Journal of Asian Earth Sciences 70–71 (2013) 179–192 Contents lists available at SciVerse ScienceDirect Journal of Asian Earth Sciences journal homepage: www.elsevier.com/locate/jseaes Causative source of Mw 6.9 Sikkim–Nepal border earthquake of September 2011: GPS baseline observations and strain analysis Rashmi Pradhan a, Sanjay K. Prajapati a,⇑, Sumer Chopra a, Ashok Kumar a, B.K. Bansal a, C.D. Reddy b a b Seismology Division, Ministry of Earth Sciences, New Delhi, India Indian Institute of Geomagnetism, Navi Mumbai, India a r t i c l e i n f o Article history: Received 22 September 2012 Received in revised form 27 February 2013 Accepted 15 March 2013 Available online 27 March 2013 Keywords: GPS baseline Fault movement Riedel shear Strain a b s t r a c t The recent earthquake of Mw 6.9 which occurred on September 18, 2011 in Sikkim–Nepal border region (epicenter 27.72°N, 88.06°E, depth 20.7 km, 68 km NW of the Capital city Gangtok) is the strongest earthquake in the instrumentally recorded history of the region. The fault plane solution of this earthquake indicates a strike-slip motion. However, the seismological and geological studies carried out so far after the earthquake could not conﬁrm the causative fault plane. In the present study, GPS observations are used to ascertain causative source in the generation of earthquake and its correlation with the observed seismic data of the region. The co-seismic displacements recorded by GPS show maximum displacement of 11 mm at Phodong and 9 mm at Taplejung station, near the epicenter. A simple rigid cross fault model using GPS baseline observations was employed to ﬁgure out the causative fault plane and seismological characteristic of the region. It is inferred that the movement represents the kinematic adjustment of the subsidiary faults as a result of the displacement along the NW–SE principal plane. Strain analysis using GPS baseline inferred that the region southeast of epicenter has undergone large deformation. In addition, a signiﬁcant part of the measured deformation across the surface fault zone for this earthquake can be attributed to post-seismic creep. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Research on co-seismic fault movement is of considerable importance not only for academic interest but also for delineation of hazards zones and earthquake prediction, since slip deﬁcit at shallow or intermediate depths at a fault could be ﬁlled by a future earthquake (Aki, 1984; Zhao, 1995). Seismic wave data can resolve temporal and spatial variation of the rupture response of an earthquake, particularly along the strike direction, and therefore the spatial distribution of seismic activity is often used as a constraint for pre-seismic or post-seismic fault movement model (Segall and Harris, 1986). In addition, geological investigations may also offer constrain on the dislocation model of the active fault segment of a fault zone. However, geological results are usually based on very long timescale and therefore these results are more qualitative than quantitative. To overcome the limitation of seismic and geological data, we have to look into the geodetic data which can detect the inter-seismic, co-seismic and post-seismic deformations, and offer a tool for monitoring fault movements with better constraints on the dimensions and amplitudes of pre- and post-seismic fault movements. ⇑ Corresponding author. Tel.: +91 11 43824434. E-mail addresses: email@example.com, firstname.lastname@example.org (S.K. Prajapati). 1367-9120/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jseaes.2013.03.012 The recent developments in space based techniques (GPS, InSAR) have made it possible to monitor the fault movements with high precision. The estimation of co-seismic rupture models and fault movements from geodetic data has been reported by numerous authors (Slade et al., 1984; Satake, 1993). Segall and Harris (1986, 1987) modeled the inter-seismic fault movements using geodetic data and estimated the fault slip distribution at depth on the Park ﬁeld segment of the San Andreas fault, California. On 18th September, 2011 an earthquake of Mw 6.9 shook the Sikkim Himalaya located approximately 68 km NW of the Gangtok city (Fig. 1) gave an opportunity to take advantage of GPS observation in the region to study the deformation pattern. This earthquake claimed several lives in Sikkim and adjoining regions and caused severe damage to the infrastructure, and initiated several landslides (Rajendran et al., 2011; Mahajan et al., 2012). The earlier GPS measurements carried out in NE India indicate that about 15–20 mm/year of convergence of Indian plate is being accommodated in the NE Himalayan wedge (Jade et al., 2007; Mukul et al., 2010). A convergence of 12.32 ± 1.16 mm/year has been observed relative to Lhasa in Darjeeling–Sikkim Himalaya (Mukul et al., 2010). However, these estimates are made from oblique baselines and represent only a component of the arc perpendicular convergence, which was estimated to be about 16 mm/year (Ni and Barazangi, 1984; Molnar and Lyon-Caen, 1989). The eastern Himalaya 180 R. Pradhan et al. / Journal of Asian Earth Sciences 70–71 (2013) 179–192 Fig. 1. The ﬁgure shows tectonic map of the Sikkim Himalaya region. The