current understanding of the seismotectonics of western nepal himalaya and vicinity

Acta Geod. Geoph. Hung., Vol. 45(2), pp. 195–209 (2010)

DOI: 10.1556/AGeod.45.2010.2.5

CURRENT UNDERSTANDINGOF THE SEISMOTECTONICS OF WESTERN

NEPAL HIMALAYA AND VICINITY

H Paudyal1, D Shanker2, H N Singh3, A Panthi3, A Kumar3,V P Singh3

1Department of Physics, Birendra Multiple Campus, Tribhuvan University,Kathmandu, Nepal, e-mail: [email protected]

2Department of Earthquake Engineering, Indian Institute of Technology Roorkee,Roorkee-247667, India, e-mail: [email protected]

3Faculty of Science, Department of Geophysics, Banaras Hindu University,Varanasi-221 005, India, e-mail: [email protected]

[Manuscript received May 5, 2009; accepted October 13, 2009]

The study of seismic activity at some stage in 1963 to 2006 in the Western NepalHimalaya and its adjoining regions (28−31◦N and 79−82.3◦E), reveal that seismicityis non-uniform in space and time. The analyses of fault-plane solutions of twenty-four earthquakes inferred that the Western part of Nepal Himalayan frontal arcis in compressed state in which seismic activity is dominated by thrust faulting.Based on orientation of P-axes, compressive stress directed north-south to northeast-southwest approximately perpendicular to the prevailing stress along the major trendof the Himalaya. Thrust faulting coupled with shallow dip of nodal planes reflectsthat the Indian continental lithosphere is under-thrusting at a shallow angle. Thisinformation suggests crustal shortening in north-south direction in which earthquakesare generated due to northward compression. In the adjoining Tibet parts earthquakeactivity is due to normal faulting with east-west extension. These might be due tothe presence of a relatively strong Main Himalayan Thrust, the plate boundary faultbelow the Himalayas, would have favored the occurrence of thrusting. While, aweak Main Himalayan Thrust below Tibet along with initiation of the Main CentralThrust can explain South Tibetan Detachment (geodynamic process) and associatedstress field in Western Nepal Himalaya and its adjoining regions.

Keywords: fault-plane solution; seismicity; seismotectonics; stress pattern;Western Nepal Himalaya

1,2 Corresponding authors

1217-8977/$ 20.00 c©2010 Akademiai Kiado, Budapest

196 H PAUDYAL et al.

1. Introduction

The Western Nepal and its adjoining region, which is seismically most activesegment of the Central Himalaya, is considered in the present study to investigatethe current status of geodynamic processes by means of the fault-plane solutions.The region has a complex tectonic history (Upreti 1999, Hodges 2000) with highlydeformed upper crust and displays all major tectonic features of the Himalayan mo-bile belt. The seismic activity in the region is confined in the plate boundary zoneand is caused by the under-thrusting of the Indian plate beneath the Eurasian plate(Molnar and Tapponnier 1978). The Main Boundary Thrust (MBT) and a num-ber of other thrusts, transverse and normal faults existing in the region control theseismicity. Most of the seismic activity of the Central Himalaya can be explainedin terms of the activity associated with major thrusts and transverse faults. Themain Himalayan seismic belt is mostly restricted between the Main Central Thrust(MCT) and the MBT (Ni and Barazangi 1984). The region is passing through byseveral faults transverse to the trend of the Himalaya which has been active sincethe Late Tertiary times (Valdiya 1976). Gansser (1964) concluded from geologicalconsiderations that the MBT of the Himalaya has shallow dip, and Molnar et al.(1973) indicated that almost all the fault-plane solutions have shallow dip (< 30◦)in Himalayan region, therefore the earthquakes are likely to have occurred in asso-ciation with the MBT or parallel to it.

