PROBABILISTIC SEISMIC HAZARD ASSESSMENT OF KARNATAKA STATE

Transcription

1 PROBABILISTIC SEISMIC HAZARD ASSESSMENT OF KARNATAKA STATE Submitted to: CiSTUP Indian Institute of Science Bangalore Investigator(s) from IISc: Prof. T. G. Sitharam Professor, Department of Civil Engineering, Indian Institute of Science, Bangalore Department of Civil Engineering, Indian Institute of Science, Bangalore , March

2 Centre for infrastructure, Sustainable Transport and Urban Planning Indian Institute of Science, Bangalore Title of the Project Probabilistic Seismic Hazard Assessment of Karnataka State 2 Scheme Code No CIST Principal Investigator-Name & Department Dr. T. G. Sitharam Professor, Department of Civil Engineering, Indian Institute of Science, Bangalore Co-Investigator (If any) none 5 Date of Commencement Jan 01, Project Duration 1 year 7 Ending Date of the Project Dec, Discussion/Summary of work carried out (Explaining Deliverables, Implementation etc. with List and future direction.) SPECIFIC AIM/ OBJECTIVES OF THE PROJECT: The main objective of the study is to identify and map linear seismic sources in the state of Karnataka Collection of earthquake events occurred in the state and surrounding area from different agencies Preparation of seismotectonic atlas for Karnataka state showing all the linear seismic sources and the earthquake events. Carrying out probabilistic seismic hazard analyses to evaluate peak horizontal acceleration at the bed rock level for the entire Karnataka state. Estimation of the peak ground acceleration at surface level for different site classes by considering the site effects. Probabilistic evaluation of SPT and CPT values required to prevent liquefaction 2

3 1. INTRODUCTION Earthquakes are one of the major natural hazards that has destroyed countless manmade structures and inflicted the death of thousands of people. Earthquakes are neither predictable to man till now nor he can evade its hazard totally. So to mitigate the seismic hazards, it is necessary to make some scientific earthquake studies for identifying the regions having high intensity of seismic risk. This work presents a detailed study on the seismic pattern of the state of Karnataka and also quantifies the seismic hazards for the entire state. The state of Karnataka is located between E and E longitude and N and N latitude. It covers an area of 1,91,791 km², or 5.83% of the total geographical area of India. Karnataka is a prime location for industrial activities as it hosts large number of small and large scale industries and a major center for Information Technology (IT) industry. The capital city of Karnataka, Bangalore, is known as the Silicon Valley of India. Many historical monuments are located in northern parts of Karnataka which makes it significant from the archeological and historical point of view. Due to rapid urbanization, cites in Karnataka are having lots of infrastructural development activities. Major projects Metro Rail projects are already initiated in Bangalore. Thus, scientific estimate of various hazards is decisive factor for urban planning with disaster management measures. 2. NEED FOR THE STUDY The state of Karnataka constitutes one of the most prominent and largest Precambrian shield areas of the world, and is tectonically termed as intraplate region or shield region. Until recently the Peninsular India was considered to be a stable land mass and a region of low seismicity. Due to the convergent movement (at 5 cm/year) of Indian plate with Eurasian plate, movements are occurring along major intraplate faults resulting in seismic activity of the region and hence the hazard assessment of this region is very important. Major earthquakes including Coimbatore (6.0 Mw in 1900) was felt over an area of about 250,000 km 2 in south India, Koyna of Mw 6.1 in 1967, also felt over a radius of 700 km, killing 177 people and 2,232 people were injured (Chandra 1977), Latur of Mw 6.1 in 1993 where over 10,000 lives were lost and several villages were destroyed (Jain et al. 1994) These earthquakes have changed the long-held image of low seismic activity of peninsular India and earth scientists became keenly interested in the investigation of seismicity and tectonics of 3

