Get 20M+ Full-Text Papers For Less Than $1.50/day. Start a 14-Day Trial for You or Your Team.

Learn More →

Depth of mid-crustal discontinuity from reflected seismic waves on local earthquake seismograms recorded at Shillong Plateau, Northeast India

Depth of mid-crustal discontinuity from reflected seismic waves on local earthquake seismograms... Geomatics, Natural Hazards and Risk Vol. 3, No. 4, November 2012, 355–364 Depth of mid-crustal discontinuity from reflected seismic waves on local earthquake seismograms recorded at Shillong Plateau, Northeast India DIPOK K. BORA*{ AND SAURABH BARUAH{ {Department of Physics, Diphu Government College, Diphu, Karbi Anglong-782 462, Assam, India {Geoscience Division, CSIR North-East Institute of Science & Technology, Jorhat-785 006, Assam, India (Received 10 June 2011; in final form 17 February 2012) In this study, an attempt is made to estimate the depth of mid-crustal discontinuity beneath the Shillong Plateau in northeast India region using broadband seismogram of local earthquakes. Principle of the technique is to relate the seismic travel times of the reflected phases (SxS) with the crustal thickness above the discontinuity. Though mid-crustal discontinuity (or Conrad discontinuity) is reported in some parts of the world, no such study was undertaken in the present study area due to complexity in analogue seismograms recorded before 2001. The digital waveforms of the local seismic events recorded by broadband digital network in the study area, however, make it possible precise detection of the seismic phases that are reflected at this discontinuity. The results show that the mid-crustal discontinuity exists at a depth 18+0.5 km beneath the Shillong Plateau, which provide a better understanding of the crustal velocity structure of the region for future study. 1. Introduction Shillong Plateau in the northeast region (NER) of India, bounded by latitude (25– 26.58N) and longitude (90–92.58E), is one of the most seismically active zones in the world where three large (M  7.0) earthquakes in 1923, 1930 and 1943 respectively, and one great earthquake (M ¼ 8.7) on June 12, 1897 (Oldham 1899) occurred during the last 115 years since 1897. The 1897 great earthquake caused extensive destruction in the region killing about 1540 lives and loss of $30 million (Tillottson 1953). The whole NER region lies in the seismic zone V of India (BMTPC 2003). The zone V is the maximum rating of zone in the National Seismicity Zoning Map of India. The Shillong Plateau in the NER is a part of the Indian shield that is separated out from the peninsular shield and moved to the east by about 300 km along the Dauki fault (Evans 1964). The gigantic E-W trending Dauki fault separates the Plateau to the north and the Bengal basin to the south (Kayal 2001) (figure 1). The mighty Brahmaputra River, on the other hand, separates the Plateau from the Himalaya to the north. The E-W segment of the river at the northern boundary of the Plateau is named Brahmaputra fault (Nandy 2001). *Corresponding author. Email: dipak23@rediffmail.com Geomatics, Natural Hazards and Risk ISSN 1947-5705 Print/ISSN 1947-5713 Online ª 2012 Taylor & Francis http://www.tandf.co.uk/journals http://dx.doi.org/10.1080/19475705.2012.668564 356 D.K. Bora and S. Baruah Figure 1. Map showing the major tectonic features of the study region. The Great earthquakes of 12 June, 1897 is shown by a larger red star. The red circles represent the epicentres of the selected earthquakes. The digital broadband seismic stations are shown by green triangles. The major tectonic features in the region are indicated: Dh F: Dudhnoi Fault; DF: Dapsi Fault; OF: Oldham Fault; CF: Chedrang Fault; BS: Borapani Shear Zone. Dauki Fault, Kopili Fault and Main Boundary Thrust are also indicated. Alongside the depth section plots of the epicentres of the earthquakes used in this study. Inset map of India indicating the study region (arrow sign). Several hundred earthquakes (M4 4.0) are recorded by the local seismic networks during the past three decade in the region since early 1980s. In addition to the national network run by the India Meteorological Department (IMD), the North- East Institute of Science and Technology-Jorhat (NEIST-J), National Geophysical Research Institute, Hyderabad (NGRI-H) and several universities have established analog networks since 1982. These stations are now upgraded to broadband (BB) digital stations with global positioning system (GPS) timing since 2001. Several authors (e.g. De and Kayal 1990, Mukhopadhya et al. 1995; Sitaram et al. 2001) have studied crustal structure in the region using the analogue data before 2001. No study is, however, made to estimate the mid-crustal discontinuity by using S-wave reflected phases (SxS) of the local earthquakes recorded by the broadband network in the region. Picking up or precise reading of reflected phases in the analogue records of earlier seismic network was difficult. With the introduction of digital seismic network and precise detection of reflected phases, it is possible to Depth of mid-crustal discontinuity beneath the Shillong Plateau 357 estimate crustal discontinuities (e.g. Sanford and Long 1965, Mizoue 1971, Sanford et al. 1973, Rinehart and Sanford 1981, Mizoue and Isao 1982). In the present study, we have utilized the broadband seismic waveforms of the local earthquakes to estimates the mid-crustal discontinuity beneath the Shillong Plateau. The principle of the technique is to relate the travel time anomalies to the thickness of the mid-crustal layer above the discontinuity. This has been done from travel times of the reflected seismic waves well recorded by the local broadband network. Since crustal structure plays an important role to understand the seismotectonics of a region, the precise location of mid-crustal discontinuity shed new light on the crustal structure beneath the Shillong Plateau. 