Fault-plane solution describes the mode in which seismic energy released in thehypocentral zone of an earthquake and facilitates understanding about the physicaland tectonic condition of a seismically active region. Such study yields informationabout a fault and the direction of movement due to an earthquake which provides aninstantaneous picture of tectonic movement. Using long period records of WWSSN,Gupta and Singh (1980) determined fault-plane solutions of Nepal Himalaya andinferred thrust faulting in general, with a few cases having a small componentof strike-slip faulting. In addition, there are several investigators who have con-tributed significantly to understand the seismotectonics and related stress patternin the Himalaya and its adjoining Tibet regions (for example Fitch 1970, Tandon andSrivastava 1975, Molnar et al. 1973, Rastogi 1974, Molnar et al. 1977, Chandra 1978,Singh and Gupta 1980, Molnar and Chen 1983, Ni and Barazangi 1984, Dasgupta etal. 1987, Verma and Kumar 1987, Verma and Reddy 1988, Molnar and Lyon-Caen1989, Rajendran et al. 1992, Singh 2000). Most of the results of fault-plane solutionsshow the earthquakes occurring between the MBT and the MCT or slightly northof the MCT have thrust solutions, with nodal planes dipping gently towards northwhereas it is normal faulting with east-west extension for the earthquakes locatednear to and north of the Indus Tsangpo suture in Tibet region. Ni and Barazangi(1984) observed shallow angle north dipping thrust environment in the Himalayancollision zone. Considering these observations, they concluded that this zone sep-arates the underthrusting Indian plate from lesser Himalayan crustal block whichappears to define a part of the detachment. Thrust faulting and crustal shorteningnormal to the southern margin of the Tibetan plateau are derived through fault-plane solutions by Molnar and Lyon-Caen (1989). A regional large scale normal

Acta Geod. Geoph. Hung. 45, 2010

SEISMOTECTONICS OF WESTERN NEPAL HIMALAYA 197

faulting pattern in Tibet, and thrust dominated faulting in the Himalayas have alsoindicated by Shanker et al. (2002) using stress simulation analysis.

Twenty-four fault-plane solutions, which include twelve new solutions and twelvecompiled from the published literatures (Chandra 1978, Baranowski et al. 1984,Centroid Moment Tensor (CMT) solution), were used to infer the prevailing stresspattern and geodynamic processes in Western Nepal region. Chandra (1978) deter-mined fault-plane solutions using WWSSN P-wave first motion data by plotting onan equal area projection of lower focal hemisphere and reported that thrust typefaulting is prevalent in Central Himalaya and confirmed underthrusting of Indianplate towards the north along the Himalayan arc. Baranowski et al. (1984) modifiedthe earlier fault-plane solutions in the Himalayan arc on comparing the syntheticseismogram with long period body waves. They inferred that slip vectors are locallyperpendicular to the Himalayan mountain range with gentle plunge in the easternportion and more steeply in western section and concluded coherent underthrust-ing of Indian plate beneath the lesser Himalaya. Nine CMT solutions used in thepresent study are supposed to be reliable since the process for the determination ofthe fault-plane solutions is uniform.

In the present work, we have attempted to infer the recent trend in the faultingpattern in relation to major tectonic features, with depth section and prevailingstress condition in the Western Nepal Himalaya and its adjoining regions usingfault-plane solutions of twenty-four events that occurred in the last forty years.Attempt has also been made to study the characteristics of seismic activity for theperiod using spatial and temporal distribution of earthquakes. Such investigationshelped in extracting additional information to improve our current understandingabout the seismotectonics of the regions.

2. Seismicity data

Earthquakes data for the period 1803–2006 of Nepal and its adjoining regionsbounded by 26−31◦N and 79−90◦E were compiled using catalogues of NEIC, NSCNepal, Bapat et al. (1983), Oldham (1882) and Chandra (1992). NEIC catalogueis the main source for our database compilation since 1963. We have extractedseismicity data for Western Nepal and its adjoining region bounded by 28–31◦N and79 − 82.3◦E with the help of above complied database and used to study seismiccharacteristics of the region. A total of 232 events occurred in the region during theperiod 1963–2006. The b-value (1.02±0.03) has been estimated using the empiricalrelationship of the magnitude and frequency of earthquake occurrence in a regiongiven by Gutenberg and Richter (1944). This gives the cutoff magnitude as mb ≥ 4.3which is estimated to be the cutoff magnitude for the region. P-wave first motiondirections data reported in the catalogue of ISC bulletin for the earthquakes withmagnitudes mb 4.9–5.6 occurred from 1970–2004 were used for determination offault-plane solutions.



