Nat Hazards (2012) 63:589–603 DOI 10.1007/s11069-012-0170-0 ORIGINAL PAPER

Macroseismic field observations of 18 September 2011 Sikkim earthquake A. K. Mahajan • Vikram Gupta • V. C. Thakur

Received: 12 December 2011 / Accepted: 26 March 2012 / Published online: 22 April 2012 Ó Springer Science+Business Media B.V. 2012

Abstract A shallow-focus damaging earthquake of magnitude 6.9 Mw struck the Sikkim Himalaya, north-east India, on 18 September 2011 at 12:40:48 UTC (06:10:48PM IST). The epicentre was located north-west of Chungthang on Indo-Nepal border of Sikkim Himalaya. The earthquake was widely felt in northern India and caused widespread damage to poorly built and framed structures in Sikkim region, northern Bihar, eastern Nepal, southern Bhutan and part of Tibet adjoining Sikkim Himalaya. A lot of secondary effects in the form of landslides, rockfalls and landslide lake outburst flood were caused due to strong shaking effect of the earthquake. Maximum intensity IX according to the European Macroseismic Scale-98 was observed in the meizoseismal zone surrounding Chungthang village. Asymmetrical distribution and heterogeneous damage pattern demonstrate intensity attenuation characteristics of the region. Although the regional tectonic framework of Sikkim region indicates compressional thrust tectonics regime, according to CMT fault-plane solution this earthquake involved predominantly strike-slip motion on a steep fault. Unlike Nepal and north-west Himalaya where microseismicity and large earthquakes indicate thrust mechanism, this Sikkim earthquake suggests that strike-slip principal component may imply transcurrent deformation. Keywords

Sikkim earthquake  Transcurrent deformation  Macroseismic observation

1 Introduction The Himalayan mountain arc and the adjoining Shillong plateau and western Assam experienced four damaging earthquakes with magnitude Mw 7.8–8.4 within the last 110 years. The four earthquakes are 1897 Western Assam, 1905 Kangra, 1934 Bihar– Nepal and 1950 Eastern Assam (Fig. 1). In addition to these earthquakes, the Himalaya has suffered several large earthquakes with magnitude 7.0 \ M \ 8.0, for example 1505 Central Himalaya, 1555 Kashmir, 1720 Kumaon, 1713 Western Bhutan, 1751 Satluj, 1803 A. K. Mahajan (&)  V. Gupta  V. C. Thakur Wadia Institute of Himalayan Geology, 33, GMS Road, Dehra Dun, India e-mail: [email protected]

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Fig. 1 a Regional tectonic map of Himalayan arc with location of great/major earthquake that occurred all along the Himalayan arc with location of Sikkim Himalayan between 1934 Bihar–Nepal meizoseismal zone and 1897 Assam earthquake meizoseismal zone. b Seismotectonic map of Central and Sikkim Himalaya showing micro-earthquake activity recorded between 1993–1999 by De and Kayal (2004) along with faultplane solution of 1965 and 1980 Sikkim earthquake. The location of 2011 Sikkim earthquake is shown as large black star with its fault-plane solution (CMT solution) given by USGS

Garhwal–Mathura and 1833 Nepal earthquakes (Bilham and Wallace 2005). Historical seismicity in the Himalaya reveals two major seismic gaps: (a) between the 1905 Kangra and the 1934 Bihar–Nepal and (b) between the 1934 Bihar and 1950 Assam, where no large earthquakes of magnitude C8 have occurred during the last three centuries (Fig. 1). The Sikkim Himalaya lies in north-eastern part of the Himalaya arc and located between the 1934 Bihar–Nepal earthquake meizoseismal zone and 1897 Assam earthquake

