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J.R. KAYAL1*, SERGEI S. AREFIEV2,SAURABH BARUAH3, RUBEN TATAVOSSIAN2,

NABA GOGOI4, MANICHANDRA SANJOUM5,J.L. GAUTAM6, DEVAJIT HAZARIKA7

AND DIPAK BORAH3

1. Department Applied Geophysics, Indian School of Mines,Dhanbad 826004, India

* Corresponding author, email: [email protected]

2. Institute of Physics of the Earth, Russian Academy of Sciences,Moscow, Russia

3. North East Institute of Science & Technology, Jorhat, Assam,India

4. National Geophysical Research Institute(Hyderabad), TezpurProject, Assam, India

5. Department Earth Sciences, Manipore University, Imphal, India

6. India Meteorological Department, New Delhi, India

7. Wadia Institute of Himalayan Geology, Dehradun, India

ABSTRACT

Seismotectonics of the two recent felt earthquakes,one Mw 6.3 in the Bhutan Himalaya on September 21,2009 and the other Mw 5.1 in the Assam valley on August19, 2009, are examined here. The recent seismicity andfault plane solutions of these two felt earthquakes suggestthat both the events occurred on the Kopili fault zone, aknown active fault zone in the Assam valley, about 300km long and 50 km wide. The fault zone is transverse tothe east-west Himalayan trend, and its intense seismicityindicates that it transgresses into the Himalaya. Thegeologically mapped curvilinear structure of the MainCentral Thrust (MCT) in the Himalaya, where theepicenter of the Bhutan earthquake is located, is possiblycaused by the transverse Kopili fault beneath the MCT.This intensely active fault zone seems vulnerable for animpending larger earthquake (M >7.0) in the region,and it needs close and continuous monitoring.

INTRODUCTION

The northeastern Himalaya and the adjoiningregion, between latitude 22-30o longitude 88-98o N, are

seismically and tectonically more complex than theeastern/central or western Himalaya segments ((Kayal,Microearthquake Seismology and Seismotectonics ofSouth Asia, 503p, Springer, 2008). The region experiencesseveral large/great earthquakes due to the Himalayancollision in the north, Indo-Burma atypical subductiontectonics to the east and due to intra-plate seismic activityin the Assam valley-Shillong Plateau-Bengal basin. Theintra-plate seismic activity is fairly intense in this regiondue to the complex stress regime (Angelia and Baruah,Geophys J. Int, doi: 10.1111/j.1365-246X.2009.04107,2009).

Since the 1897 Shillong Great Earthquake (Ms 8.7),about 22 large earthquakes (M> 7.0) are recorded in thenortheast India region including the two great earthquakes(Ms~ 8.7), the 1897 and the 1950, respectively (Kayal,Microearthquake Seismology and Seismotectonics ofSouth Asia, 503p, Springer, 2008). Most of the largeearthquakes are recorded in the Indo-Burma region, inthe inter-plate/dipping seismic zone, but four intra-platelarge earthquakes occurred much to the east of the Indo-Burma subduction zone or much south of the Himalayancollision zone. These are the 1943 event (M 7.2) on theKopili fault in the Assam valley, the 1930 event (M 7.1)on the Dhubri fault at the western boundary of theShillong plateau, and rest two in the Bengal basin (Fig.1).In the Bengal basin, the 1923 event (M 7.5) is assignedto the Hinge zone and the 1918 event (M 7.6) to theSylhet fault (Fig.1) (Nandy, Geodynamics of northeasternIndia and adjoining region, 209p., ACB Pub., 2001).The Hinge zone is a sub-surface structure, approximately500 km long and 25-100 km wide, that separates thecontinental shelf to the west and the geosynclinal basinto the east (Evans, J Geol Soc India, 5, 80-96, 1964).

Recently, we have recorded two strong feltearthquakes in this region, one Mw 6.3 on September 21,2009 that occurred on the Main Central Thrust (MCT)in the Bhutan Himalaya, and the other Mw 5.1 on August19, 2009 in the Assam valley on the Kopili fault,respectively (Fig.1). These two events occurred almostwithin a month, and are reported as shallow focus (depth~ 10 km) earthquakes in the USGS (United StatesGeological Survey) reports. The northeast India regionis well equipped with about 25 permanent broadbandseismic stations since 2001. We have studied focalmechanisms of these two felt events by waveforminversion, and examined the recent seismicity. The resultsare discussed here in the background of the knownseismotectonics of the two different tectonic domains thatare in close proximity.