3. Spatial and temporal characteristics of seismicity

In Western Nepal region, major Himalayan thrusts and some of the northeast-southwest trending transverse faults (such as Tanakpur, Karnali, Samea) are themain tectonic features responsible for seismic activity in this region. Historicalrecords show that several events with M 6–6.5 and one large earthquake of M 7.5have occurred during the period 1803–1962 in a north-south segment which wereconfined within 79.8 − 81◦E, 29.5 − 31◦N. The 1916 earthquake that occurred inthe region during the last 200 years having M 7.5 was located close to MCT towardnorth. Since 1963 till 2006, a total of 381 events were reported with mb ≥ 3.5which includes ∼61% of events with cut off magnitude mb = 4.3 and their spatialdistribution is shown in Fig. 1. For the period 1963–2006, estimated b-value for theregion is 1.02. Majority of events are located between the MCT and MBT, andalso along and in between the transverse faults indicating their combined role intriggering these earthquakes. Six medium size earthquakes (mb ≥ 6) occurred inthe region are confined to a nappe between 80◦ to 81◦E except Chamoli earthquake(Fig. 2). Bajhang earthquake of 1980 and the Chamoli earthquake of 1999 are thetwo recent medium size earthquakes which caused severe damage and casualties andboth of them were preceded by well defined anomalous seismic activity (Paudyaland Singh 2007). Clustering of events in western portion close to the MCT around79−80◦E, and further east (80−82.5◦E) between the MCT and MBT, are evidentlyrelated with the major thrusts and the transverse faults. Recent seismicity dataindicates that Western Nepal region is associated with concentrated seismic activityas compared to other parts of Central Himalayan belt. As usual the Indo-GangeticPlane, located south of the MBT is continuing to be associated with extremely lowand spatially scattered seismic activity. The Nepalese seismic network has revealeda segment with intense microseismic activity in front of the high Himalaya belt inthe Western and Eastern Nepal regions (Pandey et al. 1995, 1999). However, overthe adjoining Tibetan region, seismicity appears to be mostly related to north-southtrending normal faults/lineaments (Fig. 1).

Ninety three percent of total events are shallow and confined mostly to the southof the MCT and to the north and south of it in the western portion around Chamoli.In this region, brittle character of deeper rock strata is evident due to continuousoccurrence of shallow focus activity extending to deeper level in which about 27%events occurred in the depth range 40–80 km. Intermediate events occur betweenthe MBT and MCT and their orientation extends in NW-SE direction parallel to thesurface expression of these major thrusts. Large events located up to 60 km focaldepth might be triggered due to the compressive stress caused by continued actionof plate motion or redistribution of existing stress pattern. Whereas intermediateevents presumably are caused by sudden change in the physical properties of thematerial in the deep interiors since the tectonic features which appear on the surfaceare very unlikely to extend deeper level (Upreti 1999).

The cumulative number of events with time (CNET) exhibits almost continuousincreasing trend with 8 to 9 events per year till 2006 in four high and low anomalousphases (Fig. 3). Annual frequency of 5 to 6 events is estimated for the region with



Fig. 1. Spatial distribution of earthquakes occurred from 1963–2006 in Western Nepal and itsadjoining regions are shown in relation to major tectonic features of the region

Fig. 2. Temporal pattern of seismicity from 1963–2006 in Western Nepal and its adjoining areas.The horizontal bars shown on the top of the figure represent degree of seismicity phases

cut off magnitude 4.3. The western and the central portion trending NW-SE isassociated with highest seismic activity as compared to other parts of the region.Four identified anomalous phases are: 1. high active phase from 1963 to February1970 with 5 to 6 events per year including four moderate size earthquakes withmb ≥ 6; 2. low active phase from March 1970 to 1977 with 1 to 2 events per year;3. high active phase January 1978 to 1999 with 6 to 7 events per year including twomoderate size earthquake with mb ≥ 6; 4. and very high active phase from February2000 to 2006 with 18 to 19 events per year. Evidently, medium size earthquakes



have occurred during high active phases except the latest one initiated since 2000.Sudden jump in CNET curve observed in 1980 and 1999 are due to two separateearthquake sequences of 1980 (Bajhang) and 1999 (Chamoli). The seismic activitysince 2000 onwards are confined between the MCT and MBT in central parts ofthe region and also clustered in time which Paudyal and Singh (2007) identified asanomalous pattern for a forthcoming medium size earthquake.

4. Fault-plane solutions and stresses distribution

P-wave velocity structure models of Pandey (1985) for Nepal and Chen andMolnar (1981) for adjoining Tibetan parts were used to estimate angle of incidenceat each station whereas the value of ray parameter is taken from P-wave tableof Herrin et al. (1968). The P-wave first motion directions were plotted on anequal area projection of the lower hemisphere using estimated (i, Az) data. Wulff’sstereographic projection net was used for the determination of the orientations ofthe nodal planes and the direction of P (pressure axis), T (tension axis) and B (nullaxis) axes. For all determined solutions, a double couple source has been assumedfor interpretation of earthquake mechanism. The reliability of the solutions dependson the percentage of inconsistency of observations. In the present case, we expect∼10 percent inconsistency in the observations which may be attributed to scantydata and difficulty in reading the observations recorded on short period seismogramsat teleseismic distances.

