RESEARCH COMMUNICATIONS 5. National Council on Radiation Protection and Measurements, Exposure of the population of the United States and Canada from natural background radiation, NCRP Report No. 94, Bethesda, MD, 1987. 6. United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), Sources, effects and risks of ionizing radiation, Report to the General Assembly, United Nations, New York, 1988. 7. Sadasivan, S., Shukla, V. K., Chinnasaki, S. and Sartandel, S. J., Natural and fallout radioactivity measurements in Indian soil. J. Radioanal. Nucl. Chem., 2003, 256, 603–607. 8. UNSCEAR, Sources, effects and risks of ionizing radiation, Report to the General Assembly, United Nations, New York, 2000. 9. Tirmarche, M. et al., Lung cancer risk from radon and progeny and statement on radon. Ann. ICRP, 2010, 40(1). 10. UNSCEAR, Sources and effects of Ionizing Radiation. UNSCEAR 1993 Report to the General Assembly, with annexes, United Nations, New York, 1993, pp. 73–98. 11. Limits for intakes of radionuclides by workers. Ann. ICRP, 1988, 19(4). 12. Subba Ramu, M. C., Ramachandran, T. V., Muraleedharan, T. S. and Sheik, A. N., Indoor levels of radon daughters in some high background areas in India. Radiat. Prot. Dosim., 1990, 30(1), 41–44. 13. Esmeray, E., Natural radioactivity in various water supplies of Konya, Turkey, M Sc thesis, Selcuk University Graduate School of Natural and Applied Sciences, Konya, 2005. 14. Dunn, J. A. and Dey, A. K., The Geology and Petrology of Eastern Singhbhum and Surrounding Areas. Mem. Geol. Surv. India, 1942, 69, 281–456. 15. Deb, M., Aspects of ore mineralization in the central section of the Singhbhum copper belt, Bihar. Ph D thesis. Jadavpur University, Kolkata, 1971. 16. Sarkar, S. N. and Saha, A. K., Stratigraphy, tectonics and geochronology of the Singhbhum region and copper thrust belt shear zone. Indian School of Mines, Dhanbad Jubilee Monograph, 1986, pp. 5–17. 17. Rao, N. K. and Rao, G. V., Uranium mineralization of Singhbhum shear zone, Bihar. IV. Origin and geological time frame. J. Geol. Soc. India, 1983, 24, 615–627. 18. Kumar, R., Sengupta, D. and Prasad, R., Natural radioactivity and radon exhalation studies of rock samples from Surda copper deposits in Singhbhum shear zone. Radiat. Meas., 2003, 36, 551–553. 19. Jha, G. and Raghavayya, M., Development of a passive radon dosimeter. In Fifth National Symposium on Radiation Environment, Calcutta, 2–4 November 1983. 20. Jha, G., Raghavayya, M. and Padmanabhan, N., Radon permeability of some membranes. Radiat. Meas., 1982, 19, 307–308. 21. Subba Ramu, M. C., Muraleedharan, T. S. and Ramachandran, T. V., Calibration of a solid state nuclear track detector for the measurement of indoor levels of radon and its daughters. Sci. Total Environ., 1988, 73(3), 245–255. 22. Ramola, R. C., Rawat, R. B. S., Kandari, M. S., Ramachandran, T. V., Eappen, K. P. and Subba Ramu, M. C., Calibration of LR115 plastic track detector for environmental radon measurements. J. Indoor Built Environ., 1996, 5, 364–366. 23. ICRP Publication 65, Protection Against Radon-222 at Home and at Work, Pergamon Press, 1994. 24. Lewis, Robert, K. and Paul N. Houle, A living radon reference manual, Chemistry 2, 2009, 1. 25. Abu Jarad, F. and Fremlin, J. H., A working level monitor for radon measurements inside houses. Radiat. Prot. Dosim., 1981, 1(3), 221–226. 26. Frank, A. L. and Benton, E. V., Radon Dosimetry Using Plastic Nuclear Track Detection, 1977, 1(3/4), 149. 27. Nikolaeve, V. A. and Ilic, R., Etched track radiometers in radon measurements: a review. Radiat. Meas., 1999, 30, 1–13. 1938