star indicates the September 18, 2011 earthquake with its fault plane solution. Tista, Gangtok and Kangchenjunga lineaments are shown conﬁned to Sikkim Himalaya region. Major Himalayan thrusts viz. Main Frontal Thrust (MFT), Main Boundary Thrust (MBT), Main Central Thrust (MCT) and South Tibet Detachment System (STDS) are also marked. Focal mechanism solutions numbered 1, 2, 3 and 4 correspond to 12-1-1965 (Mb 5.8), 19-61979 (Mb 5.2), 19-11-1980 (Mb 6.1) and 5-4-1982 (Mb 5) respectively. Source of the map: environmental impact assessment of ting HE project Sikkim. including, Sikkim and Bhutan appears to be a different seismotectonic province as compared to the western Himalaya. The regional strike of Himalaya changes from EW to ENE in Sikkim and Bhutan. Fault plane solutions of medium size earthquakes in Sikkim and Bhutan indicate dominantly strike-slip faulting suggesting transcurrent deformation in this segment of the Himalaya. Seismicity of Sikkim Himalaya is of bimodal nature, occurring in mid-crustal level and at the lower crustal depth (Monsalve et al., 2008; De and Kayal, 2003; Hazarika et al., 2010). The seismological and geological inputs indicate several active faults and 2011 Sikkim event ruptured along one of the active tectonic fault. Further studies carried out by Rajendran et al. (2011), Hazarika and Kumar (2012), Kumar et al. (2012), Thakur et al. (2012) have shown concern regarding the uncertainty in epicentral location and ambiguity in correlating the earthquake with the faults/lineaments due to deﬁciency of substantial supporting evidence. R. Pradhan et al. / Journal of Asian Earth Sciences 70–71 (2013) 179–192 The present study has been undertaken to examine the co-seismic deformation due to Sikkim earthquake using data from 9 continuous GPS stations operating in and around the Sikkim–Nepal Himalaya. We have also carried out co-seismic modeling using GPS position timeseries and a simple rigid model analysis of the GPS baseline changes during the event and tried to investigate fault activity and strain pattern due to this earthquake. 2. Seismotectonics of Sikkim Himalaya The origin of Himalaya is attributed to the continent–continent collision between the Indian and the Eurasian plates. The geological provinces of the NE Indian peninsula have been characterized by complex tectonic features. The Sikkim Himalaya is part of the active Himalayan Fold-Thrust Belt (FTB) and comprises of Sub, Lesser, Higher and parts of Tibetan Himalaya which are demarcated by the Main Boundary Thrust (MBT) and the Main Central Thrust (MCT) and South Tibetan Detachment System (STDS) respectively. The low-grade metamorphic rocks of Lesser Himalaya Group are arcuately (arc like) folded with the MCT taking sinusoidal turn. The northern parts of the fold are characterized by medium- to high-grade crystalline rocks of the Higher Himalaya. The South Tibetan Detachment System (STDS) marks the northern boundary of Sikkim. Besides the major thrusts, various N–S trending gravity faults and several lineaments cut across the Himalayan belt. The prominent lineaments in Sikkim Himalaya include NW–SE trending Tista, Gangtok (GSI, 2000) and Purnea-Everest lineament, WNW–ESE trending Golapara lineament (De and Kayal, 2003), NE trending Kanchenjunga, Arun and Everest lineament (Fig. 1). The ongoing NE convergence of the Indian plate is accommodated within the crust and is released from time to time in the form of earthquakes all over the Himalayan belt. The eastern re- 181 gion of India has experienced several earthquakes in the past, the important ones being the Shillong (1897) M 8.7, Bihar–Nepal (1934) M 8.3, Assam (1950) M 8.7, Gangtok (1988) M 6.6, Phodong (2006) M 5.3 (Kumar et al., 2012). Apart from the presence of several thrusts, lineaments and faults in the study region, the fault mechanism for major earthquakes predominantly reﬂects transverse tectonics. Despite being the region of convergence, the eastern Himalaya is controlled by strike-slip faulting with close proximity to the NE, NW and NS oriented lineaments and faults that cut across the Himalayas (Dasgupta et al., 1987; Hazarika et al., 2010). De and Kayal (2003) have however reported two distinct, tectonically active zones in the Sikkim Himalaya; a north dipping, deep rooted seismogenic structure with seismicity distributed from the surface down to the Moho depth (0–45 km) presenting thrust mechanism associated with MBT and another transverse to the trend of MBT with seismicity clustered within the middle crust (10–25 km), depicting strike slip mechanism along the Golapara lineament, anticipated as the NW extension of the Golapara wedge. The observations of De and Kayal (2003) support the evolutionary model of Ni and Barazangi (1984) which suggest MBT as the active zone of convergence with the MCT being a dormant thrust. Unlike the rest of Himalaya, the MBT in Sikkim Himalaya does not converge with the plane of detachment besides it has been deﬁned as a mantle reaching thrust on the basis of gravity studies (Choudhury and Dutta,1975) and microearthquake activity (De, 2000; Kayal, 2001). The Sikkim Himalaya comes under Zone IV (BIS-1893, 2002) of the seismic zoning map of India. The epicenter of 2011 Sikkim earthquake is located NE to the