PROBABILISTIC SEISMIC HAZARD ANALYSIS AND ESTIMATION OF SPECTRAL STRONG GROUND MOTION ON BED ROCK IN NORTH EAST INDIA

4th International Conference on Earthquake Engineering Taipei, Taiwan October 12-13, 2006 Paper No. 015 PROBABILISTIC SEISMIC HAZARD ANALYSIS AND ESTIMATION OF SPECTRAL STRONG GROUND MOTION ON BED ROCK IN NORTH EAST INDIA M. L. SHARMA 1 AND SHIPRA MALIK ABSTRACT THE PRESENT PAPER CONSISTS OF THE PROBABILISTIC SEISMIC HAZARD ANALYSIS FOR THE NORTH EAST INDIAN REGION, WHICH IS ONE OF THE MOST SEISMICALLY ACTIVE REGIONS IN INDIA. THE REGION HAS BEEN DIVIDED INTO FOUR MAJOR SEISMOGENIC SOURCES NAMELY, THE REGIONAL FEATURES IN THE HIMALAYAS I.E., MAIN BOUNDARY THRUST AND MAIN CENTRAL THRUST, EASTERN SYNTAXIS, SHILLONG MASSIF AND THE NORTH SOUTH TRENDING ARAKAN YOMA SEISMIC BELT. THE PROBABILISTIC SEISMIC HAZARD ESTIMATION IS CARRIED OUT FOR TEN SEISMOGENIC ZONES WHICH ARE FURTHER SUBDIVISIONS OF THESE FOUR SEISMOGENIC SOURCES BASED ON THE SEISMOTECTONICS MODELING OF THE AREA. THE COMPLETE AS WELL AS THE EXTREME PART OF THE CATALOGUE IS USED TO MAKE MAXIMUM LIKELIHOOD ESTIMATES OF MAXIMUM PROBABLE EARTHQUAKES FOR VARIOUS RETURN PERIODS. THE MAXIMUM B-VALUE ESTIMATED IS 1.04 FOR SHILLONG PLATEAU (SEISMOGENIC ZONE III) WHILE THE RETURN PERIODS OF MAGNITUDE 6.0 HAVE BEEN ESTIMATED AS ABOUT SEVEN YEARS FOR THE EASTERN BOUNDARY THRUSTS (SEISMOGENIC ZONE VIII AND IX), WHICH ARE PART OF THE ARAKAN YOMA RANGES. THE HAZARD IN THESE INDIVIDUAL ZONES IS PRESENTED IN FORM OF SEISMIC ZONING AT THE BEDROCK LEVEL FOR 10% AND 20 % EXCEEDANCE VALUES OF STRONG GROUND MOTION IN 50 YEARS. THE EPISTEMIC ERRORS HAVE BEEN CONSIDERED USING LOGIC TREE METHOD BY USING THE SPECTRAL ATTENUATION RELATIONSHIPS DEVELOPED FOR THIS AREA AS WELL AS THOSE DEVELOPED FOR SIMILAR TECTONIC ENVIRONMENTS ELSEWHERE AND ADOPTED FOR THE REGION. THE SPECTRAL ACCELERATION AT DIFFERENT STRUCTURAL PERIODS IS PRESENTED FOR MAJOR CITIES IN THE REGION. THE PGA VALUE RANGES FROM 0.05G TO 0.6G FOR 10% EXCEEDANCE WHILE THE PGA VALUE RANGES FROM 0.01G TO 0.4G FOR THE 20 % EXCEEDANCE IN 50 YEARS. THE RESULTS OF THE PROBABILISTIC SEISMIC HAZARD ANALYSIS IN THE PRESENT STUDY MAY BE USED FOR THE SEISMIC MICROZONATION OF THE AREA AND FOR EARTHQUAKE ENGINEERING USE. Keywords: Seismic hazard, PSHA, Himalayas, Spectral ground motion INTRODUCTION The Himalayas have experienced many great earthquakes in the past namely Kangra earthquake (April 04, 1905; magnitude 8.0), Assam earthquake (August 15, 1950; magnitude 8.5), Shillong earthquake (June 12, 1897; magnitude 8.7), and Bihar-Nepal earthquake (January 15, 1934, magnitude 8.3). One of the most active regions of the Himalayas is the North eastern Indian region which has experienced many of the damaging earthquakes occurred in the whole Himalayas. Considering the seismicity rate and the return periods of the damaging earthquakes, it is of paramount importance to estimate the seismic potential of the region. The seismic potential is defined as the probability of occurrence of earthquake of maximum magnitude M max in future. Such estimate along with the knowledge of 1 Department of Earthquake Engineering, IIT Roorkee, INDIA Email: mukutfeq@iitr.ernet.in

probabilities of occurrence of earthquakes varying in size is useful in earthquake resistant design of structures and disaster mitigation. Most of the procedures for estimation of seismic potential using probabilistic seismic hazard assessment require the determination of seismic source zones, and a knowledge of their hazard parameters such as activity rate and Guttenberg- Richter parameter b. Such information is not readily available for large part of the Indian subcontinent and most Indian seismic catalogues are highly uncertain and incomplete. Most of the available earthquake catalogues usually contain two types of information: macroseismic observations of major seismic