28. Subba Ramu, M. C., Raghavayya, M. and Paul, A. C., Methods for the measurement of radon, thoron and their progeny in dwellings. In AERB Technical Manual, TM/RM-1, 1994. 29. ICRP, Protection against radon-222 at home and at work. ICRP Publication 65, 1993, Ann. ICRP, 23(2). 30. Virk, H. S. and Sharma, N., Indoor 222Rn/220 Rn survey report from Hamirpur and Uuna districts, Himachal Pradesh, India. Appl. Radiat. Isot., 2000, 52, 137–141. 31. Fujiyoshi, R., Morimoto, H., and Sawamura, S., Investigation of the soil radon variation during the winter months in Sapporo, Japan. Chemosphere, 2002, 47(4), 369–373. 32. Ramachandran, T. V., Muraleedharan, T. S., Shaikh, A. N. and Subba Ramu, M. C., Seasonal variation of indoor radon and its progeny concentration in dwellings. Atmos. Environ., 1989, 24A(3), 639–643. 33. Campos-Venuti, G., Janssens, A. and Olast, M., Indoor radon remedial action, the scientific basis and the practical implications. Proceedings of the First International Workshop, Rimini, Italy; Rad. Prot. Dos., 1994, 56(1–4), 133–135; Discriminating dosimeter. Radiat. Meas., 38, 5–17. 34. Mettler, F. A. and Upton, A. C. (eds), Medical Effects of Ionizing Radiation, W.B. Saunders, Philadelphia, 1995, 2nd edn. 35. Subba Ramu, M. C., Ramachandran, T. V., Muraleedharan, T. S., Shaikh, A. N. and Nambi, K. S. V., In Proceedings of the 7th. National Conference on Particle Tracks in Solids, Defense Laboratory, Jodhpur, 1993, pp. 11–25.
ACKNOWLEDGEMENTS. We thank Dr R. M. Tripathi, Head, Health Physics Division, Bhabha Atomic Research Centre, Mumbai for support and the officials of Uranium Corporation of India Limited. Jaduguda for providing the necessary facilities and assistance. We also thank our colleagues of the Health Physics Unit, Jaduguda for their help and encouragement. Received 16 January 2015; revised accepted 2 March 2015
The 25 April 2015 Nepal earthquake and its aftershocks S. Mitra*, Himangshu Paul, Ajay Kumar, Shashwat K. Singh, Siddharth Dey and Debarchan Powali Department of Earth Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur 741 246, India
The massive Mw = 7.8 earthquake which rocked the Nepal Himalaya on 25 April 2015 is the largest to have occurred in this region in the past 81 years. This event occurred by slip on a ~150 km long and 55 km wide, shallow dipping (~5) segment of the Main Himalayan Thrust (MHT), causing the Himalaya to lurch southwestward by 4.8 1.2 m over the Indian plate. The main shock ruptured the frictionally locked segment of the MHT, initiating near the locking line and rupturing all the way updip close to its surface expression *For correspondence. (e-mail:
[email protected]) CURRENT SCIENCE, VOL. 108, NO. 10, 25 MAY 2015
RESEARCH COMMUNICATIONS near the foothills of the Himalaya. The main shock was followed by 41 aftershocks within 26 h, among which a couple were larger than magnitude (Mw) 6.5. These two large aftershocks occurred on fault(s) which had similar orientation as the one that caused the main shock, contributing to strain release along the MHT. The rupture area of the main shock overlaps the meisoseismal zone of the 1833 Nepal earthquake and is immediately to the west of the 1934 Bihar–Nepal earthquake. This region had accumulated ~3 m of slip in the past 182 years, converging at a rate of ~18 mm/yr. The close match of the accumulated slip with the coseismic slip of the main event confirms that majority of the convergence between India and Tibet is stored as elastic strain energy and is released by brittle failure in earthquakes. This Nepal earthquake has highlighted that other segments of the Himalaya too have significant unrelieved elastic strain and may also rupture in similar or greater earthquakes in the future. Keywords: Earthquake, rupture parameters, source mechanism, seismotectonics. T HE massive Mw = 7.8 earthquake that rocked the Nepal Himalaya on 25 April 2015 and killed more than 6200 people so far, was caused by the sudden southward slip of a ~150 km long and ~55 km wide Himalayan segment at the location ~28.3N, 84.5E–27.5N, 86E. According to our calculations, this segment lurched southwestward by about 4.8 1.2 m over the Indian plate, rupturing the frictionally locked shallow segment of the Main Himalayan Thrust (MHT). The existence of this locked segment of the MHT beneath the lesser Himalaya in Nepal is known from GPS geodesy1 and has been inferred elsewhere along the Himalayan arc. This is the currently understood process whereby great earthquakes episodically originate in the Himalaya, driven by the convergence of the Indian