Frequency dependent attenuation of seismic waves for Delhi and surrounding area, India ANNALS OF GEOPHYSICS, 58, 2, 2015, S0216; doi:10.4401/ag-6636 S0216 Frequency dependent attenuation of seismic waves for Delhi and surrounding area, India Babita Sharma*, Prasantha Chingtham, Anup K. Sutar, Sumer Chopra, Haldhar P. Shukla Center for Seismology, Ministry of Earth Sciences, New Delhi, India ABSTRACT The attenuation properties of Delhi and surrounding region have been investigated using 62 local earthquakes recorded at nine stations. The fre- quency dependent quality factors Qa (using P-waves) and Qb (using S- waves) have been determined using the coda normalization method. Quality factor of coda-waves (Qc ) has been estimated using the single backscattering model in the frequency range from 1.5 Hz to 9 Hz. Wen- nerberg formulation has been used to estimate Qi (intrinsic attenuation parameter) and Qs (scattering attenuation parameter) for the region. The values Qa , Qb , Qc , Qi and Qs estimated are frequency dependent in the range of 1.5Hz-9Hz. Frequency dependent relations are estimated as Qa = 52f 1.03, Qb = 98f 1.07 and Qc = 158f 0.97. Qc estimates lie in between the values of Qi and Qs but closer to Qi at all central frequencies. Com- parison between Qi and Qs shows that intrinsic absorption is predomi- nant over scattering for Delhi and surrounding region. 1. Introduction The attenuation of seismic waves provides impor- tant information about the medium characteristics which is required for the determination of earthquake source parameters as well as for prediction of earth- quake ground motions. Attenuation of seismic waves is controlled by geometrical spreading, scattering due to inhomogeneities in the medium and damping. The at- tenuating property of a medium is described by the di- mensionless quantity called quality factor Q, which expresses the decay of wave amplitude during its prop- agation in the medium [Knopoff 1964]. Various studies have been done worldwide to understand the attenua- tion characteristics by estimating Q using P-waves (Qa), S-waves (Qb) and coda waves (Qc). Aki and Chouet [1975] gave the single backscattering model to estimate Qc. The coda normalization method to estimate Qb, was developed by Aki [1980] and later on Yoshimoto et al. [1993] extended coda normalization method to esti- mate Qb also. The estimates of Q have been found to be frequency dependent by several researchers world- wide [e.g., Aki and Chouet 1975, Rautian and Khalturin 1978, Aki 1980, Sato and Matsumura 1980, Roecker et al. 1982, Hough and Anderson 1988, Masuda 1988, Woodgold 1990, Campillo and Plantet 1991, Sekiguchi 1991, Takemura et al. 1991, Mayeda et al. 1992, Yoshi- moto et al. 1993, Gupta et al. 1995, Gupta et al. 1998, Mandal et al. 2001, Sharma et al. 2007, Sharma et al. 2008, Sharma et al. 2009, Sharma et al. 2011]. Various methods have been developed to measure the relative contribution of intrinsic attenuation Qi and scattering attenuation Qs to the total attenuation. Wu [1985] proposed a method for an estimation of the rel- ative contribution of Qs and Qi from the dependence of total S-wave energy on hypocentral distance. Frankel and Wennerberg [1987] used the energy flux model of seismic coda to obtain the separate estimates of Qs and Qi based on coda amplitude and decay. Hoshiba et al. [1991] developed a method based on Monte Carlo sim- ulations of the temporal shape of the coda envelope. Wennerberg [1993] provided the formulation to deter- mine the contribution of Qs and Qi attenuation to the total attenuation. The objective of the present study is to understand the attenuation mechanism of medium beneath Delhi and surrounding region, India by estimating Q using different parts of the seismograms and to estimate the relative contribution of Qi and Qs in the region. The ex- tended coda normalization method [Yoshimoto et al. 1993] has been used to estimate the frequency-depen- dent relations for Qa and Qb. Single backscattering model of Aki and Chouet [1975] has been used to esti- mate Qc. Wennerberg’s [1993] formulation has been used to estimate the relative contribution of Qi and Qs. 2. Seismotectonics and geology of the area The Delhi and surrounding area had active seismic history [Tandon 1975, Verma et al. 1995, Mohanty 1997, Article history Received July 7, 2014; accepted February 6, 2015. Subject classification: Coda, Intrinsic, Scattering, Attenuation, Frequency. Bansal et al. 2008, Singh et al. 2010, Bansal and Verma 2012, Prakash and Srivastava 2012]. The first reported earthquake with intensity IX occurred in the Delhi re- gion on July 15, 1720 [Tandon 1975, Chandra 1992]. The estimated intensity of these earthquakes on the