Volume I Number I February (2012) pp. 127-32 The Clarion ISSN: 2277-1697

Earthquakes and crustal deformation studies of the seismically active Kopili Fault as well as North East India: A scientific field study using Global Positioning System (GPS)

K. Mahanta¹, J. D. Chowdhury², A. Kumar⁴, A. Kumar³, I. Laskar¹, Sunil L. Singh³, P. Barman⁴

¹Cotton College, India, ²Jagiroad College, India, ³Manipur University, India, ⁴Tezpur University, India.

E-mail: ¹

Abstract

The North-East India is one of the seismically active regions of the world. It has experienced a number of great earthquakes. To know about the seismic behavior of the region, we have to study the whole region scientifically. Now a day’s Global Positioning System (GPS) is used to find out the time and velocity of tectonic plates. It gives almost perfect results upto centimeter level accuracy over a thousand kilometers of long baseline. Using GPS one can also find out the crustal deformation of fault areas. This North-East region demands a micro-seismic study to know the unknown behavior of the region. We have studied in the NNW-SSE trending Kopili Fault area in the North-East India, which is the source point of a number of earthquakes.Present day this Kopili Fault which extends from western part of Manipur upto the tri-junction of Bhutan, Arunachal Pradesh and Assam plays a great role. We have studied for the last four years (October, 2006 to February, 2011) on twelve campaign sites in this region using GPS machines and collected data for four epochs. After analysis of data, a remarkable deformation in the fault line is found which are presented in this paper.

Keywords: Crustal deformation, Kopili Fault, Global Positioning System.

Introduction

Tectonically, the whole North-East Region can be divided into six physiographic cum tectonic domains. These units are Eastern Himalayan Collision Zone, Mishmi Block of the Syntaxial Zone of Himalayan Arc and Myanmar Arc, Indo-Myanmar Subduction Zone or Mobile Belt, the Brahmaputra Valley, the Meghalaya Plateau and the Bengal Basin. The Main Boundary Fault (MBF) and the Main Central Thrust (MCT) are the two major structural element of the Eastern Himalaya which extends upto the strategic southward bend of the Himalaya at Namche Barwa. The MBF has developed during the Pliocene and represents the main central boundary between the Indian lithospheric plate and the Eurasian Plate. The Mishmi Block tectonically separated the Eastern Himalaya from the mobile Myanmar Arc, where a number of lineaments in NW-SE and N-S direction are active. The North-East Region lies at the junction of Himalayan Arc to the North and Myanmar Arc to  the East. The collision of Indian Plate and Eurasian Plate involves the large scale of active continental deformation and therefore a rather diffuse seismicity prevails in this region ( Das Chowdhury, J. 2005).

Tectonic History

Millions of years ago, the whole landmass of the earth was combined, as there was only one continent and one ocean. The Pangaea theory states that all present continents were once together and collectively known as a 'supercontinent' or Pangaea. Pangaea was the large landmass that was disintegrated to form different continents. The process of separation of this landmass was started in 225 million years ago and it is still continuing. In 100 million years ago, India was surrounded by a vast ocean. The Indian landmark was moving towards North – North East direction and collided with the Eurasian Plate, forming the mountain Himalaya. Geologically, the Tethys Sea separated India from Eurasian Landmass. With continuous plate movement, Indian plate collided with Eurasian plate face to face and the mighty Himalayas evolved from the Tethys Sea. This collision began about 45 million years ago. The stresses of the Indian plate on the Eurasian Plate is still continuing, the height of the Himalaya is still rising, which also causes the various earthquake, landslides and all other physical events on the region stretching from Pamir to  the Indo-Myanmar border (Mahanta, K. 2011). So, this region is seismically very sensitive and active.

Previous work on this region

The first scientific study of earthquake in the North East region is done by T. Oldham. In 1889, the discovery of oil in Assam also added another chapter in the scientific geological exploration of this region. R.K. Verma and M. Mukhopadhaya has extended the gravity field measurement covering the whole N-E India. R.P. Singh has done research work on geomorphology of Shillong plateau; M.N. Kulkarni has done extensive work on application of geodesy to monitor earthquake hazards. B.C Bhatia, M.R. Kumar and H.K. Gupta has also done a elaborate work on drawing seismic hazard map of India and adjoining region. But the most important investigative research work was done by D.R. Nandy. He has done a brief work of geodynamics of North East India. But in the most active Kopili Fault region, nobody has done any attempt of scientific study. So within this tectonic setup of the North East, this research work relating to study the dynamic role played by the Kopili Fault in generating any major Earthquake events in this region has great importance.