The source parameters of twenty-four earthquakes considered in the presentstudy are given in Table I and they are grouped as: 1. two events of mb 4.9; 2.ten events in the range 5.0–5.4; and 3. twelve events with mb ≥ 5.5. Twelve newfault-plane solutions were determined and analyzed along with existing solutions asgiven in Tables I and II. These solutions were put together to generate a detailedseismotectonic map of Western Nepal Himalayan region. The faulting pattern wasstudied in relation to tectonic features; and also with depth along latitudes andlongitudes. The orientation of compression and tension axes were plotted on to thelower half of the focal sphere in order to infer prevailing stress pattern.

The azimuth and the plunges of the P, T and B axes and orientation parametersof nodal planes were measured in degrees from north and horizontal respectivelyand the values thus determined are tabulated in Table II. The solution parametersfor the events reported by others and considered here, are also given in the sametable. New fault-plane solutions of twelve events determined in this study are shownin Fig. 3. The magnitude of these events range from mb 4.9–5.6 and focal depthfrom 15 to 56 km. Eight new solutions show pure thrust faulting and the remainingfour have strike-slip motion with large thrust component (Fig. 3).

The beach balls of fault-plane solutions of twenty-four earthquakes are depictedin Fig. 4 in relation to major thrusts the MCT, MBT and MFT. Over 90% ofthe solutions exhibit thrust faulting with nodal planes dipping towards north ornorth-east and a few towards northwest. Out of eighteen pure thrust solution, tenevents (1, 4, 8, 13, 14, 15, 16, 17, 18 and 23) have north dipping nodal planes,five solutions (7, 9, 19, 22 and 24) dip towards north-east and three solutions



Fig. 3. Fault-plane solutions of earthquakes occurred in and around Western Nepal Himalaya dur-ing 1970–1993 were determined using P-wave first motion data. The nodal planes were determinedon an equal area projection of the lower focal hemisphere. P is the axis of maximum compressionand T is the axis of the least compressions. The estimated fault-plane solutions data are given in

Table II, and numbers 1 to 12 attached to each solution is their respective serial number



Table I. Source parameters of earthquakes of Western Nepal Himalaya andits adjoining regions for which fault-plane solutions were studied. Solutionsnumbered from 1 to 12 were determined in the present study whereas solu-tions from 13 to 24 were complied from published literatures as referenced

in Table II

Sl. Date Origin time Lat Long Magnitude DepthNos hh:mm:ss ◦N ◦E mb km

1 12/02/1970 01:51:51 81.64 29.36 5.4 442 23/12/1974 09:45:43 81.39 29.41 5.2 453 06/09/1975 04:44:34 82.16 29.28 5.1 334 10/05/1976 18:43:53 81.46 29.28 5.2 335 29/09/1976 02:51:25 81.39 29.82 5.0 336 04/11/1977 23:54:45 81.28 29.60 4.9 157 06/03/1981 05:58:50 80.66 29.81 4.9 438 15/05/1981 17:22:43 81.94 29.50 5.1 339 19/02/1984 15:46:26 80.55 29.87 5.0 21

10 18/05/1984 04:28:52 81.87 29.58 5.6 3311 02/06/1992 22:07:45 81.91 28.98 5.2 5612 20/10/1993 16:15:59 82.28 28.72 5.1 3713 20/05/1979 22:59:15 80.31 30.03 5.8 3314 29/07/1980 12:23:16 81.26 29.33 5.7 3415 29/07/1980 14:58:51 81.09 29.60 6.1 1816 05/01/1997 08:47:25 80.53 29.84 5.6 2517 28/03/1999 19:05:18 79.42 30.51 6.6 2318 27/11/2001 07:31:57 81.75 29.55 5.4 3319 27/11/2001 08:53:59 81.75 29.61 5.6 3320 04/06/2002 14:36:08 81.44 30.59 5.6 1021 26/10/2004 02:11:37 80.97 30.88 5.9 1222 26/09/1964 00:46:03 80.46 29.96 6.2 5023 27/06/1966 10:41:09 81.00 29.70 6.0 1324 16/12/1966 20:52:16 80.90 29.70 5.8 15