2. Tectonic setting The NER, India is comprised of distinct geological units, i.e., the Himalayan frontal arc the north and the highly folded Indo-Burma mountain ranges to the east in the respective inter-plate tectonic zones, and the Brahmaputra alluvium in the Assam valley, the Shillong Plateau and the Mikir Hills. The major geotectonic structures associated with the Shillong Plateau are: the Brahmaputra fault to the north and the E-W Dauki fault to the south. There are two major thrust faults in the Shillong Plateau, namely: the Dapsi Thrust and Barapani shear zone (Kayal 1991). According to Evans (1964), the Shillong Massif has been separated from the peninsular shield and moved to the east along the Dauki fault. Seismotectonics of the region has been the subject of several studies (e.g. Tapponnier et al. 1982, Kayal et al. 2006, Angelier and Baruah 2009). Bilham and England (2001), based on geodetic and GPS data, proposed a ‘‘pop-up’’ tectonic model of the Shillong Plateau, and argued that the 1897 great earthquake was produced by a south dipping hidden fault at the northern boundary of the Shillong Plateau; they named it the ‘‘Oldham fault’’ that extends from a depth of about 9 km down to 45 km. They further suggested that the Shillong Plateau earthquakes are caused by the ‘‘pop-up’’ tectonics between the Dauki fault and the Oldham fault. Details of the seismicity and the seismotectonics of the region are reviewed by Kayal and De (1991), Rajendran et al. (2004), Baruah and Hazarika (2008), Kayal (2008) and Baruah et al. (2010). 3. Data analysis and results 3.1. Data We have selected 75 precisely located earthquakes recorded during 2002–2009 by the local broadband seismic networks operated by the NEIST-J and NGRI-H. These events are recorded with higher signal-to-noise ratio and having clear direct and reflected phases. In addition to the NEIST-J and NGRI-H network data, the data of the India Meteorological Department, Gauhati University, Manipur University and Mizoram University are also incorporated for better determination of hypocentral parameters. All the broad band seismic stations are operated both in continuous mode and in trigger mode and at a rate of 100 samples per second. The recorded seismograms have been corrected by using an instrument response based on the eloctrodynamic constant, critical damping, natural frequency of seismometers and bit weight of unit gain of each recording unit for all stations. Table 1 shows the 358 D.K. Bora and S. Baruah Table 1. Station names along with abbreviations. No Station Abbreviations Latitude (8N) Longitude (8E) Elevation (m) 1 Jogighopa JPA 26.239 90.575 42 2 Manikganj MND 25.924 90.676 40 3 Nangalbibra NGL 25.472 90.702 330 4 Gauhati University GAU 26.152 91.667 69 5 Shillong SHL 25.566 91.859 1590 6 Boko BOKO 25.969 91.244 50 7 Agia AGIA 26.066 90.464 75 8 Tura TURA 25.546 90.243 305 digital stations with the abbreviations used. About 116 seismograms were used for the selected 75 events. Epicentres of the selected earthquakes are shown in figure 1. Precision of hypocentre determination depends not only on the distribution of the recording stations but also on velocity structure between source and stations, particularly in an area where lateral heterogeneities are extreme (Okada et al. 1970). The epicentres are determined using the HYPOCENTER location program of Lienert et al. (1986) based on the crustal velocity model of Bhattacharya et al. (2005). Uncertainties involved in the estimates of epicentres show that 85 percent of the events are located with 0-2 km error in depths and epicentre and error in origin times are of the order of 0–0.3 sec. Duration magnitude (M ) of the events are estimated in the range 2–4 and the focal depth mostly at 3–15 km. 3.2. Characteristic features of the reflections of SxS The broadband seismograms of the local earthquakes recorded by stations in and around the Shillong Plateau frequently have sharp impulsive phases following the direct S-wave. These phases are identified as S to S-wave (SxS) reflections from a mid-crustal discontinuity. A few examples of seismograms and travel time plots at a station NGL are illustrated in figure 2(a) and (b), respectively. Figure 2(a) indicates the sharp reflected phases in all the seismograms with arrival time differences from 1.3 to 3.8 s. Gradual increase in travel time of these phases with S minus P difference (distance) depicts reliability of the identified phases. The S-P interval of the earthquakes used in this study ranges between 1.2 and 4.6 s. The reflected SxS phases from the mid-crustal discontinuity are observed after 2.0–4.3 s of the first S arrival. The following are the noticeable characteristic of the reflections registered on the seismograms. (i) These prominent phases are observed only in seismograms of shallow crustal (depth 15 km) earthquakes. The SxS reflection can be found prominently on the horizontal component seismograms rather than on the vertical one. The observation suggests that the later arrival can be interpreted as the deep crustal reflections of the S-wave. (ii) Moreover, the particle motion of these phases shows that these later phases arrive as shear waves. Analytical results of the particle motions of the reflected waves at the station NGL are exemplified in figure 3. Depth of mid-crustal discontinuity beneath the Shillong Plateau 359 Figure 2. (a) Examples of horizontal component seismograms recorded at NGL (Nangalbibra) station, and (b) plots of travel time versus S-P time for P, S and SxS phases. (iii) The reflections of SxS are, however, not always strong enough to be recognized on the seismograms. The amplitude of the reflections is related to the station coordinates and the physical property of the discontinuity. 