Shallow)

Alluvium

Crystalline ComplexF(FC)

(F)Cz

(F)F Older Folded Cover Sequence

Cover rocks of Frontal Belt

Older Folded Cover Sequence foldedduring Himalayan Fold Thrust Movement

Lineament / Fault

Focal mechanism solutionNormal / Strike slip

-120

80Gravity contours /Depth of basement



all these events were located in the close vicinity of the Tistalineament.

EARTHQUAKE EFFECTS AND DELINEATIONOF ISOSEISMALS

The isoseismals for the earthquake were delineated onthe basis of the study of its effects in about 35 localities.The effects were studied by exhaustively interviewing theinhabitants of different localities. The enquiries were mainlyon the following pattern.

1. Effects on persons i.e. human perception2. Effects on surroundings3. Effects on structures

Since the effects were not strong enough to cause muchdamage to structures, mainly the human perception factoredin the assessment of intensity at most of the places. Intensitieswere assigned using Medvedev - Sponheuer - Karnik (MSK-64) scale and European Macroseismic scale (EMS-92),

which, in fact, is the updated version of MSK scale (Grunthal1993). A maximum intensity of VI has been assigned to thisearthquake (Fig.3). Besides intensity VI, isoseismals of lowerintensity, viz. V and IV were also delineated. The isoseismalIV could be drawn as intermittent line, which though doesnot provide the detailed behaviour of the isoseismal, doesindicate its aerial extent. All the isoseismals, except VI,however, were left open, as they could not be followedfurther due to the inaccessible and uninhabited nature ofthe area.

Notwithstanding the incompleteness (due to the reasonsstated above), the major trend of the elliptical isoseismalscan easily be deciphered as ENE-WSW. A perusal of theisoseismal map shows that the attenuation of intensities ismaximum towards SW. Further, whereas the long axes ofall the elliptical isoseismals are extending towards ENE,southeastwards the isoseismals tend to be somewhatinflated.

There was no report of injury to any one due to theearthquake. Nor there was report of any significant damageto property, save for some instances of development ofcracks or fall of chunks of plaster representing the severesteffects of the seismic event. As such, one had to fall back onhuman perceptions for constraining the isoseismals,especially of intensity V to III. Therefore, in order to achievemaximum accuracy, several localities were visited to makean on the spot assessment of the intensity at respective areas.The following paragraphs are devoted to the brief descriptionof the effects of tremors at different localities. For the sakeof referral convenience different localities included withinan isoseist have been grouped together.Also the descriptionsof the effects in a few localities have been clubbed togetherto avoid repetition.

Isoseist VI

This isoseist, encompassing areas/localities havingexperienced the maximum intensity of the event, coveredapparently an elliptical area trending in ENE-WSWdirections with a subtle inflation towards southeast (Fig.3).

Table 2. Fault plane solutions of the earthquakes between 25°-29° N and 87°-90° E (from Seismotectonic Atlas of India and its Environs, 2000)

Year Mo Dt Mb NP1 NP2 P axis T axis B axis Source

St Dip St Dip Pl Az Pl Az Pl Az

1965 01 12 5.8 233 76 326 72 23 192 03 281 66 14 Dasgupta et al. (1987)1979 06 19 4.6 350 57 179 34 78 243 11 84 04 353 Dziewonski et al. (1988)1980 11 19 6.0 209 51 301 89 28 172 25 68 51 302 Dziewonski et al. (1988)1980 11 19 6.0 218 64 119 74 10 160 32 74 58 269 Rastogi (1992)1982 04 05 5.0 206 48 314 72 43 178 14 74 42 330 Nandy and Dasgupta (1991)1993 02 15 4.9 46 50 272 48 65 251 01 339 24 68 Dasgupta et al. (1987)

Fig.2. Epicentral map showing station locations and epicenters ofmicro earthquakes recorded between February 2007 andApril 2007 (after Chakraborty, FS- 2006-07).TistaLineament has been plotted on this map. Filled dots areepicenters; filled triangles are station locations; filledrectangles are locations of prominent places.