(3, 5 and 10) towards northwest (Fig. 4). Northward underthrusting of Indian platealong the major thrusts of Himalayas is inferred through these thrust solutions.The solutions pertaining to events 2, 6, 11, and 12 show strike-slip mechanism, thefirst one has large thrust component whereas the remaining small thrust component.The motion along nodal planes of events 2, 6 and 12 are sinistral type whereas event11 is associated with dextral type. The inferred fault planes of these four eventsshow their association with transverse faults trending NE-SW. Two events 20 and21 located in the northern part of the study region close to the ITS (Indus-TsangpoSuture) in Tibet are associated with normal faulting with north-south trendingnodal planes illustrating east-west extension. Four moderate size earthquakes withmb ≥ 6 (solutions 15, 17, 22 and 23) including Bajhang (event 15) and Chamoli(event 17) show similar thrust mechanism with shallow angled north dipping nodalplanes agreeing with the general trend of the major thrusts. The resultant faultingpattern thus inferred is in general conformity with the structural features (majorthrusts and transverse faults) present in the region. Though the nodal plane ofindividual event differs slightly in their orientation, their collective dips indicatenorthward motion of Indian plate at shallow angle.



Table II. Fault-plane solution parameters of twenty-four earthquakes from Western NepalHimalaya and its adjoining regions. Strike, Dip, Dip dir. (Dip direction), Az (Azimuth) and

Pl. (Plunges) are in degrees

Sl. Nodal plane I Nodal plane II P axis T axis B axis Reference

Nos Strike Dip Dipdir.

Strike Dip Dipdir.

Az. Pl. Az. Pl. Az. Pl.

1 081 50 350 099 42 190 001 05 118 79 261 11 This study2 007 68 278 077 48 166 308 13 052 48 207 41 This study3 056 50 326 056 40 146 326 05 146 85 056 00 This study4 111 50 010 144 50 236 212 01 123 66 303 24 This study5 041 67 312 041 23 132 312 22 132 68 041 00 This study6 012 69 284 098 80 190 147 07 054 23 253 67 This study7 130 04 040 130 86 218 218 42 040 48 130 00 This study8 113 08 012 113 82 192 192 37 012 53 113 00 This study9 146 08 056 146 82 236 236 37 056 54 146 00 This study

10 073 26 342 073 64 162 162 19 342 71 073 00 This study11 009 80 279 100 77 008 232 03 143 16 329 74 This study12 076 67 346 171 80 078 122 08 214 24 012 65 This study13 274 07 – 105 83 – 194 38 016 52 285 01 CMT14 278 25 – 098 65 – 188 20 009 70 278 00 CMT15 290 21 – 108 69 – 199 24 017 66 108 01 CMT16 279 19 – 122 73 – 206 27 42 62 300 07 CMT17 280 07 – 115 83 – 203 38 027 52 295 02 CMT18 257 04 – 104 87 – 192 42 016 48 284 02 CMT19 280 28 – 119 64 – 202 18 046 70 295 08 CMT20 159 49 – 031 55 – 00 61 096 03 187 29 CMT21 112 73 – 017 76 – 334 22 65 02 159 68 CMT22 114 73 – 305 17 – 207 28 019 62 115 03 Chandra (1978)23 277 27 – 119 65 – 202 19 049 69 295 09 Baranowski

et al. (1984)24 290 24 – 110 66 – 200 21 020 69 110 00 Baranowski

et al. (1984)

Inferred nature of faulting with focal depth along longitude and latitude in thisregion does not show any definite pattern of its relation with increasing focal depth.A vertical section of ∼30 km thick from 15 to 45 km depth section, in which over80% events are confined, is identified as the most active thrust faulting zone (Fig. 5).Four events showing strike-slip nature with thrust components are distributed fromshallow to deeper levels (15 to 56 km). Two earthquakes (events 20 and 21) havingnormal faulting are located at shallowest level (within 12 km) near the ITS in Tibetshow east-west extension. The inferred nodal planes of majority of the events showanalogous pattern of shallow dip towards north, north-east and northwest withincreasing depth (Fig. 5). The inferred northward dipping nodal planes suggestunderthrusting of Indian lithospheric plate towards north at shallow angle in thewestern parts of Nepal Himalaya.