Modified Mercalli Scale was found to be between VII and IX at Delhi and its surrounding region, as indicated by the damage pattern. The earthquake of August 27, 1960, was another significant earthquake of magnitude 6.0 with its epicenter between Delhi and Gurgaon. The seismicity of Delhi and surrounding region show max- imum concentration of epicenters in north-south trending Sonepat-Sohna fault, west of Delhi and at the tri-junction of Delhi-Haridwar ridge, Delhi-Lahore ridge and the axis of Delhi folding. It has been indicated that there are numerous hidden faults in the thick allu- vial deposits of the Indo-Gangetic plains. According to the seismotectonic studies of the region, Haridwar- Delhi ridge, Sohna fault, Aravalli Fault and Moradabad fault are the prominent tectonic features in Delhi and the surrounding areas [Mohanty 1997, Bansal and Verma 2012, Prakash and Srivastava 2012]. The entire Delhi and its surrounding area exhibit moderate seis- micity and fall under seismic zone IV of the Seismic zonation map of India [Singh et al. 2010]. It is impor- tant to consider seismic factors for urban planning, in- dustrialization, designing and construction of civil engineering structures. The rock formations exposed in the Delhi area are mainly quartzite of the Alwar series of the Delhi Su- pergroup which are 1500 million years in age and over- lain by unconsolidated Quaternary to recent sediments which are 1.65 Ma old. The terrain is generally flat ex- cept for a low NNE-SSW trending Delhi Ridge in the southern and central part of the area which consists of Quartzite while the Quaternary sediments, comprising the older and newer alluvium cover the rest of the area. The older alluvium comprises silt, clay with minor lenticular fine sand and kankar beds [Choudhary et al. 1984]. The newer alluvium mainly consists of sand, silt and clay occurring in the older and active flood plains of the Yamuna River. Thickness of the alluvium, both on the eastern and western side of the ridge, is variable but west of the ridge, it is generally thicker nearly 300m [Mohanty et al. 2009, Bansal and Verma 2012]. Figure 1 shows the area of present study along with the seismo- logical stations, earthquake locations and major tec- tonic features of the area. SHARMA ET AL. 2 Figure 1. Epicentral locations of earthquake events and seismological stations in and around Delhi area. S. No. Station with station code Latitude (Deg min) Longitude (Deg min) 1 Ayanagar (AYAN) 28°28.93 77°07.60 2 Sohana (SONA) 28°14.70 77°03.78 3 Bhadurgarh (BHGR) 28°41.26 76°56.33 4 Rataul (RTL) 28°49.93 77°20.51 5 Bisrakh (BIS) 28°34.26 77°26.34 6 Kuldal (KUDL) 28°08.65 76°29.35 7 Asauara(ASR) 28°45.35 77°46.33 8 Unchagaon (UCG) 28°18.60 77°54.60 9 New Delhi (NDI) 28°35.41 77°12.13 Table 1. Epicentral locations, origin time, date and depths of the events use in the present study. 3 3. Methodology In the present study single backscattering model of Aki and Chouet [1975] is used to estimate Qc. Ex- tended coda normalization method [Yoshimoto et al. 1993] is used to calculate Qa and Qb and Wenner- berg formulation [Wennerberg 1993] is applied to es- timate Qi and Qs. These methods are described as below. Single backscattering model The Qc has been estimated using the single backscattering model proposed by Aki and Chouet [1975]. According to this model, the coda waves are interpreted as backscattered body-waves generated by numerous heterogeneities present in the Earth’s crust and upper mantle. It implies that scattering is a weak process and outgoing waves are scattered only once before reaching the receiver. Under this assumption, the coda amplitudes, Ac( f,t), in a seismogram can be expressed for a central frequency f over a narrow bandwidth signal, as a function of the lapse time t, measured from the origin time of the seismic event, as [Aki 1980]: (1) where S( f ) represents the source function at frequency f, and is considered a constant as it is independent of time and radiation pattern, and therefore, not a func- tion of factors influencing energy loss in the medium; a is the geometrical spreading factor, and taken as 1 for body waves. The swapping of geometrical spread- ing factor and Q could give rise to unreasonable values of these parameters. To minimize the risk to get un- reasonable values an inversion method based on a par- abolic expression of the coda-normalization equation has been developed by de Lorenzo et al. [2013]. For various Indian regions the attenuation properties are estimated by several researchers [Gupta et al. 1995, Sharma et al. 2007, Sharma et al. 2008, Mohanty et al. 2009, etc.] and in all the related studies carried out for various parts of Indian subcontinent the