Body Wave Attenuation of Kumaun Himalaya M.S. Chauhan 1, A.R. Bansal 2 1 University of Naples Federico II, Naples, Italy 2 CSIR-National Geophysical Research Institute, Hyderabad, India Introduction. The Himalayan belt is formed due to the collision of Indian and Eurasian plates in the period 50-55 million years ago. This belt is seismically active and earthquake of varying magnitude are being observed. Most of these earthquakes are associated with great loss of life and destruction of property. For understanding of the nature of earthquakes and for reliable assessment of seismic risk in the Himalayan belt, knowledge and understanding of seismicity and the attenuation of strong ground motion are essential. Due to the vast natural resources in the Himalayan region, development of the region is being planned. For utilization of its resources major projects such as Tehri dam, Tunnels projects have been proposed. People living in this region are concerned about their survival due to the active seismicity of the region. In the light of the higher seismicity, an appraisal of the relation of earthquake occurrences with geology and tectonics of the region is very essential to make an assessment of the seismic potentialities, for survival of the lives and natural resources, and in designing of the major structures. The designing of earthquake resistant structures is a major challenge to the Civil Engineers. This challenge can be met if we develop ability to predict ground motion due to future earthquakes. The important structure such as nuclear power plants, dams, and high-rise buildings require estimate of ground motion for earthquake resistant designing. In the present work, we have made effort to understand the Body wave attenuation in Kumaun Himalaya using strong ground motion data (Kayal, 2008). Geology of the Kumaun Himalaya. Himalaya is a large geodynamic laboratory of nature where orogeny is still in youth to early mature phases of evolution. It is one of the most active orogens of the world and is the consequence of the collision of the Indian plate with the collage of previously sutured micro continental plates of central Asia during mid to late Eocene. The Kumaun region of the Himalaya lies near the center of the Himalayan fold-and-thrust belt and is situated between the Kali River in the east and Sutlej in the west, including a 320 km stretch of mountainous terrain. This part of Himalaya exposes all the four major litho-tectonic subdivisions of the Himalaya from South to North. They are Sub-Himalaya, Lesser Himalaya, Great Himalaya and Tethys Himalaya. All the litho-tectonic zones are bound on either side by longitudinally continuous tectonic surfaces such as Main Boundary Thrust (MBT), Main Central Thrust (MCT), South Tibetan Detachment (STD) system and Indus Tsangpo Suture Zone (ITSZ). The Sub-Himalaya includes the molassic Siwalik super group of Mio- 139
Pliocene ages. The lesser Kumaun Himalaya exposes a thick pile of highly folded Proterozoic sedimentary strata together with a few outcrops of older crystalline rocks. It is bounded by the MBT to the south and the MCT to the north. The Great Himalaya exposes a massive pile of high grade metamorphic rocks and the Tethys Himalaya includes a thick pile of sedimentary rocks of Cambrian to Lower Eocene ages. The extension of Aravalli structures into the Himalayan regions has played a role in the tectonics of the Kumaun Himalaya, and probably is the cause of the complex nature of seismicity of the region. In Kumaun Himalaya region the groups of rocks are known as Vaikriti group (Valdiya, 1980). According to the model of Srivastava and Mitra (1994), the Kumaun Himalaya evolved by an overall forelandward progression of thrusting, with some reactivation along the Munsiari thrust (MT), the Main Boundary thrust (MBT), and the Main Central thrust (MCT). In this part the maximum strain-energy release is related to the Main Central Thrust (MCT). It is among the least understood parts of the Himalayan fold-andthrust belt. Valdiya (1980) gives the most comprehensive account yet published on the geology of the region. It has been found that the large scale thrusts recognized in the Kumaun lesser Himalaya are boundary thrusts defining the limit of the various litho-tectonic units. There are large numbers of local thrusts less than 50 km in length, which have severed the tightly folded