360 D.K. Bora and S. Baruah Figure 3. (a) S-wave reflections from the midcrustal discontinuity are denoted as SxS at station NGL (Nangalbibra). (b) Particle motion of the P, S and SxS phases in the radial and vertical plane are shown. The crustal reflections at very small distance are apparently quite rare (Sanford and Long 1965, Mizoue 1971, Sanford et al. 1973) 3.3. Mapping of the mid-crustal discontinuity Travel time curves of the SxS phases are interpreted in terms of depth of the mid- crustal discontinuity, and the average velocity of the S waves in the upper crustal layer are also estimated. Theoretical travel time for a reflected ray is estimated by using the relation of Rinehart and Sanford (1981): qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 ð2H  hÞ þ d T ¼ ð1Þ where, H is the depth of the reflector, h is the depth of focus for the event, d is the epicentral distance and V is the average velocity in the upper crust. About 75 earthquakes recorded in the Shillong Plateau are used for the travel time analysis to estimate the mid-crustal discontinuity. Relations of the observed travel times versus S minus P interval are shown in figure 4(a). Theoretical travel time curves for the SxS reflection at a depth (H )of 18+0.5 km are shown along with the observational data in figure 4(b). The left hand ends of each curve are the theoretical limits of observable reflections at the specified depth of focus. Generally, it is observed that only one combination of (h) and (H ) produces the theoretical curves that fit the SxS data well. The curves of each reflected phases are Depth of mid-crustal discontinuity beneath the Shillong Plateau 361 Figure 4. (a) Observed travel times of P, S and SxS versus S-P times. (b) Observed and theoretical travel times of SxS versus S-P times. Two curves for the reflections from the midcrustal discontinuity at the depth H (solid lines) is given, one for the minimum, and the other for the maximum depth of earthquake focus h. given, one for a 3 km depth of focus (h), the other for a 15 km depth of focus. The theoretical curves were computed using the standard crustal S wave velocity of 3.53 km/s (Bhattacharya et al. 2008). 4. Results and discussion Shallow crustal reflections (SxS) are investigated to estimate the mid-crustal discontinuity beneath the Shillong Plateau. It is observed that the Shillong Plateau earthquake with S-P interval of 1.2–4.6 s. produces sharp impulsive phases. These reflected phases are observed after 2.0–4.3 s of the first S arrival. The time distance relations for the SxS reflections estimate the mid-crustal discontinuity at 18+0.5 km beneath the Shillong Plateau. Several investigations have been made to estimate crustal thickness and 1D velocity structure in the area using time distance plot (De and Kayal 1990, Sitaram et al. 2001). All these studies were based on first arrival of P and S wave data of the 362 D.K. Bora and S. Baruah analogue networks operated before 2001. The digital seismograms, recorded by the recent upgraded broadband network since 2001, are fruitfully utilized to identify the reflected phases (Baruah et al. 2010), and this study enabled us to estimate the mid- crustal discontinuity. The mid-crustal discontinuity named Conrad discontinuity,in the crust has been reported by several investigators in some other parts of the world (e.g. Sanford and Long 1965, Mizoue 1971, Sanford et al. 1973, Rinehart and Sanford 1981, Mizoue and Isao 1982). In south and central India, several DSS (Deep Seismic Sounding) surveys have been carried out (e.g. Kaila and Sain 1997). The DSS results show 2D velocity structure with a diagnostic Mohorovicic (Moho) discontinuity at the crust and mantle boundary along the profiles but the sharp Conrad discontinuity only at places are reported. In our investigation, the broadband seismograms of the local shallow crustal earthquakes have been very useful to identify the sharp mid-crustal discontinuity. In a recent study with the broadband seismograms, the Conrad depth beneath the Shillong Plateau has been estimated by Baruah et al. (2010). Positive Bouguer gravity anomaly *20–40 mGal (Verma and Mukhopadhyay 1977, Gaur and Bhattacharji 1983, Dasgupta and Biswas 2000) support the pop-up tectonic model envisaged by Bilham and England (2001) that the Plateau pops up between two reverse faults, the south-bounding Dauki fault and the north bounding Oldham fault. The gravity high over this plateau may not be entirely attributed to the absence of sedimentary overburden and hence propose a high density layer emplaced in the crust beneath the Shillong Plateau (Gokarn et al. 2008). The receiver function analysis (Mitra et al. 2005) delineate about 10 km thick high velocity crust beneath the Shillong Plateau (S-wave velocity 4 km s as against 3.6 km s in the surrounding region), which seems to the support this conjuncture. From magnetotelluric (MT) study, Gokarn et al. (2008) recorded low resistivity down to depth of *20 km beneath the Shillong Plateau, this observation is in good agreement with our observation of a discontinuity at a similar depth range. 5. Conclusions We estimated the mid-crustal discontinuity beneath the Shillong Plateau using the travel times of direct and reflected phases from 75 shallow (depth 15 km) crustal earthquakes. The SxS phases are critically observed with S-P difference of about 1.2– 4.6 s. It is found that only one combination of focal depth h and crustal depth H reproduces the theoretical curves that match the observed travel time data of the reflected phases. The estimated mid-crustal discontinuity is 18+0.5 km in the study region. The mid-crustal Conrad discontinuity in the Shillong Plateau is first reported by us based on reflected phases observed in the broadband seismograms. Acknowledgements We express our sincere gratitude to Dr. P. G. Rao, Director of the North-East Institute of Science & Technology for his encouragement to publish this work. We are thankful to Prof. J. R. Kayal, CSIR-Scientist Emeritus at Jadavpur University, Kolkota for his critical comments and review during this study. Data obtained from NEIST, Jorhat, NGRI, Hyderabad and IIG, Mumbai are acknowledged. We are thankful to all three anonymous reviewers for their valuable comments and suggestions, which helped improve the quality of the manuscript. Depth of mid-crustal discontinuity beneath the Shillong Plateau 363 References ANGELIER, J. and BARUAH, S., 2009, Seismotectonics in Northeast India: a stress analysis of focal mechanism solutions of earthquakes and its kinematic implications. Geophysical Journal International, 178, pp. 303–326. BARUAH, S., BORA, D.K. and BISWAS, R., 2010, Estimation