Earthquakes history in North-East India

The North-East part of the country is known to be one of the six most seismically active regions of the World. The other five regions are Maxico, Taiwan, California, Japan and Turkey. This region lies between the northern collision and eastern subduction margins of the Indian plate(Das Chowdhury, J., Barman, P. 2008). Out of the 5 great earthquakes of magnitude greater than 8 in Richter Scale (RS) experienced by India, two occurred in this region. These are the 1897 'Great Assam Earthquake' and '1950 Assam Earthquake'. The Great Assam Earthquake occurred in June 12, 1897 with epicenters at 26˚ N 91˚ E has devastated Shillong, which developed Chidrang and Sami faults and fractured the whole North-East. The magnitude of this earthquake was 8.7 in RS. The next one occurred with the almost same magnitude (ie. 8.7 in RS) in August 15, 1950 with epicenters at 28.5˚ N 95.5˚ E at the depth of 29 km (Tiwari. R.P.) has destroyed the whole Assam. Total numbers of about 1520 people were killed. This earthquake was one of the World's largest known earthquakes. Apart from these two, if we analyse the history of the major earthquakes in the North-East India and its neighborhood, we see that the Kopili fault is seismically very active. Two major earthquakes of magnitude more than 7 in RS occurred in this region. In January 10, 1869, an earthquake with magnitude 7.8 in RS occurred at a depth of 60 km along the Kopili Fault Zone. This earthquake has severely damaged the entire Nagaland. In October 23, 1943, there was an earthquake with epicenters at 27.5˚ N 93.5˚ E originated at Kopili Fault with magnitude 7.3 in RS. It shook the whole North-East and damaged a lot.

Geological Setting

Geologically, in North-East India the Brahmaputra Valley is built by the deposition of about more than 1000 m thick alluvium upon sag formed during the period of the rise of the Himalaya. The Brahmaputra Valley is a relatively narrow valley bounded by two mobile young mountain belts, Mishmi block to the northeast and Meghalaya Plateau to the south. Another notable formation of this region is the discontinuous exposure of metamorphic rocks along the northern most part of the ridge in Nagaland and adjacent Myanmar area (Mahanta, K. 2011). This metamorphic complex is thrusted over from east and south east and may be of pre-cambrian age.

Seismicity in N-E India

The North-East region lies at a junction of Himalayan arc to the north and the mobile Myanmar arc in the east.Zone wise, the Eastern Himalayan Collision Zone has experienced 2 large earthquakes, one in 1941 (M =7.1) and another one in 1947 (M =7.5). The Indo-Myanmar subduction zone is seismically very active, where 10 large earthquakes of M >7.0 have occurred during the last 100 years. The Mishmi Block of the syntaxial zone of the Himalaya and Myanmar Arc is the zone of the Great Assam Earthquake of 1950 (M = 8.7). A number of thrusts and faults make this region seismically more active. A cluster of earthquake events is evident in Namcha Barwa   area at the interaction of Tethyan Himalayan domain and NW Mishmi Block (Bryant, H. Richard 1999). The Shillong Plateau and its plate boundary zone is active all throughout. It was the seat of Shillong earthquake of 1897 (M = 8.7). Moreover, it is the Kopili Fault that constitutes the tectonic boundary between the two stress domains of the Meghalaya and Mikir Hills Massif. It is the Kopili Fault Zone that triggered the 1869 (M = 7.8) and 1943 (M = 7.3) events and three of (M = 6.0 to 7.0) and several of magnitude 4.5 to 6 (Chugh, R.S. & Valdiya, K.S. 1989) . This seismically active Kopili fault cuts across the Brahmaputra about 35 km NW of Nagaon between Mikir Hills and the prolongatition of Meghalaya massif (Das Chowdhury, J. 2002).

Moreover, it is also observed that the hidden basement ridge of Assam Shelf including the block from Guwahati - Dergaon have been consistently rising, the vertical velocity increasing progressively from 0.3 mm/yr  to  4.5 to 31 mm/yr (Das Chowdhury, J. 2001) at Dergaon a part of Mikir Block. North of the Dauki Fault, the Meghalaya block has been rising at the rate of 0.3 to 0.4 mm/yr.( Das Gupta, S., Mukhopadhaya, M. & Nandy, D.R. 1987).

Methodology

To understand the crustal deformation in North-East India, the work has been carried out on the latest technology using Global Positioning System (GPS). In this study, twelve (12) points in different locations of Kopili Fault region have been selected for campaign study. The first field study was initiated in October, 2006 and then after it was continued for sequent four years up to February, 2011. These GPS campaign stations are studied with Trimble 5700, Leica500 and Leica1200 receivers with choke ring and Zephyr geodetic antennae. The selection of twelve sites was based on granite and feldspar rock formations or on hard stable rocks. All the points were selected in an open area without having any disturbance to the antenna. The machines were installed in all the campaign sites at least for continuous 72 hours, so that we can collect 48 hour data without break. The GPS data so collected has been converted into RINEX observation files and quality check has been conducted using TEQC (Translations, Editing and Quality Checking) software. All these processes of quality check plots were carefully examined and data with high cycle slips, multi-path were removed from the analysis (Sunil & Kumar 2008). The data collected for the last four epochs (2006-11) processed along with the selected IGS (International GPS Service) stations KUNM, HYDE, IISC, POL2, KIT3. In the processing, we have used GAMIT/GLOBK software developed by Massachusetts Institute of Technology (MIT), USA and a reference frame ITRF 2005 is used for all time series. 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 (Sunil & Kumar 2009). The horizontal errors ellipses are calculated by GAMIT/GLOBK software from the uncertainties in the North and East coordinates/velocities using the correlation between the baseline component estimates.