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meizoseismal zone (Fig. 1). As per seismic zoning map of India, this region lies under zone IV (BIS 2002). On 18 September 2011, the Sikkim region was struck by a damaging earthquake of magnitude 6.9 Mw and caused widespread damage to adobe houses, historical monasteries and multi-storey buildings located as far as 100 km away from the epicentre. The earthquake epicentre (27.723°N, 88.064°E) was located in the high Himalaya region at Indo-Nepal border of Sikkim Himalaya. The focal depth estimates vary from 19.7 km (USGS) to 47.4 km (Global CMT catalogue). The main earthquake was followed by three aftershocks (4.0 [ M \ 5.0) on the same day. In terms of damage, this is the worst earthquake experienced in the Sikkim state during the last two decades, that is, since 1988. The earthquake killed 94 peoples in north Sikkim, 06 in Bihar and West Bengal, 07 in Nepal and one person in China and displaced more than *5,000 persons in Sikkim region. The economic losses estimated to be around 22.3 billion US dollars, out of which 70 % is attributed to the damage to the infrastructures like electricity, telecommunications and water supply. A team of scientists from the Wadia Institute of Himalayan Geology reached the affected region immediately within a week to assess the damage pattern and seismogenic effects of this earthquake. Our observations included the seismological characteristics of the event, damage to engineered and non-engineered buildings, damage to roads, damage to historical monuments like monasteries and forts and other seismogenic effects like ground cracks, landslide lake outburst flood (LLOF) and mud flows.

2 Geological setting and its seismotectonics The earthquake-affected Sikkim region of north-eastern Himalaya has exposed a variety of rocks belonging to a wide range of geological age and tectonic status. These include thin sedimentary rock cover of Tertiary age, affected by the fold thrust movement during the terminal phase of Himalayan orogeny in the south, followed northward by the pre-Tertiary meta-sedimentary, dominantly of Proterozoic age, and the intrusive; the core region exposes a vast expense of pre-Tertiary rocks arranged in a pile of thrust sheet (Ray 2000; De and Kayal 2004; Fig. 1). The earthquake-affected region is marked by three major lineaments, viz. the Kunchendzonga lineament, the Tista lineament and the Gangtok lineament (Fig. 2). Unlike north-west Himalaya, the seismic activity in the Sikkim Himalaya is not uniform (De 2000). The area had experienced two moderate earthquakes in 1965 (M 5.9) and 1980 (M 6.0), which show mainly strike-slip mechanism and were related to the transverse features present in the area (GSI 1993; De and Kayal 2003). The microseismic investigation carried out for Sikkim Himalaya during the period between March and June 2003 by De and Kayal (2004) shows the concentration of seismic activity along NNE-SSW segment of the Main Central Thrust (De 2000). The depth of the earthquakes ranges between 0 and 40 km with maximum concentration between 10 and 30 km (De and Kayal 2004), whereas in NW Himalaya, the concentration of seismic activity lies between 15 and 20 km (Ni and Barazangi 1984). Hazarika et al. (2010) showed concentration of seismic activity at 20 km depth between MBT and north of MCT using 11 broadband seismic stations. However, their depth section reveals seismicity extending to the lower crust with fewer events extending to 70 km depth. The global CMT focal mechanism solutions determined for Sikkim and adjoining Bhutan region reveal strike-slip mechanism for the 19 November 1980 (Mw 6.2), 12 February 2001 (Mw 4.8), 26 March 2005 (Mw 4.7) and 20 May 2007

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Fig. 2 Geological map of Sikkim Himalaya with main transverse tectonic features and lineaments present in the area. Location of main town/villages is marked as triangles. Black dots also mark the location of few villages covered under intensity survey (after Nath 2004)

(Mw 4.9) earthquakes. However, 14 February 2006 (M 5.3) earthquake was followed by a large number of aftershocks that were reported west of Chungthang in north Sikkim had a thrust mechanism (de La Torre et al. 2007).