4 the region. Studies by Chandra (1977) points that some of these earthquakes were felt over a much larger area than one would expect earthquakes of equivalent magnitude to be felt in most other parts of the world. Iyengar et al. (1999) listed out major earthquakes that had occurred in Peninsular India during Medieval time. From the regional earthquake data of two decades ( ), obtained from Gauribidanur seismic array, Gangrade et al. (2000) also showed that seismicity of Indian Peninsular shield is higher than what was expected. Rao (2000) showed that the increase in intraplate deformation in the lithosphere of the Indian Peninsular shield and the strain rates for the Indian Shield as a whole is found to be the second highest in stable continental region (SCR) of the world (the highest being the North America). More over remote sensing studies by Ramasamy (2006) revealed that the southern part of the Indian Peninsula is tectonically active due to the northerly to north northeasterly directed compressive force related to post collision tectonics. All these studies point to the fact that Peninsular India, which is otherwise called stable continental region, should no longer consider as a region of low seismicity. As the Karnataka state forms a part of Peninsular India, its seismicity should be properly assessed through accurate seismic hazard analysis and effective mitigation steps need to be taken. 3. SEISMIC HAZARD ANALYSIS METHODOLOGY Seismic hazard at place refers to the peak ground acceleration (PGA) at that location produced by single earthquake or contribution from multiple earthquakes of different probabilities in magnitude and occurrence. Knowing the mass of the structure and the PGA value at that location, engineers and designers can estimate the inertial force on to that structure during an earthquake, which can be used for earthquake resistant designing of the buildings. Seismic hazards for a place can be estimated using two methodologies; they are Probabilistic Seismic Hazard Analysis (PSHA) and Deterministic Seismic Hazard Analysis (DSHA). ). DSHA approach is event oriented, considering only a few (or sometimes only one) earthquakes that are estimated to produce the most severe ground motion at a site. Knowing the maximum magnitude that can occur at a source and the shortest distance between that source and the site, the peak ground acceleration (PGA) at that site is estimated using frequency dependent attenuation relation. Designs based on ground motion estimated from DSHA will be on conservative side and hence suitable for critical structures like dams, nuclear power plants etc. On the other hand, PSHA can incorporate the effect of different 4

5 events, bigger as well as smaller, on the hazard value at the site. Methodically DSHA is straight forward and simple compared to PSHA where lot of uncertainties need to be addressed. PSHA can successfully yield the seismic hazard values for various return periods and helps designer to choose a particular hazard value corresponding to the structure s life span. For evaluating seismic hazard for the state of Karnataka, probabilistic seismic hazard analysis (PSHA) methodology was employed. 3.1 Steps in PSHA Major steps in probabilistic seismic hazard analysis are as follows. a. Preparation of earthquake catalogue for the study area b. Identification of seismic sources c. Preparation of seismotectonic atlas d. Evaluation of regional recurrence relation e. Performing hazard analysis and evaluation of seismic hazard a) Preparation of earthquake catalogue for the study area: In the present study the earthquake catalogue was prepared by extracting data from different sources like, Indian Meteorological Department (IMD), National Geophysical Research Institute (NGRI) Hyderabad, Gauribidanur Array- c/o BARC, Indira Gandhi Center for Atomic Research (IGCAR) Kalpakkam etc and International agencies like United States Geological Survey (USGS), International Seismological Centre (ISC) UK, Incorporated Research Institutions for Seismology (IRIS), Northern California Earthquake Data Center (NCEDC) etc. Earthquake events within 300 km radius from the political boundary of Karnataka state were collected along with epicentral coordinates, focal depth, magnitude, date, time, and year of occurrence. All events are converted into moment magnitude scale (Mw) using suitable relations proposed by Scordilis (2006) and Heaton et al. (1986). The intensity values were converted using the equation suggested by Reiter (1990). 5

6 These events are to be declustered so that the resulting data base is free from repetitive events, foreshocks, aftershocks. After declustring there are about 1678 seismic events in the study area and out of which 555 events are of magnitude 4 and above. All these events were georeferenced and superimposed in the Karnataka map and this map was used in the future studies (Fig.1) Fig.1 Earthquake events having magnitude 4 and above in the study area b) Identification of seismic sources: The tectonic features of the study area like faults and lineaments were mapped from Seismotectonic atlas (SEISAT, 2000) published by Geological Survey of India (GSI) and also using satellite data. Sheets in SEISAT representing the features of study area were scanned 6