of Crustal discontinuities from reflected seismic waves recorded at Shillong and Mikir Hills Plateau, Northeast India. International Journal of Earth Sciences, 100, pp. 1283–1292. BARUAH, S. and HAZARIKA, D., 2008, A GIS based tectonic map of India. Current Science, 95, pp. 176–177. BHATTACHARYA, P.M., MUKHOPADHYAY, S., MAZUMDAR, R.K. and KAYAL, J.R., 2008, 3-D seismic structure of the northeast India region and its implication for local and regional tectonics. Journal of Asian Earth Sciences, 33, pp. 25–41. BHATTACHARYA, P.M., PUJOL, J., MAZUMDAR, R.K. and KAYAL, J.R., 2005, Relocation of earthquakes in the Northeast India region using joint Hypocenter determination method. Current Science, 89, pp. 1404–1413. BILHAM, R. and ENGLAND, P., 2001, Plateau ‘‘pop-up’’ in the Great 1897 Assam earthquake. Nature, 410, pp. 806–809. BMTPC, 2003, Vulnerability atlas – 2nd edn; peer group MoH & UPA; seismic zones of India IS: 1983-2002, BIS, GOI, Seismotectonic atlas of India and its environs, GSI, GOI. DASGUPTA,A.and BISWAS, A., 2000, Geology of Assam (Bangalore: Geological Society of India). DE, R. and KAYAL, J.R., 1990, Crustal P-wave velocity and velocity-ratio study in northeast India by a microearthquake survey. Pure and Applied Geophysics, 134, pp. 93–108. EVANS, P., 1964, Tectonic framework of Assam. Journal of Geological Society of India, 5, pp. 80–96. GAUR, V.K. and BHATTACHARJI, J.C., 1983, Gravimetric determination of the shape of Mohorovicic discontinuity in Peninsular and north-eastern India. In Paper presented at I.U.G.G. General Assembly. Hamburg, Germany, August 15–27. pp. 936. GOKARN, S.G., GUPTA, G., WALIA, D., SANABAM, S.S. and HAZARIKA, N., 2008, Deep Geoelectric structure over Lower Brahmaputra valley and Shillong Plateau, NE India using magnetotellurics, Journal of Geophysical International, 173, 92–104. KAILA, K.L. and SAIN, K., 1997, Variation of crustal velocity structure in India as determine from DSS studies and their implications on regional tectonics. Journal of Geological Society of India, 49, pp. 395–407. KAYAL, J.R., 1991, Earthquake prediction in northeast India – A review. Pure and Applied and Geophysics, 136, pp. 297–313. KAYAL, J.R., 2001, Microearthquake activity in some part of the Himalaya and the tectonic model. Tectonophysics, 339, pp. 331–351. KAYAL, J.R., 2008, Microearthquake Seismology and Seismotectonics of South Asia, 503 p. (Berlin: Springer). KAYAL, J.R., AREFIEV, S.S., BARUAH, S., HAZARIKA, D., GOGOI, N., KUMAR, A., CHOWDHURY, S.N. and KALITA, S., 2006, Shillong plateau earthquakes in Northeast India region: Complex tectonic model. Current Science, 91, pp. 109–114. KAYAL, J.R. and DE, R., 1991, Microseismicity and tectonic in Northeast India. Bulletin of Seismological Society of America, 81, pp. 131–138. LIENERT, B.R., BERG, B.E. and FRAZER, L.N., 1986, Hypocenter: An earthquake location method using corrected, scaled and adaptively damped least squares. Bulletin of Seismological Society of America, 76, pp. 771–783. MITRA, S., PRISTLEY, K., BHATTACHARYA, A. and GAUR, V.K., 2005, Crustal structure and earthquake focal depths beneath northeastern India and southern Tibet. Geophysical Journal International, 160, pp. 227–248. MIZOUE, M., 1971, Crustal structure from travel times of reflected and refracted seismic waves recorded at Wakayama micro-earthquake observatory and its substations. Bulletin of Earthquake Research Institute, 49, pp. 33–62. 364 D.K. Bora and S. Baruah MIZOUE, M. and ISAO, N., 1982, Mapping of an unusual crustal discontinuity by microearthquake reflections in the earthquake swarm area near Ashio, Northwestern Part of Tochigi Prefecture, Central Japan. Bulletin of Earthquake Research Institute, 57, pp. 653–685. MUKHOPADHYAY, S., KHATTRI, K.N. and CHANDER, R., 1995, Seismic velocity and related elastic parameters of the crust in the Shillong Massif. Journal of Himalayan Geology, 6, pp. 1–8. NANDY, D.R., 2001, Geodynamics of Northeastern India and the Adjoining Region, pp. 209 (Calcutta: ACB publications). OKADA, H., SUZUKI, S. and ASANO, S., 1970, Anomalous understanding structure in the Matsushiro earthquake swarm area as derived from a fan shooting technique. Bulletin of Earthquake Research Institute, 48, pp. 811–833. OLDHAM, R.D., 1899, Report on the great earthquake of the 12th June 1897, Memoirs of Geological Survey of India. 29, 1–379, reprinted 1981 (Calcutta: Geological Survey of India). RAJENDRAN, C.P., RAJENDRAN, K., DUARAH, B.P., BARUAH, S. and EARNEST, A., 2004, Interpreting the style of faulting and paleoseismicity associated with the 1897 Shillong, northeast India, earthquake: Implications for regional tectonism. Tectonics, 23, pp. 1– RINEHART, E.J. and SANFORD, A.R., 1981, Upper crustal structure of the Rio Grande rift near Socorro, New Mexico, from inversion of microearthquake S-wave reflections. Bulletin of Seismological Society of America, 71, pp. 437–450. SANFORD, A.R., ALPTEKIN, O. and TOPPOZADA, T.R., 1973, Use of reflection phases on microearthquake seismograms to map an unusual discontinuity beneath the Rio Grande Rift. Bulletin of Seismological Society of America, 63, pp. 2021–2034. SANFORD, A.R. and LONG, L.T., 1965, Microearthquake crustal reflections, Socorro, New Mexico. Bulletin of Seismological Society of America, 55, pp. 579–586. SITARAM, M.V.D., YADAV, D.K. and GOSWAMI, K., 2001, Study on crustal structure beneath Arunachal Himalaya and Assam. Journal of Geological Society of India, 58, pp. 285– TAPPONNIER, P., PELTZER, G., LE DIAN,A.Y,ARMIJO, R. and COBBOLD, P., 1982, Propagating extrusion tectonics in Asia: New insights from simple experiments with plasticine. Geology, 10, pp. 611–616. TILLOTTSON, E., 1953, The great Assam earthquake of 1950, the completion of papers on the Assam Earthquake of August 15, 1950, Compiled by M.B. Ramachandra Rao, (Calcutta, India: Central Board of Geophysics), pp. 94–96. VERMA, K.K. and MUKHOPADHYAY, M., 1977, An analysis of the gravity field in northeastern India. Tectonophysics, 42, pp. 283–317. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png "Geomatics, Natural Hazards and Risk" Taylor & Francis

