HR: 0830h
AN: S21E-0350    [PDF]
TI: Theoretical And Numerical Investigations Of Seismic Anisotropy
AU: * Chen, M
EM: mchen@gps.caltech.edu
AF: Seismo. Lab., California Institute of Technology, 252-21, Caltech 1200 E. California Blvd., Pasadena, CA 91125 United States
AU: Tromp, J
EM: jtromp@gps.caltech.edu
AF: Seismo. Lab., California Institute of Technology, 252-21, Caltech 1200 E. California Blvd., Pasadena, CA 91125 United States
AB: Azimuthally anisotropic Earth models have been determined from surface-wave dispersion measurements. Such anisotropic models have been compared to tectonic plate spreading directions and the associated mantle flow. To first order, the fractional surface-wave phase-speed perturbation $\epsilon=\delta c/c$ in an anisotropic Earth model may be written in the form of an even, degree-4 Fourier series: $\epsilon=\epsilon_0+\epsilon_1\cos2\zeta+\epsilon_3\sin2\zeta +\epsilon_3\cos4\zeta+\epsilon_4\sin4\zeta$ where $\zeta$ denotes the local azimuth. We demonstrate that the associated compressional and shear body-wave speeds~$v$ are determined by equations of the form $\rho v^2 = \alpha_0 + \sum_{n=1}^4 \big [a_n(i) \cos n\zeta + b_n(i) \sin n\zeta\big ]$, where $\zeta$ denotes the local azimuth and $i$ the local angle of incidence. Note that the azimuthal dependence is governed by a degree-4 Fourier series with even and odd terms. The coefficients $a_n(i)$ and $b_n(i)$, on the other hand, are an even, degree-4 Fourier series in the local angle angle of incidence~$i$. Surface-wave anisotropy is governed by 13 independent elastic parameters, but body-wave anisotropy involves all 21 independent elements of the elastic tensor. We use the spectral-element method to simulate full waveforms of body- and surface-wave propagating in anisotropic Earth models, and use a phase-matched-filtering method and cross-correlation to measure the effects of the major elastic parameters on surface- and body-wave phase and arrival angles. We also derive explicit expressions for the phase and arrival angle anomalies using ray theory, whose predictions are compared to measurements from numerical simulations.
DE: 1734 Seismology
SC: Seismology [S]
MN: 2003 Fall Meeting