3 Macroseismic observations The seismic shaking due to an earthquake depends on several factors such as earthquake size, the epicentral distance and the rock type. Qualitative data on ground-shaking were collected for 2 weeks immediately following the main shock to avoid the effect of human activity on the damage pattern. The intensity distribution maps have been an effective tool in evaluating the ground motion characteristics of the region as well as the earthquake magnitude and its focal depth (Hanks et al. 1975; Evernden 1975; Bakun and Wentworth 1997). The intensity values were estimated using the EMS-98 based on damage caused to different vulnerable classes of buildings/structures (Gru¨nthal 1998), effect on landforms and co-seismic changes caused due to the strong motion of the earthquake and its duration. The epicentre of the earthquake lies in the sparsely populated region with an average population density of about 5–25 persons/km2; however, the region south and south-east of the epicentre are moderately populated. In order to assign intensity in each village or town, it is important to identify building typology and the effects of earthquake on each typology. 3.1 Building types The earthquake-affected region is sparsely populated. Most of buildings in Gangtok city, Gezing, Singtam and Mangan are made up of RCC or brick with some earthquake-resistant

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Table 1 Building typology and their vulnerability class in Sikkim Himalaya Type of building

Characteristics

Vulnerability class (EMS-98)

Traditional dwellings—one to two stories

Masonry construction in field, simple stone, nonmanufactured. Mud brick, Ikra-type building made of bamboo sticks. General bad state of preservation

A

Residential buildings (modern)—two to six stories

Masonry constructions in manufactured stone units. Brick with RC floors buildings. RC structure without antiseismic design. Generally in a good state of preservation

C

measures like beams and pillars, and second type of buildings are adobe house that comprises old Buddhist monasteries and Ikra houses (walls of these houses are made of bamboo sticks piled together and plastered from outside using mud or cement, and the roof of these houses is again of bamboos). The adobe and Ikra-type houses have been categorized as building type ‘A’ as per Medvedev–Sponheuer–Karnik (MSK) scale and the European Macroseismic Scale (EMS-98). Other types of construction have been classified as type ‘C’. Detail characteristics of each type of building class present in the earthquakeaffected area are shown in Table 1. The Sikkim region witnesses low-scale damage due to the presence of earthquake-resistant structures in main towns/villages in the southern Sikkim and sparse population in north Sikkim. Figure 3 shows different typologies of building present in the area.

Fig. 3 Building typology present in the earthquake-affected region

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3.2 Earthquake effects and delineation of intensity The intensity survey was carried out from Siliguri (low-intensity zone) covering major cities/towns like Singtam, Gangtok, Mangan, Chungthang (maximum intensity village) and ended with the last village Lachung in the north. The state capital Gangtok was the first main city where major destruction could be observed to few buildings of vulnerable class ‘C’. The Chief Minister’s Office building suffered diagonal crack, indicating Grade 4 damage in spite of having framed structure (Fig. 4a). The main cause behind maximum damage to this building was site amplification. Another major damage was noticed in the Manipal Hospital that has developed diagonal cracks in one or two walls (Grade 3 damage, Fig. 4b). Multi-storey buildings in Balwakhani, a five-storey building in Lumshey Basty and a four-storey building in Dikchu Bazaar suffered heavy damage (Grade 5 damage). Two buildings that suffered Grade 5 damage located in the centre of the city (Balwakhani) were due to their location on downhill slope (Fig. 4c). One of the buildings was found tilted due to this earthquake, which is located on landslide zone as identified by earlier workers (Fig. 4d; Bhasin et al. 2002). Besides the localized damage of Grade 5, most of the buildings in Gangtok city remained intact except minor cracks (Grade 1 or Grade 2) in the ground floor. Historical buildings like Buddhist monasteries and stupas suffered heavy damage due to their old age and adobe type of construction (e.g. Rumtek Monastery near Gangtok city, Pemaynag Monastery at Pelling, Kishinga Monastery and Ringhim

