KAYAL et al

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January 2010

THE 2009 BHUTAN AND ASSAM FELT EARTHQUAKES (MW 6.3 AND 5.1) IN NORTHEAST HIMALAYA REGION AND THE ACTIVE KOPILI FAULT

J.R. KAYAL1*, SERGEI S. AREFIEV2, SAURABH BARUAH3, RUBEN TATAVOSSIAN2, NABA GOGOI4, MANICHANDRA SANJOUM5, J.L. GAUTAM6, DEVAJIT HAZARIKA7 AND DIPAK BORAH31. Department Applied Geophysics, Indian School of Mines, Dhanbad 826004, India * Corresponding author, email: jr.kayal@gmail.com 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), Tezpur Project, 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

seismically and tectonically more complex than the eastern/central or western Himalaya segments ((Kayal, Microearthquake Seismology and Seismotectonics of South Asia, 503p, Springer, 2008). The region experiences several large/great earthquakes due to the Himalayan collision in the north, Indo-Burma atypical subduction tectonics to the east and due to intra-plate seismic activity in the Assam valley-Shillong Plateau-Bengal basin. The intra-plate seismic activity is fairly intense in this region due 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 the northeast India region including the two great earthquakes (Ms~ 8.7), the 1897 and the 1950, respectively (Kayal, Microearthquake Seismology and Seismotectonics of South Asia, 503p, Springer, 2008). Most of the large earthquakes are recorded in the Indo-Burma region, in the inter-plate/dipping seismic zone, but four intra-plate large earthquakes occurred much to the east of the IndoBurma subduction zone or much south of the Himalayan collision zone. These are the 1943 event (M 7.2) on the Kopili fault in the Assam valley, the 1930 event (M 7.1) on the Dhubri fault at the western boundary of the Shillong plateau, and rest two in the Bengal basin (Fig.1). In the Bengal basin, the 1923 event (M 7.5) is assigned to the Hinge zone and the 1918 event (M 7.6) to the Sylhet fault (Fig.1) (Nandy, Geodynamics of northeastern India and adjoining region, 209p., ACB Pub., 2001). The Hinge zone is a sub-surface structure, approximately 500 km long and 25-100 km wide, that separates the continental shelf to the west and the geosynclinal basin to the east (Evans, J Geol Soc India, 5, 80-96, 1964). Recently, we have recorded two strong felt earthquakes 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 August 19, 2009 in the Assam valley on the Kopili fault, respectively (Fig.1). These two events occurred almost within a month, and are reported as shallow focus (depth ~ 10 km) earthquakes in the USGS (United States Geological Survey) reports. The northeast India region is well equipped with about 25 permanent broadband seismic stations since 2001. We have studied focal mechanisms of these two felt events by waveform inversion, and examined the recent seismicity. The results are discussed here in the background of the known seismotectonics of the two different tectonic domains that are in close proximity.13

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 August 19, 2009, are examined here. The recent seismicity and fault plane solutions of these two felt earthquakes suggest that both the events occurred on the Kopili fault zone, a known active fault zone in the Assam valley, about 300 km long and 50 km wide. The fault zone is transverse to the east-west Himalayan trend, and its intense seismicity indicates that it transgresses into the Himalaya. The geologically mapped curvilinear structure of the Main Central Thrust (MCT) in the Himalaya, where the epicenter of the Bhutan earthquake is located, is possibly caused by the transverse Kopili fault beneath the MCT. This intensely active fault zone seems vulnerable for an impending larger earthquake (M >7.0) in the region, and it needs close and continuous monitoring. INTRODUCTION The northeastern Himalaya and the adjoining region, between latitude 22-30o longitude 88-98o N, are