3.4

Magnitude

Total selected events- 31

3.7

Recent Earthquakes

GANGTOK

BLOCK-A

BLOCK-B

DAR

JTG

KAG

SILIGURI

STM

EQ MAG 4.0 on 16-05-07

EQ MAG 5.0 on 20-04-07

EQ MAG 4.6 on 18-06-07

EQ MAG 3.5 on 23-05-07

EQ MAG 4.8 on 11-06-07

Tista

Lin

eam

ent



Although the area of influence of this isoseist wasconsiderable the intensity everywhere, close to or away fromthe isoseismal, was, more or less, just on the higher side ofthe threshold of the intensity VI. Localities like Tashiding,Ralang, Rabang, Keozing, Sakyong, etc have been includedin this isoseist.

Tashiding

The serene Tashiding village with its monastery – one

of the few most sacred monasteries of the Sikkim – impartingprominence to it, is located on a gently sloping spur. Herealmost all the people, inside or outside their houses felt thetremors. Rattling of doors, windows, utensils, were few ofthe common observations. Some people, who were insidetheir houses, ran outdoors as they reportedly got frightened.Small, less stable objects were reported to have fallen duringthe ground shaking.

The ground shaking also induced damage up to grade 2

IMD20.05.07

20.05.07USGS

18.03.98

14.02.06

18.05.0727.12.03

26.05.07

23.03.96

23.05.07

02.12.01

13.09.96

USGS

USGS

USGS

IMD

SCALE

2.5 7.5km0

Yoksam

Mangan

Namok

Kabi

Ranka Gangtok

Ranipool

Pakyong

Singtam

Rabang

Ralang

Polot

SakyongTashiding

Keozing

Legship

Gezing

Rinchingpong

Rangpo

Namchi

25.05.03

Kalimpong

TIS

TA

LINEAM

EN

T

M CT

Main Boundary

Thru

st

Darjeeling

M. C. T.

(F)F

(F)F(F)F

(F)

(F)

(F)C

(F)C

(F)FC

(F)FC(F)FC

(F)FC

TIS

TAR

.

TIS

TA

R.

Al

Al

Main Central Thrust

Rangit R.

Ben

F

N

IV

IV

VI

Crystalline complex overprinted byHimalayan fold thrust movement

Older folded cover sequence overprinted byHimalayan fold thrust movementOlder cover sequence folded duringHimalayan fold thrust movement

Cover rocks of frontal belt affected byfold thrust movement during terminalphase of Himalayan orogeny

Alluvium

Isoseismal lines with intensity (MSK)

Epicentre (1964-1993) M5.0-5.9, 4.0-4.9,Not determined

Epicentre (1901-1963)M5.0-5.9

Epicentre of 20.05.2007 and RecentearthquakesLineament, Fault

Hot spring

F

Fig.3. Isoseismal map of 20th May 2007 earthquake.



in the form of widening of preexisting cracks, constructionjoints, peeling off of fairly thick chunks of plaster anddevelopment of new cracks in Type B constructions.

The Tashiding monastery, a load bearing stone masonry-set-in-mud-mortar structure is one of the most severelyaffected building in this seismic event. Enquiries revealedthat the roofing of the main monastery had been changedover the years. The wooden trusses, used in the old roofing,have been replaced by steel ones in the new one. The seismicloading of the structure – which seems to have resulted inlateral loading of the load bearing walls by the trusses –though did not cause any collapse or failure of any loadbearing structure in this building, has in fact seriouslycompromised its structural integrity in the form ofdevelopment of cracks between the beams and the walls.Almost all the walls have developed surficial to fairly deepcracks (Fig. 4). The RC floors have also been affected inthe form of cracks by the ground motions.

‘Ikra’ type or complete wood houses (Figs. 5 and 6). Ikraconstruction involves walls made up of small rectangularwooden frame, which are further filled in by bamboo mesh.This type of framed structure constitutes the core of the wall,which is then plastered from both the sides to result in alight weight load bearing structure, which can withstandlateral loading in the form of seismic forces with a betterefficiency.

Rabang

Inhabitants of this town with a gentle and rollingtopography at a height of 2100 m, felt the shock. Many ofthem ran outside in panic. A few of them reported about asound like that of ‘light blasting’ just before the groundshaking.

There were reports of fall of photo frames and otheritems kept on shelves. A few RCC constructions reportedlydeveloped cracks.