7 separately with 300 dpi (dots per inch) resolutions to get high quality digital images and these were georeferenced using MapInfo Professional Version 6.0. After this, the tectonic features were carefully picked and superimposed on to a map of study area. Fig.2 Major faults and lineaments in the study area In addition to referring Seisomotectonic atlas, the major lineaments (with length more than 100 km) of Karnataka State, South India were mapped using satellite data (Indian Remote Sensing Satellite (IRS)-1D, Wide Field Sensor (WiFS) and Landsat Multi Spectral Scanner (MSS)/Thematic Mapper (TM) data) on 1:1 million scale. This map was superimposed on physical/road network map of Karnataka to eliminate any cultural lineament. The major lineaments, which have some correlation with the occurrence of earthquakes, were identified and shown in Fig. 2. All the lineaments / faults which were associated with earthquakes of magnitude four and above were identified as active seismic sources. About 163 faults in the study area ranging from 12 km to 531 km in length were identified and mapped 7

8 c) Preparation of seismotectonic atlas A map of the study area along with the earthquake events and identified linear seismic sources are shown in Fig. 3 Fig. 3: Seismotectonic map of south India 8

9 d) Evaluation of regional recurrence relation: The Seismic activity of a region is characterized by the Gutenberg Richter (1944) earthquake recurrence law. According to this law, Log10 N = a bm (5) Where N is the total number of earthquakes with magnitude M and above which will occur in a year (mean annual rate of exceedance) and a and b are the seismicity parameters of the region. Statistical method proposed by Stepp, J.C. (1972) were used to analyze the completeness of the catalogue. The value obtained for parameters a and b were and respectively. The obtained values of a and b are comparable with that of early studies by Anbazhagan et al., (2009) for Bangalore, Vipin et. al., (2009) for South India and Menon et. al., (2010) for Tamil Nadu 1.5 y = x R² = Log(Cum no / Year) y = x R² = Magnitude (Mw) Fig. 4: Frequency magnitude relationship for study area 9

10 e) Evaluation of ground motion: Probabilistic seismic hazard analysis (PSHA) was initially developed by Cornell (1968). PSHA also consider the uncertainties associated with time of occurrences of earthquakes and its location. It also provides a frame work where these uncertainties can be combined rationally to provide more complete picture of seismic hazard (Kramer 1996). In the probabilistic approach, effects of all the earthquakes expected to occur at different locations during a specified life period are considered along with associated uncertainties and randomness of earthquake occurrences and attenuation of seismic waves with distance. But PSHA only account for the uncertainties in the parameters of a particular seismic model and the uncertainties involved in different models may make the selection of a seismic hazard models difficult for this region. Thus the use of logic tree in PSHA (Fig.5) provides a convenient framework for explicit treatment of model uncertainty. Fig 5. Logic tree model employed for the analysis Two types of seismic sources with equal weightage linear sources and smoothed gridded areal sources (Frankel 1995) were considered in the analysis. The hypocentral distance was 10

11 calculated by considering a focal depth of 15 km. A MATLAB program was developed to evaluate the peak ground acceleration (PGA) at rock level for the entire state considering a grid size of 0.05 x 0.05 (5 km X 5 km). The attenuation relations proposed by Toro et al. (1997), Atkinson and Boore (2006) and Raghukanth and Iyengar (2007) were used in the analysis with same weightages as in DSHA. Response spectra at rock level for important Tier II cities and Bangalore in Karnataka were evaluated for 8 different periods of oscillations, and the results are presented in this report. 4. EFFECT OF LOCAL SITE The effect of local geology plays a significant role in changing the characteristic of seismic wave. When earthquake waves when propagates from one medium to other, its properties like frequency, amplitude, duration, etc. are affected, which is called Local site effect. For a vast area like Karnataka state, PGA values for the entire study area for different site classes was evaluated based on nonlinear site amplification technique from the PSHA. Table 1: Site classification as per NEHRP scheme. (BSSC, 2003) NHERP site class Description 30 V s A Hard rock > 1500 m/s > 1500 m/s B Firm and hard rock m/s C Dense soil, soft rock m/s D Stiff soil m/s E Soft clays, Liquefiable soil < 180 m/s By assuming soil bed rock of the whole region belonging to site classes as per NEHRP based average shear wave velocity for top 30m (Table 1), the ground motion at surface level for each site is obtained by multiplying suitable amplification factors (Table 2) suggested for peninsular India by Raghu Kanth and Iyengar (2007), to the bedrock motion. 11