THE 2009 BHUTAN AND ASSAM FELTEARTHQUAKES (MW 6.3 AND 5.1)

IN NORTHEAST HIMALAYA REGIONAND THE ACTIVE KOPILI FAULT

January 2010DCS-DST NEWS

Fig. 1. Tectonic map of the study region (modified from Kayal et al., Curr. Sci.,91(1), 109-114, 2006); MCT: Main Central Thrust, MBT: Main Boundary Thrust,DF: Dauki Fault, DT: Dapsi Thrust; other features are named in the map. The localseismic broadband stations (blue triangles) used in this study are shown. Therecent EHB located earthquakes M > 4.5 (1995-2007) within the rectangle areaare shown by green solid circles, the past four larger (M>7.0) intra-plateearthquakes (solid yellow circles) and the two great earthquakes (larger yellowstars) are annotated with the year of occurrence. The two felt earthquakes of 2009are shown by red stars. The USGS three fault plane solutions, the HRV CMTsolution and the two solutions obtained in this study for the September 21, 2009Bhutan earthquake are illustrated by beach balls with different colours. Two HRVCMT solutions of the past two earthquakes, 1995 and 2006 respectively, in theBhutan Himalaya are shown by black beach balls. Fault plane solution of theAugust 17, 2009 Assam earthquake obtained in this study is also shown andannotated. A N-S cross section across the rectangle area, indicating earthquakefoci is shown below; the red star and cluster of red solid circles below thecurvilinear MCT indicate the September 21, 2009 Bhutan main shock andaftershocks respectively. Inset: Map of India showing the study region by arectangular box.

BHUTAN HIMALAYA EARTHQUAKES

The Bhutan Himalaya has no recordof great or large earthquake (M> 7.0)during the past 200 years. Historicalrecords of occurrence of large earthquakesbefore 1800 AD are mostly lacking. Theseismicity of Bhutan Himalaya during thelast 100 years, as reported in theInternational Seismological Centre (ISC)catalogs is, however, low compared to itsadjoining Himalayan segments in the west.The low seismicity has been attributed tolower convergence rate. It is suggested thatthe India-Eurasia convergence is largelyaccommodated by the pop-up tectonics ofthe Shillong Plateau to the south, and theBhutan Himalaya lies in the shadow zonewith less seismicity (Gahalaut, Phys EarthPlanet Interior, under pub, 2010).

The September 21, 2009 strongly feltearthquake provides an insight into theseismotectonics of the Bhutan Himalaya.We have examined the recent seismicityand a N-S cross section in this area thatincludes the Shillong plateau, Assam valleyand the Bhutan Himalaya; the EHBrelocated events (M> 4.5) since 1995 areconsidered (Fig. 1). The section shows thatthe Shillong plateau earthquakes are mostlyconfined within a depth of 40 km andbounded by two major boundary faults,Dapsi thrust (DT) and Brahmaputra fault(BF), which has been also observed bylocal broadband network data (Kayal et al.,Curr. Sci., 91(1), 109-114, 2006). Thenorth dipping DT, a conjugate of the Daukifault, is identified to be an active thrust thatdemarcates the southern boundary of theShillong plateau activity and also truncatedthe maximum isoseismal of the 1897Shillong earthquake along this thrust(Kayal and De, Bull Seism Soc Am, 81,131-138, 1991). The east-west nearvertical Dauki fault, which separates theBengal basin to the south and the Shillongplateau to the north, is much less active.The Bhutan Himalaya earthquakes arefound to be much shallower (~10 km) atthe MCT zone, particularly the 2009 mainshock and its aftershocks that occurred on

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the north-south curvilinear segment of the MCT. Theearthquakes further north of the MCT are deeper, downto 50 km (Fig.1).