geometrical spreading factor is considered to be unity. Qc is the ap- parent quality factor of coda waves representing the attenuation in a medium. The above equation can be rewritten as (2) It is a linear equation with the slope from which Qc is estimated. Extended coda normalization method This method is based on the idea that coda waves consist of scattered S waves from random hetero- geneities in the Earth [Aki 1969, Aki and Chouet 1975, Sato 1977]. The spectral amplitude, Ac( f,tc ), of the coda at a lapse time tc can be written as [Aki 1980]: (3) where f is the frequency, Ss( f ) is the source spectral am- plitude of S waves, P( f,tc ) is the coda excitation factor, G( f ) is the site amplification factor and I( f ) is the in- strumental response. The spectral amplitude of the direct S wave, As( f,r) can be expressed as (4) where Riz is the source radiation pattern and a denotes the geometrical exponent which is taken unit value as explained in the previous section. Qb( f ) is the quality factor of S waves, Vs is the average S wave velocity and } is the incident angle of S waves. On dividing Equation (4) by Equation (3), taking the logarithm and simplifying, we get [Yoshimoto et al. 1993]: (5) Using a similar equation the quality factor for the P-waves can be obtained [Yoshimoto et al. 1993]. (6) The quality factor for P waves can be obtained from the linear regression of versus r by means of least-squares method as done for S-waves. Wennerberg formulation to estimate Qi and Qs Wennerberg [1993] provided the formulation based on Zeng et al. [1991] model to estimate Qi and Qs. According to Zeng et al. [1991], we can write the observed value of Qc in terms of Qi and Qs as below: (7) where, d(x) is , ~ is the angular fre- quency and t is the lapse time. Assuming Qd as the qual- ity factor of direct wave evaluated in the Earth volume Ac f, tcQ V = Ss fQ VP f, tcQ VG fQ VI fQ V As f, rQ V = Riz Ss fQ Vr -a exp - Q b fQ VVs fr U ZG f, }Q VI fQ V ln Ac f, tcQ V As f, rQ Vr U Z r ! r =- Q b fQ VVs fr + const fQ V ln Ac f, tcQ V Ap f, rQ Vr U Z r ! r =- Q a fQ VVp fr + const fQ V Q c 1 = Q i 1 + Q s 1 - 2d xQ VQ V Ac f, tcQ V Ap f, rQ Vr U Z r ! r - 4.44 + 0.738Q V 1 , x = Q s ~t , ln Ac f, tQ VQ V = ln S fQ VQ V - Q c fQ V t - Q c fQ V Ac f, tQ V = S fQ Vt-a exp Q c - ftQ V ATTENUATION FOR DELHI, INDIA equivalent to the volume sampled by coda waves, it can be written as [Wennerberg 1993]: (9) If Qc is measured as a function of lapse time t, Qi and Qs can be estimated using Equations (7), (8) and (9), where Qd is measured as a function of distance. Data analysis Earthquake data of 62 events with Ml 2.0-4.9 recorded by the digital seismic telemetric network op- erated by India Meteorological department in and around Delhi. The data is recorded using short period sensors at 20 samples per second (sps), which limits the Nyquist frequency 10 Hz. Figure 1 shows the epicen- tral locations of events and stations considered. Loca- tions of the stations along with the station codes are given in Table 1. The vertical component of each seis- mogram have been filtered at five different frequency bands (1-2) Hz, (2-4) Hz, (4-6) Hz, (6-8) Hz and (8-10) Hz using a Butterworth band pass filter. On the filtered seismograms, the root-mean-square amplitudes of coda waves amplitude measurement starts at twice the travel time of the S-waves in a window length of 256 samples and lapse time window length of 30 seconds have been used to estimate Qc. Figure 2 shows one original and filtered seismogram recorded at SONA station on June 7, 2006, and Figure 3 shows the variation of ln(Ac( f,t).t) with lapse time t along with the least-squares-fitted line for different central frequencies at SONA for the same event. Figure 4 represents the plot of Qc values as a function of frequency obtained at 30 sec lapse time window. Data used in the present study is analysed vi- sually for the signal to noise ratio. The seismograms having signal to noise ratio less than two is not consid- ered in the present study. Table 2 shows the events se- lected for the present study. Q s 1 = 2d xQ V 1 Q d 1 - Q c xQ V 1 T Y Q i 1 = 2d xQ V 1 Q c xQ V 1 + Q d 2d xQ V - 1Q V U Z SHARMA ET AL. 4 Figure 2. A sample of original and filtered seismogram at different frequency bands for SONA station recorded on June 7, 2006. Figure 3. The variation of ln(Ac( f,t).t) with lapse time t along with the least-squares-fitted line for different central frequencies at SONA for the event of June 7, 2006. Figure 4. The plot of Qc values as a function of frequency obtained at 30 sec lapse time window. (8) 5 ATTENUATION FOR DELHI, INDIA S. No. Year Month Day Hr Min Sec Latitude (°N) Longitude (°E) Magnitude (ml) Depth (km) 1 2001 