Abstract

We estimated intrinsic attenuation forumla, scattering attenuation forumla, 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 (ML≥3.5) in 1–24 Hz.

Our results reveal that both forumla and forumla 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 ω2-source model. Site effects estimated using the fixed values of Sk, b, and g exhibit less scatter, ranging from 0.73 to 2.54 with no significant frequency dependence consistent with the rock sites.

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