Seismic hazard analysis and microzonation of Coimbatore Corporation

Transcription

1 Indian Journal of Geo Marine Sciences Vol.46 (11), November 2017, pp Seismic hazard analysis and microzonation of Coimbatore Corporation K E Viswanathan 1 & K Elangovan 2 1 Department of Civil Engineering, Builders Engineering College, Tirupur , Tamilnadu, India 2 Department of Civil Engineering, PSG College of Technology, Coimbatore , Tamilnadu, India [ 1 keviswanathan@gmail.com] Received 29 March 2016 ; revised 20 June 2016 Seismic microzonation is carried out based on the geological and geophysical characteristics of the site. The seismic data of two of the major earthquakes that occurred in these regions in the past, with the Magnitude of the Coimbatore earthquake being 5.4 and the Magnitude of the Walayar earthquake being 6. The Peak Ground Acceleration (PGA) was calculated and maximum values have been used in the PGA map 1. The regional maps on lineaments, bed rock depth and geology have been reclassed and overlaid on the PGA map. [Keywords: Hazard analysis; Microzonation; Seismic; Peak Ground Acceleration (PGA); Lineament] Introduction The most important issue affecting the applicability and the feasibility of any microzonation study is the reliability of the parameters for doing it. It is very clear based on the earthquake damage and strong motion records that there are numerous sources and site factors i.e., near field effects, directivity, duration, focusing, topographical and basin effects etc., which are important in assessing ground motion characteristics. The national seismic zoning maps are prepared in small scales like 1:1,000,000 or less neglecting all the above factors and they do not consider the geological and geotechnical site conditions. Seismic microzonation involves a very detailed field investigation to evaluate the hazard. They are also useful to evaluate the risk scenarios in the study area. This has been the most widely used method to map earthquake hazard at local scales which may incorporate a wide variety of information, including seismic response of different surface geological formations, liquefaction potential, topographic amplification of seismic waves, landslides, tsunamis etc. Seismic microzonation maps are very useful in urban planning because they help to predict the impact of future earthquakes and can also be used to locate key facilities like hospitals, fire stations, emergency operation centers etc. Microzonation studies are also very useful to save the heritage and important structures from future major earthquakes. The National Disaster Management Authority of India has suggested that microzonation study for cities with population more than 1 million or more may be carried out 2. However, since the population in India is very high, the microzonation need to be carried out for the cities based on population and macro hazard maps available in the country. For example microzonation can be taken up for the urban centres with a population of 500,000 to 1,000,000 and which falls in seismic zone five. While selecting the regions for microzonation, the population of the region, population density, its

2 2208 INDIAN J. MAR. SCI., VOL. 46, NO. 11, NOVEMBER 2017 socio-economic characteristics, gross domestic product (GDP) of the region and the strategic importance etc., also to be considered. Seismic microzonation of urban centers requires map of city with a scale of 1:5,000 to 1:25,000. Materials and Methods The Coimbatore Corporation map, illustrated figure 1, has been chosen for this seismic hazard analysis and microzonation study. Coimbatore is one of the fastest growing cities in India and a major textile, industrial, commercial, educational, information technology, healthcare and manufacturing hub of Tamil Nadu. The city has an area of 240 km 2 with a population of 12,51,237 as of 2011 census. Coimbatore falls under Class III Seismic Zone as per IS 1893 (BIS 2002) and has experienced an earthquake of Richter magnitude 6 in the past. This earthquake was reported at N, E on 8th of February Peak Ground acceleration (PGA) A seismicity study is of great importance to understand the dynamic behavior of the earth and is useful to determine the earthquake hazard in a specific region. In order to understand the seismicity of Coimbatore and its vicinity, data regarding spatial locations of earthquakes and their magnitudes have been collected for a period of 200 years ( ) from various sources and have been used for the present study. The three major earthquakes around Coimbatore region listed in Table 1 have been used for PGA calculation 4,5 Fig. 2 - Methodology adopted for weighted overlay map Fig.1 - Coimbatore corporation map Table 1 - Major Earthquakes around Coimbatore Methodology The peak ground acceleration and bed rock depth maps have been re-classed. Vector conversion was carried out on the geology and lineament maps. These maps are further reclassed and overlaid over the other maps. Finally all the maps were overlaid using an appropriate weighted overlay option 3. The sequence adopted is depicted in figure 2. Date 28/02/1890 8/2/ /7/1972 Latitude Longitude Location Ooty Walayar Coimbatore Magnitude Depth (Km)

