SEISMIC HAZARD ANALYSIS FOR AHMEDABAD CITY

Proceedings of Indian Geotechnical Conference December 22-24, 2013, Roorkee SEISMIC HAZARD ANALYSIS FOR AHMEDABAD CITY T. Bhandari, PG Student, Department of Civil Engineering, IIT Delhi, bhandaritushar1390@gmail.com T.P. Thaker, Assistant Professor, Pandit Deendayal Petroleum University, tej_p_thaker@yahoo.co.in K.S. Rao, Professor, Department of Civil Engineering, IIT Delhi, raoks@civil.iitd.ac.in ABSTRACT: Ahmedabad is an important city in the Gujarat state of India. It has experienced one of the largest earthquakes in India on 26 Jan 2001 with magnitude of Mw 7.7 that caused many destructions and human casualties. With respect to historical earthquakes in the region of Ahmedabad and existing active faults like Marginal Fault (East Cambay), Marginal Fault(West Cambay), Son Narmada Fault (SNF), Island Belt Fault (IBF), etc., this region has high seismic potential. It is needed to provide horizontal Peak Ground Acceleration (PGA) maps with different risk levels for different parts of the region. The probabilistic hazard analysis of earthquakes has been carried out for occurrence probability of 2% and 10% in 50 years along with 2% and 10% in 100 years. INTRODUCTION The necessity of seismic hazard analysis for peninsular India has been emphasized by many authors and the analysis attempted too. The devastating effects caused by several earthquakes recorded in the past call for a detailed analysis of seismic hazard with appropriate methodology. Peninsular India has witnessed many earthquakes of moderate magnitude and some major earthquakes that include, the Mahabaleshwar (1764), Kutch (1819), Son-valley (1927), Satpura (1938), Anjar (1956), Koyna (1967), Jabalpur (1997), and, the Bhuj (2001) earthquakes. The hazard caused by these earthquakes and the existing lacuna in the mitigation measures make PI a region with utmost need of proper study to generate a mitigation tool. Seismic hazard analysis involves quantitative estimation of ground shaking hazards of a particular area. Seismic hazards may be analyzed deterministically, when a particular earthquake scenario is assumed, or probabilistically, in which uncertainty in location, time, place, are explicitly considered. A critical part of seismic hazard analysis is determination of peak ground acceleration (PGA) of area. Determination of PGA is necessary in order to design earthquake resistant structures and microzonation studies. This study aims at providing a design aid for site-specific earthquake-resistant design of structures coming up in the region under consideration. STUDY AREA Ahmedabad district covers an area of abou 8086 sq km situated at latitude and longitude of 23.0170N and 72.6830E respectively with a population of above 7 million (Ahmedabad Census, 2011) and it is one of the most growing and significant economy of India. Because of high population density and presence of soft soils at many places in this region and improper construction techniques, the structures are vulnerable to damage even by average size earthquakes. In this study, regional seismological investigation has been carried out for Ahmedabad region considering an area with radius of 350km, around the centre as Ahmedabad. Study area lies between latitude of 19.88 0N to 26.18 0N and longitude of 69.260E to 76.110E. Preparation of Earthquake Catalogue Data available for past earthquake events in India is not properly arranged and is in a very lucrative manner. In order to obtain the historical data about past earthquakes events, data from different sources was collected and combined for the time interval 1668-2010. The earthquake data was collected from various sources, like, Oldham, 1883, Chandra (1977), Tandon and Srivastava (1974), Krishna, Malik et al., (1999), United States Geological Survey (USGS), Institute of Seismological Research (ISR), Gandhinagar, etc. Proper care was taken to avoid repetition of events in the catalogue and conversion to a single Page 1 of 8

