The Study On Anisotropic Scattering Process Of High Frequency S Wave Based On Radiative Transfer Theory

Recently, the strong earthquakes stroke the earth frequently and brought human society inestimable loss of possessions and life. In contrast to the historical earthquakes, one of the most remarkable features of these earthquakes is their epicenters are very close to cities. Therefore, in order to study these earthquakes more efficiently, it is the key to build the theory and methodology which consider the rupture mechanism of source and propagation effect of these near fault earthquakes. Focusing on the scattering phenomena of seismic wave which propagated in the complex medium, the anisotropic scattering theory of high frequency S wave is developed in this study.Firstly, in the framework of anisotropic scattering theory, the seismic wave energy density integral equation is improved by introducing a source characteristic time, which is used to describe the time width of initial impulse of source. Then, based on anisotropic scattering patterns, a discrete wave-number is used to solve the improved S wave energy density integral equation. And the S wave velocity envelope is obtained according to the relationship between energy density and velocity. Based the velocity envelope, the effect of scattering patterns on S wave velocity envelope is discussed. According to the numerical results, forward and backward, as well as isotropic scattering patterns, all of them can model the common decay of S coda wave velocity envelope. When hypocentral distance is small (less than100km), all the S wave velocity envelopes are almost same. However, with the increasing of hypocentral distance, the deviation between isotropic and ansotropic results will be grown. And for the result of forward scattering pattern, the velocity envelope broadening and envelope peak delay will be appeared. Thus, a new quantitative measurement for the S wave envelope broadening is provided.Secondly, based on anisotropic scattering patterns, the effect of group of direct wave and single scattering term and multiple scattering terms on S wave velocity envelope is investigated. From the analysis result, the result from the group of direct wave and single scattering term match the whole velocity envelope of S wave pretty well at small hypocentral distance (less than100km). Thus, in this case, the multiple scattering terms can be neglected and the single scattering model can be used to simulate the S wave scattering process. With increasing of hypocentral distance, multiple scattering terms will be gradually dominated over the group of direct wave and single scattering term. Compared with results from backward and isotropic scattering pattern, the contribution of multiple scattering terms from forward scattering pattern grew more rapidly. And multiple scattering terms can be used to model the whole S wave velocity envelope when the hypocentral distance is larger than200km.Finally, the multiple forward scattering pattern is used to fit the velocity record from Wenchuan aftershocks. From the fitting result, in contrast to results from backward and isotropic scattering patterns, the synthesized velocity envelopes from the forward scattering pattern not only describe the common decay of S coda wave, but also represent the envelope broadening which will be appeared when hypocentral distance is increasing. Hence, high frequency S coda wave is not traditional superposition of scattering wave from backward or isotropic scattering process, but is originated from the transfer energy from the multiple forward scattering process between scatters and S wave. This phenomenon appeared on the observed S wave velocity envelope is the attenuation of direct S wave amplitude as well as the rising of S coda wave amplitude. Therefore, the energy of direct wave is pumped by the multiple forward scattering process between S wave and scatters, and the pumped energy is transferred to the coda wave part, which form the high frequency S coda wave. Therefore, a theoretical reference is offered for the synthesis of S coda envelope by a discovery of the mechanism of high frequency S coda wave excitation.