215 Estimation of Peak Ground Acceleration for Delhi Region using Finsim, a Finite Fault Simulation Technique NEELIMA SATYAM. D* and K. S. RAO** * Earthquake Engineering Research Centre, International Institute of Information and Technology, Gachibowli, Hyderabad-500032, India **Department of Civil Engineering, Indian Institute of Technology, New Delhi110016, India Email: neelima.satyam@gmail.com, raoks@civil.iitd.ernet.in Abstract: The Delhi is situated in highly earthquake prone belt (IS: 1893-2002) near the tectonically active Himalayan region occupying an area of 1485 sq.km. An important problem in earthquake engineering is the prediction of ground motions from the future large earthquakes. Seismological input that describes the amplitude, frequency content and duration of the expected motions are required to assess the seismic performance of the structures. Several empirical relationships are available in the literature for the estimation of these ground motions. In this study, the stochastic finite fault simulation technique which implements the concept of fault discretization wherein the sub events are represented as stochastic point sources is used. The detailed description of the method is given by Beresnev and Atkinson (1997, 1998). Based on the historical seismic source data, using the FINSIM program the Peak Ground Acceleration (at bedrock level) map of the Delhi region is prepared. This map can be used further as input for microzonation of ground motion at the surface by incorporating the local soil conditions in the Delhi region. Keywords: Peak ground acceleration, FINSIM, Finite fault simulation, Stochastic method. Introduction: The damage resulting from earthquakes may be influenced in a number of ways by the characteristics of soils in the affected area. Earthquake engineers are interested in strong ground motion i.e., the motion of sufficient strength to affect people and their environment. It is important to describe the characteristics of the ground motion that are of engineering significance and to identify the ground motion parameters that reflect those characteristics. The three important characteristics of earthquake ground motions are amplitude, frequency and duration of the motion. There are various other ground motion parameters, which provide information about these ground motion characteristics. The most commonly used amplitude parameters are peak acceleration, peak velocity and peak displacement. Peak acceleration is a very useful parameter to characterize amplitude. This peak acceleration can be peak horizontal acceleration (PHA) or peak vertical acceleration (PVA). The peak horizontal acceleration for a given component of motion is simply the largest value of horizontal acceleration obtained from the accelerogram of that component. The maximum resultant PHA which is commonly used to describe ground motions can be obtained by taking the vector sum of two orthogonal components. The variation of intensity of ground motions with distance from the source i.e., attenuation relationships has been studied for many years. Plots of attenuation relationships of peak acceleration and velocity as a function of distance have been presented by numerous researchers in the seismology and earthquake engineering. But no such attenuation curve for Delhi region is available so far. A discrete finite fault model that captures the salient features of radiation from large earthquakes has been a popular seismological tool for the last two decades. Hartzell (1978) introduced this first as a finite fault plane which is subdivided into smaller elements. The contributions from all the independent sub faults are summed up #02020306 Copyright 2009 CAFET-INNOVA TECHNICAL SOCIETY. All rights reserved.
216 Estimation of Peak Ground Acceleration for Delhi Region using Finsim, a Finite Fault Simulation Technique in estimating the ground motions parameters from a large earthquake. Generally, rupture starts at hypocenter and propagates radially triggering the sub faults as it passes them. The fields from all sub events are geometrically delayed and added together at the observation point. Beresnev and Atkinson (1997) extended the original method and developed a finite fault simulation model code FINSIM. The applicability of the program to predict ground motion parameters has been tested by numerous authors in various countries. In this study an attempt is made to prepare the peak ground acceleration (PGA) map by considering all seismological sources available around Delhi. Seismicity of Delhi region: According to seismic zonation map of India, Delhi is classified in the category of moderate to high earthquake prone zone (IV), with intensity of VIII on modified Mercalli scale. The first scientifically recorded earthquake from this region was on 15th july 1720 with intensity IX. Other major earthquakes have been reported subsequently in the years 1803(IX), 1825(V), 1830 (V), 1831(VII), 1842 (VI). In the recent past earthquakes of magnitude upto 6.2 have been reported in Delhi and near by region. They are