junction of Kanchenjunga and Tista lineament. The focal mechanism of the earthquake (IMD Report, 2012) suggests strike slip faulting at 58 km depth (MT solution) with the nodal planes oriented in NW–SE and NE–SW directions and seems to correspond to Tista and Kanchenjunga respectively. Fig. 2. GPS station distribution map. The thick grey lines represent NE–SW trending Kanchenjunga fault and the NW–SE trending Tista lineament. Total 22 baselines have been shown by dashed, thick and thin black lines. The dashed and thick black lines represent the baselines which are cutting Tista lineament and Kanchenjunga fault respectively. Total 12 baselines (dashed and thick lines) are used for block model calculation. The star represents the epicentral location provided by IMD. Inset map shows the 11 IGS sites used for position coordinate analysis. Rectangular block shows the study area. 182 R. Pradhan et al. / Journal of Asian Earth Sciences 70–71 (2013) 179–192 An alternate Centroid Moment Tensor (CMT) solution by India Meteorological Department (IMD) indicates reverse faulting at a centroid moment depth of 10 km. The reported aftershocks are linearly clustered in between Tista and Golapara lineament (Kumar et al., 2012). Several landslides along Tista lineament initiated as a result of the earthquake have been reported by Chakraborty et al., 2011; Rajendran et al., 2011. 3. GPS data processing The study utilizes GPS data of three Indian stations namely, Shillong (SHIL), Tezpur (TZPR) and Phodong (PHOD), ﬁve stations from Nepal GPS network namely, Odare (ODRE), Taplejung (TPLJ), Rumjartar (RMJT), Ramite (RMTE), Biratnagar2 (BRN2) and one IGS station at Lhasa (LHAZ) (Fig. 2). The Shillong (SHIL) site was established in 2006 by Indian Institute of Geomagnetism, under the sponsored program of Multi-parametric Geophysical observatory, Ministry of Earth Sciences. Phodong (PHOD) station is operated by the Indian Institute of Geomagnetism, Navi Mumbai. Tezpur (TZPR) GPS sites were established under the mission mode project of the Ministry of Earth Science to enhance the GPS coverage in the Himalaya of NE India and is operated by Department of Physics, Tezpur University. The Nepal GPS network data is provided by the Plate Boundary Observatory (PBO) operated by UNAVCO for Earthscope. Shillong, Tezpur and Phodong GPS data were collected using dual-frequency geodetic GPS receivers with choke ring antenna at sampling interval of 30 s and an elevation mask of 11°. The GPS antennas are installed over well settled elevated concrete structures. However GPS data obtained from other network i.e. PBO, IGS have different conﬁguration i.e. Choke ring antenna with varying sampling interval of 15 s and 30 s with elevation mask of 11°. Most of IGS GPS antennas are installed on concrete pillar whereas some of the Nepal network GPS antennas are installed on basement rock using bipod/tripod. The data were organized into 24-h segments covering a UTC day to facilitate integration of more data from eleven surrounding International GNSS Service (IGS) sites, viz. IISC, HYDE, DGAR, PIMO, KUNM, WUHN, LHAZ, GUAO, SELE, POL2 and KIT3. GPS data was processed in a two-step procedure using the GAMIT/GLOBK software (King and Bock 1998, 2000) with IGS precise orbits and IGS earth rotation parameters. In the ﬁrst step, daily loose GAMIT solutions are obtained which accounted for error contributions due to signal path delay caused by atmosphere, orbital accuracy, antenna phase center variations, signal multi-path and scattering by receiver environment, satellite and receiver clock errors. Ambiguity free and ambiguity ﬁxed solutions were performed with ionosphere free linear combination to account for carrier phase ambiguities and signal delay due to ionosphere. Precise satellite orbits provided by Scripps Orbit and Permanent Array Center (SOPAC, http://sopac.ucsd.edu) were used. GPS site coordinates, satellite orbits, tropospheric zenith delays, earth orientation parameters and phase ambiguities of the carrier waves were estimated daily and independently by weighted least square technique. Zenith tropospheric delay for each GPS station was estimated by incorporating a piecewise linear model with stochastic constraints and then corrected for the signal delay due to troposphere. Loosely constrained daily solutions from global tracking sites were combined with daily local solutions using GLOBK, resulting in a loosely constrained position time series for the entire survey span. The combined solutions were then passed to a Kalman ﬁlter, through GLOBK software, to estimate network adjusted site coordinates and velocities. Some of the IGS sites used in the analysis were constrained according to reported values of reference station positions and velocities with standard errors provided by IGS (Dow et al., 2009). ITRF2008 reference frame was realized through GLORG using local generated H-ﬁles. Typical precision of position solution was 2–3 mm for north component, 3–5 mm for east component, and 10–15 mm for vertical component. 