12 Table 2: Values for regression coefficients the error term for various site classes (Raghu Kanth and Iyengar 2007) Site class a 1 a 2 ln δ s A B C D Figures 10 to 17 show the spatial variation of PGA values at ground level for various site classes for a return period of 475 years and 2500 years. 5. PROBABILISTIC EVALUATION OF SPT AND CPT VALUES REQUIRED TO PREVENT LIQUEFACTION Liquefaction potential of soil at a particular can be evaluated by laboratory and field testing. It is very difficult to get an undisturbed soil sample from the field, laboratory testing becomes uneconomical and unpractical in majority of the cases. Thus field testing is much easier way of evaluating liquefaction procedure and in India the most popular field testing is standard penetration test (SPT). The evaluation of liquefaction potential involves two stages (i) evaluation of earthquake loading based on cyclic stress ratio (CSR) and (ii) evaluation of soil strength against earthquake loading based on cyclic resistance ratio (CRR). Seed and Idriss (1971) suggested a simplified of evaluating earthquake loading as per equation 1. CSR = 0.65(a max /g)(σ vo /σ vo )(r d /MSF) (2) Where a max peak ground acceleration (surface level for site class D); σ vo and σ vo total effective over burden pressure; r d - depth reduction factor used to account for the flexibility of the soil and MSF - magnitude scaling factor (for quantifying the effect of earthquake having magnitude other than Mw = 7.5). A performance based approach suggested by Karmer and Mayfield (2007) where the contributions from all magnitudes and all acceleration levels are considered. CRR was estimated in terms of SPT (N) values. The SPT resistance 12

13 required to prevent liquefaction, Nreq, at a given location in the site is obtained using equation 3. (3) Where CSR eq,i is the cyclic stress ratio for an acceleration a i and this will be calculated for all the acceleration levels. θ 1 -θ 6 are the regression coefficient. The value of req N is the corrected N value (for energy, overburden pressure and percentage of fines) required to prevent liquefaction with an annual frequency of exceedance of λ N*req, where λ N*req is given by equation 4. (4) Δλ ai,mj - incremental annual frequency of exceedance for acceleration ai and magnitude mj. Similar methodology was adopted for evaluating the CPT values required to prevent liquefaction throughout the state of Karnataka. A probabilistic performance based relation can be derived from Eq. 5 for evaluating the CPT values required to prevent liquefaction for any return period. (5) Where λ q*c1-req annual rate at which q c1 will be higher than q * c1-req; q * c1-req - targeted values of corrected CPT values; q c1 - corrected CPT value required to prevent liquefaction; N M - number of magnitude increments; a N a - number of peak ground acceleration increments; Δλ ai,mj - incremental annual frequency of exceedance for acceleration ai and magnitude mj. Standard penetration and cone penetration test (SPT and CPT) values required to prevent liquefaction, were estimated for the entire state of Karnataka using was done using probabilistic approaches. Spatial variations of SPT and CPT values required to prevent liquefaction at 3m and 6m depth is presented here in figures 20 to

14 6. RESULTS The Peak horizontal acceleration were evaluated for the entire study area and the results are show in Figs. 6 Fig.6a Fig.6b Fig.6 (a, b): Spatial variation of PGA value for 10% &2% probability in 50 years Table.3: The exact location of major cities and corresponding PGA value Major cities Location Mean PGA value Longitude( o E) Latitude( o N) 10% 2% Bangalore Belgaum Bellary Gulbarga Hubli Kaiga Mangalore Mysore

15 Fig.7: Response spectrum for important cities in Karnataka (return period 475 years) Fig.8: Response spectrum for important cities in Karnataka (return period 2500 years) 15

16 1.00E E 01 Mean Annual Rate of Exceedance 1.00E E E E E E 07 Bangalore Mysore Belgaum Hubli Bellary Gulbarga Mangalore Kaiga 1.00E E E E E E E E E+00 PGA (g) Fig. 9 Hazard curve for various cities in Karnataka Fig. 10 Site class A (475 years) 16 Fig. 11 Site class B (475 years)

17 Fig. 12 Site class A (475 years) Fig. 13 Site class B (475 years) Fig. 14 Site class A (2500 years) Fig. 15 Site class B (2500 years) 17

