MODELING THE TELESEISMIC P-CODA ENVELOPE - DEPTH-DEPENDENT SCATTERINGAND DETERMINISTIC STRUCTURE

The P-coda of teleseismic events contains information on the small-sca le random structure of the near-receiver lithosphere. Various scatteri ng theories have been suggested to model the time and frequency depend ence of the coda envelopes for a given random structure. A specially s imple model is the energy-flux model that describes P-coda excitation in terms of scattering attenuation of the direct P-wave front without dealing with the details of the scattering process. It is based on ene rgy conservation and is valid for both weak and strong scattering case s. Here, the energy-flux model is extended to include a depth-dependen ce of the scattering structure. A variation of the depth range of the random medium manifests itself mainly in different decay rates of the coda, whereas the coda amplitudes at small lapse times behind the dire ct arrival are sensitive to the scattering strength of the shallow str ucture. Strongly heterogeneous layers at greater depths yield more com plicated coda shapes which cannot be fully described by parameters lik e decay rate or coda e. Full waveform simulations in random structures superimposed onto a deterministic layered model show that (1) the ene rgy-flux model yields reliable results, and (2) the effects of determi nistic layering on the coda envelopes are relatively small for waves w ith small incidence angles and realistic scattering strength and there fore may be neglected in coda inversions for teleseismic waves. (C) 19 97 Elsevier Science B.V.