Sakyong

This locality is aerially about 5 km north of Geyzingand about the same distance WNW of Tashiding. Here, mostof the people felt the shock. Many of them rushed outsideas the tremors frightened them.

There was also damage to structures, which include RCCframed structures, Ikra constructions and wooden houses.Sakyong secondary school, developed oblique as well ashorizontal cracks in the walls. Opening of construction jointsbetween the beams and walls was a relatively more commonphenomenon.

Keozing (Samsing)

The twin localities – Keozing being a cluster of newlybuilt predominantly RCC structures, situated on the road to

Fig.4. Cracks developed in one of the walls of the TashidingMonstery as a result of earthquake shaking

The other effects include a little shifting of the heavyfurniture and the fall of some of the articles placed in thehands of the statue of Lord Buddha, encased in a glasscovered shelf, which resulted in breaking of another statue.

Ralang

Situated at a distance of about 10 km from Rabangla(Rabang) on the road to Brang and founded on gentle slopesof deeply weathered rocks, this locality experienced strongground shaking that most of the people felt the shock. Manypeople ran outdoors in panic. Articles and utensils kept inshelves fell in good numbers.

Some of the stone masonry structures in the villagereportedly suffered damage in the form of fairly deep cracksin the walls. However, the monastery here remainedunscathed. Most of the constructions in the villages are either

Fig.5. Typical dwelling of a Sikkim village using wood as theconstruction material for the entire structure renders itseismically resistant.



Rabangla and Samsing being its rural part located aeriallyabout 1 km north of it – experienced considerably strongground shaking, which resulted in the development ofcracks between beam and the walls in even Type B to Cstructures. The damage in the poor quality constructionsof rubble masonry set in mud mortar (Type A) was ofsomewhat higher grade in the form of fall of thick chunks ofplaster and development of deeper cracks in the walls(Fig.6)

Isoseist V

This isoseist, like others, has also an elliptical shape withthe long axis trending in ENE-WSW directions. Due tounavailability of the data towards west, it had to be leftopen-ended. The isoseismal of intensity V passes throughplaces just north of Yuksam, in the north, just aroundNamok in the ENE, through Ben (Thalabari) and Rishi, andjust south of Rinchingpong in the south. Interestingly, theisoseismal extends more towards ENE to impart an elementof asymmetry in this direction. Observations, as recorded atsome of the localities within this isoseist, are describedbelow.

Polot

Located about 2 km south of Brang along the road onthe eastern flank of the Rangit River, the landscape here isin the form of gently sloping surface made up of deeplyweathered rocks. The area boasts of the hottest hot watersprings among the many such springs located all along thelength of the Rangit River.

The earthquake effects in this locality were strong enoughto be felt by most of the inhabitants. However, the shockwas not frightening enough to cause panic among the

residents. There were reports of stray cases of fine cracks ina few constructions, majority of which is constituted by theIkra houses – the tremors affecting mostly those parts of thewalls, which used random rubble masonry.

Yuksam

Yuksam, a tourist destination being the first capital ofSikkim where first Chogyal of the Kingdom of Sikkim wasconsecrated in 1642 A.D. is located at a distance of about40 km from Pemayangtse –where one of the oldestmonasteries of the state is situated.

The earthquake effects were felt by most of theinhabitants. Many of those who were standing out in theopen also felt it. Few of those inside their dwellings cameout in fear. However some who were inside their vehiclesdid not feel the shaking. Although people informed of fallof articles kept in shelves there was no report of damage toany structure, most of which are either wood constructionsor Ikra type.

Isoseist IV

The isoseist follows more or less the same ENE-WSWtrend of higher isoseists. It encompasses localities likeGangtok, Namchi, Naya Bazar, Kabi and Mangan in thesouthwest. In these places most of people inside their housesfelt the tremors. The shocks were not frightening.

Places like Mangan, Rangpo and Pakyong have beenkept in Isoseist III, where only few people reported of slighttrembling.

Hypocentral Depth Calculated from Macroseismic Data

By using maximum intensity value Io = VI in variousempirical relations relating magnitude (M), maximumintensity (Io) and depth of hypocenter (h), we get thefollowing results.