events that occurred over a period of few hundred years, and complete instrumental data for relatively short periods of time, say the last fifty years at the most. In the present study use of complete part of the catalogue as well as extreme part of the data is used for estimation of seismic hazard employing the methods as developed by Kijko and Sellevoll (1989). The seismotectonic, geological and other information is mostly collected from the Seismotectonic Atlas of India by Geological Survey of India (GSI) while the earthquake catalogue (updated from other sources like USGS open files, ISC bulletins and other published reports) from India Meteorological Department, New Delhi has been used. SEISMOTECTONIC SETUP The northeastern region in the present study sprawls between latitude 20 to 29 N and longitude 87 to 96 E (Fig. 1). This area is one of the seismically very active areas of the world and that of the Himalayas in particular. This region has experienced two great earthquakes in the past viz., Assam earthquake of August 15, 1950 and Shillong earthquake of June 12, 1897. Tectonically the northeastern parts of Indian region is characterized by the Archean landmass which is surrounded by the Tertiary Himalayan mountain belt in the north and Indo-Burman fold belt in the east and southeast. These mountain belts are the result of continent-continent collisions due to convergent plate tectonism in terms of which the Northeast India has been affected from two different directions. As a result, the whole region has suffered multiple phases of deformational processes, which have resulted into numerous geological structures. It exhibits rugged terrain, the only exception being Brahmaputra valley (Assam plains), which is ravaged annually by floods. Regionally, north-east India can be subdivided into various geotectonic units, namely the Shillong massif and tectonic features bounding it, Halflong-Disang-Naga thrusts and other thrusts (comprising the Belt of Schuppen) trending NE-SW and merging with the Arakan-Yoma fold belt, the complex folded, faulted and jointed meta sedimentaries of Arunachal Himalaya having a north-easterly orographic trend, and the Assam basin with Tertiary sediments in Brahmaputra valley, surrounded by the above four lithotectonic units along its margins. The Shillong plateau region, the central part of the study area, has suffered predominantly compressional tectonic forces in N-S, NE-SW and NW-SE directions (Das, 1996). The E-W trending Dauki Fault (DF) forming steep scarps, is a very prominent linear feature and marks the southern edge of the Shillong Massif. The region is divided into four major divisions (as given in Table 1) namely the (i) East west trending zone comprising of Main Boundary Thrust (MBT) and Main Central Thrust (MCT), (ii) Eastern Syntaxis, (iii) Shillong Massif and (iv) north south trending tectonic features consisting of Arakan Yoma ranges and eastern boundary Thrust. The Eastern-Himalaya Zone comprises of Main Central Thrust (MCT), Main Boundary Thrust (MBT) and Main Frontal Thrust (MFT). Since 1762 the region has experienced 119 earthquakes of which 60 are below magnitude 5. There are 38 earthquakes of magnitude 5 M<6. There are 9 earthquakes of magnitude 6 M<7. Earthquake of maximum magnitude which is registered in this zone is of magnitude 6.7 which occurred on August 16,1950. For 12 earthquakes no magnitude could be assigned. The Eastern Syntaxis Zone comprises of MBT (Eastern region), MFT, Lohit Thrust, Mishmi Thrust, Bame Tuting Fault. Since 1762 the region has experienced 158 earthquakes of which 98 are below magnitude 5. There are 37 earthquakes of magnitude 5 M<6. There are 17 earthquakes of magnitude M<7. Earthquake of greatest magnitude which is registered in this zone is of magnitude 7.7 which occurred on July 29, 1947. For 5 earthquakes no magnitude is assigned.