1 28 15 22 42.4 28.74 76.61 2.4 5 2 2001 4 28 13 28 29.3 28.62 76.55 2.3 4 3 2001 8 10 12 19 32.8 28.91 77.24 3.2 13 4 2001 8 12 21 9 30.2 28.96 77.69 2.7 26 5 2001 8 23 18 23 20.3 29.08 76.97 2.4 9 6 2001 9 14 14 31 29.9 29.22 77.23 2.1 15 7 2001 10 9 23 18 15.5 29.17 77.37 2.2 10 8 2002 10 22 21 43 12.9 28.68 77.11 2.7 15 9 2002 10 23 20 52 31.7 28.91 76.81 2.2 15 10 2002 10 30 5 10 10 28.77 77.06 2.1 23 11 2002 10 31 0 59 23.2 28.8 76.95 2.1 22 12 2002 11 6 2 12 0.5 28.97 76.84 2.9 10 13 2003 2 15 6 37 6.1 29.3 77.21 2.6 5 14 2004 5 3 16 55 5.7 29.25 76.46 3 5 15 2004 5 15 3 12 53.9 29.19 76.39 2.3 13 16 2004 5 22 14 45 55 29.14 76.46 2.6 18 17 2004 5 3 16 55 5.7 29.25 76.46 3 5 18 2004 5 15 3 12 53.9 29.19 76.39 2.3 13 19 2004 5 22 14 45 55 29.14 76.46 2.6 18 20 2004 8 22 22 47 53.6 28.44 77.38 2.1 12 21 2004 10 9 11 34 41.5 29.23 76.43 2.8 5 22 2004 12 18 15 39 16.9 29 76.62 2.7 4 23 2004 12 20 1 11 57.5 28.92 77.11 2.6 28 24 2004 12 23 19 21 1.7 28.56 76.69 2.5 6 25 2005 6 10 23 53 20.6 29.32 77.02 2.6 9 26 2005 6 13 17 31 48.6 28.3 76.19 2.2 5 27 2006 1 19 8 12 38.3 28.82 76.57 2.4 5 28 2006 3 15 5 5 54.5 28.91 76.7 2.3 17 29 2006 4 7 18 56 40.3 28.93 76.92 3.3 5 30 2006 4 10 14 11 58.7 29.12 76.64 3 5 31 2006 4 11 23 21 8.5 29.29 76.9 2.4 40 32 2006 4 11 23 26 23.7 28.83 76.81 2.7 10 33 2006 5 7 16 1 0.2 28.72 76.6 4.2 4 34 2006 5 11 7 0 10.4 28.67 76.66 2.7 15 35 2006 5 12 1 30 55.2 28.89 76.65 2.3 10 36 2006 7 9 2 30 34.1 28.91 76.74 2.5 25 37 2006 10 31 12 59 3.1 28.75 76.67 2.1 5 38 2006 12 9 18 52 4.3 28.95 76.64 2.8 10 Table 2 (continues on next page). Coordinates of the stations along with their station codes in and around Delhi region. In order to estimate Qa and Qb, the rms amplitudes of P- and S-waves have been taken from the filtered seis- mograms and normalized by the coda wave amplitude. Figure 5 shows a plot of ln((As/Ac)r) with respect to hypocentral distance r (km) and corresponding plots of S-waves at five different central frequencies for NDI. The slopes of the best-fitted lines are used to estimate Qa and Qb by using Equations (5) and (6). The average velocities of 7.02 km/sec and 4.06 km/sec for P and S waves respectively, have been used in the present study [IMD 2000]. Afterwards, Qs and Qi are estimated ac- cording to Equations (8) and (9) with the help of val- ues of Qc and Qb calculated using single backscattering and coda normalization methods assuming the Qb as quality factor of direct wave. 4. Results and discussions In the present study the attenuation properties of the crust for Delhi, India have been estimated. For this purpose, 62 local earthquakes recorded by 9 stations in Delhi and surrounding area have been used. The esti- mated mean values of Qa, Qb, Qc, Qi and Qs along with the standard deviation error at different central fre- quencies for the region considered in the present study are given in Table 3. The average value of Qc varies from 274 at 1.5 Hz to 1656 at 9 Hz. The average values of Qa and Qb vary from 77 and 156 at 1.5 Hz. to 538 and 969 at 9 Hz, respectively. The increase in Q values with increasing frequency indicates the frequency-depen- dent nature of the Q estimates in the region. In order to obtain the frequency-dependent relations, the esti- mated average Qa, Qb and Qc values as a function of fre- quency are plotted in Figure 6. The frequency-dependent relationships estimated for the region along with the standard deviation are: Qa = (52±4) f (1.03±.07), Qb = (98±7) f (1.07±.09) and Qc = (158±9) f (0.97±.08). The small lateral variation found in the estimated Q values may be attributed to the heterogeneities present in the re- SHARMA ET AL. 6 39 2006 12 23 5 9 32.1 28.31 76.05 2.4 10 40 2006 12 28 13 40 46.8 28.98 76.67 2 10 41 2006 12 30 18 31 52.4 28.3 76.09 2.5 5 42 2007 1 8 12 1 0.6 28.81 76.76 2.5 10 43 2007 1 23 1 48 26.1 28.33 76.82 2.3 20 44 2007 1 29 20 26 0.4 28.9 76.6 2.2 10 45 2007 2 27 20 37 58.5 29.24 77.26 2.5 10 46 2007 3 6 3 14 4.4 28.93 76.62 2.2 9 47 2007 3 12 5 51 26.9 28.6 76.96 2.2 13 48 2007 4 3 15 35 11.2 28.94 76.62 2.8 10 49 2007 5 14 7 22 47.9 28.92 76.64 3.2 5 50 2007 5 14 10 51 22 28.81 76.77 2.2 33 51 2007 5 16 12 56 7.9 28.78 76.64 2.3 18 52 2008 5 27 21 51 59.4 29.8 77.07 2.0 15 53 2008 8 29 3 43 27.1 28.8 77.34 2.3 15 54 2008 10 19 7 56 48.4 29 76.76 3.9 6 55 2008 10 28 20 29 48.1 28.8 76.52 4.9 10 56 2009 01 07 14 53 29.5 29.57 77.8 3.9 5 57 2009 02 01 18 32 9.6 29.5 77.88 2.8 10 58 2009 03 10 12 22 26.7 29.5 77.9 3.6 10 59 2009 03 14 2 22 54.7 29.27 77.2 4.2 10 60 2009 05 30 8 00 00 29.2 77.65 2.3 7 61 2009 06 17 12 07 31.7 29.53 77.82 4.2 11 62 2011 09 07 17 58 16.3 28.6 77.05 4.3 10 S. No. Year Month Day Hr Min Sec Latitude (°N) Longitude (°E) Magnitude (ml) Depth (km) Table 2 (continued from previous page). Coordinates of the stations along with their station codes in and around Delhi region. 