and Eurasian plates. The current rate of convergence between India and Tibet across the Nepal Himalaya is fairly well constrained from GPS geodetic measurements at ~18 mm/yr (ref. 2). This convergence is accommodated along a dipping interface (the MHT), which separates the underthrusting Indian plate beneath the Himalaya and southern Tibet from the overriding mountains. Since rocks within the upper 15– 20 km of the surface are colder and brittle, the shallow segment of the MHT is frictionally locked and accumulates strain energy. Further downdip, the MHT transits to a deeper and warmer regime (T > 350C) and allows aseismic sliding (creep). The zone of transition from frictionally locked to aseismic creep on the MHT is known as the locking line. The locked zone lies updip of the locking line and has been found to be ~100 km wide beneath the Nepal Himalaya1. As Tibet converges with India, elastic strain builds within the locked zone, updip of the locking line. Once this strain exceeds the rock CURRENT SCIENCE, VOL. 108, NO. 10, 25 MAY 2015
failure strain limit, estimated to be ~10 −4, the locked segment is ruptured and the Himalayan mountains lurch forward over the Indian plate, releasing the accumulated strain energy. This leads to mega-thrust events, like the present Nepal earthquake. The slow build-up of elastic strain in various segments of the Himalaya mediated by the above process and its eventual catastrophic release, constitutes an earthquake cycle whose knowledge is extremely valuable in quantifying earthquake hazard. The last major event in the central Himalaya occurred 81 years ago and has no instrumental records. Therefore, we have not been able to definitively decipher the mechanism and cyclicity of such events, which could act as a template to study major historical earthquakes in the Himalaya. In its absence, every new event tells us a unique story of its own, even as one endeavours to fit it in a generalized framework of Himalayan tectonics. Thus, from a knowledge of the strain energy, annually accumulated in the entire 2200 km long and 100 km wide Himalayan locked zone, and comparing it with those released by all M > 6 Himalayan earthquakes (since AD 1500), it was found that over two-thirds of the accumulated energy still remains to be released3. This excess stored energy is capable of driving a few magnitude 8.2–8.6 earthquakes in the Himalaya. The accumulated slip in different segments of the Himalaya has been calculated by accounting for all great earthquakes since AD 1800 (ref. 4). This showed that either side of the meisoseismal zone of the 1934 Bihar– Nepal earthquake (with the exception of the 1905 Kangra and 1950 Assam earthquake regions), has enough stored strain energy within the locked segment, to drive a magnitude 8 earthquake. This would have the potential of rupturing 200–300 km long segment of the MHT and produce a slip of ~4 m. In this perspective, the recent Nepal earthquake comes as no surprise. Occurring 81 years after the 1934 Bihar–Nepal earthquake, its 150 55 km rupture zone (Figure 1), spanning the Kathmandu valley, extends right up to the western edge of the 1934 meisoseismal zone. The rupture area also covers the 1833 Nepal earthquake (Mw 7.7), where a ~3 m of potential slip had additionally developed since its occurrence. Within 26 h, the main shock was followed by 41 aftershocks: two of them were of magnitude greater than 6.5 (Table 1). Aftershocks are believed to result from the balancing of residual strains on the already lubricated rupture plane revealing its extent and the state of stress. In this study we determine the fault plane geometry, slip vector, depth, seismic moment, rupture area, stress drop and coseismic slip of the Nepal main shock and the largest aftershocks. Our results are then discussed in the context of Himalayan tectonics and their implications to seismic hazard. We use waveform data obtained from the Global Digital Seismic Network (GDSN) stations in the epicentral distance range 30–80 for modelling the source mechanism 1939
RESEARCH COMMUNICATIONS Table 1.