The tectonics of Western Nepal Himalaya and its adjoining regions is complex;however, more or less similar faulting pattern dominated by thrust environment oc-curs as evident from Table III. The normal faulting pattern in the adjoining SouthCentral Tibet is predominant with north-south trending nodal planes representing



Fig. 4. Nature of faulting patterns in Western Nepal and its adjoining regions with epicenters of theearthquakes. Shaded quadrants represent compressional first motion, and opens are dilatations.Solutions 1 to 12 were determined in the present study and remaining twelve compiled from thepublished literatures (Table II). Two earthquakes located near ITS (Nos 20 and 21) show normal

faulting with east-west extension

Table III. Frequency of different types of fault-plane solutions observed in Western NepalHimalaya and its vicinity. Please note that the number shown within parentheses under

“Normal” column is the earthquakes actually located in the adjacent Tibetan part

Seismogenic Number of Frequency of nature of fault-plane solutionsregion fault-plane Thrust Strike-slip Normal Strike-slip Strike-slip

solutions +Thrust +Normal

Western Nepal 24 18 4 (2) – –

east-west flow of materials and it is totally different than that of Himalayan com-pressed belt. The data furnished in Table III also suggest that most predominantmode of energy release in the Western Nepal Himalayan belt is by thrust faulting,whereas mechanism for energy release is normal faulting in the adjoining SouthCentral Tibet.



Fig. 5. Relationship of nature of faulting patterns with focal depth along longitude (top) andlatitude (bottom) in Western Nepal and its adjoining regions for the earthquakes occurred during1964–2004. Shaded quadrants represent compressional first motion, and opens are dilatations.

The numbers attached to each solution is their respective serial number as given in Table II

5. Stress orientation pattern

Orientation of compressive (P) and tension (T) axes of the fault plane solutionsare used to infer information about the stress pattern in a region. For a homogenousmedium and under the assumption that both principal stress axes make angles of 45◦

with slip direction, the P and the T axes show the direction of maximum compres-sion and maximum tension respectively. The thrust faulting is generally associatedwith compressive stress prevailing in an area, while normal faulting causes tension.If P plunge is less than 45◦ and the T plunge is greater than 45◦, it represents acompressive regime, and reverse condition is the indication of extensional regime.The composite focal mechanisms yield more robust and more general characteristicof the stress field than using individual event mechanisms (Fig. 6).

The nature of stress pattern and their influence on tectonics in the WesternNepal Himalaya region were studied from the composite stereographic projection oforientations of the compression and tension axes of twenty two earthquakes (excepttwo events 20 and 21; Table I). The estimated values of plunges and azimuth of P-and T-axes are furnished in Table II and their composite plot is shown in Fig. 6.It is evident that the plunges of P-axis are shallow and almost horizontal whereasthat of the T-axis is almost vertical showing thrust mechanism in this part of theNepal Himalaya. All of the P-plunges are less than 45◦ with around 63 percent



Fig. 6. Lower hemisphere projection of P-axes and T-axes in Western Nepal Himalaya region.Note that the roughly NE-SW to NW-SE orientations of the P-axes implying thrusting in the

Himalayan front

within 25◦. On the other hand, over 86 percent of T-plunges are greater than 45◦

(Table II). This observation suggests clearly a compressive regime in the WesternNepal Himalaya. It is apparent that the compressive stress is generally acting in N-Sto NE-SW directions which are approximately perpendicular to the major trend ofthe Himalaya. It also reveals that the earthquake generation process in the regionis due to the north-northeast compressive stress exerted by the Indian plate to theTibetan plate. However, plunges of P-axis of a few events show compression fromapproximately northwest direction.

6. Discussion

The Western Nepal and its adjoining regions are most active parts of CentralHimalaya in the recent time in which shallow and intermediate focus earthquakesoccur mostly in between the surface manifestation of the MCT and MBT. It isobserved that the seismic activity fluctuates both in space and time in the form ofhigh and low phases and inferred that these are triggered by the MBT and transversefeatures of the region. Most of the events are associated with 1980 Bajhang and1999 Chamoli earthquakes. An alarming increase in seismic activity is observedbetween the MCT and MBT since 2000 in the central part of the region.

It is generally accepted that the faulting pattern occurred in Himalaya and itsadjoining regions after its formation is the result of collision of the northward movingIndian plate with the Eurasian plate that occurred in the Late Tertiary (Powell andConaghan 1973, Molnar and Tapponnier 1978). The present geotectonic activityin Himalayan collision zone is the result of post collisional incident. The currentinformation derived through fault-plane solution testifies that the processes which



were responsible for the formation of Himalaya still continue. Thrust faulting inan area is normally caused by compressive stress due to collision and subductionof lithospheric plates whereas normal faulting indicates an extensional zone likespreading centers in which materials flow perpendicular to the fault plane. Fault-plane solutions of earthquakes in the Himalayan front zone are of thrust type andpredominantly normal faulting in the adjoining Tibet.

