Nonlinear Inversion for a Multilayer Seismic S‐Wave Attenuation Model Using Radiative Transfer Theory

Abstract

AbstractWe numerically solve the acoustic radiative transfer equation for seismic S‐waves via Monte Carlo simulation. By assuming a von Kármán‐type random medium with anisotropic scattering, we are able to simulate a realistic medium and determine its attenuation properties. In this study, we present an improved method, called QEST, to determine the frequency‐dependent intrinsic and scattering attenuation by nonlinear envelope inversion for a 1‐D multilayer model. Additionally, the spectral source energy of earthquakes and the energy site amplification of stations are determined. The code was applied to real data from the northern and southern Leipzig‐Regensburg fault zone (LRZ), Germany, as well as fluid‐induced earthquakes at the Insheim geothermal reservoir, Germany. The attenuation was analyzed in several frequency bands between 4.2 and 33.9 Hz and between 6.0 and 67.9 Hz, respectively. The inversion results reveal that the crystalline crustal subsurface along the LRZ shows little to no depth dependence, but there are differences in attenuation between the north and south. At Insheim, the near‐surface sedimentary basin exhibits significantly greater absorption and scattering than the crystalline basement. The inversion also shows that isotropic scattering can be an oversimplification and thus underestimate attenuation.