Fig. 4 Development of diagonal crack in Chief Minister’s Office building, Gangtok city, b development of Grade 2 damage crack in Manipal Hospital, Gangtok, c Grade 5 and Grade 4 damage to buildings at Balwakhani, Gangtok city, and d tilt of building due to this earthquake with Grade 3 damage inside the building due to slope failure in the area already identified as prone to slides by experts

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Table 2 Expected damage as per European Macroseismic Scale-98 (EMS-98) and the observed damage in each area with their corresponding intensity level Intensity value

Expected damage according to scale

Observed/examined damage

IX

General panic; many buildings of class ‘A’ sustain damage of Grade 5, class ‘B’ sustain damage of Grade 4, and class ‘C’ may suffer damage of Grade 4

General panic among the community; many buildings of vulnerability class ‘C’ suffered Grade 4 damage, for example Chungthang Theng, Khedum, many buildings. Coseismic features like huge landslides between Theng and Chungthang village

VIII

Many buildings of class ‘A’ suffer damage of Grade 4 but few may suffer Grade 5 damage, and class ‘C’ suffers Grade 3 damage; furniture overturns

Almost all buildings suffered Grade 3 damage to vulnerability class ‘C’. Landslides and rockfalls along the road

VII

Most people are frightened, furniture is shifted and buildings of class ‘A’ suffer damage of Grade 3

Slight damage to old monasteries and cracks in adobe buildings, damage to Rumtek Monastery, Pemayang Monastery (Grade 3) damage); fissures in road, small-scale rockfall

VI

Slightly damaging, Grade 1 damage is sustained in many buildings of classes ‘A’ and ‘B’

Slight damage reported from Singtam, Jorthang, Meri; debris slides reported from Singtam were followed after rainfall. Grade 1 damage observed from Singtam

V

Earthquake was felt indoors by most, hanging objects swing; doors and windows swing

The event was felt up to Siliguri in Darjeeling, Calcutta in West Bengal, Bihar and north India

Total collapse of building is Grade 5, partial collapse of some walls is Grade 4, and opening of wide cracks in walls is Grade 3 damage. Damage Grade 2 refers to the development of visible cracks in walls and Grade 1 refers to hairline cracks in walls

Monastery at Mangan. Based on our observations, the overall damage pattern in the area is shown in Table 2. The Mangan town, located in north Sikkim district, also registered almost the same damage to multi-storey buildings as observed in Gangtok; however, few adobe houses suffered Grade 5 damage and cause casualties (i.e. the church located before Mangan town, Fig. 5). Ringhim Monastery at Mangan suffered Grade 4 damage. This town has also been assigned intensity VII. North of Mangan, damage was increased in terms of landform changes and damage to buildings. Few buildings in Theng village register Grade 4 damage. However, Chungthang village that is located at the confluence of two tributaries of Tista River coming from Lachung and Lachen villages, respectively, registered heavy to very heavy damage. Almost 90 % of the buildings suffered Grade 4 damage to vulnerable building type ‘C’. A four-storey missionary school building suffered a very heavy damage (Grade 5 damage) where first floor was sandwiched between upper three floors and the ground floor; fortunately, no one was killed in the building since the earthquake occurred in the evening (Fig. 6). Buildings constructed on the terrace deposits suffered diagonal cracks of 4–5 inch width on north-facing wall and complete shattering of east- and west-facing walls. The iron bars of a two-storey building were also found twisted towards SE and SW direction (Fig. 7). Figure 7e shows the nature of deposits present in the area where these buildings have suffered heavy damage. This has given the realization that ground motion in this area has increased to intensity IX. Heavy to very heavy damage has also been

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Fig. 5 Grade 5 damage to church located at Mangan, north Sikkim