4. Co-seismic displacement using GPS observation We have used GPS position coordinate to compute the co-seismic displacement within 100 km of the earthquake epicenter. The time plot of the position coordinates at permanent GPS site SHIL, TZPR and LHAZ (Supplementary material) do not reveal any change in the position coordinates (in NS, EW and UP components) either during the period of stress build up (precursory) or during sudden release of strain (co-seismic) in association with the earthquake of September 18, 2011. Therefore, we consider SHIL as a stable site (SHIL is having longer timeseries since 2006) to remove the plate motion effect from GPS sites namely TPLJ, PHOD, RMJT, RMTE, and ODRE. Further, by averaging and subtracting the pre- and post-seismic daily positions during three days before and after the earthquake, we are able to detect sub-centimeter co-seismic displacements (Table 1). In order to determine the causative fault, we have used the elastic dislocation model (Okada, 1992) to estimate the co-seismic displacements in a region around the epicenter area of the Sikkim earthquake. Constrained by GPS derived displacement, we simulated the spatial disribution of co-seismic displacement for both the Nodal Planes (NP1-N216°E, dip 79°, Rake 15° and NP2N124°E, dip 75°, Rake 169°) and fault parameters (L = 50 km and W = 15 km) with varying depth. We found that the NP2 parameters at 20 km depth provided co-seismic displacement ﬁeld close to the observed (shown in Fig. 3 and Table 1). While, for NP1 the predicted displacements mismatches with the observed displacements (Table 1). 5. Estimation of fault movement using baseline changes 2011 Sikkim earthquake is believed to be caused by strike slip faulting. The microseismic and geomorphologic studies infer a dextral strike-slip faulting, possibly along a NW–SE oriented fault (Rajendran et al., 2011). The landslides triggered by this earth- Table 1 The table shows the observed and calculated co-seismic displacements at ﬁve GPS sites using Okada dislocation model for both nodal plane (i.e. NP1 and NP2). Station TPLJ RMTE RMJT ODRE PHOD NP1 NP2 Observed displacement (cm) Calculated displacement (cm) Observed displacement (cm) Calculated displacement (cm) N E U N E U N E U N E U 0.275 0.164 0.286 0.055 1.103 0.807 0.147 0.0187 0.206 0.997 0.836 0.389 0.399 0.065 2.442 0.1635 0.04 0.03 0.1 0.02 0.5853 0.2 0.25 0.09 0.7 0.41 0.01 0.02 0.01 0.14 0.275 0.164 0.286 0.055 1.103 0.807 0.147 0.0187 0.206 0.997 0.836 0.389 0.399 0.065 2.442 0.2966 0.0425 0.264 0.0293 2.117 0.9914 0.2151 0.0331 0.2368 0.8395 0.2993 0.0231 0.0271 0.0072 0.0875 183 R. Pradhan et al. / Journal of Asian Earth Sciences 70–71 (2013) 179–192 Fig. 3. Co-seismic displacements calculated for 18th September 2011 Sikkim Himalaya earthquake using Okada (1992) dislocation model. The left and right panel shows the observed and calculated displacement considering NP1 and NP2 fault parameter respectively. Red star shows the location of Sikkim earthquake. The rectangular blue block shows the fault model. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.) quake are concentrated along Tista lineament (Chakraborty et al., 2011; Thakur et al., 2012; Rajendran et al., 2011). On the basis of the aftershock distribution, Kumar et al. (2012), have suggested possible NW orientation of the causative fault plane. In summary, the studies so far carried out by various researchers (Rajendran et al., 2011; Hazarika and Kumar, 2012; Kumar et al., 2012; Thakur et al., 2012) have shown concern regarding the epicenter location and ambiguity in correlating the earthquake with the faults/lineaments due to deﬁciency of substantial supporting dataset/evidence. As the earthquake reﬂects a complex source, inputs from other datasets are required to facilitate the understanding of the source characteristics and its relation to the seismogenic structures. Therefore, the present study was undertaken to examine the co-seismic deformation due to the earthquake using data from 9 continuous GPS stations operating in the nearby region of epicenter. In this study, GPS baseline changes and cross-fault baseline observations are made to investigate the fault activity and strain behavior before and after the Sikkim earthquake 2011. 6. GPS baseline analysis In this study, we follow a different approach. Instead of operating directly on the station coordinates, we ﬁrst calculated baseline lengths between stations, which are inherently free from the common systematic effects in the station coordinates such as reference frame errors. Twenty-two (22) baselines (Table 2, Fig. 4) were analyzed to study the characteristics of deformation pattern before and after the Sikkim event. Fig. 2 shows that the longest baseline is B7 (537 km, approx.) and the smallest is B10 (35 km approx.). GPS timeseries analysis indicate that stations namely LHAZ, TZPR and SHIL (Supplementary material) which are approx. 