HR: 1340h
AN: S23B-0316    [Abstracts]
TI: Synthesis of Scalar-Wave Envelopes in Anisotropic Random Media Using the Markov Approximation
AU: * Saito, T
EM: tatsu-saito@aist.go.jp
AF: AIST,GREEN, Central #7, Tsukuba, 305-8567 Japan
AB: Due to the Earth's inhomogeneity, seismic waves impulsively radiated from a source are distorted with increasing travel distance, characterized by increase of the duration and decrease of the maximum amplitude. Those phenomena have been studied on the basis of the forward scattering in random media. The waveform envelopes are theoretically calculated by using a stochastic approximation referred to as Markov approximation. Although the conventional studies have assumed isotropic random media, they are not realistic enough to represent the anisotropic lithosphere. The present study formulates a method of envelope synthesis using the Markov approximation in anisotropic random media: two-dimensional media and a special case of three-dimensional media. For two-dimensional media, random inhomogeneity are characterized by the root-mean-square value of the fractional velocity fluctuation and the two correlation distances a$_{x}$ and a${_z}$ that are the characteristic scale-length of the inhomogeneity in the x and z directions, respectively. The formulation is made for the Gaussian-type and the von Karman-type random media, where the horizontal correlation-distance is larger than the vertical correlation-distance. The reliability of the formulation is confirmed by the finite-difference numerical simulations of wavefield; the envelopes based on the Markov approximation are in good agreement with the envelope obtained from the wave traces of the finite-difference simulations. As is the case of isotropic random media, the envelopes by the Markov approximation become scale-free when the time axis and the amplitude are normalized by using a characteristic time. It should be noted that the characteristic time is a function of propagation direction in addition to travel distance, frequency and the parameters of random media. It predicts that envelopes are more collapsed when propagating in the horizontal direction than in the vertical direction. For three-dimensional random media (x-y-z space), the case of transverse isotropy characterized by two correlation distances, a$_{h}$ (= a$_{x}$ = a$_{y}$) and a$_{z}$, is studied. The formulation based on the Markov approximation is given when waves propagate along the z direction. When a$_{z}$ $<$ a$_{h}$, the envelopes are less collapsed than in the case of the isotropic random media with the correlation distance of a$_{z}$ or a$_{h}$. This indicates that the intensity of the inhomogeneity is underestimated when one assumes isotropic random media. In past studies, the intensity of the lithospheric inhomogeneity estimated from the envelope duration of the intermediate-depth events are small compared to that estimated from the coda excitation of shallow events. The anisotropic inhomogeneity in the lithosphere may be a key to solve this discrepancy.
DE: 7260 Theory and modeling
DE: 7200 SEISMOLOGY
DE: 7203 Body wave propagation
DE: 7218 Lithosphere and upper mantle
SC: Seismology [S]
MN: 2004 AGU Fall Meeting