29 SZ II 28 SZ I 27 SZ VI 26 SZ III 25 SZ IV SZ VII SZ V Latitude 24 SZ X SZ VIII 23 22 21 20 + Unassigned Mag Mag. < 5.0 7 5.0 Mag < 6.0 6.0 Mag < 7.0 Mag > 7.0 SZ IX 87 88 89 90 91 92 93 94 95 96 Longitude Fig. 1 Seismotectonic setup of the North East Indian region. The solid (thicker) lines shows major seismogenic zones with their further subdivision in seismogenic zones SZ I to SZ X The Shillong Massif Zone comprises of Dhansiri Fault, Kopili Fault, Dauki Fault, Dudhnoi Fault and Sylhet Fault. Since 1762 the region has experienced 156 earthquakes of which 99 are below magnitude 5. There are 32 earthquakes of magnitude 5 M<6. There are 6 and 3 earthquakes of magnitude 6 M<7 and 7 M<8 respectively. Earthquake of greatest magnitude which is registered in this zone is of magnitude 8.7 which occurred on June 12, 1897. For 15 earthquakes no magnitude is assigned.

Table 1 Division of Seismogenic Sources in North Eastern Indian Region Sl. Seismogenic Major division Subdivision No. Zone 1 SZ-I East West Trending Features Eastern Himalaya 2 SZ-II Eastern Syntaxis Eastern Syntaxis 3 SZ-III Shillong Massif Shillong Plateau 4 SZ-IV Shillong Massif Dauki Fault 5 SZ-V Shillong Massif Sylhet Fault 6 SZ-VI North South Trending features Naga Disang Thrust 7 SZ-VII North South Trending features Eastern Boundary Thrust (I) 8 SZ-VIII North South Trending features Eastern Boundary Thrust (II) 9 SZ-IX North South Trending features Eastern Boundary Thrust (III) 10 SZ-X North South Trending features Mat Fault The North-South trending features comprises of Naga- Disang Thrust zone, Eastern Boundary Thrust and Mat Fault. Since 1762 the region has experienced 787 earthquakes of which 531 are below magnitude 5. There are 113 earthquakes of magnitude 5 M<6. There are 41 and 6 earthquakes of magnitude 6 M<7 and 7 M<8 respectively. Earthquake of greatest magnitude which is registered in this zone is of magnitude 7.3 which occurred on August 16, 1938 and March 21, 1954. For 96 earthquakes no magnitude is assigned. For the purpose of carrying out the estimations for seismic hazard in the region and for estimating the seismic hazard employing probabilistic methodology, these four major regions are further subdivided into smaller seismogenic sources. This subdivision is based on the tectonic and geological similarities, seismicity and its association with the geological features of the subdivided part, the amount of seismic energy released by the area and other geophysical parameters like gravity and magnetic anomalies, depth of bed rock etc (refer Seismotectonic Atlas of India by GSI). The four major seismogenic zones and the subdivisions as above are given in Table 1 and shown in Fig. 1. ESTIMATION OF SEISMIC POTENTIAL The whole NE region undertaken in the present study is divided into ten seismogenic zones i.e. SZ I to SZ X. For the purpose of carrying out the seismic hazard estimation for this region the earthquake catalogue from 1762 to 2001 provided by India Meteorological Department has been used. A wellknown technique (Kijko and Sellevoll, 1989) for the estimation of seismic hazard parameters has been used. Poissonian occurrence of earthquakes with doubly truncated Gutenberg-Richter distribution is assumed. A computer program based on this method and written by Kijko and Sellevoll is also used for this analysis. Table-2 summarizes the input data used for the present study. TABLE 2 INPUT DATAUSED FOR SEISMIC HAZARD ANALYSIS ZONES NUMBER OF EVENTS MAX. OBS. MAGNITUDE EXTREME(N E ) COMPLETE(N C ) I 12 87 6.70 II 13 125 7.70 III 2 50 5.60 IV 10 55 8.70 V 5 10 7.60 VI 6 10 7.20 VII 7 106 7.00 VIII 18 307 7.30 IX 6 208 7.30 X 0 8 4.90