Fig. 6 Grade 4 damage is observed in missionary school in Chungthang. In this building, second floor is seen sandwiched between the ground floor and upper three floors

reported in different adobe houses located west of Chungthang village. The Kishinga Monastery located west of Chungthang also suffered Grade 4 damage as per reports from local residents of that village. The damage decreased further north as at Lachen and Lachung most of the structures of vulnerability class ‘C’ and ‘A’ registered only Grade 1 and Grade 2 damages, respectively, whereas few class ‘A’ structures suffered Grade 3 damage, thus assigned intensity VII. It is important to mention that both villages, that is, Chungthang (Fig. 7e) and Lachung (Fig. 8), are located on the fan sediments, but the damage noticed in Chungthang was much higher as compared to Lachung; thus, it has been ascertained that the damage at Chungthang village was due to the higher ground motion as compared to Lachung. Unlike Himalayan earthquakes, which have occurred in NW Himalaya, this earthquake has caused very less damage to buildings, probably due to three reasons: (a) multi-storey structures are well built and that too on hard rock basement, although few well-built structures located far away in Gangtok suffered a Grade 4 damage, but the damage has been attributed to slope failure; (b) very sparse population density in meizoseismal zone; and (c) earthquake-resistant traditional houses known as Ikra houses.

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Fig. 7 a Grade 4 damage is seen in buildings located on terrace deposits at Chungthang village, b bending of steel bars due to earthquake motion, c view of the terrace deposits and d blow-up of the terrace sediments

Fig. 8 Grade 2 damage in buildings at Lachung, located on terrace deposits sediments

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4 Secondary changes and collateral effects The Tista catchment areas in the north Sikkim was badly affected due to landslides. The landslides disturbed the road network and cutting north Sikkim region from rest of the Sikkim that hampered the rescue and relief work in this difficult terrain. During September 2011, the area experienced heavy rainfall, and it continued when the damaging earthquake occurred. The continued rainfall followed by damaging earthquake triggered more than 500 landslides. Damage to building and development of ground cracks dominated due to strong shaking in Gangtok city, which is situated on very steep slopes. The co-seismic changes were monitored using traverses along four different sectors, namely south of Gangtok, within the Gangtok city, from Gangtok and Theng, and north of Theng. The traverses were made along road sections and accessible footpaths along the valley up to Theng, and further north airborne survey was carried out using helicopter. However, the ground fractures and fissures were observed at a number of places, and these were associated with the mass movement activities and slope failure. The farthest rockfalls/landslides on the southern (*100 km) end of the epicentre were at Lohapul on Siliguri-Gangtok highway. Most of the slides and rockfalls on this highway were from very unstable slopes. Within the Gangtok city, most of the landslides are either the reactivation of the old landslide zones or in the zone already demarcated as unstable zone by earlier workers (Verma 1993; Bhasin et al. 2002). Almost all the landslides were observed along the road cuts and the lower valley slope. The collapses of the buildings were mainly due to failure of steep slope. Small-scale rockfalls noted along the road cuts were due to failure of intersection of foliation and joints on the steep slopes. It has been noted that during the month of September, the area has received heavy rainfall that continued even after the occurrence of the earthquake. Rainfall acted as a secondary agent in triggering debris slides and mudflows in the area (e.g. Singtam area, Theng, Chungtang, Singik, Tingdha, etc., Fig. 9a). The highest number ([500 slides) and densest concentration of landslides produced by this earthquake were located in the area between Theng (south of Chungthang) and Chungthang and Bichhu (north of Chungthang) in north Sikkim. Most of the landslides were located on the right bank of the Tista River, facing east and south-east. A fewer landslides on the left bank of the river were also observed (Fig. 9b). Landslides, unlike around Gangtok and further south, are located all along the valley slopes. Most of the landslides were classified as debris slide, soil slides, soil falls, rock slides and rockfalls. They are shallow with depths less than 5 m, though deep-seated landslides like translational block slide have also been noted in this sector. North of the Chungthang, the number of landslides decreased, with most of the landslides in the form of typical rockfalls and debris slides. North of the Chungthang village (27°360 18.700 N; 88°380 54.600 E), typical rockfall occurred on south-east-facing steep slopes that have completely destroyed the government dispensary office and the piggery farm (Fig. 9c). The size of the rolled boulders was as big as 7 m in diameter. The ground fissures and cracks were observed at a number of places in the secretariat complex (27°190 28.600 N; 88°360 51.600 E), Gangtok city, and Pelling Helipad. These fissures were developed in unconsolidated sedimentary cover, suggesting site amplification. The structural elements of fissures and cracks show NNE–SSW trend with right-lateral right step. Landslide lake outburst flood was observed in the Lachung village (27°410 39.700 N; 88°440 47.600 E) (Fig. 10). The flood was caused as a result of breaching of an earlier blocked channel due to earthquake shaking (Fig. 10).