400 km away from the epicenter have not observed co-seismic effect due to the Sikkim earthquake. In view of this, these three stations were considered as stable stations for comparison of the movement along various baselines. We found that baselines B1, B3, B5 and B18 (Fig. 4, Table 2) clearly reﬂect the changes where as other station closer to epicenter (RMJT, RMTE, ODRE) are not showing any change in baseline with respect to stable GPS station (B9, B7, B14, B17). Apart from the above mentioned baselines (B1, B3, B5, B18), longer stable baselines could not reﬂect clearly any changes with respect to GPS stations near to epicenter (Table 2, Fig. 4). TPLJ is the closest GPS station to the epicenter (42 km) of Sikkim earthquake (Fig. 2). Taking into consideration the positional proximity of TPLJ station with the epicenter, we focused on the baseline changes from TPLJ to the surrounding stations. Fig. 4 shows an increase in the baseline B1, B2 and B3 indicating extension in the region. However, the baseline B8 and B15 represent shortening in the region. Assuming all other stations undeformed and considering the baseline changes, the south-west displacement of TPLJ station satisﬁes the lengthening and shortening of the baselines. The deformation towards south of TPLJ seems to have ceased beyond ODRE, as evident from negligible change in the baseline B22 (Fig. 4). Since, TPLJ station lies in the middle of the Kanchenjunga and Tista, its displacement may reﬂect the movement along anyone of them. Hence, for further validation of the movement in the epicentral region, we applied rigid block model for cross-fault observation along and across the existing nearby faults/lineaments. Table 2 The table shows 22 baselines in which 12 baselines used for cross fault observation and 16 used for strain analysis. The table shows the angles between each baseline and the fault direction (a), measured w.r. to Tista lineament (B1, B2, B3, B4, B5, B6, B7) and Kanchenjunga fault (B11, B12, B14, B15, B16) which were used for cross fault studies. The Baseline azimuth (h) was used for strain computation. Baseline (BL) Stations Angle from BL to fault (a) Azimuth of BL (h) Baseline (BL) Stations Angle from BL to fault (a) Azimuth of BL (h) B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11 TPLJ–LHAZ TPLJ–PHOD TPLJ–TZPR TPLJ–SHIL ODRE–LHAZ ODRE–PHOD ODRE–TZPR ODRE–TPLJ ODRE–SHIL RMTE–RMJT RMTE–TPLJ 92 62 49 35 95 83 55 – – – 148 55 86 – 112 53 65 – 33 106 171 72 B12 B13 B14 B15 B16 B17 B18 B19 B20 B21 B22 RMTE–ODRE RMTE–PHOD RMTE–LHAZ RMJT–TPLJ RMJT–ODRE RMJT–LHAZ PHOD–LHAZ PHOD–SHIL SHIL–LHAZ RMJT–PHOD ODRE–BRN2 125 – 162 134 105 – – – – – – 98 78 59 87 117 62 48 120 170 – – 184 R. Pradhan et al. / Journal of Asian Earth Sciences 70–71 (2013) 179–192 Fig. 4. GPS baseline changes observed along 22 baselines. Star shows the day of the 18th September 2011 Sikkim earthquake. where a and b are the angles between baselines AC, BC and the fault direction (Fig. 5). Then the horizontal movements could be 7. Characterstics of deformation using baseline changes observation We investigated fault movement in epicentral area by observing the horizontal displacement along and across the fault. Huang et al. (2010) have introduced several models for cross-fault observation using short baseline method for crustal deformation studies. Since the studied region has Kanchenjunga fault and Tista lineament as the major structural features in the vicinity of epicenter, the rigid block model was considered favourable for the present study, which aimed at determining the horizontal displacements along (U) and across (V) the faults. For calculating U and V at least two baselines AC and BC are required which cut across the fault (Fig. 5). AC ¼ U cos a þ V sin a BC ¼ U cos b þ V sin b ð1Þ U ¼ ðDAC cos b BC cos aÞ=ðcos a sin b cos b sin aÞ ð2aÞ V ¼ ðDAC sin b BC sin aÞ=ðcos a sin b cos b sin aÞ ð2bÞ For three baselines, U and V could be calculated by 1 0 cos a1 Db1 B C B ¼ D b2 @ A @ cos a2 cos a3 Db3 0 sin a1 1 C U sin a2 A V sin a3 ð3Þ For this study, out of 22 baselines, twelve baselines were chosen, in which ﬁve cut across Kanchenjunga fault and seven cut across the Tista lineament (Fig. 2). The baselines were chosen in R. Pradhan et al. / Journal of Asian Earth Sciences 70–71 (2013) 179–192 185 Fig. 4. (continued) such a manner that it cuts only single lineament/fault. Different baseline combinations (listed in Table 3) were substituted in Eq. (3) to obtain U and V values for Kanchenjunga fault and Tista lineament. a and b are angles for baselines as listed in Table 2. For NE oriented Kanchenjunga fault (following the right hand rule), the U and V are positive in NE and NW directions respectively. Similarly, for Tista lineament the positive U and V point towards NW and SW direction respectively. Fig. 6a and b correspond to the horizontal displacement obtained along and across the Tista and Kanchenjunga respectively. From Fig. 6b, no clear movement is depicted (group 7 and 8) along and across the Kanchenjunga fault except for group 6 indicating negligible positive change along the fault which might be the reﬂection of SW displacement of TPLJ station whereas for Tista we noticed prominent positive changes perpen- dicular to the orientation of the lineament (Fig. 6a). As evident from Fig. 6c, the displacement obtained using all the baselines cutting across Kanchenjunga (Fig. 6c, left) and Tista (Fig. 6c, right) depict similar pattern as observed using different baseline combinations (Fig. 6a and b). 8. Strain analysis As a result of tectonic plate movements, enormous forces are applied to the earth’s crust. These forces when applied to rocks, results in the alteration of position and shape. In general, strain analysis is used for the determination of deformation pattern. As illustrated by Deniz and Ozener (2010), the ratio of baselines is independent from the translation and rotation effect of the datum. 