HR: 1340h
AN: S23B-0304    [Abstracts]
TI: Modeling of SH Wave Envelopes in Media With Many Cavities: Wave Simulations vs. Radiative Transfer Theory
AU: * Kawahara, J
EM: junk@mx.ibaraki.ac.jp
AF: Department of Environmental Sciences, Faculty of Science, Ibaraki University, 2-1-1 Bunkyo, Mito, 310-8512 Japan
AU: Yomogida, K
EM: yomo@ep.sci.hokudai.ac.jp
AF: Division of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, North 10 West 8, Kita-ku, Sapporo, 060-0810 Japan
AB: Since Aki's (1969) pioneering work on the origin of coda waves, synthesis of seismic wave envelopes in the randomly heterogeneous earth has been attracting seismologists' interest. Several models have been proposed for this purpose, but their validation has been mainly restricted to continuous random media. Here, instead, we deal with 2-D circular cavities as a tractable example of discrete heterogeneities. We randomly distribute many cavities within a rectangular area with the concentration varying from 0.004 to 0.2. We then let a plane SH Ricker wavelet impinge on its one side, and synthesize rigorously the RMS seismogram envelopes along the opposite side, using a boundary integral method (Benites, Aki and Yomogida 1992). In our previous work (Eos Trans. AGU, 84 (46), Fall Meet. Suppl., Abstract S11E-0346, 2003), we compared these synthetics with the predictions by a few models, such as the single isotropic scattering model (SISM), thus discussing their validity ranges. In the present study, we now examine other two models: the radiative transfer theory (RTT) and the diffusion model (DM). RTT is numerically solved using a Monte Carlo method (Yoshimoto 2000). Here anisotropic scattering is replaced for simplicity by isotropic one with the aid of the momentum transfer scattering coefficient; this approximation is valid when multiple scattering is dominant. Note that the analytical solutions of SISM and DM are the asymptotic solutions of RTT in the limits of weak and strong multiple scattering, respectively. It is shown that RTT agrees with the wave simulation-based coda envelopes as a whole in many cases examined. The agreement tends, however, to be worse for the early coda part with which single scattering is dominant, as the anisotropy of scattering (depending on frequencies) gets stronger. Concerning the direct wave part, RTT systematically underestimates the envelopes and also cannot reproduce the time delay occurring at low frequencies, probably because of neglecting the constructive interference. It is also shown that DM works as well as RTT does when the distribution area is thicker than the transport mean free path, as expected.
DE: 7260 Theory and modeling
DE: 7203 Body wave propagation
SC: Seismology [S]
MN: 2004 AGU Fall Meeting