3-COMPONENT SEISMOGRAM ENVELOPE SYNTHESIS IN RANDOMLY INHOMOGENEOUS SEMIINFINITE MEDIA BASED ON THE SINGLE SCATTERING APPROXIMATION

We present a method for synthesizing three-component seismogram envelo pes of local earthquake in a randomly inhomogeneous semi-infinite medi um. The method extends the single scattering model of Sate [Sate, H., 1984. Attenuation and envelope formation of three-component seismogram s of small local earthquakes in randomly inhomogeneous lithosphere. J. Geophys. Res. 89: 1221-1241.] by incorporating the effects of a free surface, frequency-dependent non-spherical radiation from a double-cou ple point source, and non-isotropic scattering including wave-type con version. We synthesize seismogram envelopes-the root-mean-square (rms) amplitude of seismograms-at a receiver located on the free surface by dividing an inhomogeneous medium into many small cells and summing th e energy of scattered waves from the cells on the isochronal scatterin g shells of different scattering modes for a given lapse time. Our mai n focus is the free surface effect, which alters the shape of isochron al scattering shells and the amplitudes of incident waves. Synthesizin g three-component seismogram envelopes for different source-receiver c onfigurations, we find that these effects on seismogram envelopes are not negligible and are pronounced at early S coda. P coda envelope sha pe is not very sensitive to free-surface incorporation because of domi nant SP scattered waves which come from the source direction. (C) 1997 Elsevier Science B.V.