Event date, location, focal depth, scalar seismic moment, moment magnitude, rupture area and source mechanism of the earthquakes used in the present study. Source mechanism for these earthquakes is plotted in Figures 1 and 3.
Event date and time 25–04–2015 06:11:26 26–04–2015 07:09:08 25–04–2015 06:45:21 25–04–2015 23:16:15 25–04–2015 17:42:49
Mw
Rupture area (sq. km)
1.30E21 1.98E20 3.69E19 4.98E18 1.4E19
7.8
8376
6.9
3755
6.6
–
6.20E17 1.23E17 4.94E17
5.7 5.6
Latitude (N)
Longitude (E)
Depth (km)
M0 (N-m)
28.14
84.70
17 3
27.794
85.974
16 3
28.193
84.865
20
27.805
84.874
–
27.794
85.974
–
Figure 1. a, Topographic map of the Himalaya, with plot of the major historical earthquakes in the past 200 years. The region of the 2015 Nepal earthquake is outlined by a box and locations of the main shock and aftershocks (m b > 4.0 occurred within 26 h of the main shock) are plotted as white circles. b, Detailed topographic map of the central Nepal Himalaya with plot of the focal mechanism of the main shock (red) and the two largest aftershocks (black – taken from GFZ – German Research Center for Geosciences and blue – modelled in the present study). The other aftershocks are plotted as yellow circles (5 < m b < 6) and white circles (4 < mb < 5). The rupture area of the main shock (red), the 1934 Nepal–Bihar earthquake (blue) and the 1833 Nepal earthquakes (black) are plotted as ovals (broken lines) to reveal their spatial relationship. The profile A–B is plotted in Figure 3.
and 0–30 for modelling the fault rupture parameters. We also combine data from the IISER Kolkata Seismological Observatory (Mohanpur – 22.96N, 88.53E), for computing the rupture parameters of the aftershocks. 1940
Strike ()
Dip ()
Rake ()
1833
299 100 339 111 285 110 –
5 85 7 85 7 83 –
108 88 138 85 86 90 –
1586
–
–
–
(bar) 34.75 3.79 4.76 1.17 –
Slip (m) 4.84 1.2 0.30 –
–
–
–
–
Comments Main shock (present study) Aftershock (present study) Aftershock (from GFZ) Aftershock (present study) Aftershock (present study)
We model the source mechanism of the main shock and the largest aftershock using waveform inversion technique. The broadband teleseismic waveform data are deconvolved from the instrument response and reconvolved with a filter to produce the 15–100 s response of the long-period World Wide Standard Seismograph Network (WWSSN) instrument. Windowed P- and SHwaveforms from the vertical and tangential components respectively, were used for the analysis. Moment tensor inversion algorithm5 was used to estimate the strike and dip of the fault plane, rake of the slip vector, earthquake focal depth and the seismic moment (i.e. the energy released). The algorithm adopts an iterative approach to minimize the least squares misfit between the observed and synthetic P- and SH-waveforms. Further details of the algorithm6,7 and detailed methodology have been discussed in previous studies8,9. An example of the analysed data for the main shock is plotted in Figure 2 a. We compute the fault rupture parameters (e.g. seismic moment, fault rupture area, stress drop and amount of slip) by fitting the observed P-wave displacement spectra assuming a symmetric rupture model10 . The amplitude and shape of the P-wave displacement spectrum are a function of the seismic moment (M0 ), corner frequency ( f0), average P-wave velocity in the crust, density of the rocks at the source, geometrical spreading (as a function of epicentral distance (R) and earthquake focal depth), and the quality factor of the crust11. The observed P-wave is windowed on the vertical component and fast Fourier transformed to obtain the frequency spectrum. The P-wave frequency spectrum is first corrected for geometrical spreading (using a 1/R model for body waves), and then corrected for attenuation using a frequency-dependent relationship Q( f ) = Q0 f . We use an average value of Q0 = 300 and = 1.04, obtained for the Sikkim Himalaya 12. The corrected P-wave spectrum represents the spectrum at the source and is fitted using a theoretical spectrum of the form 0/(1 + ( f/f0)2 ); where 0 is the spectral amplitude and f0 is the corner CURRENT SCIENCE, VOL. 108, NO. 10, 25 MAY 2015