1. Gzovsky’s formulaIo = 1.5 M - h/15 gives h = 22.5 km

2. Shebalin’s formulaIo = 1.5 M - 3.5 log h + 3 gives h = 19.3 km

3. Gutenberg and Richter’s formulaM = 0.6 Io + 1.8 log h – 1.0 gives h = 21.54 km

These values of depth of hypocenter are somewhathigher than the instrumental depth of 17.0 km and 15.0 kmprovided by USGS and IMD, respectively.

DISCUSSION ON SOURCE MECHANISM

Regarding the seismotectonics of the region, Seeber and

Fig.6. Peeling off of chunks of plaster from a thick ground floorwall of random rubble masonry set in mud mortar. Theupper storey is made up of the ubiquitious Ikra typeconstruction in the village. Location Samsing village.



Armbruster (1981) and Ni and Barazangi (1984), haveproposed the most convincing models for the Himalaya.According to them there exists a gently dipping decollement/detachment surface underlying the entire Himalaya. Thisplane, which controls the major seismic activity in the Sub,Lesser and Higher Himalaya, represents the upper surfaceof the under thrusting Indian Plate, and the MCT, MBT,FHT and their subsidiary thrusts and faults, merge with thisplane at depth. Srinivasan and Khar (1996) have supportedthe existence of decollement under the outer Himalaya.According to them, the depth of this northward dippingsurface under Lesser and Higher Himalaya, constrained withthe help of accurately determined depth of thrust type events,varies from 10 to 20 km below surface.

Mattauer (1986) has proposed a model, which presentsthe Himalaya as a crustal stacking accretionary wedge, whichis related to inter continental subduction. He proposed formore than one decollements occurring along boundaries oflithological contacts such as basement cover contact, lowerand upper crust, the crust mantle boundary or anisotropieswithin the lithosphere.

As mentioned earlier there is a broadly NW-SE trendingTista lineament, which passes through just east of NayaBazar and aerially about 6 km west of Geyzing (Figs. 1 and3). Recently a micro earthquake study carried out by GSI inthe southern part of Sikkim and northern part of West Bengalthrough a four-station network between 19th Feb. and 19th

April 2007 revealed that the occurrence of majority of theseismic events and thus the seismic activity was concentratedeast of the Tista lineament (Fig. 2).

Additionally, presence of a number of hot water springsalong the Rangit River valley, which also falls on the easternside of the said lineament further strengthens the notion thatthis part of the area is under contemporary tectonic strain(Fig.3).

Interestingly, there was a spate of seismic activity justprior to and after the instant event of 20th May 2007. Twoevents of M 4.0 and 4.7 occurred on 16th and 18th Mayrespectively, and another event of M 3.5 took place on 23rd

May 2007 (Figs. 2 and 3). It was even more interestingthat all these events were located in the close vicinity ofthe Tista lineament. In view of this observation alongwith the revelations of the micro earthquake survey, theauthors conclude that this may be the transverse feature,laterally controlling the tectonic activity in this region,which is taking place under the influence of regionalprincipal stresses acting in northerly direction. It may bementioned here that with the help of geological, geo-thermal, macroseismic, seismological, gravity and magneticdata, Narula and Shome (1992) have concluded that the

transverse Himalayan features have a significant role inthe generation of the earthquakes. This, in other words,implies that such transverse features deciphered on theground extend downwards at least to the detachment surfaceor still lower. There are many earthquakes e.g. 1978Dharamshala Earthquake; 1980 Dharchula Earthquake;1996 Chamoli Earthquake and 2005 DharamshalaEarthquake, whose isoseismal trends are transverse to theHimalayan grain.

The area exposes traces of ENE-WSW trending MBTand MCT disposed parallel and close to each other. Thesurface disposition of these regional tectonic discontinuitiesindicates that the probable gently dipping decollements mayalso be, by and large, having a similar trend. The MCT,however, makes a reentrant here. It is a well known fact thatthe areas of reentrant, be it Kangra reentrant or Ravireentrant, are the locales of tectonic strain.

Presence of major thrusts like MBT and MCT, and theavailable fault plane solutions of the two past earthquakes,which occurred east of the present event, indicating strikeslip mechanism suggest that the area is under compressionalstresses.