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Fig. 9 a Debris slide on way to Singtam from Gangtok, b mass-scale landslides north of Chungthang, and c rockfalls in Chungthang village destroying piggery farm house. Huge boulder is seen inside the house, which has broken all the walls of the house, and thus causes Grade 5 damage in the area

5 Results The study of macroseismic effect related to the 18 September 2011 earthquake has permitted a description of the strong motion fields generated by the earthquake. As per intensity distribution map, maximum intensity observed is IX (EMS-98); it implies a maximum peak ground acceleration of 0.45 g at Chungthang, and it reduced towards north, south and southwest from intensity VIII (0.25 g at Theng, Bichhu, west of Chungthang, etc.) to intensity VII (0.15 g at Mangan, Gangtok city, Gezing, etc.). A number of hydropower projects are coming up in north Sikkim region, but none of the dams developed any crack due to this earthquake. However, most of the tunnels under construction were affected by landslides and rockfalls. The main damage was along the river to the habitation located on terrace or fan deposits and that is the main reason of having high-intensity earthquakes only along the river channel habitations. North Sikkim region lying west of Chungthang (locally known as Dzonga area) suffered maximum number of landslides due to strong ground motion, which can be categorized under intensity zones IX and VIII (Fig. 9b). The intensity distribution map provides a fair picture of the distribution of ground-shaking effects to meizoseismal zone and to far distant places in Sikkim region (Fig. 11). On the basis of damage survey, an intensity distribution map is prepared, which shows an almost north-east–south-west trend with its longest axis in the north-east–south-west direction. The fault-plane solution (CMT solution) provided by USGS shows strike-slip mechanism with two nodal plane trending in north-east– south-west or north-west–south-east direction; incidentally, both these nodal planes match with the existing transverse features. From the tectonics of the area and intensity distribution

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Fig. 10 Landslide lake outburst flood (LLOF) at Lachung, an after-effect of 18 September 2011 earthquake

and fault-plane solution (CMT solution, USGS), it can be inferred that the causative fault of this earthquake was trending north-east–south-west.

6 Discussion and conclusion The Sikkim region occupies a part of the north-eastern Himalaya, which has not been shaken by earthquake except in 1934 (M 8.4) and in 1988 (Ms 6.7), which occurred to the south of Main Boundary Thrust (Fig. 1). However, the region had a number of moderate earthquakes showing predominantly strike-slip mechanism with nodal plane striking in NW–SE and NE–SW directions (GSI 1993; De and Kayal 2003). De and Kayal (2003), while presenting the seismotectonic model of Sikkim Himalaya, concluded that the earthquake activity in Sikkim Himalaya was concentrated mainly along the MBT and NW– SE trending lineament. A composite fault-plane solution of the cluster of micro-earthquakes recorded just north of MCT during the survey conducted by the GSI indicated a predominantly thrust-type mechanism with nodal plane oriented in E–W direction (De et al. 1995). Similar inference was derived by Jounne et al. (2004) while studying the micro-seismicity in central and eastern Nepal. However, the micro-seismic investigation carried out for Sikkim Himalaya during the period between March and June 2003 by De and Kayal (2004) shows bimodal seismic activity with two clusters at different depth levels, that is, first cluster is concentrated at a depth of 20 km along the Himalayan trend in