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P-waveform
SH-waveform
Figure 2. a, P (top) and SH (bottom) focal mechanism and waveforms (observed – bold, synthetic – dashed) for our minimum-misfit solution of the Nepal main shock. The station code for each waveform is accompanied by a letter corresponding to its position in the focal sphere. The time window used for the inversion is marked by vertical lines on each waveform. The pressure and tension axes are plotted as solid and open circles on the P-wave focal sphere15 . b, Plot of average P-wave spectra for the Nepal main shock using data from 36 global stations within a radius of 30. The best fitting spectra and its 1– bound are plotted as a grey band.
frequency. To obtain the best-fitting theoretical spectrum, we implement a grid search algorithm to explore over a range of spectral amplitude (0) and corner frequency ( f0) values. The observed best-fitting spectrum is obtained by minimizing the misfit between the observed and theoretical spectra in a least-squares sense. From this analysis we use the most probable value of 0 to obtain the seismic scalar moment (M0) and that of f0 in order to estimate the fault radius (a) for a circular fault 13: a = 0.32/f0, where is the S-wave velocity. Using the seismic scalar moment (M0 ) and fault radius (a), we calculate the stress drop, rupture area and the amount of slip, assuming crustal shear modulus of 3.2 1010 Pa. The results of the analysis for the main shock is plotted in Figure 2 b and for all events has been tabulated in Table 1. CURRENT SCIENCE, VOL. 108, NO. 10, 25 MAY 2015
Using the above methods we obtain the strike, dip and rake of the main shock fault plane as 299, 5 and 108 respectively. The main shock originated at a depth of 17 3 km and ruptured an area of ~8376 sq. km. The scalar seismic moment released by the main shock is 1.30E21 1.98 1020 N-m and the fault slipped by 4.8 1.2 m. This was followed by a number of magnitude 5+ aftershocks on the same day. The largest among them was the 6.6 magnitude aftershock, which occurred within 34 min of the main shock and originated close to the point of initiation of the main shock (Figure 1 b). The fault plane solution of this event has been taken from the German Research Center for Geosciences (GFZ) catalog (Table 1), as the data from global network of stations were overprinted by the surface waves of the main shock and could not be used for our analysis. The largest aftershock 1941
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Figure 3. Cross-section plot along A–B (Figure 1) showing focal mechanism of the main shock and the two largest aftershocks, colour-coded as in Figure 1. The main shock ruptured a significant portion of the locked detachment (MHT), which is highlighted with a red line on the detachment. The direction of motion on the fault is marked with a pair of arrows. The topography is projected along the profile (note the vertical exaggeration) and the outcrop of the major thrust faults (viz. MFT, Main Boundary Thrust (MBT) and Main Himalayan Thrust (MCT)) are plotted as curved lines and arrows denoting their sense of thrust motion 16 .