The September 2009 Felt Earthquake (Mw 6.3)

The September 21, 2009 earthquake was widelyfelt; 11 casualties, more than 18 injuries and about 1,100damaged houses were reported in Bhutan (Fig.2). Themaximum intensity reached to VIII the epicenter area(Dowchu, D., 2010, pers com). The epicenter of the eventis placed at latitude 27.34ºN and longitude 91.41ºE, anddepth at ~10 km in the USGS report. The tremor waswell felt in the Bhutan Himalaya and in the adjoiningnortheast India region including Sikkim, Assam, ArunachalPradesh, Shillong plateau and in Bangladesh; minordamages to a few Guwahati city houses were alsoreported in the local news papers.

Three moment tensor solutions are given by theUSGS and a centroid moment tensor (CMT) solution bythe HRV (Fig. 1). All the four solutions are compatiblewith Mw 6.1-6.3 and depth ~ 8-10 km, inferring that theevent occurred on a shallow north dipping plane assignedto the plane of detachment as the fault plane for thisevent. This seismotectonic model that the Himalayanearthquakes occur on the plane of detachment is the mostwidely accepted tectonic model as envisaged by (Seeberet al., Geodyn Series 3, AGU pub. 215-242, 1981), andthe Himalayan earthquake solutions are mostly biased tothis model. Kayal (Tectonophys. 339, 331-351, 2001),however, argued that this model fits fairly well in thewestern Himalaya, but not in the eastern or in thenortheastern Himalaya.

We have reanalyzed the teleseismic waveforms ofabout 30 global digital seismic stations and obtained asolution of thrust faulting with strike slip component forthis event (Fig.1). We infer that the east dipping north-south nodal plane is the fault plane that is compatiblewith the north-south trending curvilinear segment of theMCT where the main shock occurred at a shallow depth(Fig.1). A similar solution is reported in the HRV catalogfor the event Mw 5.4 of February 26, 2006 near to the2009 main shock epicenter; this event possibly alsooccurred on the same seismogenic structure. Anothersimilar solution is observed for the event Mw 5.4 thatoccurred on February 17, 1995 within 50 km of the mainshock epicenter (Fig. 1).

Further, we have used the waveform data of fivebroadband seismic stations of the local network in thenortheast India region, which is running to the immediatesouth of the epicenter of the 2009 Bhutan earthquake, andwe obtained a pure thrust faulting mechanism by inversion(Fig.1). In this pure thrust faulting solution (Fig. 1), wealso infer the east dipping north-south nodal plane as thefault plane, comparable to that obtained by the teleseismicdata of the 30 global stations in this study (Fig.1). Themoment magnitude Mw is found to be 6.2 in both thesolutions in our study. Thus the two solutions obtainedin this study by the teleseismic and by the local networkdata respectively are unbiased and compatible. These twosolutions are also comparable with the HRV solutions ofthe past two events (M> 5.0) in immediate vicinity.

ASSAM VALLEY EARTHQUAKES

We focus our observation on the Kopili faultearthquakes in the Assam valley. The Kopili fault zone,approximately 300 km long and 50 km wide, separatesthe Shillong plateau and the Mikir massif by strike slip

Fig. 2. Damaged houses in the meizoseismal area of theSeptember 21, 2009 earthquake, Narang village, BhutanHimalaya (courtesy: Dowchu, D., 2010, pers com).

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movement (Fig.1), and it is identified as the most activefault in the Assam valley area (Kayal et al., Curr. Sci.,91(1), 109-114, 2006; Bhattacharya et al., J Asian EarthSci., 33, 25-41, 2008). The Shillong-Mikir massif arebelieved to be part of the Indian shield, and transportedto the east by the Dauki fault (Evans, J Geol Soc India,5, 80-96, 1964) (Fig.1). Further, the Mikir massif is afragmented part of the Shillong massif separated by theKopili fault. A seismicity map is prepared by relocationof earthquakes recorded by the local networks in thenortheast India region (Fig 3). The seismicity trendsclearly indicate that the Shillong plateau and the Kopilifault zone are the two main intra-plate earthquake sourcezones in the area. The b-value, fractal dimension andseismic tomography images revealed the Shillong plateauas a circular source zone whereas the Kopili fault as agigantic NW-SE trending transverse structure with intenseseismic activity down to 40 km (Bhattacharya et al., CurrSci, 82(12), 1486-1491, 2002; Bhattacharya et al., JAsian Earth Sci., 33, 25-41, 2008). It is further noted thatthe Kopili fault transgresses into the Himalaya (Fig. 3).No local network is presently running in the BhutanHimalaya (Dowchu, D., 2010, pers com) that would helpto incorporate its local earthquake data in the seismicitymap of Fig 3, but the global network data in the BhutanHimalaya clearly shows that the Kopili fault active zoneis extending up to the MCT zone (Figs 1 and 3). Kayal