Bulaneshahr earthquake (1956) of magnitude 6.2, earthquake near Sohna (1960) of magnitude 6.2 and Moradabad earthquake (1966) of magnitude 5.6. Figure 1 shows the detailed lineamental map with faults and fractures. Similarly as mentioned earlier, Delhi has far field sources (~200 km) in the Himalayas, hich can produce greater magnitude earthquakes. Figure 2 shows the epicenters ranging in magnitude from 2 to >6 with 10 and 100 years of probability. The distribution of earthquake epicenters in Delhi region indicates that the clusters of seismic events are located to the west of Delhi and particularly between Sonepat- Rohtak- Gurgaon. These earthquakes are shallow focus events. The map also indicates that the maximum concentration of epicenters occurring along NNE-SSW direction and around the intersection of margin of Delhi- Lahore ridge and Mahendragarh-Dehradun fault. Several attenuation laws are available in the literature for assessing the seismic ground motion parameters. Greater acceleration can be observed on the stations located on alluvium than compared to the stations located on hard rock for event of same magnitude. Figure 1. Lineamental map of Delhi with the location of the sources considered Figure 2 Epicenters In and Around Delhi (Rao and Mohanty, 2001)
NEELIMA SATYAM. D and K. S. RAO 217 Method adopted: The computer code FINSIM, a finite fault simulation technique (Beresnev and Atkinson, 1998) is used in this study to generate the PGA map at bedrock for five different near field sources in Delhi. As stated earlier, the method implements the concept of discretization of the major fault plane into minor faults and estimation of the earthquake response by summing the contributions from all the sub elements. In general, the rupture starts at the hypocentral point on the fault plane and triggers all sub faults by propagating in radial direction. The information on the orientation and dimension of the fault plane, location of the hypocenter and the dimensions of the sub faults are the major inputs for its analysis. The discretization of the fault plane can be done using the empirical equation developed by Beresnev and Atkinson (1998) as:log l = 0.4 M 2 (1)where l is the sub fault dimension and M is the earthquake magnitude. The depth to the top of the fault plane surface is assumed a value of 1 km in this study. The other important input is the stress parameter σ. This stress parameter is known with large uncertainties, a constant and average value of 50 bars is recommended by the authors. Lower values are possible but higher values should not generally be used. In this study also a value of 50 bars is used. The authors of this program cleared that a fixed value of this parameter primarily affects the number of elementary sources that need to be summed in order to conserve the seismic moment of the target event and not the radiation amplitudes. The material properties like shear wave velocity and density are taken a value of 3.6 km/sec and 2.8 g/cc. Table 1 gives the input earthquake parameters of all the five sources considered for estimating peak ground acceleration at bedrock. The first three are simulated earthquakes on the particular faults and the last two are actual ones recorded in the year 2001. In the implementation of the stochastic method, the attenuation effects of the propagation path are modeled through the empirical Q and geometric attenuation models. The ground motions are strongly influenced by the value of Q factor, strength of the subfault radiation and σ. For the geometric attenuation, the geometric spreading operator of 1/Rb, where b=1.0 for R less than 50 km, b=0 for 50 R<150 km, and b=0.5 for R greater than or equal to150 km is assumed. The frequency dependent Q is interpreted as a tectonic parameter and regions of high seismic activity are characterized by less value compared to stable regions. The relationship for this factor will be in the form of Q(f) = Q0 fn, generally Q0 is Q(f) at a frequency of 1 Hz and n, power of frequency dependence, represents the level of tectonic activity of the region. Higher n value shows that the regions with high seismic activity and vice versa. The frequency dependent quality factor Q estimated by Singh et al. (2004) for the Indian shield is used in this analysis as given in the Eqn. 2. This equation is based on a larger data and was modeled for a wide frequency range of 1 to 20 Hz. Based on the Jabalpur data, Singh et al. (2003) proposed the Q0 and n values as 508 and 0.48. The number of iterations made is 6 in the present study. Table 2 shows the values of the input parameters used in the FINSIM program. Q (f) = 800 f 0.42 (1 Hz < f < 20 Hz) (2) The program is executed for all the five selected potential earthquake sources in which are located with in Delhi region. Table 3 gives the values of the estimated PGA value at bedrock and the corresponding closest distance from all the five sources. Figures 3 (a) to 3 (e) show the twodimensional contour maps of the peak ground acceleration at bedrock level generated for all the sources considered.