3 VISWANATHAN & ELANGOVAN : SEISMIC HAZARD ANALYSIS AND MICROZONATION 2209 Coimbatore base map was created and overlaid in Google earth imagery. The locations of earthquake were marked in the map using the latitude and longitude data. The epicentral distances were calculated and by knowing the depth of the earthquake, the hypocentral distance was also determined for various locations around Coimbatore. Attenuation relationship developed by Iyengar and Raghukanth (2004) 6,7 for Peninsular India applicable to Coimbatore is used for the present study. The attenuation relation used to calculate PGA is given 1 : ln(y) = c 1 +c 2 M 6 + c 3 (M 6) 2 ln R c 4 R + ln( ) -- (1) Where refers to PGA in g, refers to magnitude and refers to hypocentral distance. Since PGA is known to be attributed nearly as a log normal random variable would normally ln(y) distributed with the average of ln( ) being almost zero. Hence, coefficients for the southern region are shown in table 2. Table 2 - Coefficients for the Southern Region Coefficients Value c Lineaments Fig.3 - Peak Ground Acceleration Map Lineaments are narrow ruptures in the underlying geologic structure 8. The earthquake can cause relatively more damage along these lineaments. The regional lineaments map exposed in figure 4. The lineaments were subsequently buffered about 500 meters on either side, assuming the earthquake damage intensity reduces with the distance from the lineaments (figure 5). c c c (taken as zero) The determined PGA for the 3 identified potential seismic Sources for the city of Coimbatore are in the range of m/s 2 to m/s 2 (Table 3). The PGA calculated for Coimbatore earthquake is maximum, so the same PGA values have been used for generating PGA map (figure 3). Fig. 4 - Regional Lineament Map

4 2210 INDIAN J. MAR. SCI., VOL. 46, NO. 11, NOVEMBER 2017 Table 3 - PGA for Coimbatore Earthquake and Walayar Earthquake of Magnitude 5.4 and 6.0. Location Distance (km) Hypocentral Distance (km) PGA (m/s 2 ) (Figure 1) I II I II I II

5 VISWANATHAN & ELANGOVAN : SEISMIC HAZARD ANALYSIS AND MICROZONATION 2211 Geology The geology of Coimbatore (figure 7) mainly consists of hornblende gneiss covering major portions of the Coimbatore region. Fluvial covers the western regions of Coimbatore and garnet and limestone are concentrated in small regions in south of Coimbatore. Bed Rock Depth Fig.5 - Buffered Lineament Map Geology is an important factor in seismic hazard analysis 9, 10 &11. Harder the rock the seismic wave velocity will be more and thus this reduces earthquake intensity in areas having hard rocks. Whereas seismic wave velocity will be low in areas having fluvial deposits, so the earthquake hazard is more in those regions. Borehole data are the only reliable data to estimate the depth to bedrock, which is an important key parameter in assessing the soil column thickness over it. The bedrock configuration of a site will give an idea about the basement topography. The basement topography will help in the study of frequencies and amplitudes of ground motions. This will help to identify high (as well as low) seismic risk areas from the point-of-view of wave propagation influence (figure 6). The basement is somewhat shallow in the southeast side and mid regions of Coimbatore from 0 to 6m. The basement is relatively high in the northwest part of the city from 23to 35m. The remaining part of city has moderate bed rock depth from 12 to 23m. Fig.7 - Geology Map Weightage Calculation The factors discussed in the above, each contain alternatives. The weightage have to be given to both the factors and alternatives. To serve this purpose Thomas L Saaty 12 suggested a multi criteria analysis method called analytical hierarchical process. Saaty gave an importance scale shown in table 5. Fig.6 - Bed Rock Depth Map

6 2212 INDIAN J. MAR. SCI., VOL. 46, NO. 11, NOVEMBER 2017 Table 5 - Saaty 'S Importance Scale Intensity of Definition Importance 1 Equal Importance 3 Moderate Importance 5 Strong Importance 7 Very Strong Importance 9 Extreme Importance 2,4,6,8 For Compromises Between The Above Reciprocals In Comparing Elements i And j of Above - If i Is 3 Compared To j - Then j Is 1/3 Compared To i Comparison Matrix In the matrix the diagonal is always kept as unit. If the judgment value of C1 is more than C2, followed by C1 is given importance value as per Saaty importance scale 12. When comparing C1 and C2, if they are of equal importance, than it is given as unit value. If C1 is less important than C2, after that reciprocal of the important value is given. An example is exposed in the table 6. Table 6 - Comparison Matrix Criteria C1 C2 C3 C4 C5 C /2 2 C2 1/3 1 1/3 1/2 5 C3 1/ C /2 1 2 C5 1/2 1/5 1/2 1/2 1 Totally there are five criteria, therefore the matrix formed is 5x5 matrices. The upper triangular matrix is filled by using the Saaty s scale of priority Thomas L. Saaty (2008) 12. Comparing C1 and C2, C1 has moderate important to C2, thus 3 is given in first row, second column in the matrix. Comparing C1 and C4, C1 is slightly less important than C4, thus reciprocal of the actual judgment value 2 (i.e. 1/2) is given in fourth row, fourth column in the matrix. Similarly the matrix is filled. The lower triangular matrix is filled by the reciprocal of the upper triangular matrix. The normalized principle eigen vector is obtained by summing up column values and dividing each column values by its column total. Then each row values are added. Now the total row sum gives unit. The eigen vector is obtained for the matrix. The largest eigen value of the eigen vector is the principle eigen value, λmax. The below value is weightages for the five criteria s in the table 7. Table 7 - Weightage Table C1 C2 C3 C4 C Consistency Check To check the consistence of the judgment the consistence ratio is calculated. If the Consistency Ratio (CR) <0.10, the decision makers pair-wise comparisons are relatively consistent. If the Consistency Ratio (CR) > 0.10, the decision makers should seriously consider re-evaluating the pairwise comparisons Thomas L. Saaty (2008) 12.To check the consistency of the criteria s the following calculations are done. Table 8 - Random Index Table N RI Consistency Index (CI) = λmax N / N -1 = ( ) / (5 1) = Random index is obtained from the table 8 for N = 5, Random Index (RI) =1.12 Consistency Ratio (CR) = CI / RI = / 1.12 = < 0.1 (Therefore the judgment is consistent.) Weightage Calculation for the Factors The above method has been used for calculation of weightage for the factors to be used in this study. Consistency check has also been done and shown in the following tables 9, 10, 11 and 12. Type Table 9 - Weightage Calculation for Geology Fluvial Limeston e Garnet Hornblende Fluvial Limestone 1/ Hornblende 1/5 1/3 1/2 1 Weightage Consistency ratio < 0.1