The results of seismic hazard analysis of North East India are given for SZ I to SZ X in terms of seismic hazard parameters namely, λ, β, b value, M max and R 6.0 (return period for magnitude 6.0) in Table-3. The major division namely, the East west trending features consist of only one subdivision i.e., SZ-I. The seismic hazard parameters estimated for this zone are b = 0.65±0.04 and M max = 6.85±0.36. The return period of magnitude 6.0 earthquakes is 16.4 years. The data contribution by the complete and extreme parts is 47% and 53% for β and 13.2% and 86.8% for λ, respectively. TABLE3 SEISMIC HAZARD PARAMETERS Zones λ β b-value M max R 6.0 Years I 7.73±1.04 1.59±0.09 0.65±0.04 6.85±0.36 16.4 II 13.42±1.59 1.74±0.08 0.76±0.03 9.05±1.39 11.7 III 10.03±1.58 2.39±0.07 1.04±0.03 6.11±0.61 564.7 IV 3.70±0.59 1.13±0.08 0.49±0.04 9.09±0.51 10.3 V 0.73±0.20 0.98±0.07 0.43±0.03 8.37±0.84 23.9 VI 0.99±0.26 0.96±0.07 0.42±0.03 7.62±0.53 20.4 VII 11.21±1.23 1.66±0.05 0.72±0.02 7.65±0.56 13.8 VIII 32.72±2.40 1.79±0.05 0.78±0.02 7.53±0.40 7.1 IX 24.08±2.34 1.71±0.08 0.74±0.03 7.89±0.68 7.3 X 1.13±0.41 1.16±0.11 0.50±0.05 5.31±0.53 - The second major division namely, Eastern Syntaxis also consist of only one subdivision i.e., SZ II. The difference between M max and X max is highest i.e. 1.35 in SZ II. Other parameters are b = 0.76±0.03 and M max = 9.05±1.39. The return period of magnitude 6.0 earthquakes is 11.7 years. The data contribution by the complete and extreme parts is 50.2% and 49.8% for β and 10% and 90% for λ, respectively. The third major division which is Shillong Massif consist of three subdivisions namely, SZ III, SZIV and SZV. The parameters estimated for the zone SZ-III are b = 1.04±0.03 (which is highest of all the zones) and M max = 6.11±0.61. The return period of magnitude 6.0 earthquakes is 564.7 years. The data contribution by the complete and extreme parts is 43.8% and 56.2% for β and 4.4% and 95.6% for λ, respectively. In Zone-IV we have extended the catalogue to the past. We have assumed that there is an earthquake of magnitude 8.7 in pasts too (around 1500 A.D.). In this zone the difference between M max and X max is 0.39. Other parameters are b = 0.49±0.04 and M max = 9.09± 0.51(which is highest of all the zones). The return period of magnitude 6.0 earthquakes is 10.3 years. The data contribution by the complete and extreme parts is 46.4% and 53.6% for β and 16.9% and 83.9% for λ, respectively. The parameters estimated for zone SZ-V are b = 0.43±0.03 and M max = 8.37±0.84. The return period of magnitude 6.0 earthquakes is 23.9 years. The data contribution by the complete and extreme parts is 52.2% and 47.8% for β and 33.4% and 66.6% for λ, respectively. The Arakan Yoma seismicity belt is subdivided into five seismogenic zones SZ VI to SZ X. The b- value estimated for the zone SZ-VI is 0.42±0.03 and M max is 7.62±0.53. The return period of magnitude 6.0 earthquakes is 20.4 years. The data contribution by the complete and extreme parts is 53.2% and 46.8% for β and 37.5% and 62.5% for λ, respectively. In the zone SZ-VII, the estimated b- value is 0.72±0.02 and M max is 7.65±0.56. The return period of magnitude 6.0 earthquakes is 13.8 years. The data contribution by the complete and extreme parts is 45.8% and 54.2% for β and 5.7% and 94.3% for λ, respectively. The parameters estimated for the zone SZ-VIII are b = 0.78±0.02 and M max = 7.53±0.40. The return period of magnitude 6.0 earthquakes is 7.1 years (which is least of all zones). The data contribution by the complete and extreme parts is 46.2% and 53.8% for β and 6.1% and 93.9% for λ, respectively. In zone SZ-IX the estimated parameters are b = 0.74±0.03 and M max = 7.89±0.68. The return period of magnitude 6.0 earthquakes is 7.3 years. The data contribution by the complete and extreme parts is 23.7% and 76.3% for β and 3% and 97% for λ, respectively. The zone-x consists only of Mat fault.