(Mw = 6.9) occurred the next day at the farthest end of the aftershock zone in the SE. Both these aftershocks, similar to the main shock, occurred on shallow NE dipping faults (Table 1, Figures 1 b and 3). Using global network waveform data, we also computed the aftershock energy released and the rupture area (Table 1). The Nepal main shock occurred on a shallow NE dipping thrust fault at a depth of ~17 km, which coincides with the MHT beneath the Nepal Himalaya. The main shock rupture initiated ~80 km NW of Kathmandu at (28.3N, 84.5E) and propagated southeastward right underneath the city to the edge of the meisoseismal zone of the great Bihar–Nepal earthquake of 1934 (Figure 1 b). The main shock rupture area has been estimated to be ~8376 sq. km. Considering a symmetric rupture, it would have broken an area of ~91 91 km. However, given the distribution of aftershocks, which generally span the main shock fault, the rupture area is assumed to be rectangular (~150 55 km). The rupture initiated close to the downdip end of the locked portion of the MHT, and propagated updip on the shallow dipping (~5) detachment (Figure 3). However, it appears to have stopped short of the surface or died, in a diffused slow deformation towards the surface as no significant damage has been reported from the foothills. The average coseismic slip associated with the main shock is 4.84 1.2 km. The total stress drop associated with the main shock is 34 3.79 bar, which is typical of interplate earthquakes. The distribution of aftershocks outlines the main shock 1942
rupture area on the MHT (Figure 1). The two largest aftershocks (Table 1) originated on either end of the main shock rupture area and have similar source mechanism as the main shock. This confirms that they too participated in strain release on the MHT. The main shock rupture occurred over the meisoseismal zone of the 1833 Nepal earthquake. However, this is not surprising, as the region had since accumulated an additional slip of ~3 m, which is in good agreement with the average coseismic slip associated with the present event. This provides additional evidence that the GPS geodesy-derived convergence rate across the Himalaya, which is similar to the long-term geologic slip rates, is primarily stored as elastic strain energy to be eventually released by the earthquakes. Earthquakes of this magnitude occurring in one segment of the plate boundary can cause propagation of stresses away from the rupture zone within the adjacent regions. Therefore, the possibility of this event driving future major earthquakes in adjacent regions NW and SE of the rupture area cannot be ruled out. It is to be noted that the region to the SE of this rupture zone (i.e. the zone of the 1934 Bihar–Nepal earthquake) has already accumulated ~1.5 m of potential slip since the last big event. Therefore, if seismic slip occurs and releases this entire slip deficit, it will result in a magnitude ~7 earthquake. The segment immediately NW of the rupture area of the present event, has not produced a major earthquake in the past ~500 years (the last known mega-thrust event was in CURRENT SCIENCE, VOL. 108, NO. 10, 25 MAY 2015
RESEARCH COMMUNICATIONS ca. 1505). This segment has a slip deficit of ~9 m, which can potentially rupture in an earthquake larger than the present one. The recent Nepal earthquake was the largest to have occurred in the Himalaya with an average slip comparable to that of the great Bihar–Nepal earthquake of 1934. This event clearly demonstrates that Himalayan megaquakes are produced by the rupture of the locked segment of the MHT, initiating near the locking line and rupturing all the way updip close to its surface exposure near the foothills. This earthquake occurred in the meisoseismal zone of the 1833 earthquake and produced an average coseismic slip close to the expected additional slip accumulated since that event. This confirms that majority of the convergence between India and Tibet is stored as elastic strain energy only to be released by brittle failure in earthquakes. This assumption had been used to calculate the potential slip deficit along all segments of the Himalayan arc4, by summing up the rates of convergence over the time elapsed since the last major earthquake. The magnitude and slip of this Nepal earthquake confirm their predicted values and vindicate this approach to seismic hazard analysis for the Himalaya. This also highlights that other segments of the Himalaya have significant unrelieved elastic strain 4. It would be imprudent to ignore the possibility that these segments could also rupture in similar or greater earthquakes in the future. Note added in proof: An earthquake of magnitude (Mw) 7.3 struck Nepal Himalaya on 12 May 2015, 16 days after the 25 April 2015, Mw 7.8 Nepal earthquake. This event originated ~18 km NE of Kathmandu, at the eastern edge of the rupture area of the 25 April event. Rupture plane of the event spans an area of ~6226 sq. km, spreading farther eastward into the meisoseismal zone of the 1934 Bihar–Nepal earthquake, and slipped by ~0.9 m. This event was followed by one 6.3 and several 5+ aftershocks in the next 24 h. Our preliminary analyses reveal that this latest earthquake occurred on the same fault plane as the 25 April Nepal main shock and has a similar source mechanism. Its occurrence thus confirms our suggestion that the 25 April event, could result in propagation of stresses on adjacent segments of the fault and cause future damaging earthquakes. 1. Ader, T. et al., Convergence rate across the Nepal Himalaya and interseismic coupling on the Main Himalayan Thrust: implications for seismic hazard. J. Geophys. Res., 2012, 117, B04403.
