High-Frequency Seismogram Envelope Inversion Using a Multiple Nonisotropic Scattering Model: Application to Aftershocks of the 2008 Wells Earthquake

Abstract

Abstract In this paper, we use the Sato (1995) nonisotropic scattering formulation to describe an intensity spectral density with a spherical impulse wave radiated from the source. A characteristic source time is introduced to define the initial impulse width. We apply the modified multiple nonisotropic scattering model to analyze velocity seismogram envelopes of the 2008 Wells earthquake in Nevada. A series of scattering patterns is generated by a nonisotropic scattering pattern function. Higher‐order spherical functions are included in the expansion of the scattering pattern function so that stronger nonisotropic scattering patterns can be represented. A variety of synthesized envelopes, from backward‐dominated, forward‐dominated, and isotropic scattering processes, are investigated. By comparing synthesized envelopes with observed data, we found that the backscattering pattern fails to predict the observations of high‐frequency S waves, and envelopes from a forward‐dominated scattering pattern provide the best fit to the data. Based on these results, we offer a new interpretation for the origination of high‐frequency S ‐coda waves. The broadening effect of the envelope is successfully explained by the process being dominated by forward scattering. For all the aftershocks, the frequency dependence of intrinsic attenuation and scattering attenuation is discussed for frequency bands of 1–2, 2–4, 4–8, and 8–16 Hz. Both types of attenuation decrease when frequency increases, and the intrinsic attenuation dominates over scattering attenuation at higher frequencies.