The b-value estimated for this zone is 0.50±0.05 and M max is 5.31±0.53. For this zone only complete part of data is used for estimating the parameters. The strong ground motion at the surface is estimated using the CRISIS program (Ordaz, 2001). The hazard parameters estimated as above has been used to compute the strong ground motion in terms of spectral acceleration at various periods. The epistemic errors have been considered using logic tree method by using the spectral attenuation relationships developed for this area as well as those developed for similar tectonic environments elsewhere and adopted for the region. The two attenuation relationships used for this study are- Sharma and Bungum (2006) and Youngs et al (1997). The relationship by Sharma and Bungum (2006) has been developed based on the data acquired in the Himalayas. The attenuation relationship by Youngs et al (1997) has been developed for subduction zone and therefore applied in the present case. Thus estimated PGA value for 10% and 20% exceedance in 50 years is shown in Fig. 2. The results of the probabilistic seismic hazard analysis in the present study may be used for the seismic microzonation of the area and for earthquake engineering use. The spectral acceleration values for major cities is given in Table 4. Fig. 2 PGA values for 10% and 20% exceedance values in 50 years for NE Indian region Table 4 Spectral acceleration in g at major cities in NE India Periods Agaratala Guwahati Itanagar Jorhat Shillong 0.03 0.33316 0.46888 0.44388 0.42908 0.48367 0.10 0.71480 0.95714 0.90816 0.89031 0.99388 0.20 0.84337 1.03469 0.98265 0.98520 1.07296 0.30 0.66531 0.85153 0.75204 0.75816 0.91837 0.40 0.45459 0.58980 0.50714 0.51327 0.64643 0.50 0.36429 0.47653 0.40714 0.41327 0.52653 0.75 0.24541 0.32500 0.27857 0.28163 0.35765 1.00 0.17500 0.23418 0.20255 0.20306 0.25459 1.50 0.11276 0.15561 0.13776 0.13571 0.16531 2.00 0.07816 0.11122 0.09918 0.09679 0.11684

DISCUSSIONS AND CONCLUSIONS The seismic hazard estimation has been carried out for the north east Indian region using the complete and the extreme part of the earthquake catalogue. The return period for various magnitudes in SZ I to SZX are shown in Fig. 3. In this fig. 3(a) shows the return periods for SZ I, III, V, VI and X which are having relatively higher return periods for magnitude 6.0 in comparison to SZ II, IV, VII, VIII and IX which are shown in 3(b). The PGA value ranges from 0.05g to 0.6g for 10% exceedance while the PGA value ranges from 0.01g to 0.4g for the 20 % exceedance in 50 years. Similarly, the probability of occurrence of magnitudes for return period 50, 100 and 1000 are shown in Fig. 4 (a), (b) and (c) respectively. The comparison of the observed and predicted maximum magnitudes is shown in Fig. 5. The b-values are also shown along with the two types of magnitudes for SZ I to SZ X. The difference in the observed and estimated magnitude is maximum in Zone II while the b-values is highest in SZ III and lowest for SZ VI. The 10% and 20 % exceedance values in terms of maximum probable magnitudes for SZ s are shown in Fig. 6. The λ values are relatively very less for SZ V and VI and is highest for SZ VIII showing that lot of earthquakes with relatively lower magnitude range are occurring in this zone (the observed maximum and estimated magnitudes are comparable see fig. 5). It may be noted that the maximum magnitudes for 10 % and 20 % exceedance values are for SZ IV. Fig. 3. Return period vs magnitude plots for the seismogenic zones SZ I to SZ X. The SZ s with relatively higher return periods are shown in (a) and other five SZ s are shown in (b) The seismic hazard analysis has been carried out by various research workers in this area (Kaila et al., 1972; Rao and Rao, 1979; Gupta and Srivastava, 1990; Shanker and Sharma, 1997 & 1998; Arora and Sharma, 1998, Bhatia et al., 1999; Sharma and Sahnker, 2001) This region is covered by Zone