rock formations along the axial plane and brought the older rocks over the younger. This sector evidenced reactivation of some of the faults and thrusts during Quaternary times. This is amply evident by the recurrent seismicity patterns, geomorphic developments and by geodetic surveys (Valdiya, 1999). A generalized tectonic sequence for the Lesser Kumaun Himalaya (Valdiya, 1978) is tabulated below and shown in the Fig. 1 (after Célérier et al., 2009). Vaikrita Group...Vaikrita Thrust... Munsiari Formation...Main Central Thrust... Almora-Dudhatoli Nappe (with Askot, Baijnath, Chiplakot and Satpuli Klinne)...Almora Thrust... Outer Sedimentary Belt Inner sedimentary Belt...Main Boundary Fault... Siwalik Group Methodology. In this work we used coda normalization method which provides a reliable way to estimate the frequency dependence of important parameters quantifying the seismic source radiation and receiver site amplification, both of which are used in seismic risk assessment. It also allows the investigation of propagation effects. Most of seismology is focused on characterizing one of these three influences, (1) source radiation propagation, (2) site amplification, on seismograms and (3) propagation effect. The coda normalization method is based on the idea that at lapse time, the seismic energy is uniformly distributed in some volume surrounding the source (Sato and Fehler, 1998). In this method spectral amplitude of the earthquake source is normalized by coda waves at a fixed lapse time. It is based on the idea that coda waves consist of scattered S waves from random heterogeneities in the Earth (Aki, 1969; Aki and Chouet, 1975; Sato, 1977). Roughly lapse time is taken twice of the direct S-wave travel time and spectral amplitude of coda at a lapse time t c, A c (f, t c )is independent of hypocentral distance r in the regional distance range, and can be described (Aki, 1980) 140
Where f is the frequency, S s (f) is the spectral amplitude of S waves, P(f, t c ) is the codaexcitation factor and I(f) is the instrument response. The code excitation factor P(f, t c ) represents how the spectral amplitude of coda waves decays with lapse time. After solving mathematically, we get following equation (Aki, 1980): (1) where A s (f, r) is the spectral amplitude of the direct P-wave, Q s (f) is the quality factor of S wave and V s is the average S-wave velocity. Eq. (1) was first proposed by Aki (1980). Yoshimoto et al. (1993) extended this method for the measurement of Q P by assuming that earthquakes within a small range of magnitude have the same spectral ratio of P- to S-wave radiation within a narrow frequency range f ± Δf for different spectral shapes of P and Swaves (Molnar et al., 1973; Rautian et al., 1978). We are writing same symbol as Yoshimoto et al. (1993) used. (2) where A p (f, r) is the spectral amplitude of the direct P-wave, Q p (f) is the quality factor of P wave and V p is the average P-wave velocity. In our study we used Eqs. (1) and (2). Data. A network of nine strong motion accelerographs of Kinemetrics, USA, have been installed in the Kumaun Himalaya under the major research project sponsored by Department of Science and Technology/MOES, Government of India, in March 2006. Location of the stations are shown in Tab. 1. Fig. 1 Generalized tectonic sequence for the Lesser Kumaun Himalaya (from Valdiya, 1978). 141
Tab. 1 Location of the stations. Name of Station Latitude Longitude Elevation Dharchula 29.845 80.532 929 Didihat 29.801 80.252 1644 Lohaghat 29.404 80.083 1640 Munsiari 30.066 80.293 2105 Narayan Ashram 29.970 80.655 2535 Pithoragarh 29.584 80.212 1576 Thal 29.841 80.173 1576 Sobla 30.068 80.293 2105 Tejam 29.950 80.120 951 Result and discussion. By using extended coda normalization method of Yoshimoto et al. (1993) as described in previous section. Frequency dependent Q P are estimated for the Kumaun Himalaya which can be obtained from the slope of the linear fitted lines for different frequencies. The estimated values of Q P with coefficient of determination are presented in Tab. 2 and Fig. 2. Tab. 2 Average value of Q P at different central frequencies. Frequency (Hz) Q P Q S R p 2 (%) R s 2 (%) 1.5 34.09 87.00 95 69 3 61.00 173.92 91 63 6 105.77 379.76 78 57 12 126.07 416.63 66 76 24 205.44 530.39 72 79 Q = Q 0 f n Q p = (24 ± 3) f (0.9 ± 0.3) (1.04 ± 0.07) Q s = (64 ± 3) f 142 Fig. 2 Q P fits.