Results

GPS velocity vectors of all twelve sites with the IGS sites have been estimated in ITRF05 (GLOBK version 5.16 & 5.19). The analysis of data gives a clear picture that the North-East Region is moving in North-North-East (NNE) direction with a average velocity of 42.36E(�2.20mm) and 29.00N(�1.66mm). A remarkable convergence is seen between the points NATU-KUNM, NATU-POL2, UMRA-IISC, UMRA-POL2, UMRA-KIT3, PANI-IISC, PANI-KIT3, RAJA-POL2 and KHER-POL2. Between the campaign points KHER and RAJA, a clear divergence is seen at a rate of 2.245 mm/yr. (Table1 & 3).

Table 1: Estimated velocities of campaign sites of North-East India (using GLOBK Version 5.16)

Sl. No.	Name of the Stations	Station Code	Long (�E)	Lat (�N)	East (mm/yr)	North (mm/yr)
1	Natun Bazar	NATU	92.602	25.521	44.50(�2.36)	28.17(�1.90)
2	Umrangsho	UMRA	92.725	25.526	41.97(�2.47)	27.26(�1.70)
3	Sokra Pam	SOKR	92.736	25.596	44.58(�2.34)	27.07(�1.80)
4	Panimura	PANI	92.825	25.717	42.03(�2.47)	30.66(�1.92)
5	Raja Gaon	RAJA	92.629	26.073	43.98(�2.27)	29.96(�1.88)
6	Kheroni	KHER	92.857	25.812	49.44(�2.45)	30.85(�1.98)
7	Kumoi	KUMO	92.248	26.196	41.09(�1.04)	28.22(�0.92)
8	Jagiroad	JAGI	92.204	26.117	39.51(�1.37)	27.98(�1.24)
9	Bura Mayong	BURA	92.013	26.246	39.59(�1.26)	28.69(�1.12)
10	Amsoi	AMSO	92.484	26.156	36.74(�2.29)	30.01(�2.08)
11	Kampur	KAMP	92.650	26.163	43.36(�2.72)	30.93(�2.34)
12	Khetri	KHET	92.080	26.126	41.54(�1.21)	28.28(�1.07)

 

The average velocity of IGS stations KIT3 and POL2 in Eurasian plate is 5.19N(�0.395mm) and 26.96E(�0.385mm). The IGS sites in India IISC and HYDE have the average velocity of 35.28N(�0.47mm) and 41.125E(�0.48mm); means the Southern part of the Eurasian plate is more stable than the Indian plate(Table 1 & 2 and figure1)

Table 2: Estimated velocities of IGS sites of different plates (using GLOBK Version 5.16)

Sl. No.	Name of the Stations	Station Code	Long (�E)	Lat (�N)	East (mm/yr)	North (mm/yr)
1	Kunming	KUNM	102.797	25.030	29.11 (�.43)	-18.51 (�.43)
2	Lhasa	LHAZ	91.104	29.657	47.87 (�.36)	15.61 (�.34)
3	Hyderabad	HYDE	78.551	17.417	41.03 (�.50)	34.61 (�.50)
4	Bangalore	IISC	77.570	13.021	41.22 (�.46)	35.95 (�.44)
5	Bishkek	POL2	74.694	42.680	26.77 (�.38)	5.13 (�.38)
6	Kitab	KIT3	66.885	39.135	27.15 (�.39)	5.25 (�.41)

We have estimated the deformation rate between the sites by dividing the convergence rate by their average baseline length. A remarkable deformation is seen between the campaign points SOKR-POL2, UMRA-POL2, KHER-POL2 and PANI-POL2. The deformation rate of SOKR-POL2 is -8.978�10^-10 per year where as UMRA-POL2 is -6.399�10^-09. But the both UMRA and SOKR campaign points are situated very nearby. The baseline length between them is 7897.33 m and is separated by the river Kopili. The point SOKR which is in the North bank of the river Kopili is more stable than UMRA in the South bank of the river. The convergence rate of SOKR and UMRA are -2.24 mm/yr and -16.0025 mm/yr respectively; which means about 13.7625 mm/yr is accommodated in the Kopili Fault that is passing through the river. A notable deformation between KHER and PANI is also seen. The difference of the both points from POL2 is almost 6377.06 m and the convergence rate of PANI-POL2 and KHER-POL2 are -20.34 mm and -13.80 mm respectively; means about 6.5375 mm/yr is accommodated in between the Kopili Fault. The points KHER and RAJA are separated from each other by a rate of 2.245 mm/yr, with the rate of deformation 6.10050 �10^-08. If we carefully see the deformation rate of all points of Kopili Fault in the North-East India, a clear picture of deformation on the fault line is seen (Table 3).