The results derived in the present study through fault-plane solutions of 24events indicate that predominantly thrust environment exists in Western NepalHimalaya leading to compression towards north, and normal faulting with east-west extension in the adjoining south central Tibet region. Singh (2000) inferredP-axes orientations in N-S to NE-SW for the Himalayas and from N-S to E-Wfor Tibet region whereas T-axes changes from N-S to E-W in both the regions.Approximately horizontal tension axis observed in southern Tibet is an indicativeof crustal thinning due to east-west flow of Tibetan landmass (Molnar and Chen1983) which is interpreted as weak crust as well as upper mantle beneath the Tibetanplateau (Molnar and Tapponnier 1978, Molnar and Lyon-Caen 1989). Based oncomposite plot of P and T-axes, Molnar and Lyon-Caen (1989) observed roughly E-W to SE-NW crustal extension in both southern Tibet and higher parts of Tibetanplateau.

The examined fault-plane solutions in Western Nepal Himalaya region showthrust mechanism with NW to NE dipping nodal planes indicating northward un-derthrusting of Indian plate along the major thrusts of the Himalayas. A fewfault-plane solutions showed sinistral and dextral motions along their inferred faultplanes indicating that a portion of stress is also being released along some of theexisting transverse tectonic features. In spite of the inconsistency in the orienta-tion of nodal planes of individual event the collective dip of the nodal planes shownorthward underthrusting at shallow angle. Further, a clear picture of underthrust-ing phenomenon of Indian lithosphere at shallow angle is inferred from the faultingpattern of moderate size earthquakes (mb ≥ 6). The upper crust of this region isobserved to be the most active thrust faulting zone.

In the Western Nepal Himalayan region, we infer predominant N-S to NE-SWdirected compressive stress based on orientation of P-axes which is approximatelyperpendicular to the major trend of the Himalaya. It is evident that the plungesof P-axis are almost horizontal coupled with almost vertical T-axis showing thrustenvironment in the region. A considerable difference observed in the plunges of P-and also in T-axes between Western and Eastern Nepal Himalaya appears to be dueto dominant strike-slip faulting with thrust component in eastern sector. The strikesof the northward gently dipping nodal planes in Western Nepal Himalaya region areobserved normally parallel to the major thrust/local tectonic features. The presentinvestigation suggests that the Nepal Himalayan frontal arc is in compressed statein which seismic activity is controlled by thrust faulting whereas the adjoining southcentral Tibet reflects an extensional zone with dominantly normal faulting.



7. Conclusions

It is implicit that a single earthquake represents the stress state at the hypocenterand the compression and tension axes of a single fault-plane solution denote moreor less the principal directions of the greatest and the least stress component. Mostof the fault-plane solutions indicate predominantly compressive environment withgently northward dipping nodal planes in the Western Nepal Himalayan regionwhereas in the adjoining South Central Tibet region it is predominantly east-westextension with normal faulting. The seismic activity in Western Nepal region from1963 to 2006 fluctuates in the form of high and low seismic phases and these eventswere probably triggered by the MBT and transverse features existing in the region.An extremely high seismic activity is observed between the MCT and MBT since2000 in the central part of the region which may have some bearing on future seismichazard. It is deduced that the Indian continental lithosphere continues to under-thrust at a shallow angle; and the process of earthquake generation in the Himalayanregion is due to northward compression. Using these fault-plane solutions, it isstudied here the faulting pattern in relation to the major tectonic features, thenature of faulting in the vertical sections along east-west and north-south directionsand the prevailing stress condition using the orientation of pressure and tensionaxes. Such investigations helped in extracting the additional information which, inturn, improved the current understanding of the seismotectonics of the Nepal andits adjoining region.

The following conclusions can be drawn from this study:

1. The inferred nature of faulting with a focal depth along longitudes and lat-itudes in the Western Nepal and its adjoining region does not show up anydefinite pattern. However, the thrust faulting environment is observed to bedominating throughout the thick crust of the Himalaya. A vertical section of∼30 km thick from 15 to 45 km depth section is identified as the most activethrust faulting zone in which over 80 percent events are confined.

2. The inferred nodal planes of the majority of the events show analogous patternof shallow dip towards north or north-east or northwest at all the depths inthe crustal block. The above information may be viewed as under thrusting ofIndian plate towards north at shallow angle in the western parts of the NepalHimalaya. Concentrated pattern of steeply dipping tension axes show thatthere is no mass movement like Tibet in the Himalayan region.