7 VISWANATHAN & ELANGOVAN : SEISMIC HAZARD ANALYSIS AND MICROZONATION 2213 Table 10 - Weightage Calculation for Peak Ground Acceleration m/s 2 (PGA) PGA weightage Consistency ratio < 0.1 Table 11 - Weightage Calculation for Buffered Lineaments in meters Meters weightage Consistency ratio < 0.1 Table 12 - Weightage Calculation for Bed Rock Depth in Meters Meters weightage Consistency ratio < 0.1

8 2214 INDIAN J. MAR. SCI., VOL. 46, NO. 11, NOVEMBER 2017 Results and Discussion The final hazard maps are illustrated in the zones viz., Very high, High, Moderate Low and Very Low. The final seismic hazard map of Coimbatore Corporation is exposed in figure 8. Fig. 8 - Seismic Hazard map of Coimbatore Corporation Conclusion The hazard map has been drawn as per the methodology discussed. Based on the map central part of Coimbatore is more prone particularly in Singanallur, Peelamedu, R.S.Puram, Town hall and western regions. These areas are mostly covered in commercial buildings. Most of the regions in Coimbatore are comes under moderate seismic hazard. The outer fringes of Coimbatore are classified under low seismic hazard. The areas are Vellakinar, Saravanampatti, Vilankurichi, Kalapatti and Vadavelli. This map could be used for city planning and selecting site for infrastructure development purpose. Seismic hazard analysis and microzonation map is the first step in mitigation of earthquake, it is recommended to microzonation a city with a population of one million or more 13. Acknowledgements Authors are thankful to Principal and HoD, Builders Engineering College and PSG College of Technology, Tamilnadu, for providing facilities to carrying out the above research work. References 1. Ganapathy, G., First level seismic microzonation map of Chennai city a GIS approach, Natural Hazards and Earth System Sciences, 11(2011), Technical report on Geotechnical / Geophysical Investigations for Seismic Microzonation Studies of Urban Centers in India by National Disaster Management Authority Government of India (August 2011). 3. Elangovan, K.. GIS: fundamentals, applications and implementations: New India Publishing (2006). 4. Anbazhagan, P., Gajawada, P., & Smitha, C., Rupture based seismic hazard analysis for future seismic zonation-a case study of coimbatore, Proceeding of indian geotechnical conference (2011), Sitharam, T., & Anbazhagan, P., Report on seismic microzonation of Bangalore Urban Centre, Seismology Division, Main Volume, Ministry of Earth Sciences, Government of India, Printed in Bangalore, India, 174p (2009). 6. Raghukanth, I. a., Estimation of seismic hazard in peninsular India, Proceedings of structural engineering convention, proceedings of structural engineering convention (2005), Elangovan K, Gopalakrishnan. S. a., Generation of Geologic Database for Seismic Microzonation of Coimbatore Corporation, India, International Journal of Earth Sciences and Engineering, 3(2010), Wong, I., Thomas, P., Bendimerad, F., Silva, W., & Gregor, N.. Seismic microzonation and development of seismic design ground motions for the Stanford University campus, California. Paper presented at the Proceedings of the 4th IASPEI/IAEE international symposium: effects of surface geology on seismic motion. University of California: Santa Barbara (2011). 9. Pelekis, P., & Athanasopoulos, G., Seismic microzonation of Chania, Crete (Greece) based on SASW measurements and non-linear site response analyses, Soil Dynamics and Earthquake Engineering, 53(2013), Papadimitriou, A. G., Antoniou, A. A., Bouckovalas, G. D., & Marinos, P. G., Methodology for automated GIS-aided seismic microzonation studies, Computers and Geotechnics, 35(2008), Cadet, H., Macau, A., Benjumea, B., Bellmunt, F., & Figueras, S., From ambient noise recordings to site effect assessment: The case study of Barcelona microzonation, Soil Dynamics and Earthquake Engineering, 31(2011), Saaty, T. L., Decision making with the analytic hierarchy process, International journal of services sciences, 1(2008), India, G. S. o., Dasgupta, S., Narula, P., Acharyya, S., & Banerjee, J.. Seismotectonic atlas of India and its environs: Geological Survey of India (2000).