SIMULATED ENVELOPES OF NON-ISOTROPICALLY SCATTERED BODY WAVES AS COMPARED TO OBSERVED ONES - ANOTHER MANIFESTATION OF FRACTAL HETEROGENEITY

Envelopes of scalar waves are simulated at various distances from an i nstant point source embedded in a random uniformly scattering medium b y means of direct Monte-Carlo modelling of wave-energy transport. Thre e types of scattering radiation pattern ('indicatrix') are studied, fo r media specified by (1) a Gaussian autocorrelation function of inhomo geneities, (2) a power-law ('fractal', k(-alpha)) inhomogeneity spectr um and (3) the mix of case (1) and the isotropic indicatrix (very smal l + large inhomogeneities). We look for a model that can qualitatively reproduce the two most characteristic features of real S-wave envelop es of near earthquakes, namely (1) the broadening of the 'direct' wave group with distance and (2) the monotonously decaying shape of the co da envelope that does not deviate strongly from that expected in the i sotropic scattering case. Both properties are observed for any band ov er a wide frequency range (1-40 Hz). The well-studied isotropic scatte ring model realistically predicts the appearance of codas but fails to predict pulse broadening. The model of large-scale inhomogeneity real istically predicts the mode of pulse broadening but fails to predict c odas. We have found that, for a particular frequency band, within each class of inhomogeneity studied, both requirements can be qualitativel y satisfied by a certain choice of parameters. In the Gaussian-ACF cas e, however, this match can be obtained only for a narrow frequency ran ge. In contrast, the fractal case (with a value of exponent alpha of a bout 3.5-4) reproduces qualitatively the observed wide-band behaviour, and we consider it a reasonable representation of the gross propertie s of the earth medium.