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

Abstract

We estimated intrinsic attenuation ![Graphic][1] , scattering attenuation ![Graphic][2] , 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 ![Graphic][3] and ![Graphic][4] 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 ω 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. [1]: /embed/inline-graphic-1.gif [2]: /embed/inline-graphic-2.gif [3]: /embed/inline-graphic-3.gif [4]: /embed/inline-graphic-4.gif