Cumulative No. of Earthquakes Standard Deviation (σ λ ) T. Bhandari, T.P.Thaker, & K.S.Rao magnitude scale. The raw catalogue consisted of 978 events from the year 1668 to 2010, distributed with latitude ranging from 10.00 N to 26.32 N and longitude ranging from 68.00 E to 89.00 E. For the present study, the widely used Urhammer method was used to decluster these events. The final catalogue with single magnitude scale and declustered events contains a total of 791 earthquakes varying in magnitude from Mw 2.0 to 7.8 recorded from the year 1668 to 2010. PROBABILISTIC APPROACH FOR SEISMIC HAZARD ANALYSIS Completeness of Catalogue As one goes backward in time the earthquake catalogue gets sparser and the uncertainty of its content increases. Earthquakes in the ancient times have not been recorded unless they were large and destructive. Catalogue incompleteness can be defined as recorded seismicity that differs from the real seismicity (Mulargia et al, 1985). There are different techniques used to account for catalogue incompleteness. To determine the completeness periods for different magnitude classes, two different methods namely Visual Cumulative (CUVI) method (Tinti and Mulargia, 1985) and the method by Stepp (1973) were used to calculate the completeness periods for different magnitude classes as shown in Fig.1and Fig.2. Table 1 summarizes completeness periods estimated with these two methods. 70 60 50 40 30 20 10 1964 Mw 3.5-3.9 0 1600 1700 1800 1900 2000 2100 Years Fig. 1 Cumulative Visual Interpretation Method 1 0.1 0.01 10 100 Time Period, T s (Years) SQRT(1/Ts) 2-2.9 3-3.4 3.5-3.9 4.0-4.4 4.5-4.9 5.0-5.4 5.5-5.9 >6 Fig. 2 STEPP Method Table 1 Completeness Periods M w CUVI Method STEPP (1973) Period Years Period Years 2.0-2.9 1999-2010 11 2001-2010 10 3.0-3.4 1970-2010 41 1961-2010 50 3.5-3.9 1964-2010 47 1971-2010 40 4.0-4.4 1958-2010 53 1971-2010 40 4.5-4.9 1958-2010 53 1961-2010 50 5.0-5.4 1958-2010 53 1961-2010 50 5.5-5.9 1919-2010 92 1941-2010 70 >=6 1848-2010 163 1911-2010 100 Recurrence Relationship Definition of the seismicity recurrence characteristics for each source is the next fundamental step in the probabilistic seismic hazard assessment. Each source is characterized by an earthquake probability distribution or recurrence relationship. In present analysis, a simple and widely used Gutenberg and Richter (1944) relationship has been adopted to evaluate the seismic hazard parameter b which is; where, λ m = cumulative number of earthquakes greater than or equal to a particular magnitude M per year and parameters a and b describe the seismic activity of the region. The completeness periods from both the analyses (Table 1) have been used to obtain the G-R magnitude- frequency relationship as shown in Fig. 3. The recurrence relations obtained from both the methods is, Page 2 of 8

Annual Rate of Events Mw log N 0.4 0.2 0-0.2-0.4-0.6-0.8-1 -1.2-1.4 CUVI Method y = -0.7788x + 3.3646 R² = 0.9833 STEPP Method y = -0.7238x + 3.1058 R² = 0.9833 4.0 4.5 5.0 5.5 6.0 Magnitude (Mw) Fig. 3 Regional Magnitude-Frequency Relation Deaggregation and Selection of Sources for Probabilistic Hazard Analysis Out of the many faults, 14 faults were considered for PSHA that could influence PGA (g) values, namely Allah Bund Fault (ABF), Island Belt Fault (IBF), Kutch Mainland Fault (KMF), Marginal Fault of East Cambay (ECF), Marginal Fault of West Cambay (MF(WC)), Son Narmada Fault (SNF), Tapti North Fault (TNF), Neotectonic Fault, West Coast Fault (WCF), Kishangarh Chipri Fault (KMF), Katrol Bhuj Fault (KBF), Barwani Sukta Fault (BSF), Left Down Neotectonic Fault (LDN), North Kathiawar Fault (NKF) and Upper Godavari Fault (UGF). The recurrence relation derived is for the entire region and not necessarily applicable to any particular fault. Therefore, the process of deaggregation is required i.e., the mean annual rate Seismic hazard analysis for Ahmedabad city of exceedance be expressed as a function of magnitude or distance. An approach given by Iyenger and Raghukanth (2004) derived on heuristic basis invoking the principle of conservation of seismic activity is adopted. The maximum magnitude (mmax) for each source was estimated based on the maximum reported magnitude at the source plus 0.5 (Kijko and Graham, 1998; Raghukanth and Iyengar, 2006; Anbazhagan et al., 2008). The recurrence relation for each fault capable of producing earthquake magnitude in the range mo to mmax was calculated using the truncated exponential recurrence model developed by McGuire and Arabasz (1990) and an annual rate of event of magnitude greater than or equal to M w (λ m ) is given by the following expression; λ m = v for m o m m max where, m o is threshold magnitude, β equals 2.303b and N i (m 0 ) or ν is weightage factor for a particular source based on deaggregation. The deaggregation of regional hazards in terms of fault recurrence is shown in Fig. 4. Finally, probability density function f M (m) is calculated for each fault based on the following expression; f M (m) = P[M<m/m o m m max ] = 1.000 0.100 0.010 0.001 4 5 6 7 8 Magnitude (Mw) ABF IBF KMF East Cmabay BSF LRL West Cambay SNF TNF LDN WCF KCF CJL KB NK UGF Region Fig. 4 Deaggregation of Regional Hazard Page 3 of 8