18 Fig. 16 Site class A (2500 years) Fig. 17 Site class B (2500 years) Fig. 20 at 3m depth Fig. 21 at 6m depth SPT values required to prevent liquefaction for a return period of 2500 years 18

19 Fig. 22 at 3m depth Fig. 23 at 6m depth CPT values (MPa) required to prevent liquefaction for return period of 475 years Fig. 24 at 3m depth Fig. 25 at 8m depth CPT values (MPa) required to prevent liquefaction for a return period of 2500 years 19

20 7. CONCLUSIONS Major conclusions from the hazard studies are given below. Spatial variations of PGA values from probabilistic are shown in this report. From probabilistic analysis, places in Bidar district have hazard values ranging above 0.14g for a return period of 475 years and 0.3g above for a return period of 2500 years. Moreover from the site response studies, this region is expected to have a PGA value up to 0.25g for 475 year return period and 0.55g for 2500 year return period. These regions are very much close to Latur fault that has produced an earthquake of magnitude 6.1 in Places between Bangalore and Mysore also found to have significant value of PGA of above 0.1g and for critical case it can go up to 0.25g also. Hazard at ground level by considering results from different site classes, can have value 0.25g for 475 year return period and 0.4g for a return period of 2500 years. Analysis also shows for Mangalore Udupi regions, a hazard value up to 0.08g at rock level for return period of 475 year and can go up to 0.2g for 2500 year return period above. Hazard at the surface level is expected to have 0.1g to 0.15g for 475 year return period and 0.2g to 0.25g for 2500 year return period. Hazard analysis also points out interior regions in Karnataka having low hazard value of around 0.04g for both return periods. Kaiga, which is the location for nuclear power plant has the least value of hazard, compared other cities. Hence it s best suited for critical structures like nuclear structures. Response spectra of some important cities in Karnataka are also presented. From probability analysis a hazard curve is also presented for each city in Karnataka. Using this hazard curve, hazard value for any return period can be estimated. The PHA values obtained in the present study matches well with the values obtained by other researchers for different parts of the study area Liquefaction hazard analysis shows that Bangalore Mysore region and the Bidar region has the highest liquefaction. In these regions, SPT value of more than 10 for 475 year return period and 15 for 2500 year return period is required to prevent liquefaction at 3m 20

21 and 6m depth. CPT values, to prevent liquefaction in these areas are also high at 3m (9 to 13 MPa) and 6m depth (8 to 11MPa). Regions near Mangalore Udupi have moderate liquefaction hazard and required SPT value of more than 5 for 475 year return period to 10 for 2500 year return period is required to prevent liquefaction at 3m and 6m depth. The interior parts of Karnataka have the lowest liquefaction hazard, with required SPT value of less than 5 for 475 year return period and more than 5 for 2500 year return period is required to prevent liquefaction at 3m and 6m depth. 8. MAJOR ACIEVEMENTS Identify and map linear seismic sources in the state of Karnataka is done Earthquake events occurred in the state and surrounding area from different agencies were collected and compiled Preparation of seismotectonic atlas for Karnataka state showing all the linear seismic sources and the earthquake events was done Probabilistic seismic hazard analysis was carried out to evaluate peak horizontal acceleration at the bed rock level for the entire Karnataka state. Peak ground acceleration at surface level for different site classes by considering the site effects was done using nonlinear site amplification technique Probabilistic evaluation of SPT and CPT values required to prevent liquefaction for the entire state of Karnataka was done 21