7 gion and difference in distances of the events from the recording stations. The estimate of Qc is found to be higher than Qb in this region. The effect of intrinsic and scattering attenuation combine in a manner that Qc is more than Qb as shown in Figure 6. This supports the Zeng et al.’s [1991] model which predicts the idea of coda enrichment over Qb. According to Wennerberg [1993] formulation, Qc is separated in terms of scatter- ing and intrinsic attenuation in the present study. The estimated Qi values vary from 472 at 1.5 Hz to 2525 at 9 Hz. The estimated Qi values vary from 232 at 1.5 Hz to 1937 at 9 Hz. It has been reported in literature and using laboratory measurements that coda-Q is very close to Qi [Frankel and Wennerberg 1987, Matsunami 1991]. However, Mayeda et al. [1992] have found that this observation is valid at higher frequencies while Qc is intermediate between Qi and Qs. It has been observed ATTENUATION FOR DELHI, INDIA C.F. Qa Qb Qc Qi Qs 1.5 77±14 156±11 274±25 232±25 472±22 3 185±17 363±19 730±79 591±33 941±23 5 277±12 583±21 931±94 816±18 2040±25 7 382±19 764±16 1265±216 1096±16 2520±32 9 538±26 969±12 1656±91 1937±31 2525±46 Table 3. Values of different quality factors estimated at different central frequencies. Figure 5. Plot of ln((As/Ac)r) with respect to hypocentral distance r (km) and corresponding plots of S-waves at five different central fre- quencies for NDI station. Figure 6. Plot of estimated average Qa, Qb and Qc values as a func- tion of frequency. from the present study that Qc values lie in between Qi and Qs at all frequencies (Table 3). A comparison be- tween estimates of Qi and Qs in this study shows that in- trinsic absorption is predominant over scattering for the frequency range (1.5 Hz - 9 Hz) considered here. It has been found that the value of Q0 (Qc at 1 Hz) varies from 47 to 200 and that of n varies from 0.70 to 1.10 for the active regions including Parkfield [Hellweg et al. 1995], Friuli, Italy [Rovelli 1982]. Singh et al. [2004] have esti- mated a relation Q( f ) = 800f 0.42 for the Indian shield region using the dataset of four earthquakes recorded in the distance range of 240-2400 km. Using the ac- celerograms of the aftershocks of 2001 Bhuj earth- quake, Bodin and Horton [2004] have obtained a relation Q( f ) = 790f 0.35 for the Kachchh basin. The coda-based method used in this study gives Q of a very shallow portion of the crust, while Q estimates ob- tained by Singh et al. [2004] and Bodin and Horton [2004] sample deeper in the crust. Mohanty et al. [2009] have estimated coda wave attenuation for Delhi using local earthquakes and obtained frequency dependent relationship as Qc = 142f 1.04. The frequency dependent relationship obtained using coda waves in the present study as: (158±9) f (0.97±.0), is comparable to that of Mo- hanty et al. [2009]. The study region of Mohanty et al. [2009] is same but they have computed only Qc for the region and we have extended the attenuation study by separating the total attenuation parameter in terms of intrinsic (Qi) and scattering parameters (Qs). For this purpose we have also estimated Qb, Qa and Qb/Qa, which represent the attenuation of seismic waves for Delhi and surrounding region in a better way. Figure 7 shows the comparison of present estimates of Qc with some attenuation studies of India and worldwide, which in turn shows a similar trend for Delhi Capital area as other tectonic regions. This shows that the at- tenuation characteristics of seismic waves in the Delhi region are similar to the seismically active regions of the world. In Figure 7, if we compare the Delhi Qc with Indian regions, it is clear that Qc values lie close to Koyna region at lower frequencies and match with Qc values of Kachchh region at higher frequencies. This may lead to the conclusion that crust of Delhi and surrounding areas is less attenuative as compared to Kachchh and Koyna regions. Using the aftershocks of 2001 Bhuj earthquake and Multiple Lapse Time Window Analysis, Hoshiba et al. [1991], Fehler et al. [1992] and Ugalde et al. [2006] have shown that intrinsic absorption is predominant over scat- tering for all frequencies except for 1-2 Hz in Kachchh region. Similarly for Delhi and surrounding region we found that intrinsic absorption is dominating over scat- SHARMA ET AL. 8 Figure 7. Comparison of present estimates of Qc with some attenuation studies in India and worldwide. 