T. Bhandari, T.P.Thaker, & K.S.Rao Predictive Relationship Magnitude, distance and site conditions are the principal variables used in predicting future ground motions. Since the study area is located in peninsular India, the attenuation relation developed by Iyengar and Raghukanth (2004) through statistically simulated seismological model for rock site is used for this study. Normal cumulative function has a value which is more efficient in terms of standard normal variables (z) which can be computed for any random variable using a transformation as: z = where, Y* = various targeted PGA/Sa levels which will be exceeded; lny = value calculated using predictive relationship and ln ε = uncertainty in predictive relation expressed in terms of standard deviation. Development of Seismic Hazard Curves The approach involves the development of seismic hazard curves indicating the annual probability of exceedance of different values of a selected ground motion parameter. The seismic hazard curves can be used to compute the probability of exceeding selected ground motion parameter in a specified period of time. Seismic hazard curves can be obtained for the individual source zones and are combined to express aggregate hazard at a particular site. For given earthquake occurrence, the probability that a ground motion parameter Y will exceed a particular value y* can be computed easily in total probability theorem; i.e.: P[Y>y*]=P[Y>y* X] P[X]= where, X is a vector of random variables that influences Y. In most cases, quantities in X are limited to magnitude M, and distance R. Assuming that M and R are independent, the probability of exceedance can be given as; P[ Y> y* ] = where, obtained from the predictive relationship; and are probability density functions for magnitude and distance respectively. If the site of interest is a region of N s potential earthquake sources, each of which has an average rate of threshold magnitude exceedances,, the total average exceedance rate will be given by: λ y * = The complexity of the components to be integrated makes the analytical evaluation difficult. Therefore, to simplify the numerical integration, the possible ranges of magnitude and distance are divided into N M and N R segments respectively. The average exceedance rate can be estimated by; λ * y= [ ] where, m j = m 0 + (j-0.5)(m max m 0 )/N M; r k = r min + (k-0.5)(r max r min )/N R; m = (m max m 0 )/N M and ; r = (r max r min )/N R This is equivalent to assuming that each source capable of generating only N M different earthquakes of magnitude, mj, at only N R different source-to-site distances, r k Eq. is then equivalent to λ * y= [ ] [ ] With the assumption that number of earthquakes occurring on a fault follows a stationary Poisson s process, the probability that the control variable Y exceeds level y*, in terms of a final time interval T years is given by: P[Y T > y*] = 1 - The seismic hazard curves can be obtained by computing the mean annual rate of exceedance λ y for different specified ground motion values y*. These are obtained individually for all the 14 faults and combined to estimate the aggregate hazard at the site. The PGA(g) obtained for different probabilities were used to prepare the PGA(g) contours; which are presented in Fig.5 to Fig.8. Page 4 of 8

Seismic hazard analysis for Ahmedabad city Fig. 5 PGA(g) Contours for Return Period 500 years Fig. 6 PGA(g) Contours for Return Period 1000 years Page 5 of 8

T. Bhandari, T.P.Thaker, & K.S.Rao Fig. 7 PGA(g) Contours for Return Period 2500 years Fig. 8 PGA(g) Contours for Return Period 5000 years Page 6 of 8

