Attenuation of P- and S-waves in the Chamoli Region, Himalaya, India

The attenuation properties of the crust in the Chamoli region of Himalaya have been examined by estimating the frequency-dependent relationships of quality factors for P waves (Q(alpha)) and for S waves (Q(beta)) in the frequency range 1.5-24 Hz. The extended coda normalization method has been applied on the waveforms of 25 aftershocks of the 1999 Chamoli earthquake (M 6.4) recorded at five stations. The average value of Q(alpha) is found to be varied from 68 at 1.5 Hz to 588 at 24 Hz while it varies from 126 at 1.5 Hz to 868 at 24 Hz for Q(beta). The estimated frequency-dependent relations for quality factors are Q(alpha) = (44 +/- A 1)f((0.82 +/-.04)) and Q(beta) = (87 +/- A 3)f((0.71 +/-.03)). The rate of increase of Q(f) for P and S waves in the Chamoli region is comparable with the other regions of the world. The ratio Q(beta)/Q(alpha) is greater than one in the region which along with the frequency dependence of quality factors indicates that scattering is an important factor contributing to the attenuation of body waves in the region. A comparison of attenuation relation for S wave estimated here (Q(beta) = 87f(0.71)) with that of coda waves (Q(c) = 30f(1.21)) obtained by Mandal et al. (2001) for the same region shows that Q(c) > Q(beta) for higher frequencies (> 8 Hz) in the region. This indicates a possible high frequency coda enrichment which suggests that the scattering attenuation significantly influences the attenuation of S waves at frequencies > 8 Hz. This observation may be further investigated using multiple scattering models. The attenuation relations for quality factors obtained here may be used for the estimation of source parameters and near-source simulation of earthquake ground motion of the earthquakes, which in turn are required for the assessment of seismic hazard in the region.