Depth of mid-crustal discontinuity from reflected seismic waves on local earthquake seismograms recorded at Shillong Plateau, Northeast India

Download PDF
10 pages

Loading next page...
 
/lp/taylor-francis/depth-of-mid-crustal-discontinuity-from-reflected-seismic-waves-on-nWfa0OPiBa

References

References for this paper are not available at this time. We will be adding them shortly, thank you for your patience.

Publisher
Taylor & Francis
Copyright
Copyright Taylor & Francis Group, LLC
ISSN
1947-5713
eISSN
1947-5705
DOI
10.1080/19475705.2012.668564
Publisher site
See Article on Publisher Site

Abstract

Geomatics, Natural Hazards and Risk Vol. 3, No. 4, November 2012, 355–364 Depth of mid-crustal discontinuity from reflected seismic waves on local earthquake seismograms recorded at Shillong Plateau, Northeast India DIPOK K. BORA*{ AND SAURABH BARUAH{ {Department of Physics, Diphu Government College, Diphu, Karbi Anglong-782 462, Assam, India {Geoscience Division, CSIR North-East Institute of Science & Technology, Jorhat-785 006, Assam, India (Received 10 June 2011; in final form 17 February 2012) In this study, an attempt is made to estimate the depth of mid-crustal discontinuity beneath the Shillong Plateau in northeast India region using broadband seismogram of local earthquakes. Principle of the technique is to relate the seismic travel times of the reflected phases (SxS) with the crustal thickness above the discontinuity. Though mid-crustal discontinuity (or Conrad discontinuity) is reported in some parts of the world, no such study was undertaken in the present study area due to complexity in analogue seismograms recorded before 2001. The digital waveforms of the local seismic events recorded by broadband digital network in the study area, however, make it possible precise detection of the seismic phases that are reflected at this discontinuity. The results show that the mid-crustal discontinuity exists at a depth 18+0.5 km beneath the Shillong Plateau, which provide a better understanding of the crustal velocity structure of the region for future study. 1. Introduction Shillong Plateau in the northeast region (NER) of India, bounded by latitude (25– 26.58N) and longitude (90–92.58E), is one of the most seismically active zones in the world where three large (M  7.0) earthquakes in 1923, 1930 and 1943 respectively, and one great earthquake (M ¼ 8.7) on June 12, 1897 (Oldham 1899) occurred during the last 115 years since 1897. The 1897 great earthquake caused extensive destruction in the region killing about 1540 lives and loss of $30 million (Tillottson 1953). The whole NER region lies in the seismic zone V of India (BMTPC 2003). The zone V is the maximum rating of zone in the National Seismicity Zoning Map of India. The Shillong Plateau in the NER is a part of the Indian shield that is separated out from the peninsular shield and moved to the east by about 300 km along the Dauki fault (Evans 1964). The gigantic E-W trending Dauki fault separates the Plateau to the north and the Bengal basin to the south (Kayal 2001) (figure 1). The mighty Brahmaputra River, on the other hand, separates the Plateau from the Himalaya to the north. The E-W segment of the river at the northern boundary of the Plateau is named Brahmaputra fault (Nandy 2001). *Corresponding author. Email: dipak23@rediffmail.com Geomatics, Natural Hazards and Risk ISSN 1947-5705 Print/ISSN 1947-5713 Online ª 2012 Taylor & Francis http://www.tandf.co.uk/journals http://dx.doi.org/10.1080/19475705.2012.668564 356 D.K. Bora and S. Baruah Figure 1. Map showing the major tectonic features of the study region. The Great earthquakes of 12 June, 1897 is shown by a larger red star. The red circles represent the epicentres of the selected earthquakes. The digital broadband seismic stations are shown by green triangles. The major tectonic features in the region are indicated: Dh F: Dudhnoi Fault; DF: Dapsi Fault; OF: Oldham Fault; CF: Chedrang Fault; BS: Borapani Shear Zone. Dauki Fault, Kopili Fault and Main Boundary Thrust are also indicated. Alongside the depth section plots of the epicentres of the earthquakes used in this study. Inset map of India indicating the study region (arrow sign). Several hundred earthquakes (M4 4.0) are recorded by the local seismic networks during the past three decade in the region since early 1980s. In addition to the national network run by the India Meteorological Department (IMD), the North- East Institute of Science and Technology-Jorhat (NEIST-J), National Geophysical Research Institute, Hyderabad (NGRI-H) and several universities have established analog networks since 1982. These stations are now upgraded to broadband (BB) digital stations with global positioning system (GPS) timing since 2001. Several authors (e.g. De and Kayal 1990, Mukhopadhya et al. 1995; Sitaram et al. 2001) have studied crustal structure in the region using the analogue data before 2001. No study is, however, made to estimate the mid-crustal discontinuity by using S-wave reflected phases (SxS) of the local earthquakes recorded by the broadband network in the region. Picking up or precise reading of reflected phases in the analogue records of earlier seismic network was difficult. With the introduction of digital seismic network and precise detection of reflected phases, it is possible to Depth of mid-crustal discontinuity beneath the Shillong Plateau 357 estimate crustal discontinuities (e.g. Sanford and Long 1965, Mizoue 1971, Sanford et al. 1973, Rinehart and Sanford 1981, Mizoue and Isao 1982). In the present study, we have utilized the broadband seismic waveforms of the local earthquakes to estimates the mid-crustal discontinuity beneath the Shillong Plateau. The principle of the technique is to relate the travel time anomalies to the thickness of the mid-crustal layer above the discontinuity. This has been done from travel times of the reflected seismic waves well recorded by the local broadband network. Since crustal structure plays an important role to understand the seismotectonics of a region, the precise location of mid-crustal discontinuity shed new light on the crustal structure beneath the Shillong Plateau. 