9 tering. Figure 8 shows the comparison of the ratio Qb/Qa estimated here at different frequencies with those of other tectonic regions worldwide. We note that Qb/Qa ≥ 1 obtained in the present study for the fre- quencies considered here is comparable with other re- gions of the world. Mandal [2006] estimated the Qb vs. Qa relation for the Kachchh rift zone using the Sp con- verted phases on the accelerograms. He estimated that the ratio Qb/Qa lies in between 0.41 to 2.99 in Kachchh region. Also Padhy [2009] estimated Qb/Qa ≥ 1 for Bhuj region. To interpret the results, we compared our results with the laboratory measurements of Qb and Qa. Vassil- iou et al. [1982] have given general observations of Qb and Qa relations in sedimentary rocks. Qb = Qa for dry rocks, Qb ≥Qa for partially saturated rocks and Qb ≤Qa for fully saturated rocks. In our case, we obtained Qb > Qa, which shows that the region is comprised of partially satu- rated rocks or crustal pore fluids [de Lorenzo et al. 2013]. According to Figure 8, we analyze that Delhi and surrounding area are comprised of partially saturated sediments. Also, it is seen in Figure 8 that at lower fre- quencies Qb/Qa valves are lower than Bhuj region, but at frequencies between 5 and 6 Hz, Qb/Qa are nearly equal to that of Bhuj region of India and after that there is a decrease in Qb/Qa ratio. The bump at frequency 5-6 Hz in this figure may correspond to the sediments present in the subsurface of the Delhi region. It is known that if the Qc values are lower than 200 then it depicts the seismically active region [Aki and Chouet 1975]. Our results for Delhi region show low Q (Qc is 158), which corresponds to high attenuation and is compa- rable with other seismically active regions of India and world. The area has a considerable thick layer of par- tially saturated sediments demonstrated by Qb/Qa > 1, due to which most part of the energy gets dissipated in the medium. 5. Conclusions The present study is an attempt to understand the attenuation properties of Delhi, India and surrounding region, India. For this purpose quality factors Qa, Qb, Qc, Qi and Qs are estimated. The analysis shows their dependence on the frequency in the range from 1.5 Hz to 9 Hz in the region. Power law relationships for the region along with the standard deviation are obtained as: Qa = (52±4) f (1.03±.07), Qb = (98±7) f (1.07±.09) and Qc = (158±9) f (0.97±.08). The attenuation characteristics of coda waves in the Delhi region are close to other simi- lar and tectonically active regions of the world. The es- timates of Qc are found to be higher than Qb in the studied region. This observation shows that the effects of intrinsic and scattering attenuation combine in such a manner that Qc is more than Qb. The Qc is separated in terms of scattering and intrinsic attenuation param- eter Qi and Qs. The Qc estimates lie in between the es- timates of Qi and Qs but are closer to Qi at all frequencies. This is in agreement with the theoretical as well as lab- oratory observations/measurements. A comparison between Qi and Qs shows that intrinsic absorption is predominant over scattering in Delhi and surrounding region. Qb/Qa ≥ 1 obtained for the frequency range of 1.5 Hz to 9 Hz shows that the area of the present study is mainly comprised of partially saturated sediments. The results of present study indicate high attenuation which also corroborates well with the regional geology of Delhi and surrounding areas. Acknowledgements. Authors are thankful to the Secretary, Ministry of Earth Sciences, Government of India for support and encouragement to do this study. Authors are grateful to Director General of Meteorology, Indian Meteorological Department (IMD), New Delhi for providing earthquake waveform data. The authors also acknowledge continued guidance and support of Head, Center for Seismology, Ministry of Earth Sciences. References Aki, K. (1969). Analysis of seismic coda of local earth- quakes as scattered waves, J. Geophys. Res., 74, 615- 631. Aki, K., and B. Chouet (1975). Origin of the Coda waves: Source attenuation and Scattering effects, J. Geophy. Res., 80, 3322-3342. Aki, K. (1980). Attenuation of shear waves in the litho- sphere for frequencies from 0.05 to 25 Hz, Phys. Earth Planet. In., 21, 50-60. Bansal, B.K., S.K. Singh, R. Dharmaraju, J.F. Pacheco, M. Ordaz, R.S. Dattatrayam and G. Suresh (2008). ATTENUATION FOR DELHI, INDIA Figure 8. Comparison of the ratio Qb/Qa estimated at different fre- quencies with those of other tectonic regions of India and worldwide. Source study of two small earthquakes of Delhi, India, and estimation of ground motion from future moderate, local events, J. Seismol.; doi:10.1007/s10 950-008-9118-y. Bansal, B.K., and M. Verma (2012). The M 4.9 Delhi earthquake of 5 March 2012, Curr. Sci. India, 102, 1704-1708. Bodin, P., and S. Horton (2004). Source