Inversion of Seismogram Envelopes Using a Multiple Isotropic Scattering Model in Garhwal Himalaya

We estimated intrinsic attenuation Q(i)(-1), scattering attenuation Q(s)(-1), site amplification Z, and source energy S from inversion of three-component coda envelopes of the 1999 Chamoli earthquake of India for central frequencies 1.5, 3, 6, 12, and 24 Hz. The multiple isotropic scattering of S waves was numerically simulated by using a Monte Carlo method based on the radiative transfer theory. Isotropic sources and acoustic scattering in a full space were assumed. Adapting a grid search for scattering coefficient g and least-squares inversion for intrinsic attenuation parameter b, and source energy S, we inverted the observed envelopes of ten aftershocks (M(L) >= 3.5) in 1-24 Hz. Our results reveal that both Q(s)(-1) and Q(i)(-1) are weakly frequency dependent with the power-law forms of (0.006 +/- 0.004)(f-(0.89 +/- 0.33)) and (0.003 +/- 0.0005)(f-(0.84 +/- 0.08)), respectively. High scattering loss can be interpreted to be due to the presence of large lateral velocity heterogeneities in the crust. The total attenuation Q(-1) decreases with frequency, taking the power-law form of (0.009 +/- 0.003)(f-(0.87 +/- 0.19)). The mean free path ranges from 30 to 300 km, with an average value of 100 km, and the intrinsic attenuation parameter b ranges from 0.01 to 0.05 sec(-1), with an average value of 0.03 sec(-1). Our estimates of source energy are in good agreement with the values obtained assuming an omega(2)-source model. Site effects estimated using the fixed values of S(k), b, and g exhibit less scatter, ranging from 0.73 to 2.54 with no significant frequency dependence consistent with the rock sites.