186 R. Pradhan et al. / Journal of Asian Earth Sciences 70–71 (2013) 179–192 Fig. 4. (continued) Table 3 Baseline combination used for calculating horizontal U and V movement. Fig. 5. Rigid block model crossed by a strike-slip fault F–F00 (Bold black line). Black ﬁlled circles mark the GPS points on the rigid blocks. a and b are the angles measured clockwise from the baselines AC and BC to the fault respectively. Group no. Tista lineament Group no. Kanchenjunga lineament 1. 2. 3. 4. 5. B1, B1, B1, B1, B1, 6. 7. 8. B15, B11, B16 B14, B11, B12 B14, B15, B16 B2, B2, B5, B3, B3, B6 B4 B4 B4 B7 tion of strain near the two possible rupture zone namely, Kanchenjunga and Tista. Deniz and Ozener (2010) have avoided the intersection of the baseline with any fault. In our study area, such criterion could not be followed due to very complex tectonics. However, the aim of the study is to focus on determining the strain change with respect to the faults and lineaments in the region, in order to get the strain pattern which strongly depends on neighboring structure, namely, existence of the fault. The strain tensors for each triangle were calculated from the following equation: e ¼ exx cos2 h þ exy sin2 h þ eyy sin2 h Therefore, the baseline data was used for studying the strain variations in the Sikkim region before and after the earthquake. The strain was calculated using the triangulation method. The triangles T1–T12 (Table 4) were constructed so as to facilitate the calcula- ð4Þ where exx, exy and eyy are the strain tensor parameters, e is the strain rate and h is the azimuth of the baseline. The strain rate is calculated from the following equation: R. Pradhan et al. / Journal of Asian Earth Sciences 70–71 (2013) 179–192 187 Fig. 6. (a) U–V changes for Group 1–5 show the displacement calculated for Tista lineament (Group combinations are listed in Table 3). All the baselines used here cut across Tista lineament. (b) U–V changes for Group 6–8 show the displacement calculated for Kanchenjunga fault (Group combinations are listed in Table 3). All the baselines used here cut across Kanchenjunga fault. (c) U and V changes show the displacements obtained by combining all baselines (with observations during earthquake) for Kanchenjunga (left) and Tista (right). Fig. 6. (continued) e ¼ ðBL2 BL1 Þ=ðDt BL1 Þ ð5Þ where BL1 is the initial baseline at epoch t1 and BL2 is the baseline at epoch t2, Dt is the time interval (t2–t1). The exx, exy and eyy for each triangle was calculated for the available data of year 2011. The values were then substituted in Eqs. (6)–(14) to obtain the strain parameters. 188 R. Pradhan et al. / Journal of Asian Earth Sciences 70–71 (2013) 179–192 Fig. 6. (continued) D ¼ exx þ eyy c1 ¼ exx eyy c2 ¼ 2exy pﬃﬃﬃ c ¼ c21 þ c22 E1 ¼ 0:5ðD þ cÞ E2 ¼ 0:5ðD cÞ b ¼ arctanðexy =ðE1 exy ÞÞ ESHEAR ¼ 0:5ðE1 E2 Þ EINTER ¼ 0:5ðE1 þ E2 Þ ð6Þ ð7Þ ð8Þ ð9Þ ð10Þ ð11Þ ð12Þ ð13Þ ð14Þ where D is dilatancy, c1 is principal shear strain, c2 is engineering shear strain, c is total shear strain, E1 and E2 are maximum and minimum principal strain, b is direction of maximum principal strain arc. ESHEAR and EINTER are the maximum shear strain and maximum normal strain respectively. The data from DOY 200–267 was used for calculating the strain variation in the region. Table 2 represents the azimuths (h) of various baselines. The principal strain parameters values so obtained were averaged three days before and after the earthquake, excluding the earthquake day data. Table 5 shows the principal strain parameters for the twelve triangles. Fig. 7a shows comparative plot of E1 and E2 variation for triangles T1–12. The dot, circles and triangles in the plot denote the principal strain values for the whole dataset, 3 days before and after the earthquake respectively. Fig. 7b shows plots of twelve triangulations (T1–T12) where the left, middle and right panel represents the strain obtained from whole dataset (200–252), 3 days before and 3 days after the earthquake respectively. As seen from Fig. 7b triangle T7 and T3 have undergone larger strain change followed by decreasing trend in triangles T10, and T6. The equilibrium point for triangles T7 lie closer to Kanchenjunga and the values obtained for E1 and E2 have undergone major change after the earthquake whereas for triangle T3 which is mid way between Kanchenjunga and Tista shows similar pattern as T7. 