HR: 1340h
AN: S23B-0312    [Abstracts]
TI: Synthesis of Vector-Wave Envelopes in Random Elastic Media Based on the Stochastic Ray Method
AU: * Sato, H
EM: sato@zisin.geophys.tohoku.ac.jp
AF: Geophysics, Science, Tohoku University, Aoba-ku, Sendai-shi, 980-8578 Japan
AB: High-frequency seismograms of local earthquakes are mostly composed of incoherent waves that are scattered in the inhomogeneous lithosphere. Although their phase variations are complex, their wave-envelopes are smooth and apparent durations increase with travel distance increasing because of diffraction due to velocity inhomogeneity. When the wavelength is much smaller than the correlation distance of medium inhomogeneity, each potential field of P- and S-wave is independently governed by the parabolic wave equation. The stochastic treatment of the parabolic equation, the Markov approximation, gives the temporal trace of the sum of mean square amplitudes of vector waves for a given frequency band. The use of angular spectrum makes it possible to calculate the mean square amplitude of each vector component in the framework of the Markov approximation. Diffraction of waves can be interpreted as a ray bend by velocity inhomogeneity, where the probability of ray bending is controlled by the power spectra of velocity inhomogeneity. In the case of 2-D random elastic media characterized by the Gaussian autocorrelation function, the equivalence of the stochastic ray method and the Markov approximation is numerically shown for plane waves and cylindrical waves isotropically radiated from a point source. The ratio of transverse-component amplitude to radial-component amplitude gives a quantitative measure of diffraction effect for both P- and S-waves, and the ratio turns over as the lapse time increases. We note that the stochastic ray method is extendable even for the case of a point shear dislocation source: mean square envelopes just after their onsets are different between different components reflecting the source radiation pattern; however, mean square envelopes become equal to each other with lapse time increasing. Vector envelope simulations will be practically useful for studies of earthquake source radiation and medium inhomogeneity in high frequencies.
UR: http://zisin.geophys.tohoku.ac.jp/~sato/
DE: 7260 Theory and modeling
DE: 7200 SEISMOLOGY
DE: 7203 Body wave propagation
DE: 7212 Earthquake ground motions and engineering
DE: 7218 Lithosphere and upper mantle
SC: Seismology [S]
MN: 2004 AGU Fall Meeting