HR: 08:00h
AN: S51D-01 INVITED    [Abstracts]
TI: Envelope Synthesis of High-Frequency Seismic Waves in the Lithospheric Inhomogeneity
AU: * Sato, H
EM: sato@zisin.geophys.tohoku.ac.jp
AF: Geophysics, Science, Tohoku University, Aramaki-Aza-Aoba 6-3, Aoba-ku, Sendai-shi, 980- 8578, Japan
AU: Korn, M
EM: mikorn@uni-leipzig.de
AF: Geophysics and Geology, University of Leipzig, Talstr. 35, Leipzig, D-04103, Germany
AB: High-frequency seismograms of earthquakes are complex and mostly composed of incoherent waves scattered by distributed inhomogeneities in the lithosphere; however, their wave-envelopes are smooth, systematic, frequency dependent, and vary regionally. Therefore, it is useful to analyze bandpass-filtered seismogram envelopes disregarding phase information for the study of medium inhomogeneity. If we focus on seismogram envelopes around the onset and the maximum peak, the envelope broadening and peak delay increase with travel distance increasing; the transverse-component amplitude of P waves and the longitudinal- component amplitude of S waves increase with travel distance increasing. We can interpret these phenomena as the result of multiple scattering around the forward direction by random velocity inhomogeneities. When the wavelength is shorter than the characteristic scale of random media, the Markov approximation, a stochastic extension of the phase screen method for the parabolic wave equation, is a powerful theoretical method for the synthesis of wave envelopes in random media in relation with their power spectrum. We have theoretically developed the envelope synthesis for vector waves in isotropic random elastic media as an extension of that for scalar waves for both plane-and spherical-wave cases. A closed form analytical solution is obtained especially in random media characterized by a Gaussian power spectrum. The developed method well explains how the wave envelope of each vector-component is broadened with travel distance increasing. The envelopes synthesized agree well with ensemble-average wave envelopes calculated from finite difference simulations of the elastic wave equation for a suite of 2-D random media. We also show the vector-wave envelope synthesis in nonisotropic random media, which are appropriate to represent the lithospheric inhomogeneity. The broadening of synthesized vector-wave envelopes with travel distance depends on the ray direction relative to the nonisotropy of randomness. These envelope syntheses will be a mathematical base for the study of lithospheric inhomogeneity from the analyses of local earthquake seismograms and teleseismic records.
DE: 7203 Body waves
DE: 7218 Lithosphere (1236)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 7260 Theory
SC: Seismology [S]
MN: 2007 Fall Meeting