parameters and tectonic implications of aftershocks of the Mw 7.6 Bhuj earthquake of January 26, 2001, B. Seismol. Soc. Am., 94, 818-827. Campillo, M., and J.L. Planet (1991). Frequency de- pendence and spatial distribution of seismic attenu- ation in France: Experimental results and possible interpretations, Phys. Earth Planet. In., 67, 48-64. Castro, R.R., C.J. Rebollar, L. Inzunza, L. Orozco, J. Sanchez, O. Galvez, F.J. Farfan and I. Mendez (1997). Direct body-wave Q estimates in northern Baja Cal- ifornia, Mexico, Phys. Earth Planet. In., 103, 33-38. Castro, R.R., C. Condori, O. Romero, C. Jacques and M. Suter (2008). Seismic attenuation in northeastern Sonora, Mexico, B. Seismol. Soc. Am., 98, 722-732. Chandra, U. (1992). Seismotectonics of the Himalaya, Curr. Sci. India, 62, 40-71. Choudhary, A.K., K. Gopalan and C.A. Sastry (1984). Present status of geochronology of Precambrian rocks of Rajasthan, Tectonophysics, 105, 131-140. Chung, T.W., and H. Sato (2001). Attenuation of high- frequency P and S waves in the crust of southeastern South Korea, B. Seismol. Soc. Am., 91, 1867-1874. de Lorenzo, S., F. Bianco and E. Del Pezzo (2013). Fre- quency dependent Qa and Qb in the Umbria-Marche (Italy) region using a quadratic approximation of the coda-normalization method, Geophys. J. Int., 193 (3), 1726-1731. Fehler, M., M. Hoshiba, H. Sato and K. Obara (1992). Separation of scattering and intrinsic attenuation for the Kanto-Tokai region, Japan using measurements of S-wave energy vs hypocentral distance, Geophys. J. Int., 108, 787-800. Frankel, A. (1982). The effects of attenuation and site response on the spectra of micro-earthquakes in the northeastern Caribbean, B. Seismol. Soc. Am., 72, 1379-1402. Frankel, A., and L. Wennerberg (1987). Energy-flux Model of Seismic Coda: Separation of Scattering and Intrinsic Attenuation, B. Seismol. Soc. Am., 77, 1223-1251. Gupta, S.C., V.N. Singh and A. Kumar (1995). Attenua- tion of coda waves in the Garhwal Himalaya, India, Phys. Earth Planet. In., 87, 247-253. Gupta, S.C., S.S. Teotia, S.S. Rai and N. Gautam (1998). Coda Q estimates in the Koyna region, India, Pure Appl. Geophys., 153,713-731. Gupta, S.C., and A. Kumar (2002). Seismic wave atten- uation characteristics of three Indian regions: A comparative study, Curr. Sci. India, 82, 407-413. Hellweg, M., P. Spandich, J.B. Fletcher and L.M. Baker (1995). Stability of coda Q in the region of Parkfield, California: view from the U.S. Geological survey Parkfield dense seismograph array, J. Geophys. Res., 100, 2089-2102. Hoshiba, M., H. Sato and M. Fehler (1991). Numerical basis of the separation of scattering and intrinsic ab- sorption from full seismogram envelope - a Monte- Carlo simulation of multiple isotropic scattering, Pap. Meteorol. Geophys., 42, 65-91. Hough, S.E., and J.G. Anderson (1988). High frequency spectra observed at Anza, California: Implications for Q structure, B. Seismol. Soc. Am., 78, 672-691. Ibanez, J.M., E. Del Pezzo, F. De Miguel, M. Herraiz, G. Alguaicil and J. Morales (1990). Depth dependent seismic attenuation in the Granda zone (southern Spain), B. Seismol. Soc. Am., 80, 1232-1244. IMD (2000). A report on the Chamoli Earthquake of March 29, 1999 and its aftershocks, Seismology, 2/2000, 70. Kanao, M., and K. Ito (1991). Attenuation of S-waves and coda waves in the inner zone of southwestern Japan, Disaster Prev. Res. Inst., Kyoto Univ., Bull. 41 2, 356, 87-107. Kim, K.D., T.W. Chung and J.B. Kyung (2004). Attenu- ation of high frequency P and S waves in the crust of Choongchung provinces, central South Korea, B. Seismol. Soc. Am., 94, 1070-1078. Knopoff, L. (1964). Q, Reviews in Geophysics, 2, 625- 660. Kvamme, L.B., and J. Havskov (1989). Q in southern Norway, B. Seismol. Soc. Am., 79, 1575-1588. Mandal P., S. Padhy, B.K. Rastogi, H.V.S. Satyanarayana, M. Kousalaya, R. Vijayraghavan and A. Srinivasan (2001). Aftershock activity and frequency-dependent low coda Qc in the epicentral region of the 1999 Chamoli Earthquake of magnitude Mw 6.4., Pure Appl. Geophys., 158, 1719-1735. Mandal, P. (2006). Sedimentary and crustal structure be- neath Kachchh and Saurashtra regions, India, Phys. Earth Planet. In., 155, 286-299. Masuda, T. (1988). Corner frequencies and Q values of P waves by simultaneous inversion technique, Sci. Rep. To Univ. Ser.5, Geophy, 31, 101-125. Matsunami, K. (1991). Laboratory tests of excitation and attenuation of coda waves using 2-D models of scat- tering media, Phys. Earth Planet. In., 67, 36-47. Mayeda K., S. Koyangi, M. Hoshiba, K. Aki and Y. Zeng (1992). A comparative study of scattering, intrinsic SHARMA ET AL. 10 11 and coda Q for Hawaii, Long Valley and Central Cal- ifornia between 1.5 and 15 Hz, J. Geophys. Res., 