2. Tectonic setting The NER, India is comprised of distinct geological units, i.e., the Himalayan frontal arc the north and the highly folded Indo-Burma mountain ranges to the east in the respective inter-plate tectonic zones, and the Brahmaputra alluvium in the Assam valley, the Shillong Plateau and the Mikir Hills. The major geotectonic structures associated with the Shillong Plateau are: the Brahmaputra fault to the north and the E-W Dauki fault to the south. There are two major thrust faults in the Shillong Plateau, namely: the Dapsi Thrust and Barapani shear zone (Kayal 1991). According to Evans (1964), the Shillong Massif has been separated from the peninsular shield and moved to the east along the Dauki fault. Seismotectonics of the region has been the subject of several studies (e.g. Tapponnier et al. 1982, Kayal et al. 2006, Angelier and Baruah 2009). Bilham and England (2001), based on geodetic and GPS data, proposed a ‘‘pop-up’’ tectonic model of the Shillong Plateau, and argued that the 1897 great earthquake was produced by a south dipping hidden fault at the northern boundary of the Shillong Plateau; they named it the ‘‘Oldham fault’’ that extends from a depth of about 9 km down to 45 km. They further suggested that the Shillong Plateau earthquakes are caused by the ‘‘pop-up’’ tectonics between the Dauki fault and the Oldham fault. Details of the seismicity and the seismotectonics of the region are reviewed by Kayal and De (1991), Rajendran et al. (2004), Baruah and Hazarika (2008), Kayal (2008) and Baruah et al. (2010). 3. Data analysis and results 3.1. Data We have selected 75 precisely located earthquakes recorded during 2002–2009 by the local broadband seismic networks operated by the NEIST-J and NGRI-H. These events are recorded with higher signal-to-noise ratio and having clear direct and reflected phases. In addition to the NEIST-J and NGRI-H network data, the data of the India Meteorological Department, Gauhati University, Manipur University and Mizoram University are also incorporated for better determination of hypocentral parameters. All the broad band seismic stations are operated both in continuous mode and in trigger mode and at a rate of 100 samples per second. The recorded seismograms have been corrected by using an instrument response based on the eloctrodynamic constant, critical damping, natural frequency of seismometers and bit weight of unit gain of each recording unit for all stations. Table 1 shows the 358 D.K. Bora and S. Baruah Table 1. Station names along with abbreviations. No Station Abbreviations Latitude (8N) Longitude (8E) Elevation (m) 1 Jogighopa JPA 26.239 90.575 42 2 Manikganj MND 25.924 90.676 40 3 Nangalbibra NGL 25.472 90.702 330 4 Gauhati University GAU 26.152 91.667 69 5 Shillong SHL 25.566 91.859 1590 6 Boko BOKO 25.969 91.244 50 7 Agia AGIA 26.066 90.464 75 8 Tura TURA 25.546 90.243 305 digital stations with the abbreviations used. About 116 seismograms were used for the selected 75 events. Epicentres of the selected earthquakes are shown in figure 1. Precision of hypocentre determination depends not only on the distribution of the recording stations but also on velocity structure between source and stations, particularly in an area where lateral heterogeneities are extreme (Okada et al. 1970). The epicentres are determined using the HYPOCENTER location program of Lienert et al. (1986) based on the crustal velocity model of Bhattacharya et al. (2005). Uncertainties involved in the estimates of epicentres show that 85 percent of the events are located with 0-2 km error in depths and epicentre and error in origin times are of the order of 0–0.3 sec. Duration magnitude (M ) of the events are estimated in the range 2–4 and the focal depth mostly at 3–15 km. 3.2. Characteristic features of the reflections of SxS The broadband seismograms of the local earthquakes recorded by stations in and around the Shillong Plateau frequently have sharp impulsive phases following the direct S-wave. These phases are identified as S to S-wave (SxS) reflections from a mid-crustal discontinuity. A few examples of seismograms and travel time plots at a station NGL are illustrated in figure 2(a) and (b), respectively. Figure 2(a) indicates the sharp reflected phases in all the seismograms with arrival time differences from 1.3 to 3.8 s. Gradual increase in travel time of these phases with S minus P difference (distance) depicts reliability of the identified phases. The S-P interval of the earthquakes used in this study ranges between 1.2 and 4.6 s. The reflected SxS phases from the mid-crustal discontinuity are observed after 2.0–4.3 s of the first S arrival. The following are the noticeable characteristic of the reflections registered on the seismograms. (i) These prominent phases are observed only in seismograms of shallow crustal (depth 15 km) earthquakes. The SxS reflection can be found prominently on the horizontal component seismograms rather than on the vertical one. The observation suggests that the later arrival can be interpreted as the deep crustal reflections of the S-wave. (ii) Moreover, the particle motion of these phases shows that these later phases arrive as shear waves. Analytical results of the particle motions of the reflected waves at the station NGL are exemplified in figure 3. Depth of mid-crustal discontinuity beneath the Shillong Plateau 359 Figure 2. (a) Examples of horizontal component seismograms recorded at NGL (Nangalbibra) station, and (b) plots of travel time versus S-P time for P, S and SxS phases. (iii) The reflections of SxS are, however, not always strong enough to be recognized on the seismograms. The amplitude of the reflections is related to the station coordinates and the physical property of the discontinuity. 