9:15 AM - 9:30 AM
[SSS07-02] 3-D ray tracing in P-wave azimuthal anisotropic media
Keywords:P-wave azimuthal anisotropy, 3-D ray tracing, seismic tomography
Seismic tomography is the most powerful geophysical technology that can be used to explore the 3-D structure of the Earth's interior. In the past 40 years, researchers have used seismic tomography to reveal significant structural heterogeneities in the Earth's interior, such as subducting slabs, mantle plumes, fluids in seismogenic zones, and magma chambers (e.g., Zhao, 2015). To conduct travel-time tomography, we need to formulate a system of observation equations for travel times along ray paths. Hence, the accuracy of theoretical travel times and ray paths strongly affects the final tomographic results. Recently, many researchers have investigated seismic anisotropy, which is a phenomenon that the propagation velocity of seismic waves varies depending on the direction of wave propagation or polarization. Seismic anisotropy reflects the structure, deformation, mantle convection, and dynamics of the Earth's interior.
In conventional seismic tomography studies, researchers often assume isotropy when they conduct ray tracing. Zhao et al. (1992) developed an isotropic 3-D ray tracing technique that adopts the pseudo-bending scheme (Um & Thurber, 1987) in continuous media and Snell's law at complexly shaped velocity discontinuities. Gou et al. (2018) improved the method of Zhao et al. (1992) to conduct 3-D ray tracing in P-wave radial anisotropic media. They suggested that, at this stage P-wave radial anisotropic tomography can be conducted using the isotropic ray tracing technique because of the limited resolution and the effect of damping and smoothing regularizations, but anisotropic ray tracing will be necessary when seismic stations are installed more densely for higher-resolution anisotropic tomography in the near future.
In this study, we develop a 3-D ray tracing technique for P-wave azimuthal anisotropic (AAN) media based on the method of Zhao et al. (1992). We assume the AAN medium with hexagonal symmetry and take advantage of the property that the AAN symmetry axis, phase velocity vector, and group velocity vector exist in the same plane (e.g., Gou et al., 2018). Then, we evaluate the new ray tracing code and compare two rays: one is in the isotropic velocity medium (we call it an isotropic ray), and the other is in the AAN medium (we call it an anisotropic ray). We construct four 3-D P-wave AAN models for evaluating our ray tracing code. In model 1, the AAN amplitude has a gradient in the north-south direction, and the fast-velocity direction (FVD) is east-west. In model 2, the AAN amplitude is 5%, and FVD has a gradient in the north-south direction. In model 3, the AAN amplitude has a gradient in the depth direction, and FVD is east-west. In model 4, the AAN amplitude is 5%, and FVD has a gradient in the depth direction.
Our results show that in each of the four AAN models, the anisotropic ray bends in reasonable directions and satisfies Fermat's principle, indicating that the theory and approximations adopted in our calculations are appropriate. Spatial variations in the AAN amplitude and FVD lead to ray-path and travel-time differences between the isotropic and anisotropic rays. For some long rays (> 350 km), depending on the model, the ray-path difference between the two rays is up to 20 km or greater, and the travel-time difference between them is up to 0.1 s or greater. These results indicate that it is important and necessary to take into account AAN in the 3-D ray tracing.
We plan to evaluate our new ray tracing code in a realistic 3-D P-wave AAN model of the East Japan subduction zone (Jia & Zhao, 2023). To investigate the effect of anisotropic ray tracing on tomography, we determine a new 3-D AAN tomographic model using our new ray tracing code and compare the new model with the Jia & Zhao (2023) model that was obtained with the isotropic ray tracing technique of Zhao et al. (1992).
References
Gou, T., D. Zhao, Z. Huang, L. Wang (2018). Anisotropic 3-D ray tracing and its application to Japan subduction zone. J. Geophys. Res. 123, 4088-4108.
Jia, R., D. Zhao (2023). Anisotropic tomography of the East Japan subduction zone: influence of inversion algorithms. Geophys. J. Int. 234, 2199-2213.