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Fig. 11 Intensity distribution map of 18 September 2011 Sikkim earthquake

response to the strain accumulation and other cluster around 40 km depth, which is located further north of shallow earthquake cluster. Monsalve et al. (2006) also reached the almost same inference after analysing the earthquake data from a close network that was operated during 2001–2003 in the western Nepal and China region. CMT fault-plane solutions obtained from the similar tectonic setting in Sikkim and Bhutan indicate strike-slip faulting (de la Torre et al. 2007; Drukpa et al. 2006; Hazarika et al. 2010). A more recent study made by Hazarika et al. (2010) using 11 broadband station network along the north–south profile from the MBT to the north of MCT in Sikkim indicated the distribution of microseismicity between the MBT and the MCT and indicated the concentration of seismic activity at 20 km depth south of the higher Himalaya. However, their depth section reveals seismicity extending to the lower crust with few events extending to 70 km depth. The present earthquake was probably the largest earthquake since 1988 Bihar–Nepal earthquake experienced by the Sikkim Himalaya. The focal depth estimates of this earthquake varied from 19.7 km (USGS) to 47.4 km (Global CMT catalogue). The earthquake was widely felt in northern India with very limited aftershock activity after the main event on 18 September 2011, which favours a relatively deeper focal depth. The damage distribution is very heterogeneous and isolated, which might also favour the deeper depth hypothesis. From the previous studies, it is evident that the Sikkim Himalaya is characterized by biomodal seismicity, occurring in mid-crustal level and the lower crust depth. The large earthquakes originating in mid-crust are generated on the MHT (Main Himalayan Thrust) rupturing the locked segment and propagating south to the Himalayan front. The surfaceruptured earthquake reported in the trenches at Chalsa in the Sikkim Himalayan front (Kumar et al. 2010) provides evidence for southward propagated rupture of such earthquakes. The strike-slip fault environment of the 2011 Sikkim earthquake was generated in

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the lower crust, which is in a different seismotectonic setting than the thrust mechanism in mid-crust earthquakes in the north-western Himalaya and Nepal. This may suggest transcurrent deformation that occurs at deeper depth (Thakur et al. 2012). Minimum damage was caused by the Sikkim earthquake compared with the lower-magnitude earthquakes of Garhwal (1991, 1993) as observed (Kumar and Mahajan 1993; Mahajan and Virdi 2001) also suggest deeper focal depth than mid-crustal level. However, using the present intensity distribution pattern, it is difficult to discern whether the rupture occurred on the Kanchendzonga lineament or along any other hidden NE–SW trending lineament until near-field station data are analysed and a more reliable rupture model is proposed. Acknowledgments The authors thank Prof. Anil Kumar Gupta, Director, Wadia Institute of Himalayan Geology, Dehra Dun, for supporting the field investigation immediately after the occurrence of earthquake, as well as his constant encouragement to publish the results of the study. The authors thank Sikkim state administration especially District Magistrate, Mangan, North Sikkim, Sub-Divisional Magistrate, Chungthang, Army Unit at Chungthang, and Managing Director, Tista Urja, and Pawan Hans authorities for providing Heli facility to visit north Sikkim earthquake-affected region.

