The 25 April 2015 Nepal earthquake and its aftershocks

RESEARCH COMMUNICATIONS

CURRENT SCIENCE, VOL. 108, NO. 10, 25 MAY 2015 1938

*For correspondence. (e-mail: [email protected])

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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 south-westward 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 rup-turing all the way updip close to its surface expression



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 over-laps the meisoseismal zone of the 1833 Nepal earth-quake and is immediately to the west of the 1934 Bihar–Nepal earthquake. This region had accumu-lated ~3 m of slip in the past 182 years, converging at a rate of ~18 mm/yr. The close match of the accumu-lated 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. THE 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 Himala-yan 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 else-where along the Himalayan arc. This is the currently understood process whereby great earthquakes episo-dically originate in the Himalaya, driven by the conver-gence 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 accumu-lates 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 fric-tionally 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

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 quantify-ing 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 en-deavours to fit it in a generalized framework of Himala-yan 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 earth-quakes (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 mag-nitude 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 after-shocks: 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 dis-tance range 30–80 for modelling the source mechanism



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.

Rupture Event date Latitude Longitude Depth M0 area Strike Dip Rake Slip and time (N) (E) (km) (N-m) Mw (sq. km) () () () (bar) (m) Comments

25–04–2015 28.14 84.70 17 3 1.30E21 7.8 8376 299 5 108 34.75 4.84 Main shock 06:11:26 1.98E20 100 85 88 3.79 1.2 (present study) 26–04–2015 27.794 85.974 16 3 3.69E19 6.9 3755 339 7 138 4.76 0.30 Aftershock 07:09:08 4.98E18 111 85 85 1.17 (present study) 25–04–2015 28.193 84.865 20 1.4E19 6.6 – 285 7 86 – – Aftershock 06:45:21 110 83 90 (from GFZ) 25–04–2015 27.805 84.874 – 6.20E17 5.7 1833 – – – – – Aftershock 23:16:15 1.23E17 (present study) 25–04–2015 27.794 85.974 – 4.94E17 5.6 1586 – – – – – Aftershock 17:42:49 (present study)

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 (mb > 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 < mb < 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 com-puting the rupture parameters of the aftershocks.

We model the source mechanism of the main shock and the largest aftershock using waveform inversion tech-nique. The broadband teleseismic waveform data are deconvolved from the instrument response and recon-volved with a filter to produce the 15–100 s response of the long-period World Wide Standard Seismograph Net-work (WWSSN) instrument. Windowed P- and SH-waveforms 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 re-leased). 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 dis-cussed 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 fre-quency 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 Himalaya12. 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



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 ver-tical 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 fault13: a = 0.32/f0, where is the S-wave velocity. Us-ing 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.

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 initia-tion of the main shock (Figure 1 b). The fault plane solu-tion 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 over-printed by the surface waves of the main shock and could not be used for our analysis. The largest aftershock

P-waveform

SH-waveform



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 de-tachment (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 motion16.

(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 wave-form data, we also computed the aftershock energy released and the rupture area (Table 1). The Nepal main shock occurred on a shallow NE dip-ping 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 un-derneath 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) detach-ment (Figure 3). However, it appears to have stopped short of the surface or died, in a diffused slow deforma-tion 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

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 meisosei-smal 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 seg-ment 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 accu-mulated ~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



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 compara-ble to that of the great Bihar–Nepal earthquake of 1934. This event clearly demonstrates that Himalayan mega-quakes 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 co-seismic slip close to the expected additional slip accumu-lated 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 Hima-layan 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 un-relieved elastic strain4. 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 far-ther eastward into the meisoseismal zone of the 1934 Bi-har–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 mecha-nism. 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.

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ACKNOWLEDGEMENTS. Seismograms used in this study were ob-tained 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

The 25 April 2015 Nepal earthquake and its aftershocks

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