V and Zone VI selected on a on a regional basis by Shanker and Sharma (1998) as North East India (NEI) and Burma-Andaman-Nicobar (BAN). The M max, R 6.0 and β are estimated to be 8.5, 7 and 1.89 ± 0.15 for NEI and 9.2, 5 and 1.49 ± 0.12 for BAN. While the β given by Rao and Rao (1979) is 1.66 and 1.93 for NEI and BAN respectively; b-value given by Kaila et al. (1972) is 0.7<b 1.30 and 0.8<b 1.30 for NEI and BAN respectively and value of R 6.0 given by Gupta and Srivastava (1990) is 4.44 and 3.97 for NEI and BAN respectively. The maximum likelihood estimates of earthquake hazard were made by Shanker and Sharma (1997) for the North East India region. The seismic hazard parameters namely M max, b, beta and lambda are estimated as 9.20, 0.92±0.05, 2.16±0.12 and 6.70± 0.61 respectively. The same region was considered by Arora and Sharma (1998) for carrying out the seismic hazard estimation in the North East India using Artificial Neural Network approach. The whole region was divided into four seismogenic zones. Qualitative interpretations could be made regarding the probability of occurrence of moderate

earthquakes in the region. In most of the cases the energy accumulation cycle is over and the seismic energy release will be taking place in this region for next 30 to 50 years. (a) (b) (c) Fig. 4 Probability vs magnitude curves for (a) 50 years (b) 100 years and (c) 1000 years.the SZ s are divided into two figures based on their probability of occurrences.

Fig. 5 Histograms showing variation of the b-values, Observed and estimated M max for seismogenic zones SZ I to SZ X Fig. 6 Histograms of showing variation of λ, 10% exceedance in 50 years and 20% exceedance in 50 years for seismogenic zones SZ I to SZ X The same region is also covered in the GSHAP exercise for India region which started with the exercises in the GSHAP Test Area #8 covering parts of India, China, and Nepal, bounded by 20 o N- 40 o N and 85 o E-105 o E during the workshop held at NGRI in February-march 1996 (Bhatia et al. 200). Using the probabilistic hazard assessment approach of McGuire adopted by GSHAP, the Peak Ground Accelerations (PGA) were computed using the FRISK88M software for 10 % probability of exceedance in 50 years, at locations defined by a grid of 0.5 o X 0.5 o in the region 0 o N-40 o N and 65 o E - 100 o E. The hazard map depicts that a majority of the plate boundary region and the Tibetan plateau region have hazard levels of the order of 0.25g with prominent highs of the order of 0.35-0.4g in the seismically active zones like the Burmese arc, Northeastern India and North-west Himalaya. The present study has the advantage of using the complete as well as extreme data from the earthquake catalogue. In the probabilistic seismic hazard analysis the source definition in terms of its extent plays an important role. In the present study the major divisions are further subdivided into seismogenic zones SZ I to X to have more realistic seismic hazard estimation. ACKNOWLEDGMENT The earthquake catalogue have been provided by India Meteorological Department, New Delhi. The help extended by Department of Science and technology, New Delhi is thankfully acknowledged. Thanks are due to Andrzej Kijko for providing computer program for estimation of seismic hazard. REFERENCES Arora, M., and Sharma, M.L. (1998), Seismic hazard analysis: An artificial neural network approach, Current Science 75(1), 54-59. Bhatia, S.C., Kumar, R., and Gupta, H.K. (1999), The Probabilistic Seismic Hazard Map of India and adjoining regions, Annali Di Geofisicia 42(6) 1153-1166. Das, J. D. (1996). Tectonics of the Shillong plateau and adjoining regions in Northeast India using field and remotely sensing data. Ph. D. Thesis, University of Roorkee, India, 201 pp, (Unpublished). GSI (2000), Seismotectonic Atlas of India and its Environ, Geological Survey of India, India

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