Like in almost all the past earthquakes of Himalaya thefault rupture in this event also did not extend up to thesurface. However, the ENE-WSW trending typical elliptical

Fig.7. Isoseismal and source area delineation (red dashed line) ofthe 14th Feb. 2006 earthquake (from Som et al. 2008). Inpresent authors’ view an ENE-WSW trend is alsodiscernable from the isoseismal pattern.



shape of the isoseismals suggests that a rupture roughlyalong this trend would have caused this event. The isoseismalpattern also shows that the intensity dies out or attenuatesfaster towards south-southwest. The hypocentral depthfigure of the event as determined by the United StatesGeological Survey (USGS) is 17 km. The IMD’s figure forthe same is 15 km, and that calculated empirically throughmacroseismic study by the authors is in the range of 19.3 to22.5 km.

Incidentally, there was an earthquake of Mb 5.4 on 14th

February 2006, which had its macroseismic epicentral tractaround Gangtok – Tadong area. The macroseismic study ofthis event by GSI concluded that an E-W trending lineamentcould be the source area of this event (Som et al. 2008).However, in present authors’ view, a keener look on theisoseismal pattern of this event reveals that a generalENE-WSW trend could also be made out from theirdisposition (Fig.7).

The authors, on the basis of isoseismal pattern and theconsiderations regarding principal regional stress vis-a-visthe orientations of tectonic discontinuities, opine that the20th May 2007 event was generated by the initiation of arupture extending in ENE-WSW directions under theinfluence of regional principal stresses along the TistaLineament at an asperity formed by its intersection with oneof the decollements within the crust. The attenuation of theintensities towards southwest and somewhat inflated patterntowards southeast also suggest that the lineament has playeda role in the strain release.

CONCLUSIONS

�Sikkim Earthquake of 20th May 2007 with a magnitude of5.0 induced a maximum intensity of VI (MSK) in partsof West District.

� The trend of isoseismals, which is more or less similarto the orientation of the MBF and MCT in this part ofthe Himalaya, indicated a rupture along one of thedecollements extending in ENE-WSW directions asthe source of this event.

� The observation that a number of seismic events haveoccurred along or in the vicinity of the Tista lineamentindicates that the lineament has an important bearingon the accumulation of stress and release of tectonicstrain in this part of the Sikkim Himalaya.

� Trends as given out by the macroseismic study of theinstant event and the 14th February 2006 earthquake,and the strike slip mechanism with NE-SW and NW-SE nodal planes as revealed by the fault plane solutionsof some of the earlier events suggest that tectonicfeatures – along as well as transverse to the Himalayangrain – are seismically active.

Acknowledgements: The authors express their gratitudeto Deputy Director General, Geological Survey of India,Northern Region for according permission to publish thispaper. They are also thankful to Shri Harsh Gupta, Director,Geological Survey of India, discussions with whom hashelped in the improvement of this paper.

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(Received: 5 May 2009; Revised form accepted: 3 September 2009)

Announcement

Geological Society of India

Annual General Meeting - 2010

and

National Seminar on “Tectonics of the Himalaya withFocus on the Seismotectonics of NW Himalaya"

At the invitation of the Department of Geology and Geophysics, University of Kashmir, Hazratbal,Srinagar - 190 006, Jammu and Kashmir, the Annual General Meeting (AGM) of the Geological Societyof India for 2010 will be held at Srinagar (J & K). A National Seminar organized by the Department ofGeology and Geophysics on “Tectonics of the Himalaya with Focus on the Seismotectonics of NWHimalaya” will be held concurrently.

Scientists interested in participating in the National Seminar may please contact: Prof. Mohammad IsmailBhat, Professor and Head, Department of Geology and Geophysics, University of Kashmir, Hazratbal,Srinagar - 190 006, Jammu and Kashmir. Phone: 0194-2422543 (O); 09419093690 (M); Fax: 0194-2421357;Email: [email protected].

During the Annual Convention, it is customary to hold a session for the presentation of interestingresults of recent and ongoing research, especially by younger researchers, who are requested tocontact: Shri R.H. Sawkar, Secretary, Geological Society of India, No.63, 12th Cross, Basappa Layout,Gavipuram P.O., P.B.No. 1922, Bangalore - 560 019; Telefax: 080-26613352, Phone: 080-22422943;Email: [email protected]

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