360 D.K. Bora and S. Baruah Figure 3. (a) S-wave reflections from the midcrustal discontinuity are denoted as SxS at station NGL (Nangalbibra). (b) Particle motion of the P, S and SxS phases in the radial and vertical plane are shown. The crustal reflections at very small distance are apparently quite rare (Sanford and Long 1965, Mizoue 1971, Sanford et al. 1973) 3.3. Mapping of the mid-crustal discontinuity Travel time curves of the SxS phases are interpreted in terms of depth of the mid- crustal discontinuity, and the average velocity of the S waves in the upper crustal layer are also estimated. Theoretical travel time for a reflected ray is estimated by using the relation of Rinehart and Sanford (1981): qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 ð2H  hÞ þ d T ¼ ð1Þ where, H is the depth of the reflector, h is the depth of focus for the event, d is the epicentral distance and V is the average velocity in the upper crust. About 75 earthquakes recorded in the Shillong Plateau are used for the travel time analysis to estimate the mid-crustal discontinuity. Relations of the observed travel times versus S minus P interval are shown in figure 4(a). Theoretical travel time curves for the SxS reflection at a depth (H )of 18+0.5 km are shown along with the observational data in figure 4(b). The left hand ends of each curve are the theoretical limits of observable reflections at the specified depth of focus. Generally, it is observed that only one combination of (h) and (H ) produces the theoretical curves that fit the SxS data well. The curves of each reflected phases are Depth of mid-crustal discontinuity beneath the Shillong Plateau 361 Figure 4. (a) Observed travel times of P, S and SxS versus S-P times. (b) Observed and theoretical travel times of SxS versus S-P times. Two curves for the reflections from the midcrustal discontinuity at the depth H (solid lines) is given, one for the minimum, and the other for the maximum depth of earthquake focus h. given, one for a 3 km depth of focus (h), the other for a 15 km depth of focus. The theoretical curves were computed using the standard crustal S wave velocity of 3.53 km/s (Bhattacharya et al. 2008). 4. Results and discussion Shallow crustal reflections (SxS) are investigated to estimate the mid-crustal discontinuity beneath the Shillong Plateau. It is observed that the Shillong Plateau earthquake with S-P interval of 1.2–4.6 s. produces sharp impulsive phases. These reflected phases are observed after 2.0–4.3 s of the first S arrival. The time distance relations for the SxS reflections estimate the mid-crustal discontinuity at 18+0.5 km beneath the Shillong Plateau. Several investigations have been made to estimate crustal thickness and 1D velocity structure in the area using time distance plot (De and Kayal 1990, Sitaram et al. 2001). All these studies were based on first arrival of P and S wave data of the 362 D.K. Bora and S. Baruah analogue networks operated before 2001. The digital seismograms, recorded by the recent upgraded broadband network since 2001, are fruitfully utilized to identify the reflected phases (Baruah et al. 2010), and this study enabled us to estimate the mid- crustal discontinuity. The mid-crustal discontinuity named Conrad discontinuity,in the crust has been reported by several investigators in some other parts of the world (e.g. Sanford and Long 1965, Mizoue 1971, Sanford et al. 1973, Rinehart and Sanford 1981, Mizoue and Isao 1982). In south and central India, several DSS (Deep Seismic Sounding) surveys have been carried out (e.g. Kaila and Sain 1997). The DSS results show 2D velocity structure with a diagnostic Mohorovicic (Moho) discontinuity at the crust and mantle boundary along the profiles but the sharp Conrad discontinuity only at places are reported. In our investigation, the broadband seismograms of the local shallow crustal earthquakes have been very useful to identify the sharp mid-crustal discontinuity. In a recent study with the broadband seismograms, the Conrad depth beneath the Shillong Plateau has been estimated by Baruah et al. (2010). Positive Bouguer gravity anomaly *20–40 mGal (Verma and Mukhopadhyay 1977, Gaur and Bhattacharji 1983, Dasgupta and Biswas 2000) support the pop-up tectonic model envisaged by Bilham and England (2001) that the Plateau pops up between two reverse faults, the south-bounding Dauki fault and the north bounding Oldham fault. The gravity high over this plateau may not be entirely attributed to the absence of sedimentary overburden and hence propose a high density layer emplaced in the crust beneath the Shillong Plateau (Gokarn et al. 2008). The receiver function analysis (Mitra et al. 2005) delineate about 10 km thick high velocity crust beneath the Shillong Plateau (S-wave velocity 4 km s as against 3.6 km s in the surrounding region), which seems to the support this conjuncture. From magnetotelluric (MT) study, Gokarn et al. (2008) recorded low resistivity down to depth of *20 km beneath the Shillong Plateau, this observation is in good agreement with our observation of a discontinuity at a similar depth range. 5. Conclusions We estimated the mid-crustal discontinuity beneath the Shillong Plateau using the travel times of direct and reflected phases from 75 shallow (depth 15 km) crustal earthquakes. The SxS phases are critically observed with S-P difference of about 1.2– 4.6 s. It is found that only one combination of focal depth h and crustal depth H reproduces the theoretical curves that match the observed travel time data of the reflected phases. The estimated mid-crustal discontinuity is 18+0.5 km in the study region. The mid-crustal Conrad discontinuity in the Shillong Plateau is first reported by us based on reflected phases observed in the broadband seismograms. Acknowledgements We express our sincere gratitude to Dr. P. G. Rao, Director of the North-East Institute of Science & Technology for his encouragement to publish this work. We are thankful to Prof. J. R. Kayal, CSIR-Scientist Emeritus at Jadavpur University, Kolkota for his critical comments and review during this study. Data obtained from NEIST, Jorhat, NGRI, Hyderabad and IIG, Mumbai are acknowledged. We are thankful to all three anonymous reviewers for their valuable comments and suggestions, which helped improve the quality of the manuscript. Depth of mid-crustal discontinuity beneath the Shillong Plateau 363 References ANGELIER, J. and BARUAH, S., 2009, Seismotectonics in Northeast India: a stress analysis of focal mechanism solutions of earthquakes and its kinematic implications. Geophysical Journal International, 178, pp. 