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January 2010

BHUTAN HIMALAYA EARTHQUAKES The Bhutan Himalaya has no record of great or large earthquake (M> 7.0) during the past 200 years. Historical records of occurrence of large earthquakes before 1800 AD are mostly lacking. The seismicity of Bhutan Himalaya during the last 100 years, as reported in the International Seismological Centre (ISC) catalogs is, however, low compared to its adjoining Himalayan segments in the west. The low seismicity has been attributed to lower convergence rate. It is suggested that the India-Eurasia convergence is largely accommodated by the pop-up tectonics of the Shillong Plateau to the south, and the Bhutan Himalaya lies in the shadow zone with less seismicity (Gahalaut, Phys Earth Planet Interior, under pub, 2010). The September 21, 2009 strongly felt earthquake provides an insight into the seismotectonics of the Bhutan Himalaya. We have examined the recent seismicity and a N-S cross section in this area that includes the Shillong plateau, Assam valley and the Bhutan Himalaya; the EHB relocated events (M> 4.5) since 1995 are considered (Fig. 1). The section shows that the Shillong plateau earthquakes are mostly confined within a depth of 40 km and bounded by two major boundary faults, Dapsi thrust (DT) and Brahmaputra fault (BF), which has been also observed by local broadband network data (Kayal et al., Curr. Sci., 91(1), 109-114, 2006). The north dipping DT, a conjugate of the Dauki fault, is identified to be an active thrust that demarcates the southern boundary of the Shillong plateau activity and also truncated the maximum isoseismal of the 1897 Shillong earthquake along this thrust (Kayal and De, Bull Seism Soc Am, 81, 131-138, 1991). The east-west near vertical Dauki fault, which separates the Bengal basin to the south and the Shillong plateau to the north, is much less active. The Bhutan Himalaya earthquakes are found to be much shallower (~10 km) at the MCT zone, particularly the 2009 main shock and its aftershocks that occurred on14

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 local seismic broadband stations (blue triangles) used in this study are shown. The recent EHB located earthquakes M > 4.5 (1995-2007) within the rectangle area are shown by green solid circles, the past four larger (M>7.0) intra-plate earthquakes (solid yellow circles) and the two great earthquakes (larger yellow stars) are annotated with the year of occurrence. The two felt earthquakes of 2009 are shown by red stars. The USGS three fault plane solutions, the HRV CMT solution and the two solutions obtained in this study for the September 21, 2009 Bhutan earthquake are illustrated by beach balls with different colours. Two HRV CMT solutions of the past two earthquakes, 1995 and 2006 respectively, in the Bhutan Himalaya are shown by black beach balls. Fault plane solution of the August 17, 2009 Assam earthquake obtained in this study is also shown and annotated. A N-S cross section across the rectangle area, indicating earthquake foci is shown below; the red star and cluster of red solid circles below the curvilinear MCT indicate the September 21, 2009 Bhutan main shock and aftershocks respectively. Inset: Map of India showing the study region by a rectangular box.

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January 2010

the north-south curvilinear segment of the MCT. The earthquakes further north of the MCT are deeper, down to 50 km (Fig.1). The September 2009 Felt Earthquake (Mw 6.3) The September 21, 2009 earthquake was widely felt; 11 casualties, more than 18 injuries and about 1,100 damaged houses were reported in Bhutan (Fig.2). The maximum intensity reached to VIII the epicenter area (Dowchu, D., 2010, pers com). The epicenter of the event is placed at latitude 27.34N and longitude 91.41E, and depth at ~10 km in the USGS report. The tremor was well felt in the Bhutan Himalaya and in the adjoining northeast India region including Sikkim, Assam, Arunachal Pradesh, Shillong plateau and in Bangladesh; minor damages to a few Guwahati city houses were also reported in the local news papers.

Three moment tensor solutions are given by the USGS and a centroid moment tensor (CMT) solution by the HRV (Fig. 1). All the four solutions are compatible with Mw 6.1-6.3 and depth ~ 8-10 km, inferring that the event occurred on a shallow north dipping plane assigned to the plane of detachment as the fault plane for this event. This seismotectonic model that the Himalayan earthquakes occur on the plane of detachment is the most widely accepted tectonic model as envisaged by (Seeber et al., Geodyn Series 3, AGU pub. 215-242, 1981), and the Himalayan earthquake solutions are mostly biased to this model. Kayal (Tectonophys. 339, 331-351, 2001), however, argued that this model fits fairly well in the western Himalaya, but not in the eastern or in the northeastern Himalaya. We have reanalyzed the teleseismic waveforms of about 30 global digital seismic stations and obtained a solution of thrust faulting with strike slip component for this event (Fig.1). We infer that the east dipping northsouth nodal plane is the fault plane that is compatible with the north-south trending curvilin