Um, J., C. Thurber (1987). A fast algorithm for two-point seismic ray tracing. Bull. Seismol. Soc. Am. 77, 972–986.
Zhao, D. (2015). Multiscale Seismic Tomography. Springer, 304 pp.
Zhao, D., A. Hasegawa, S. Horiuchi (1992). Tomographic imaging of P and S wave velocity structure beneath northeastern Japan. J. Geophys. Res. 97, 19909-19928.
In conventional seismic tomography studies, researchers often assume isotropy when they conduct ray tracing. Zhao et al. (1992) developed an isotropic 3-D ray tracing technique that adopts the pseudo-bending scheme (Um & Thurber, 1987) in continuous media and Snell's law at complexly shaped velocity discontinuities. Gou et al. (2018) improved the method of Zhao et al. (1992) to conduct 3-D ray tracing in P-wave radial anisotropic media. They suggested that, at this stage P-wave radial anisotropic tomography can be conducted using the isotropic ray tracing technique because of the limited resolution and the effect of damping and smoothing regularizations, but anisotropic ray tracing will be necessary when seismic stations are installed more densely for higher-resolution anisotropic tomography in the near future.
In this study, we develop a 3-D ray tracing technique for P-wave azimuthal anisotropic (AAN) media based on the method of Zhao et al. (1992). We assume the AAN medium with hexagonal symmetry and take advantage of the property that the AAN symmetry axis, phase velocity vector, and group velocity vector exist in the same plane (e.g., Gou et al., 2018). Then, we evaluate the new ray tracing code and compare two rays: one is in the isotropic velocity medium (we call it an isotropic ray), and the other is in the AAN medium (we call it an anisotropic ray). We construct four 3-D P-wave AAN models for evaluating our ray tracing code. In model 1, the AAN amplitude has a gradient in the north-south direction, and the fast-velocity direction (FVD) is east-west. In model 2, the AAN amplitude is 5%, and FVD has a gradient in the north-south direction. In model 3, the AAN amplitude has a gradient in the depth direction, and FVD is east-west. In model 4, the AAN amplitude is 5%, and FVD has a gradient in the depth direction.
Our results show that in each of the four AAN models, the anisotropic ray bends in reasonable directions and satisfies Fermat's principle, indicating that the theory and approximations adopted in our calculations are appropriate. Spatial variations in the AAN amplitude and FVD lead to ray-path and travel-time differences between the isotropic and anisotropic rays. For some long rays (> 350 km), depending on the model, the ray-path difference between the two rays is up to 20 km or greater, and the travel-time difference between them is up to 0.1 s or greater. These results indicate that it is important and necessary to take into account AAN in the 3-D ray tracing.
We plan to evaluate our new ray tracing code in a realistic 3-D P-wave AAN model of the East Japan subduction zone (Jia & Zhao, 2023). To investigate the effect of anisotropic ray tracing on tomography, we determine a new 3-D AAN tomographic model using our new ray tracing code and compare the new model with the Jia & Zhao (2023) model that was obtained with the isotropic ray tracing technique of Zhao et al. (1992).
References
Gou, T., D. Zhao, Z. Huang, L. Wang (2018). Anisotropic 3-D ray tracing and its application to Japan subduction zone. J. Geophys. Res. 123, 4088-4108.
Jia, R., D. Zhao (2023). Anisotropic tomography of the East Japan subduction zone: influence of inversion algorithms. Geophys. J. Int. 234, 2199-2213.
Um, J., C. Thurber (1987). A fast algorithm for two-point seismic ray tracing. Bull. Seismol. Soc. Am. 77, 972–986.
Zhao, D. (2015). Multiscale Seismic Tomography. Springer, 304 pp.
Zhao, D., A. Hasegawa, S. Horiuchi (1992). Tomographic imaging of P and S wave velocity structure beneath northeastern Japan. J. Geophys. Res. 97, 19909-19928.