9. Summary and discussion The seismological and geological studies carried out so far after the Sikkim earthquake could not ascertain the causative source due to limited availability of observation. The inaccessibility due to tough terrain conditions and earthquake initiated landslides mired the geological studies to collect ﬁeld evidences in the region. The availability of GPS stations in the region, near to the epicenter provided the opportunity to carry out the study to ascertain the role of tectonics in the generation of the earthquake and its correlation with the seismicity of the region. Based on GPS study we tried to determine the fault movement in Sikkim–Nepal region as a result of the 2011 Sikkim earthquake. The available baseline changes clearly signify that TPLJ station has undergone maximum deformation due to the earthquake, however the deformation seems to have ceased southwards beyond ODRE and westwards beyond Kanchenjunga fault as evident from negligible change in the baseline between the stations namely ODRE– BRN2 and RMJT–RMTE. The motion of TPLJ suggests nearly SW movement as visualized from the prominent change in B1 baseline (Fig. 4). Further as an additional constraint, we modeled the coseismic displacement using Okada dislocation model (Okada, 1992). The computed displacements estimated using both the nodal planes with varying depth suggest that NP2 is the possible source fault of the Sikkim earthquake. The cross-fault observation studies using GPS baseline show no signiﬁcant change for U component in group 1–5 (Table 3) while the prominent positive changes in V component indicate considerable movement perpendicular to Tista lineament in SW direction (Fig. 6a). There is negligible change observed from group 7–8 for Table 4 Baseline combinations used for strain calculations. Triangulation Baseline combinations Triangulation Baseline combinations T1 T2 T3 T4 T5 T6 B15, B11, B10 B11, B8, B12 B8, B2, B6 B15, B8, B16 B10, B12, B16 B12, B6, B13 T7 T8 T9 T10 T11 T12 B11, B2, B13 B16, B5, B17 B15, B1, B17 B2, B18, B1 B6, B19, B9 B19, B20, B18 189 R. Pradhan et al. / Journal of Asian Earth Sciences 70–71 (2013) 179–192 Table 5 Principal strain parameters before and after the sikkim earthquake computed using various baseline combination listed in table number 4. Triangle T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12 Before earthquake Coordinates of equilibrium point Principal strain components Longitude Latitude E1 (lstrain) E2 (lstrain) 86.95 87.23 87.89 87.22 86.85 87.52 87.63 88.35 88.45 89.13 89.27 90.51 27.22 27.07 27.21 27.17 27.05 27.09 27.25 27.94 28.10 28.14 26.61 27.55 0.02 0.11 0.72 0.02 0.16 0.49 1.51 0.002 0.01 1.01 0.48 0.14 0.56 0.16 0.66 0.16 0.52 0.39 2.36 0.13 0.09 0.36 0.84 0.75 Kanchenjunga except for an unclear positive change in the U component from Group 6 (Table 3, Fig. 6b). Using the fault coordinate system, and assuming that the SE wall of Kanchenjunga is ﬁxed, the positive U indicates the dextral movement along the fault. However, the focal mechanism determined by IMD suggest dextral motion along NW nodal plane and sinistral motion along the NE After earthquake Angle of the principal strain (b) Principal strain components E1 (lstrain) E2 (lstrain) 53.15 25.76 30.83 41.83 15.80 30.95 32.02 51.02 20.53 28.19 22.22 57.75 0.08 0.29 1.98 0.03 0.30 1.27 3.93 0.001 0.09 1.93 1.40 0.08 0.41 0.37 1.28 0.22 0.34 0.25 5.80 0.08 0.18 1.94 0.62 0.63 Angle of the principal strain (b) 32.47 27.40 13.16 64.82 64.14 51.75 25.54 21.56 21.52 9.95 63.67 29.61 nodal plane. The positive change in U component therefore might be the reﬂection of extension of TPLJ from LHAZ. The study is based on rigid cross fault model and may contain nominal error due to lack of dense GPS network observations. However we argue that model estimated from the GPS data can still throw light on the main characteristics of the fault motion. Fig. 7. (a) Figure shows E1, E2 plot for triangle T1–T12. For E1 maximum changes are observed in T 7, 3, 6, 10 and 11 while for E2 the changes are observed in E7 followed by T10 and T3. The region east to the Kanchenjunga fault has undergone much deformation. (b) Strain plots computed for 16 baseline combination as shown in Table 4. Left panel shows the strain behaviour for whole data set (200–280 days) whereas Middle and right panel shows the 3 days of dataset before the and after the Sikkim earthquake respectively. The Star denotes the epicenter of 18th September, 2012 earthquake and blueline shows the triangle used for strain computation. 190 R. Pradhan et al. / Journal of Asian Earth Sciences 70–71 (2013) 179–192 Fig. 7. (continued) In addition to the baseline observation, strain analysis for various triangulations reveal that signiﬁcant strain changes occurred due to the earthquake around the source area of Sikkim earthquake. For all triangulations, the trend of the maximum and minimum strain axis obtained from the data 3 days prior to the earthquake matches well with that obtained from the processing of whole baseline timeseries (200–252 DOY) (Fig. 7b, Left and middle panel). The maximum and minimum strain axes for T3, T7 and T10 have undergone a major change after the earthquake (Fig. 7b, right panel). T6 has shown reversal in the maximum and minimum strain axis however, the orientation of strain axis prior to the earthquake matches with that for nearby triangles (Fig. 7b). It appears that the SW deformation started ceasing, creating localized change in the strain behavior of this region. The maximum displacement has occurred in the central region (near the epicenter) decreasing gradually towards the end of the fault/lineament. The strain changes are high in the region between Kanchenjunga and Tista, more prominently near to the epicenter. The fault plane solution provided by IMD, (IMD Report, 2012) New delhi denotes dextral strike-slip motion along the N124°E oriented nodal plane. Hazarika et al. (2010) have also shown NNW trend of P-axis orientations in the Sikkim Himalaya