Characteristics of coda waves and inhomogeneity of the earth

During the last twenty years, the characteristics of seismic waves scattered by inhomogeneous structure in the earth have been gradually clarified by many studies. The composition of P- and S-coda waves has been investigated using seismograms observed by seismic arrays. S to P and P to S wave conversions play an important role in the excitation of P-coda waves. S-coda waves at times later than twice the S-wave travel time are mainly composed of S-waves incoherently scattered by inhomogeneities. In the early part of S-coda waves, the contributions of forward scattering and of surface waves excited by topographic relief are large. Application techniques using coda waves have been developed by many researchers. Regional and temporal variations of coda Q^-1 values are related to tectonic activity, although the physical meaning of coda Q^-1 values is not always clear. A method for the separation of site amplification and source spectra using coda wave amplitudes has been proposed. Multiple scattering models for coda wave energy distribution have been developed for randomly distributed isotropic scatterers. Based on radiative energy transfer theory, analytical solutions have been obtained. Recently, the contribution of scattering loss to the total attenuation factor has been studied. Theoretical studies for coda waves suggest that coda Q^-1 values reflect intrinsic attenuation, since multiple scattering energy largely contributes to coda waves. Observational facts, however, imply that the single scattering assumption is still valid. To escape this dilemma, it is necessary to study in more detail the anisotropic scattering process and obtain more realistic models of the inhomogeneous distribution of scatterers.