97, 6643-6659. Mohanty, W.K. (1997). Seismicity and related studies for Delhi and the surrounding region, Unpublished Thesis submitted to Delhi University, 139 pp. Mohanty, W.K., R. Prakash, G. Suresh, A.K. Shukla, M.Y. Walling and J.P. Srivastava (2009). Estimation of Coda Wave Attenuation for the National Capital Region, Delhi, India Using Local Earthquakes, Pure Appl. Geophys.; doi:10.1007/s00024-009-0448-7. Padhy, S. (2009). Characteristics of Body-Wave Attenu- ations in the Bhuj Crust, B. Seismol. Soc. Am., 99, 3300-3313. Prakash, R., and J.P. Shrivastava (2012). A review of the seismicity and seismotectonics of Delhi and adjoin- ing areas, J. Geol. Soc. India, 79, 603-617. Pulli, J.J. (1984). Attenuation in New England, B. Seis- mol. Soc. Am., 74, 1149-1166. Rautian, T.G., and V.I. Khalturin (1978). The use of the coda for the determination of the earthquake source spectrum, B. Seismol. Soc. Am., 68, 923-948. Rocker, S.W., B. Tucker, J. King and D. Hatzfeld (1982). Estimates of Q in central Asia as a function of fre- quency and depth using the coda of locally recorded earthquakes, B. Seismol. Soc. Am., 72, 129-149. Rovelli, A. (1982). On the frequency dependence of Q in Friuli from short period digital records, B. Seismol. Soc. Am., 72, 2369-2372. Sato, H. (1977). Energy propagation including scatter- ing effects, single isotropic scattering approxima- tion, J. Phys. Earth, 25, 27-41. Sato, H., and S. Matsumura (1980). Q-1 value for S-waves under the Kanto district in Japan, Zisin, 33, 541-543. Sekiguchi, S. (1991). Three dimensional Q structure be- neath Kanto-Tokai district, Japan, Tectonophysics, 195, 83-104. Sharma, B., S.S. Teotia and K. Dinesh (2007). Attenua- tion of P, S and coda waves in Koyna region, India, J. Seismol., 11, 327-344. Sharma, B., A.K. Gupta, D.K. Devi, K. Dinesh, S.S. Teo- tia and B.K. Rastogi (2008). Attenuation of High-Fre- quency Seismic Waves in Kachchh Region, Gujarat, India, B. Seismol. Soc. Am., 98, 2325-2340. Sharma, B., S.S., Teotia and K. Dinesh (2009). Attenua- tion of P- and S-waves in the Chamoli region, Hi- malaya, India, Pure Appl. Geophys.; doi:10.1007/s00 024-009-0527-9. Sharma, B., K. Dinesh, S.S. Teotia, B.K. Rastogi, A.K. Gupta and S. Prajapati (2011). Attenuation of Coda Waves in the Saurashtra Region, Gujarat (India), Pure Appl. Geophys.; doi:10.1007/s00024-011-0295-1. Singh, S.K., D. Garcia, J.F. Pachew, R. Valenzuela, B.K. Bansal and R.S. Dattatrayam (2004). Q of Indian shield, B. Seismol. Soc. Am., 94, 1564-1570. Singh, S.K., A. Kumar, G. Suresh, M. Ordaz, J.F. Pacheco, M.L. Sharma, B.K. Bansal, R.S. Dattatrayam and E. Reinoso (2010). Delhi Earthquake of 25 November 2007 (Mw 4.1): Implications for seismic Hazard, Curr. Sci. India, 99, 939-947. Takemura, M., K. Kato, T. Ikeura and E. Shima (1991). Site amplification of S waves from strong motion records in special relation surface geology, J. Phys. Earth, 39, 573-552. Tandon, A.N. (1975). Some typical earthquakes of north and western UP, Bulletin of Indian Society of Earthquake Technology, 12 (2). Ugalde, A., J.N. Tripathi, M. Hoshiba and B.K. Rastogi (2006). Intrinsic and scattering attenuation in western India from aftershocks of the 26 January, Kachchh earthquake, Tectonophysics, 429, 111-123. Vassiliou, M., C.A. Salvado and B.R. Tittman (1982). Seismic attenuation, In: R.S. Carmichael (ed.), CRC Handbook of Physical Properties of Rocks, CRC Press, 547-581; ISBN 0-8493-3703-8. Verma, R.K., G.S. Roonwal, V.P. Kamble, W.K Mohanty, U. Dutta, Y. Gupta, D. Chatterjee, N. Kumar and P.K.S. Chauhan (1995). Seismicity of Delhi and its surrounding region, Journal of Himalayan Geology, 6, 75-82. Wennerberg, L. (1993). Multiple-scattering interpreta- tions of coda-Q measurements, B. Seismol. Soc. Am., 83, 279-290. Woodgold, R.D.C. (1990). Estimation of Q in Eastern Canada using coda waves, B. Seismol. Soc. Am., 80, 411-429. Wu, R.S. (1985). Multiple scattering and energy transfer of seismic waves: separation of scattering effect from intrinsic attenuation, I, theoretical model, Geophys- ical Journal of the Royal Astronomical Society, 82, 57-80. Yoshimoto, K., H. Sato and M. Ohtake (1993). Frequency dependent attenuation of P and S waves in the Kanto area, Japan, based on the coda normalization method, Geophys. J. Int., 114, 165-174. Zeng, Y., F. Su and K. Aki (1991). Scattered wave en- ergy propagation in a random isotropic scattering medium, I, theory, J. Geophys. Res., 96, 607-619. Corresponding author: Babita Sharma, Center for Seismology, Ministry of Earth Sciences, New Delhi, India; email: babita_s@rediffmail.com. © 2015 by the Istituto Nazionale di Geofisica e Vulcanologia. All rights reserved. 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