 

Figure 1: Velocity of various campaign sites along the IGS sites

Table 3: Estimated Baseline lengths of campaign sites and IGS sites for four years and deformation rate

From Station	To Station	Baseline Length 2007-08 (m)	Baseline Length 2008-09 (m)	Baseline Length 2009-10(m)	Baseline Length  2010-11(m)	Convergence Rate between first and fourth epoch (mm/yr)	Deformation Rate (yr-1)
UMRA	NATU	12387.01187	12387.06905	12387.07388	12386.99608	-3.9475	-3.18681�10-07
KHER	RAJA	36800.27833	36800.28107	36800.28156	36800.28731	2.2450	6.10050 �10-08
KUMO	JAGI	9818.19435	9818.18257	9818.22700	9818.19298	-0.3425	-3.48842 �10-08
AMSO	BURA	48178.25627	48178.13204	48178.00854	48178.25224	-1.0075	-2.09119 �10-08
KAMP	KHET	57092.91790	57092.91671	57092.94546	57092.91462	-0.8200	-1.43626 �1008
NATU	KUNM	1027193.80016	1027193.80893	1027193.79702	1027193.75797	-10.5475	-1.02683 �10-08
NATU	IISC	2087536.92697	2087536.89292	2087536.97656	2087536.90626	-5.1775	-2.4802 �10-09
NATU	POL2	2493764.20584	2493764.17029	2493764.17200	2493764.15385	-12.9975	-5.212 �10-09
NATU	KIT3	2814617.20855	2814617.17598	2814617.19552	2814617.18252	-6.5075	-2.31204 �10-09
UMRA	IISC	2097424.50216	2097424.15828	2097423.95525	2097424.45460	-11.8900	-5.66886 �10-09
UMRA	POL2	2500513.13754	2500512.72713	2500512.39583	2500513.07353	-16.0025	-6.39969 �10-09
UMRA	KIT3	2823738.88858	2823738.43554	2823738.07702	2823738.84443	-11.0375	-3.90882 �10-09
SOKR	IISC	2102999.61881	2102999.28951	2102999.10269	2102999.60584	-3.2425	-1.54185 �10-09
SOKR	POL2	2494925.91432	2494925.52098	2494925.20530	2494925.90536	-2.2400	-8.97822 �10-10
PANI	IISC	2118075.89568	2118075.54774	2118075.34069	2118075.84903	-11.6625	-5.50618 �10-09
PANI	POL2	2489562.35615	2489561.93409	2489561.59432	2489562.27479	-20.3400	-8.17011 �10-09
PANI	KIT3	2818531.99147	2818531.52476	2818531.15727	2818531.93020	-15.3175	-5.43457 �10-09
RAJA	IISC	2127503.74338	2127503.72510	2127503.79341	2127503.71831	-6.2675	-2.94594 �10-09
RAJA	POL2	2447084.38356	2447084.35363	2447084.32492	2447084.32029	-15.8175	-6.46381 �10-09
RAJA	KIT3	2779851.52491	2779851.49582	2779851.48331	2779851.48115	-10.9400	-3.93546 �10-09
KHER	IISC	2127037.22916	2127037.21977	2127037.29566	2127037.20774	-5.3550	-2.51759 �10-09
KHER	POL2	2483185.2694	2483185.24962	2483185.22491	2483185.21419	-13.8025	-5.55839 �10-09

Conclusion

The whole North-Eastern Region is seismically very active due to the various faults present in this region.The NNW-SSE trending Kopili Fault which extends from western part of  Manipur upto the tri-junction of Bhutan, Arunachal Pradesh and Assam covers a distance of about 400km. The Kopili fault bisects the Meghalaya Plateau and isolated the Mishmi block from the main part of the plateau. In the last two years more than four notable earthquakes (M>5.0) occurred in the two endpoint of the Kopili Fault (Manipur and Bhutan), which signify that present day the Kopili Fault is very active. Our results also show the extreme deformation in the fault area as well as the North-East India.

Acknowledgement

We acknowledge Ministry of Earth Sciences (MoES), Government of India for providing funds vide Project no. MoES/P.O.(Seismo)/GPS/48/2005 to Department of Geography, Jagiroad College for carrying out the GPS surveys in the North-East Region.

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