The Q values increase from about 34 (P waves) and 87 (S waves) at 1.5 Hz to 205 (P waves) and 530 (S waves) at 24 Hz respectively. The frequency dependence is comparable to those obtained in other tectonic areas such as Kanto (Japan) (Yoshimoto et al., 1993), Bhuj (India) (Padhy, 2009), Koyna (India) (Sharma et al., 2007), NE India (Padhy and Subhadra, 2010) and Cairo Metropolitan area (Egypt) (Abdel-Fattah, 2009). To obtain the frequency-dependent relations, the estimated average Q values as a function of frequency are fitted by a power law in the form Q = Q 0 f n (where Q 0 is Q c at 1 Hz and n is the frequency relation parameter) (Fig. 2). The power law forms of Q P = (24 ± 3) f (0.9 ± 0.3) = (64 ± 3) f (1.04 ± 0.07) for the Kumaun Himalaya region. The low Q P and Q s correspond to those of the seismically active areas in the world. The values of Q obtained for the S-waves in this study agree with the coda Q estimated in previous studies (Paul et al., 2003; Singh et al., 2011) for this region. We find that P waves attenuate more strongly than S waves ) for the entire frequency ranges. The obtained is observed in the upper crust of many other regions with a high degree of lateral heterogeneity (Bianco et al., 1999; Sato and Fehler, 1998). High degree of structural heterogeneities may be expected in the crust of Kumaun Himalaya as revealed by travel time tomography result of Sharma (2008) and by Mukhopadhyay et al. (2008) for the adjoining Garwhal Himalaya region (western part of our study area). According to Paul et al. (2003) Kumaun Himalaya is more heterogeneous and less stable compared to Garwhal Himalaya. So this underlying heterogeneity may have brought notable changes in seismic attenuation properties in the crust of Kumaun Himalaya. On the other hand from MLTW (multiple lapse time window) analysis by Mukhopadhyay et al. (2010), it is revealed that dominating attenuation mechanism for the Garwhal Himalaya is scattering attenuation. The crustal level folding and faulting in this region are also evident from tomography results (Mukhopadhyay and Sharma, 2010). Therefore it may suggest that the scattering is likely to be an important factor contributing to the attenuation of body waves in the Kumaun region. Hough and Anderson (1988) pointed out that is expected for most kinds of scattering. Padhy (2009) suggested that a high value in is expected to be due to scattering from shallow heterogeneities in the crust. The observed high value in is anticipated to be due to scattering from shallow heterogeneities in the crust beneath the study area. Conclusion. The strong motion data of digital network in Kumaun Himalaya is analyzed from 2006 to 2008 in this study. The modified coda normalization method (Yoshimoto et al., 1993) is used for estimating of Q P. Q P in the Kumaun Himalaya region are found to be strongly frequency dependent. The Q P increase with frequency. The Q S / Q P 1 is found for all frequency range. The low values of Q P correspond to seismically active areas with tectonic complexity due to the ongoing convergence between Indian and Eurasian plate. It is found that the attenuation is stronger for P wave than S waves for the entire frequency range and this probably reflects the high degree of heterogeneity presence in the crust of Kumaun Himalaya. Our results are well comparable to the other tectonically active regions characterized by high degree of heterogeneity reported globally. Acknowledgement. First of all I, Mahak Singh Chauhan, would like to express my sincere gratitude to Dr. Dinesh Kumar, Chairman, Department of Geophysics, Kurukshetra University for allowing me to take up my dissertation at CSIR- National Geophysical Research Institute (NGRI), Hyderabad and cultivating my interest in Computational Seismology, which culminated in this humble effort. I am highly indebted to Dr. Abhey Ram Bansal, Senior Scientist, CSIR-NGRI, my dissertation supervisor, for his invaluable guidance, overwhelming enthusiasm and affection. I am grateful to Prof. Mrinal K. Sen, Director, CSIR-NGRI, for his kind permission to work with Non Linear Processing in Geophysics Group and for providing a conductive environment. I am also very grateful to Prof. V.P. Dimri, Distinguished Scientist and former Director, CSIR-NGRI for his kind encouragement and fruitful discussion at various stages of the work. His philosophy of merging enjoyment with work is very much inspiring and need to be followed in life. 143
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