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2. Bilham, R. et al., GPS measurements of present-day convergence across the Nepal Himalaya. Nature, 1997, 386, 61–64. 3. Bilham, R. and Ambraseys, N., Apparent Himalayan slip deficit from the summation of seismic moments for Himalayan earthquakes, 1500–2000. Curr. Sci., 2005, 88(10), 1658–1663. 4. Bilham, R., Gaur, V. and Molnar, P., Himalayan seismic hazard. Science, 2001, 293, 1442–1444. 5. McCaffrey, R. and Abers, J., SYN3: a program for inversion of teleseismic body wave form on microcomputers. Technical Report AFGL-TR-0099, Air Force Geophysical Laboratory. Hanscomb Air Force Base, Massachusetts, USA, 1988. 6. Nabelek, J., Determination of earthquake source parameters from inversion of body waves. Ph D thesis. Massachusetts Institute of Technology, USA, 1984. 7. McCaffrey, R. and Nabelek, F., Earthquakes, gravity and the origin of the Bali Basin: an example of a nascent continental foldand-thrust belt. J. Geophys. Res., 1987, 92, 441–460. 8. Mitra, S., Wanchoo, S. and Priestley, K., Source parameters of the 1 May 2013 m b 5.7 Kishtwar earthquake: implications for seismic hazards. Bull. Seismol. Soc. Am., 2014, 104, 1013–1019; doi: 10.1785/0120130216. 9. Paul, H., Mitra, S., Bhattacharya, S. and Suresh, G., Active transverse faulting within underthrust Indian crust beneath the Sikkim Himalaya. Geophys. J. Int., 2015, 201, 1070–1081. 10. Brune, J., Tectonic stress and the spectra of seismic shear waves from earthquakes. J. Geophys. Res., 1970, 75, 4997–5009. 11. Havskov, J. and Ottemoller, L., Routine Data Processing in Earthquake Seismology: With Sample Data, Exercises and Software, Springer Science and Business Media, 2010. 12. Ajaay, T. and Mitra, S., Earthquake relocation and lateral variation of coda Q in Sikkim Himalaya. In Eos Transactions, Fall Meeting Supplementary Abstract, American Geophysical Union, 2013, pp. T23I–06. 13. Madariaga, R., Dynamics of an expanding circular fault. Bull. Seismol. Soc. Am., 1976, 66, 639–666. 14. Goldstein, P., Dodge, D., Firpo, M. and Minner, L., Sac2000: signal processing and analysis tools for seismologists and engineers. In The IASPEI International Handbook of Earthquake and Engineering Seismology (eds Lee, W. et al.), Academic Press, London, 2003. 15. Wessel, P. and Smith, W. H. F., New, improved version of the generic mapping tools released. EOS Trans. AGU, 1998, 79, 579. 16. Torre, T. L., Monsalve, G., Sheehan, A. F., Sapkota, S. and Wu, F., Earthquake processes of the Himalayan collision zone in eastern Nepal and the southern Tibetan plateau. Geophys. J. Int., 2007, 171, 718–738.
ACKNOWLEDGEMENTS. Seismograms used in this study were obtained from IRIS-DMC (www.iris.edu/dms/dmc/). Data preprocessing and part of the analysis were performed using Seismic Analysis Code 2000, version 100 (ref. 14). All plots were made using the Generic Mapping Tools version 4.0 (www.soest.hawaii.edu/gmt) 15 . We thank IISER Kolkata for support and Vinod Gaur (CSIR-Fourth Paradigm Institute – formerly CSIR-CMMACS) for discussions and comments that helped improve the manuscript.
Received 28 April 2015; revised accepted 12 May 2015
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