et al. (Curr. Sci.,91(1), 109-114, 2006) made a detailedstudy on the Kopili fault zone earthquake sourceprocesses and reported that the earthquakes are generatedby strike slip faulting on this northeast dipping fault.

The August 2009 Felt Earthquake (Mw 5.1)

The August 19, 2009 earthquake Mw 5.1 in theAssam valley originated at 10h 45m 14s at latitude:26.56ºN, longitude: 92.48ºE and at a depth ~10 km(USGS report). It has been a well felt earthquake in theregion including the Guwahati, Jorhat, Tezpur andShillong cities in northeast India. No casualties werereported; the maximum intensity reached to VI (GeolSurv India, unpub report).

The event is well recorded by the local broadbandseismic stations in the area. A well constrained fault planesolution is obtained by waveform inversion usingbroadband seismograms of five close seismic stations.The estimated seismic moment is compatible with themoment magnitude Mw 5.1. A right lateral strike-slipsolution is obtained; the beach ball representation of thesolution is given in Fig.1. The NNW oriented nodal planeis the inferred fault plane, which is compatible with theKopili fault. A similar solution is reported in the HRVcatalog for the event Mw 5.4 that occurred at the northernend of the Kopili fault on February 23, 2006 (Fig 1).

DISCUSSION AND CONCLUSIONS

It is reported that the Himalayan earthquakesare greatly influenced by transverse structures inthe eastern and northeastern region (Kayal,Microearthquake Seismology and Seismotectonicsof South Asia, 503p, Springer, 2008). Unlike inthe western Himalaya where the earthquakesmostly occur on the plane of detachment andfairly fit with the envisaged seismotectonic model(Seeber et al., Geodyn Series 3, AGU pub., 215-242, 1981), the eastern or northeastern Himalayanearthquakes do not fit into this model (Kayal,Tectonophys. 339, 331-351, 2001 and Kayal,Geomat Nat Haz and Risk, 1 (1), 51-62, 2010).Long transverse structures across the easternand northeastern Himalayas, like those of theEast Patna fault and Tista Lineament in theeastern Himalaya, and the Kopili fault andDhubri/Jamuna fault in the northeasternHimalaya play a major role in generatingearthquakes in the foredeep and foothillsregion (Mukhopadhyay, Tectonophys, 109, 227-240, 1984; Dasgupta et al., Tectonophys, 136,

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Fig. 3. The recent seismicity map prepared by the relocated events using thelocal network data (1993-1999) in northeast India showing high seismicactivity in the Shillong plateau and intense seismicity along the NW-SEKopili fault zone (modified from Bhattacharya et al., J Asian Earth Sci., 33,25-41, 2008). The two felt earthquakes of 2009 are shown by red stars.


255-264, 1987). The East Patna fault is argued to bethe causative fault for the 1934 great earthquake in theeastern Himalaya foredeep (GSI Sp Pub., 32, 1993;Kayal, Geomat Nat Haz and Risk, 1 (1), 51-62, 2010),and the Dhubri fault for the 1930 large earthquake in thewestern border of the Shillong plateau (Nandy,Geodynamics of northeastern India and adjoiningregion, 209p., ACB Pub., 2001). It is further interestingto note that the long lineaments or faults that transgressinto the Himalaya cause curvilinear structure on theMCT; the Tista lineament caused such structure in theSikkim Himalaya (De and Kayal, Tectonophys, 386, 243-248, 2004), and the Kopili fault in the Bhutan Himalaya(Fig.1). In Bhutan Himalaya we also observe anothercurvilinear structure on the MCT along the Jamunalineament (Fig. 1). Although the MCT is believed to beseismically dormant (Ni and Barajangi, J. Geophys Res,89, 1147-1163, 1984), the north-south segments of suchcurvilinear structures on the MCT possibly reflectseismogenic transverse structures transecting them. Thezones of insertion seem to be localizing the shallowstrike-slip faulting events (De and Kayal, Tectonophys,386, 243-248, 2004).