Seismic hazard analysis for Ahmedabad city RESULTS AND DISCUSSION Seismic hazard analysis results for Ahmedabad city are presented. After a review of the seismo-tectonic set-up around Ahmedabad, a controlling region of 350 km radius is considered for detailed study. The regional seismicity is distributed essentially among fourteen faults of varied lengths and potential. Probabilistic seismic hazard analysis has been carried out to arrive at the mean annual probability of exceedance of PGA value at any site. Detailed results are presented in the form of a contour map covering Ahmedabad city and its environs for an extent of 1375 km 2 on a grid of about 7 km 2. The major limitation has been the non-availability of field measurements for shear-wave velocity in the top layers of the local soils. Nevertheless, the present results form a sound basis for future extensions to cover local soil effects, liquefaction susceptibility, and vulnerability analysis of buildings. REFERENCES 1. A.K. Mathur, A. K. Saxena, Surendra Prasad, D. N. Fulzele, V. N. Singh and R. L. Regar- Seismic Hazard and Risk Microzonation of Ahmedabad City, Gujarat- Geological Survey of India. 2. Gutenberg B. and Richter C.F. (1956). Earthquake magnitude, intensity, energy and acceleration. Bull. Seismol. Soc. Am. 46, 105 145. 3. Iyengar R.N. and Ghosh S. (2004). Microzonation of earthquake hazard in greater Delhi area Curr. Sci. 87, 1193 1202. 4. Iyengar R.N. and Raghukanth S.T.G. (2004). Attenuation of strong ground motion in Peninsular India Seismol. Res. Lett. 75, 530 540. 5. Jaiswal K. and Sinha R. (2007). Probabilistic seismic-hazard estimation for Peninsular India. Bull. Seismol. Soc. Am. 97(1B), 318 330. 6. Kijko A., and Graham G. (1998). Parametrichistoric procedure for probabilistic seismic hazard analysis, Part I: Estimation of maximum regional magnitude Mmax. Pure Appl. Geophy. 152, 413 442 7. Kramer S.L. (1996). Geotechnical earthquake engineering, Prentice- Hall International Series in Civil Engineering and Engineering Mechanics, Washington, 106 129. 8. Krishna J. (1959). Seismic zoning of India. In Earthquake Engg., Seminar, Roorkee University, India, 32 38. 9. Malik J.N., Sohoni P.S., Karanth R.V., Merh S.S. (1999). Modern and historic seismicity of Kachchh Peninsula, western India. J. Geol. Soc. India 54, 545 550. 10. Oldham T.A. (1883). Catalogue of Indian earthquakes. Mem. Geol. Surv. India, 19, 163 215. 11. Raghukanth S.T.G., and Iyengar R.N. (2007). Estimation of seismic spectral acceleration in Peninsular India J. Earth Sys. Sci. 116(3), 199 214. 12. Rao K.S., Thaker T.P., Aggarwal A., Bhandari T., Kabra S. (2012), Deterministic seismic hazard analysis of Ahmedabad Region, Gujarat, International Journal of Earth Sciences and Engineering, 5 (2), pp. 206-213. 13. Sitharam T.G., and Anbazhagan P. (2007). Seismic hazard analysis for the Bangalore region. Nat. Hazards. 40, 261 278. 14. Stepp J.C. (1973). Analysis of completeness of the earthquake sample in the Puget Sound area, ed. by in Seismic Zoning, Harding S.T. NOAA Tech. Report ERL 267-ESL30, Boulder, Colorado. 15. T. P. Thaker, Ganesh W. Rathod, K. S. Rao, and K. K. Gupta. Use of Seismotectonic Information for the Seismic Hazard Analysis for Surat City, Gujarat, India: Deterministic and Probabilistic Approach 16. Tandon A.N. (1956). Zoning of India liable to earthquake damage. Indian J. Meteorol. Geophys. 10, 137 146. 17. Tinti S., and Mulargia F. (1985). Completeness analysis of a seismic catalogue Ann. Geophysics. 3, 407 414. 18. Anbazhagan P., Vinod J.S., and Sitharam T. G. (2008). Probabilistic seismic hazard analysis for Bangalore. Nat. Hazards. 8, 145 166. 19. Faccioli E. (2003). Basic of Engineering Seismology and Seismic Hazard Analysis. Lecture notes at ROSE School- Winter Term, with the cooperation of C.G. Lai and M. Stupazzini. Page 7 of 8

T. Bhandari, T.P.Thaker, & K.S.Rao 20. Prantik Mandal, N. Kumar, C. Satyamurthy, and I. P. Raju (2009), Ground-motion Attenuation Relation from Strong-motion Records of the 2001 Mw 7.7 Bhuj Earthquake Sequence (2001 2006), Gujarat, India. 21. Criteria for Earthquake Resistant Design of Structures (Part 1) - General Provisions and Buildings - (Fifth Revision) Page 8 of 8