which differentiate it further from the rest of the Himalaya with well-known north–northeast trend. The focal mechanisms for several earthquakes in the NE Himalaya suggest ongoing transcurrent deformation in Sikkim and adjoining regions (Drukpa et al., 2006; Hazarika et al., 2010). The NNW orientation of P-axis (Hazarika et al., 2010) and its variation to NE have been interpreted as the accommodation of the oblique convergence along deep rooted vertical planes (Torre et al., 2007). The GPS study in Sikkim Himalaya reveals that the minimum principal strain axis is oriented in the WNW direction both before and after the earthquake (Fig. 7b, middle and right panel), indicating the orientation of the direction of maximum compression. The pre-existing cross-structures can act as segment barriers during the rupture of a single fault segment or they can be activated as transfer zones inducing the activation of the adjacent segments that belong to the same fault system (Pizzi and Galadini, 2009). Fragmentation barrier (King, 1986), cross-structure, structural barriers (Pizzi and Galadini, 2009) are some of the terms describing near-end fault branching, fracturing or intersection with other faults. Such parallel or conjugate sets of strike-slip faults accommodate crustal deformation over broad areas often rotating the crustal blocks about vertical axes (Luyendyk, 1991). The geometric arrangement of these structures has been studied in detail by various researchers (King, 1986; Bhat et al., 2007) and found that such fault structures affect the fault dynamics, rupture directivity, background and aftershock concentration. The Sikkim Himalaya region is dissected by several parallel, sub-parallel and intersecting faults and lineaments chieﬂy trending N–S, E–W, NW–SE, NE–SW. Extending the Gangtok, Golapara, and Tista lineaments in NW direction to intersect the NE trending Kanchenjunga fault close to epicenter region, it appears that the lineaments/fractures in the study region are arranged en echelon, inclined from 10° to 30° to the relative movement direction like riedel shear (Riedel, 1929). In such case, if the main fault undergoes strike slip motion, the adjoining block moves down due to kinematic adjustment (Fig. 8). The occurrence of the aftershocks aligned in WNW–ESE direction may be attributed to the transfer of the energy along such a preexisting fault/fracture/subsidiary set of fault. The results of R. Pradhan et al. / Journal of Asian Earth Sciences 70–71 (2013) 179–192 191 Fig. 7. (continued) strain observations, co-seismic displacement, and focal mechanism solution, conﬁrmed that the movement has taken place along a NW to WNW oriented plane. 10. Conclusion Fig. 8. The Sketch shows the geological arrangement of faults/fractures inclined at angles 10–30° and represent that the cross-faults/fractures in present study region may undergo movement in similar pattern as the major fault. The 18th September, 2011 Sikkim earthquake connote the complexities and diverse tectonic setting of the Himalayas. This study focuses on co-seismic displacement and investigation of the relative movement of GPS baselines along and across the existing geological fault/lineament around the Sikkim event. It has been observed that the region east of Kanchenjunga fault has undergone large deformation (10 mm) while the region on the west appears moreover stable. The deformation is localized with most of it occurring within 100 km radius as observed in the case of TPLJ, the deformation almost ceased beyond it as evident from negligible change in the baseline B22. It has been observed from the strain analysis, supported by co-seismic displacement calculation and the aftershock alignment, that a NW to WNW oriented plane may be the contributing source of the 18th September, 2011 Sikkim earthquake. However, absence of any GPS station north and north-west of the epicenter may put some uncertainties in the result. Further, to delineate the pre-existing fractures and to better understand the behavior of the seismogenic faults, the seismological data must be compared with the detailed geological, geodetic and structural data. Since the entire higher Himalaya region has rugged topography and vastly inaccessible, prohibiting detail studies in the region, it is the need of the hour to carry out studies jointly with various 192 R. Pradhan et al. / Journal of Asian Earth Sciences 70–71 (2013) 179–192 institutes to provide inputs through geophysical and geological observations in order to delineate all possibilities of the seismogenic potential of the region. Acknowledgements We are greatly thankful to Secretary, Ministry of Earth Sciences, Govt. of India for giving permission to publish this work and extending all facilities. We acknowledge Indian Institute of Geomagnetism, Navi Mumbai, India, Prof. Ashok Kumar, Department of Physics, Tezpur University, Assam, India and Plate Boundary Observatory (PBO), California, USA for providing the GPS data of Shillong MPGO, Tezpur, and Nepal newtork respectively. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.jseaes.2013.03.012. Reference Aki, K., 1984. Asperities, barriers, characteristic earthquakes and strong motion prediction. 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