22 9. REFERENCES 1. Anbazhagan P, Vinod, J S, and Sitharam T G (2009). Probabilistic seismic hazard analysis for Bangalore. Journal of Natural Hazards, 48, Atkinson, G.M. and Boore, D.M. (2006). Earthquake Ground-Motion prediction equations for Eastern North America. Bulletin of the Seismological Society of America, 96(6), BIS-1893 (2002). Indian Standard criteria for earthquake resistant design of structures, Part 1 - General provisions and buildings. Bureau of Indian Standards, New Delhi. 4. Chandra U (1977). Earthquakes of Peninsular India--A Seismotectonic Study. Bulletin of the Seismological Society of America, 67(5), Cornell CA (1968) Engineering seismic risk analysis; Bulletin of the Seismological Society of America, 58, Frankel A, 1995 Mapping seismic hazard in the Central Eastern United States; Seismological Research Letters, 66(4), Gangrade B K and Arora S K 2000 Seismicity of the Indian Peninsular Shield from regional earthquake data; Pure and Applied Geophysics, 157, Gutenberg B and Richter C F, 1944 Frequency of earthquakes in California; Bulletin of the Seismological Society of America, 34, Heaton T H, Tajima T F and Mori A W (1986). Estimating Ground motions using recorded accelerograms. Surveys in Geophysics, 8, Iyengar R N, Sharama D and Siddiqui J M (1999). Earthquake History of India in Medieval Times. Indian Journal of History of Science, 34(3), Jain S.K, Murty C V R, Chandak N, Seeber L and Jain N K (1994). The September 29, 1993, M6.4 Killari, Maharashtra Earthquake in Central India. EERI Special Earthquake Report, EERI Newsletter, 28(1). 12. Kramer S L (1996). Geotechnical Earthquake Engineering Pearson Education Pvt. Ltd, Reprinted 2003, Delhi, India. 13. Kramer, S.L., Mayfield, R.T. (2007). Return period of soil liquefaction. Journal of Geotechnical and Geoenvironmental Engineering, 133(7),

23 14. Menon A, Ornthammarath T, Corigliano M and Lai C G (2010). Probabilistic Seismic Hazard Macrozonation of Tamil Nadu in Southern India. Bulletin of the Seismological Society of America, 100(3), Raghu Kanth, S.T.G. and Iyengar, R.N. (2007). Estimation of seismic spectral acceleration in Peninsular India. Journal of Earth System Sciences, 116(3), Ramalingeswara Rao B (2000). Historical seismicity and deformation rates in the Indian peninsular shield. Journal of Seismology, 4, Ramasamy S M (2006). Remote sensing and active tectonics of South India. International Journal of Remote Sensing, Vol. 27, No. 20, Scordilis E M (2006). Empirical global relations converting Ms and mb to moment magnitude. Journal of Seismology, 10, Seed, H.B. and Idriss, I.M. (1971) Simplified procedure for evaluating soil liquefaction potential. Journal of Soil Mechanics and Foundation, 97, SEISAT (2000). Seismotectonic Atlas of India. Geological Survey of India, New Delhi. 21. Stepp J C (1972). Analysis of the completeness of the earthquake sample in the Puget Sound area and its effects on statistical estimates of earthquakes hazard. Proc. Inter. Conf. Microzonation for safer construct. Res. Appl., Seattle, Washington, 64(4), Toro, G.N., Abrahamson, N. and Schneider, J. (1997). Model of strong ground motions from earthquakes in central and eastern North America: Best estimates and uncertainties. Seismological Research Letters, 68, Vipin K.S (2010). Assessment of Seismic Hazard with Local Site Effects: Deterministic and Probabilistic Approaches PhD Thesis IISc Bangalore. 24. Vipin K. S, Anbazhagan P and Sitharam T. G (2009). Estimation of peak ground acceleration and spectral accelerationfor South India with local site effects: probabilistic approach. Natural Hazards Earth System Science., 9, ,

24 10. PUBLICATIONS OUT OF THIS WORK Journal Paper 1. Sitharam T.G., James N., Vipin K.S and Ganesha Raj K (2010). A Study On Seismicity And Seismic Hazards For The Karnataka State Journal of Earth System and Sciences (manuscript submitted on 20/11/2010) Conference Papers 1. Sitharam T.G., James N and Vipin K.S (2010). Seismic Hazard Map for the State of Karnataka with Local Site Effects: Probabilistic Seismic Hazard Analysis. 14 th Symposium on Earthquake Engineering 2010 at IIT Roorkee 2. Sitharam T.G., James N and Vipin K.S (2010). Seismic Hazard Map for the State of Karnataka with Local Site Effects: Deterministic Seismic Hazard Analysis. Indian Geotechnical Conference IIT Bombay 3. James N, Kolathayar S, Vipin K.S and Sitharam T.G (2010). Seismic Hazard Scenario of Karnataka State: An Input for Planning of Urban Centers. Conference on Infrastructure, Sustainable Transportation And Urban Planning CiSTUP, IISc, Bangalore. 16 March Signature of the Principal Investigator/Investigator. 24