218 Estimation of Peak Ground Acceleration for Delhi Region using Finsim, a Finite Fault Simulation Technique Table1. Earthquake Parameters of the Five Different Sources SIMULATED EARTHQUAKE ACTUAL EARTHQUAKE Earthquake Parameters Moment Magnitude, M W Mathura Fault Moradabad Fault Sohna Fault 28 th Feb 2001 Earthquake 28 th April 2001 Earhquake (near Sonepat) 6.8 5.8 6.7 4.2 3.8 Latitude 27.5 28.67 28.15 28.55 28.61 Longitude 77.7 78.93 77.67 76.19 77.04 Strike( 0 ) 77 49 51 48 84 Dip( 0 ) 48 84 76 71 54 Table.2 Input Parameters Used in the Analysis Parameter Value Depth of the upper edge of the fault (km) 1 Subfault dimensions (km) 10 4 Stress parameter σ (bars) 50 Radiation strength factor 1 Q (f) 800 f 0.42 Geometric spreading 1/R b b =1 for R<50 km b=0 for 50 R<150 km b=0.5 for R 150 km Crustal shear wave velocity (km/s) 3.6 Crustal density (g/cm 3 ) 2.8 Rupture velocity (km/sec) 0.8 (shear wave velocity) No of iterations 6
NEELIMA SATYAM. D and K. S. RAO 219 Figure 3. Peak Ground Acceleration map at bedrock for five different sources (a: Mathura fault, b: Moradabad Fault, c: Sohna fault, d: 28th feb 2001 earthquake, e: 28th April 2001 earthquake)
220 Estimation of Peak Ground Acceleration for Delhi Region using Finsim, a Finite Fault Simulation Technique Table 3 Closest Distance and the Corresponding Peak Ground Acceleration Values for the Five near Field Sources Considered
NEELIMA SATYAM. D and K. S. RAO 221
222 Estimation of Peak Ground Acceleration for Delhi Region using Finsim, a Finite Fault Simulation Technique Results and discussions: Since Delhi is seismically very active, estimation of ground motion parameters is atmost important. Using the computer program FINSIM, considering five seismic sources in Delhi peak ground accelerations are calculated. The contour maps generated are shown in Fig 3. From the results it is clear that the PGA generated by Mathura fault and Sohna fault are almost equal. It is high in the southeastern side and decreased towards north west. The location of these two sources is almost close. The values of the peak ground accelerations generated by Moradabad fault are comparatively high among the first three sources. This fault produced maximum PGA of 0.115g. The 28th February 2001 earthquake generated maximum of 0.012g in the eastern side of Delhi where as, the 28th April 2001 earthquake produced a quiet high value of 0.44g. Based on this analysis five different PGA maps at bedrock level are generated. That is five PGA values are determined corresponding to five sources at all locations. These maps can be used further as input for microzonation of ground motion at the surface by incorporating the soil properties. The peak ground acceleration at bedrock level is ranging from 0.055 to 0.115 g. The first three sources are simulated earthquakes on Mathura fault, Morababad fault and Sohna fault and the next two are the actual earthquakes recorded in 2001. The value of peak ground acceleration depends not only on the epicentral distance but also on site conditions, which is an important input parameter in seismic design of structures. Thus this stochastic method has become a popular tool in estimating ground motion parameters like PGA, especially for the regions like Delhi with insufficient instrumental data. Khattri (1999) has estimated 170gals of amax on each of the three components using synthesized accelerograms at nine critical sites including Delhi from a postulated earthquake of MW = 8.5 in the western part of the central seismic gap. Singh et al. (2003) estimated the probable ground motions in Delhi region from possible future large/great earthquakes. It is observed that the maximum expected acceleration in the horizontal and vertical components ranges from 50-60g and 25-40g respectively. Parvez et al. (2004) estimated the peak acceleration of 1.6g at an epicentral distance of 10km by considering 15th July 1720 earthquake. The epicenter and magnitude of this event are taken from Global Seismic Hazard Assessment Program (GSHAP) catalogue. The peak ground acceleration at bedrock is generated (DST Report, 2004) by considering six sources i.e., Himalayan source, Delhi Haridwar ridge, Moradabad Fault, Rajasthan great boundary fault, Mathura fault and Sohna fault. According to
NEELIMA SATYAM. D and K. S. RAO 223 this, the maximum PGA value considering all the six sources is 0.34g generated by Mathura fault. The PGA values for Delhi region at bedrock are ranging from 0.15g to 0.27g where as in our study it is ranging from 0.055g to 0.115g. References: [1] Beresnev, I. and G. Atkinson (1997). Modeling finite fault radiation from the ωn spectrum Bull. Seism. Soc. Am., 87, 67-84. [10] Singh S. K., Pacheco J. F., Bansal B. K., X.erez-Campos, Dattatrayam R. S., and Suresh, G. (2004) A Source Study of the Bhuj, India, Earthquake of 26 January 2001 (M7.6), Bull. Seism. Soc. Am., 94, 1195-1206. [2] Beresnev, I. and G. Atkinson (1998). FINSIM - a FORTRAN program for simulating stochastic acceleration time histories from finite faults. Seism. Res. L., 69, 27-32. [3] DST Report, 2004 DST Report. (2004) Geoscientific Studies In and Around Delhi, p 74. [4] Hartzell, S. (1978). Earthquake aftershocks as Green s functions, Geophys.Res. Lett. 5, 1 14. [5] IS: 1893-2002, Criteria for Earthquake Resistant Design of Structures. [6] Khattri K N (1999) An Evaluation of Earthquakes Hazard and Risk in Northern India Himalayan Geology, 201-46 [7] Parvez I.A., Vaccari F and Panza G.F., (2004) Site specific microzonation study in Delhi Metropolitan City by 2D Modelling of SH and P-SV Waves J of Pure appl. Geophys,116, 1165-1184 [8] Rao K.S. and Mohanty W.K. (2001) Microzonation of Delhi Region: An Approach J. Indian Building Congress, VIII, 102-114 [9] Singh S. K., Bansal B. K., Bhattacharya S. N., Pacheco J. F., Dattatrayam R. S., Ordaz M., Suresh G., Kamal and Hough S. E. (2003) Estimation of Ground Motion for Bhuj (26 January, 2001; Mw = 7.6) and for Future Earthquakes in India Bull. Seism. Soc. Am., 93(1), 353-370