The September 21, 2009 strong earthquake (Mw6.3) and its aftershocks in the Bhutan Himalaya occurredon such a north-south intersection segment of thecurvilinear MCT (Fig.1). Several studies have reportedthat the 300 km long Kopili fault transgresses into theBhutan Himalaya up to the MCT (Nandy, Geodynamicsof northeastern India and adjoining region, 209p., ACBPub., 2001; Bhattacharya et al., Curr Sci., 82(12), 1486-1491, 2002). The two fault plane solutions of the mainshock in this study and the seismic cross section of themain shock and aftershocks clearly indicate that thesource zone is below the N-S intersecting Kopili segment(NSIKS) of the curvilinear MCT (Fig 1). We believe thatthe main shock and the aftershocks occurred on thenorthern extension of the Kopili fault below this NSIKS.

The August 19, 2009 earthquake in the foredeepAssam valley, about 100 km south of the BhutanHimalaya earthquake occurred at a similar depth (~10km) and with a right lateral strike-slip fault mechanismon the Kopili fault. The two fault plane solutions thatare obtained for the September 21 Bhutan earthquake inthe present study are compatible with the August 19Assam valley earthquake solution. The September 21Bhutan earthquake solution obtained by the teleseismicdata also shows a right lateral strike-slip movement. Webelieve that the August 19 Assam valley earthquake on

the Kopili fault possibly triggered the September 21Bhutan Himalaya earthquake at the northern end of thefault at a similar depth and with similar sourcemechanism. Kayal et al. (Curr. Sci., 91(1), 109-114,2006) strongly argued that the Kopili fault is intensivelyactive in the region, and is vulnerable for an impendinglarge earthquake in the northeast India/Himalaya region.The August 19 event could be the foreshock for theSeptember 21, 2009 Bhutan earthquake. A larger andmore significant question is whether both the events couldbe foreshocks (?) for a larger impending earthquake (M>7.0) in the region.

Although, apparently the Bhutan Himalaya falls inthe Himalayan collision zone and the Assam valley in theforedeep / intra-plate zone, the long transverse Kopilifault zone links these two tectonic zones, and this gigantictransverse structure is capable of generating largerearthquakes in the Bhutan Himalaya as well as in theAssam valley. The January 10, 1869 Cachar earthquake(M 7.5, depth ~ 50 km, intensity VIII+) occurred at thesoutheastern end of the Kopili fault that caused severedamages in northeast India region (Nandy, Geodynamicsof northeastern India and adjoining region, 209p., ACBPub., 2001). The October 23, 1943 earthquake (M 7.2)occurred almost at the centre of the Kopili fault zone(Fig.1). The present seismicity recorded by the localnetwork shows intensive activity along the Kopili faultzone (Fig. 3). We thus conclude that the gigantic Kopilitransverse structure is seismically most active in theregion; it caused the two recent felt earthquakes in 2009and is possibly vulnerable for an impending largerearthquake. Our observations suggest that seismicityalong the Kopili Fault and its intersection with the MCTneeds to be closely and continuously monitoredseismicity and geophysical and other precursors alongthis active fault needs to be closely monitored so thatour preparedness for facing a large magnitudeearthquake in this region is scientifically enhanced.

ACKNOWLEDGEMENT

This research is undertaken under the ILTP(Integrated Long Term Programme), an Indo-Russiancollaboration, supported by the Department of Scienceand Technology, New Delhi, India and by the RussianAcademy of Sciences, Moscow, Russia. We express oursincere thanks to the all Heads of the concernedInstitutes for their kind support to carry out this research.We sincerely thank Sri. T M Mahadevan, Editor, DSTNL for his keen interest in the early publication of thispaper.

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