Acknowledgements

The authors are grateful to Dr. M Banerjee, Department of Geophysics, Banaras

Hindu University, Varanasi, and Dr. T Radhakrishna, Centre for Earth Science Studies,

Thiruvanathapuram, for critically examining the manuscript and offering useful comments

and suggestions. The anonymous reviewers for offering very valuable comments and

suggestions and Judit Szendroi for her efforts throughout publication are also acknowl-

edged. The first author (HP) was on sabbatical leave from Tribhuvan University, Nepal.



He is indebted to the University Grant Commission, Nepal for financial support in the

form of fellowship. Second author is thankful to Prof. and Head Department of Earth-

quake Engineering, IIT Roorkee for providing excellent computational facility.

References

Bapat Anup Kulkarni R C, Guha S K eds 1983: Catalogue of earthquakes in India andneighbourhood. Indian Society of Earthquake Technology, Roorkee

Baranowski J, Armbruster J, Seeber L, Molnar P 1984: J. Geophys. Res., 89, 6918–6928.Chandra U 1978: Phys. Earth Planet. Int., 16, 109–131.Chandra U 1992: Current Science, 62, 40–71.Chen W P, Molnar P 1981: J. Geophys. Res., 86, 5937–5962.Dasgupta S, Mukhopadhaya M, Nandy D R 1987: Tectonophysics, 136, 255–264.Fitch T J 1970: J. Geophys. Res., 75, 2699–2709.Gansser A 1964: Geology of the Himalaya. Wiley Inter-Science, LondonGlobal Centroid Moment Tensor database, formerly known as the Harvard CMT catalogGupta H K, Singh D D 1980: Tectonophysics, 62, 53–66.Gutenberg B, Richter C F 1944: Bull. Seism. Soc. Am., 34, 185–188.Herrin E, Arnold E P, Bolt B A, Clawson C E, Engdahl E R, Freedman H W, Gordon D W,

Hales A L, Lobdell J L, Nuttli O, Romney C, Taggart J, Tucker W 1968: Bull. Seism.Soc. Am., 58, 1193–1241.

Hodges K V 2000: GSA Bulletin, 112, No. 3, 324–350.Molnar P, Chen W P 1983: J. Geophys. Res., 88, 1180–1196.Molnar P, Lyon-Caen H 1989: Geophys. J. Int., 99, 123–153.Molnar P, Tapponnier P 1978: J. Geophys. Res., 83, 5361–5375.Molnar P, Fitch T J, Wu F T 1973: Earth Planet. Sci. Lett., 19, 101–112.Molnar P, Chen W P, Fitch T J, Tapponnier P, Warsi W E K, Wu F T 1977: In: Himalaya.

Sciences de la Terre, Paris, 269–294.NEIC, National Earthquake Information Center, USGS, USA: The catalogues of earth-

quake from 1963 to 2006.Ni J, Barazangi M 1984: J. Geophys. Res., 89, 1147–1163.Oldham T 1882: Memoir. Geol. Surv. India, 19, 163–215.Pandey M R 1985: J. Nepal Geological Society, 3, 1–11.Pandey M R, Tandukar R P, Avouac J P 1995: Geophy. Res. Let., 22, 751–754.Pandey M R, Tandukar R P, Avouac J P, Heritier T 1999: J. Asian Earth Sci., 17, 703–712.Paudyal H, Singh H N 2007: In: Proc. of National seminar on Modern Trend in Geophysical

Sciences and Techniques, Indian School of Mines University, Dhanbad, 40–43.Powell C M, Conaghan P J 1973: Earth Planet. Sci. Lett., 20, 1–12.Rajendran K, Talwani P, Gupta H K 1992: Current Science, 62, 86–93.Rastogi B K 1974: Tectonophysics, 21, 47–56,Shanker D, Kapur N, Singh B 2002: J. Geological Society, London, 159, 264–273.Singh D D 2000: J. Geodynamics, 30, 507–537.Singh D D, Gupta H K 1980: Bull. Seism. Soc. Am., 70, 757–773.Tandon A N, Srivastava H N 1975: Bull. Seism. Soc. Am., 65, 963–969.Upreti B N 1999: J. Asian Earth Science, 17, 577–606.Valdiya K S 1976: Tectonophysics, 32, 353–386.Verma R K, Kumar G V R K 1987: Tectonophysics, 134, 157–175.Verma R K, Reddy Y S K 1988: Tectonophysics, 156, 107–131.


current understanding of the seismotectonics of western nepal himalaya and vicinity

Documents