GNGTS 2013 - Atti del 32° Convegno Nazionale

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) and Q S = (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 and Q S . • Q P and Q S in the Kumaun Himalaya region are found to be strongly frequency dependent. • The Q P and Q S increase with frequency. • The Q S / Q P ≥ 1 is found for all frequency range. • The low values of Q P and Q S 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 GNGTS 2013 S essione 1.1