References Bakun WH, Wentworth CM (1997) Estimating earthquake location and magnitude from seismic intensity data. Bull Seism Soc Am 87(6):1502–1521 Bhasin R, Grimstad E, Larsen JO, Dhawan AK, Singh R, Verma SK, Venkatachalam K (2002) Landslide Hazards and mitigation measures at Gangtok, Sikkim Himalaya. Eng Geol 64:351–368 Bilham R, Wallace K (2005) Future Mw [ 8 earthquakes in the Himalaya: implications from the 26th December 2004 Mw ? 9.0 earthquake on India’s eastern plate margin. Geol Surv India Spec Publ 85:1–14 BIS Code 1893 (2002) Earthquake hazard zoning map of India. http://www.bis.org.in De R (2000) A microearthquake survey at the MBT zone: Sikkim Himalaya. J Geophys 22:77–84 De R, Kayal JR (2003) Seismotectonic model of the Sikkim Himalaya: constraint from microearthquake surveys. Bull Seism Soc Am 93(3):1395–1400 De R, Kayal JR (2004) Seismic activity at the MCT in Sikkim Himalaya. Tectonophysics 386:243–248 de La Torre TL, Monsalve AF, Sheehan S, Sapkota S, Wu F (2007) Earthquake processes of the Himalaya collision zone in eastern Nepal and southern Tibet plateau. Geophys J Int 171:178–738 De R, Saha DK, Chakraborty P (1995). A microearthquake survey in the Himalayan foredeep region, north Bengal area, Unoub. Geol Surv India Report Drukpa D, Velasco AA, Doser DI (2006) Seismicity in the Kingdom of Bhutan (1937–2003): evidences for crustal transcurrent deformation. J Geophys Res 111, B06301:1–14 Evernden JF (1975) Seismic intensities, ‘‘size’’ of earthquakes and related parameters. Bull Seism Soc Am 65:1287–1313 Gru¨nthal G (1998) European macroseismic scale 1998 (EMS-98). Cashiers de centre Europe´en de Geodynamique et de Sesimologie 15:99 GSI (1993) The Bihar-Nepal earthquake of 1934. Mem Geol Surv India 73:287–288 Hanks TC, Hileman JA, Thatcher W (1975) Seismic moments of the larger earthquakes of the southern California region. Geol Soc Am Bull 86:1131–1139 Hazarika P, Kumar MR, Srijayanthi G, Raju PS, Rao NP, Srinagesh D (2010) Transverse tectonics in the Sikkim Himalaya: evidence from seismicity and focal mechanism data. Bull Seism Soc Am 100:1816–1822 Jounne F, Mugnier JL, Gamond JF, Le Fort P, Serrurier L, Flouzat M, Avouac J (2004) Current shortening across the Himalayas of Nepal. Geophys J Int 157:1–14 Kumar S, Mahajan AK (1993) The Uttarkashi earthquake of 20th October, 1991: field observations. Terra Nova 6(2):95–99 Kumar S, Wesnousky SG, Jayangondaperumal R, Nakata T, Kumhara Singh V (2010) Palaeoseismological evidence of surface faulting along the northern Himalayan front, India: timing, size and spaded extent of great earthquake. J Geophys Res 115:B12422. doi:1029 Mahajan AK, Virdi NS (2001) Macroseismic field generated by the 29th March Chamoli Earthquake, 1999 and its seismotectonics. J Asian Earth Sci 19(4):507–516

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Monsalve G, Sheehan A, Schulate-Pelkum V, Rafaure S, Pandey MR, Wu F (2006) Seismicity and one dimensional velocity structure of the Himalayan collision zone: earthquake in the crust and upper mantle. J Geophys Res 111, B10301:1–19 Nath SK (2004) Seismic hazard and microzonation in the Sikkim Himalaya through GIS integration of site effects and strong motion attributes. Nat Hazard 31:319–342 Ni J, Barazangi M (1984) Seismotectonics of the Himalayan collision zone: geometry of the under thrusting Indian plate beneath the Himalaya. J Geophys Res 89:1147–1163 Ray SK (2000) Culmination zones in Eastern Himalaya. Geol Surv India Spec Publ 55:85–94 Thakur VC, Mahajan AK, Gupta V (2012) Seismotectonics of 18 September 2011 Sikkim earthquake: a component of transcurrent deformation in eastern Himalaya. Him Geol 33(1):89–96 Verma PN (1993) Geotechnical report on September, 1983 Landslide in North Sikkim. Geol Surv India (unpublished) Rep 1983–1984, FS

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