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Title: Determination of Q ?(f) in Different Parts of Kumaon Himalaya from the Inversion of Spectral Acceleration Data
Authors: Joshi A.
Kumar P.
Mohanty M.
Bansal A.R.
Dimri V.P.
Chadha R.K.
Published in: Pure and Applied Geophysics
Abstract: This paper presents the results of a modified two-step inversion algorithm approach to find S wave quality factor Q ?(f) given by Joshi (Bull Seis Soc Am 96:2165-2180, 2006). Seismic moment is calculated from the source displacement spectra of the S wave using both horizontal components. Average value of seismic moment computed from two horizontal components recorded at several stations is used as an input to the first part of inversion together with the spectra of S phase in the acceleration record. Several values of the corner frequency have been selected iteratively and are used as inputs to the inversion algorithm. Solution corresponding to minimum root mean square error (RMSE) is used for obtaining the final estimate of Q ?(f) relation. The estimates of seismic moment, corner frequency and Q ?(f) from the first part of inversion are further used for obtaining the residual of theoretical and observed source spectra which are treated as site amplification terms. The acceleration record corrected for the site amplification term is used for determination of seismic moment from source spectra by using Q ?(f) obtained from first part of inversion. Corrected acceleration record and new estimate of seismic moment are used as inputs to the second part of the inversion scheme which is similar to the first part except for use of input data. The final outcome from this part of inversion is a new Q ?(f) relation together with known values of seismic moment and corner frequency of each input. The process of two-step inversion is repeated for this new estimate of seismic moment and goes on until minimum RMSE is obtained which gives final estimate of Q ?(f) at each station and corner frequency of input events. The Pithoragarh district in the state of Uttarakhand in India lies in the border region of India and Nepal and is part of the seismically active Kumaon Himalaya zone. A network of eight strong motion recorders has been installed in this region since March, 2006. In this study we have analyzed data from 18 local events recorded between March, 2006 and October, 2010 at various stations. These events have been located using HYPO71 and data has been used to obtain frequency-dependent shear-wave attenuation. The Q ?(f) at each station is calculated by using both the north-south (NS) and east-west (EW) components of acceleration records as inputs to the developed inversion algorithm. The average Q ?(f) values obtained from Q ?(f) values at different stations from both NS and EW components have been used to compute a regional average relationship for the Pithoragarh region of Kumaon Himalaya of form Q ?(f) = (29 ± 1. 2)f (1. 1 ± 0. 06). © 2011 Springer Basel AG.
Citation: Pure and Applied Geophysics (2012), 169(10): 1821-1845
Issue Date: 2012
ISSN: 334553
Author Scopus IDs: 55682260000
Author Affiliations: Joshi, A., Indian Institute of Technology, Roorkee, Roorkee 247 667 Uttaranchal, India
Kumar, P., Indian Institute of Technology, Roorkee, Roorkee 247 667 Uttaranchal, India
Mohanty, M., Department of Science and Technology, Government of India, New Delhi, India
Bansal, A.R., National Geophysical Research Institute, Uppal Road, Hyderabad, India
Dimri, V.P., National Geophysical Research Institute, Uppal Road, Hyderabad, India
Chadha, R.K., National Geophysical Research Institute, Uppal Road, Hyderabad, India
Funding Details: The study area in the present work lies in the Kumaon Himalaya region which is considered to be seismically active. Frequent seismic activity and thrust systems present in this region demonstrate the seismotectonic importance of the region. The Kumaon sector manifests strong deformation and reactivation of some of the faults and thrusts during Quarternary times. This is amply evident by the recurrent seismicity patterns, geomorphic developments and geodetic surveys (VALDIYA 1999). This region shows development of all the four morphotectonic zones, which are demarcated by intracrustal boundary thrust of regional dimension. Morphotectonics take into account crustal anisotropy and the state of stress due to plate tectonics, erosion and isostatic rebound (SMITH et al. 1999). These zones from south to north are: Siwalik or Sub Himalaya, Lesser Himalaya, Great Himalaya and Tethys Himalaya (PAUL and GUPTA 2003). Under a major seismicity project funded by the Ministry of Earth Sciences, Government of India, a network of eight strong motion stations has been installed in the highly mountainous terrain of Kumaon Himalaya. Locations of these eight stations along with the geology of the region is shown in Fig. 1. A three-component force balance, short period accelerometer has been installed at all eight stations. In order to have a nearly continuous digital recording mode, the threshold level of
Corresponding Author: Joshi, A.; Indian Institute of Technology, Roorkee, Roorkee 247 667 Uttaranchal, India; email:
Appears in Collections:Journal Publications [ES]

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