303–326. BARUAH, S., BORA, D.K. and BISWAS, R., 2010, Estimation of Crustal discontinuities from reflected seismic waves recorded at Shillong and Mikir Hills Plateau, Northeast India. International Journal of Earth Sciences, 100, pp. 1283–1292. BARUAH, S. and HAZARIKA, D., 2008, A GIS based tectonic map of India. Current Science, 95, pp. 176–177. BHATTACHARYA, P.M., MUKHOPADHYAY, S., MAZUMDAR, R.K. and KAYAL, J.R., 2008, 3-D seismic structure of the northeast India region and its implication for local and regional tectonics. Journal of Asian Earth Sciences, 33, pp. 25–41. BHATTACHARYA, P.M., PUJOL, J., MAZUMDAR, R.K. and KAYAL, J.R., 2005, Relocation of earthquakes in the Northeast India region using joint Hypocenter determination method. Current Science, 89, pp. 1404–1413. BILHAM, R. and ENGLAND, P., 2001, Plateau ‘‘pop-up’’ in the Great 1897 Assam earthquake. Nature, 410, pp. 806–809. BMTPC, 2003, Vulnerability atlas – 2nd edn; peer group MoH & UPA; seismic zones of India IS: 1983-2002, BIS, GOI, Seismotectonic atlas of India and its environs, GSI, GOI. DASGUPTA,A.and BISWAS, A., 2000, Geology of Assam (Bangalore: Geological Society of India). DE, R. and KAYAL, J.R., 1990, Crustal P-wave velocity and velocity-ratio study in northeast India by a microearthquake survey. Pure and Applied Geophysics, 134, pp. 93–108. EVANS, P., 1964, Tectonic framework of Assam. Journal of Geological Society of India, 5, pp. 80–96. GAUR, V.K. and BHATTACHARJI, J.C., 1983, Gravimetric determination of the shape of Mohorovicic discontinuity in Peninsular and north-eastern India. In Paper presented at I.U.G.G. General Assembly. Hamburg, Germany, August 15–27. pp. 936. GOKARN, S.G., GUPTA, G., WALIA, D., SANABAM, S.S. and HAZARIKA, N., 2008, Deep Geoelectric structure over Lower Brahmaputra valley and Shillong Plateau, NE India using magnetotellurics, Journal of Geophysical International, 173, 92–104. KAILA, K.L. and SAIN, K., 1997, Variation of crustal velocity structure in India as determine from DSS studies and their implications on regional tectonics. Journal of Geological Society of India, 49, pp. 395–407. KAYAL, J.R., 1991, Earthquake prediction in northeast India – A review. Pure and Applied and Geophysics, 136, pp. 297–313. KAYAL, J.R., 2001, Microearthquake activity in some part of the Himalaya and the tectonic model. Tectonophysics, 339, pp. 331–351. KAYAL, J.R., 2008, Microearthquake Seismology and Seismotectonics of South Asia, 503 p. (Berlin: Springer). KAYAL, J.R., AREFIEV, S.S., BARUAH, S., HAZARIKA, D., GOGOI, N., KUMAR, A., CHOWDHURY, S.N. and KALITA, S., 2006, Shillong plateau earthquakes in Northeast India region: Complex tectonic model. Current Science, 91, pp. 109–114. KAYAL, J.R. and DE, R., 1991, Microseismicity and tectonic in Northeast India. Bulletin of Seismological Society of America, 81, pp. 131–138. LIENERT, B.R., BERG, B.E. and FRAZER, L.N., 1986, Hypocenter: An earthquake location method using corrected, scaled and adaptively damped least squares. Bulletin of Seismological Society of America, 76, pp. 771–783. MITRA, S., PRISTLEY, K., BHATTACHARYA, A. and GAUR, V.K., 2005, Crustal structure and earthquake focal depths beneath northeastern India and southern Tibet. Geophysical Journal International, 160, pp. 227–248. MIZOUE, M., 1971, Crustal structure from travel times of reflected and refracted seismic waves recorded at Wakayama micro-earthquake observatory and its substations. Bulletin of Earthquake Research Institute, 49, pp. 33–62. 364 D.K. Bora and S. Baruah MIZOUE, M. and ISAO, N., 1982, Mapping of an unusual crustal discontinuity by microearthquake reflections in the earthquake swarm area near Ashio, Northwestern Part of Tochigi Prefecture, Central Japan. Bulletin of Earthquake Research Institute, 57, pp. 653–685. MUKHOPADHYAY, S., KHATTRI, K.N. and CHANDER, R., 1995, Seismic velocity and related elastic parameters of the crust in the Shillong Massif. Journal of Himalayan Geology, 6, pp. 1–8. NANDY, D.R., 2001, Geodynamics of Northeastern India and the Adjoining Region, pp. 209 (Calcutta: ACB publications). OKADA, H., SUZUKI, S. and ASANO, S., 1970, Anomalous understanding structure in the Matsushiro earthquake swarm area as derived from a fan shooting technique. Bulletin of Earthquake Research Institute, 48, pp. 811–833. OLDHAM, R.D., 1899, Report on the great earthquake of the 12th June 1897, Memoirs of Geological Survey of India. 29, 1–379, reprinted 1981 (Calcutta: Geological Survey of India). RAJENDRAN, C.P., RAJENDRAN, K., DUARAH, B.P., BARUAH, S. and EARNEST, A., 2004, Interpreting the style of faulting and paleoseismicity associated with the 1897 Shillong, northeast India, earthquake: Implications for regional tectonism. Tectonics, 23, pp. 1– RINEHART, E.J. and SANFORD, A.R., 1981, Upper crustal structure of the Rio Grande rift near Socorro, New Mexico, from inversion of microearthquake S-wave reflections. Bulletin of Seismological Society of America, 71, pp. 437–450. SANFORD, A.R., ALPTEKIN, O. and TOPPOZADA, T.R., 1973, Use of reflection phases on microearthquake seismograms to map an unusual discontinuity beneath the Rio Grande Rift. Bulletin of Seismological Society of America, 63, pp. 2021–2034. SANFORD, A.R. and LONG, L.T., 1965, Microearthquake crustal reflections, Socorro, New Mexico. Bulletin of Seismological Society of America, 55, pp. 579–586. SITARAM, M.V.D., YADAV, D.K. and GOSWAMI, K., 2001, Study on crustal structure beneath Arunachal Himalaya and Assam. Journal of Geological Society of India, 58, pp. 285– TAPPONNIER, P., PELTZER, G., LE DIAN,A.Y,ARMIJO, R. and COBBOLD, P., 1982, Propagating extrusion tectonics in Asia: New insights from simple experiments with plasticine. Geology, 10, pp. 611–616. TILLOTTSON, E., 1953, The great Assam earthquake of 1950, the completion of papers on the Assam Earthquake of August 15, 1950, Compiled by M.B. Ramachandra Rao, (Calcutta, India: Central Board of Geophysics), pp. 94–96. VERMA, K.K. and MUKHOPADHYAY, M., 1977, An analysis of the gravity field in northeastern India. Tectonophysics, 42, pp. 283–317.

Journal

"Geomatics, Natural Hazards and Risk" – Taylor & Francis

Published: Nov 1, 2012

There are no references for this article.