10:15 AM - 10:30 AM
[SSS09-03] 3-D ray tracing for P-wave azimuthal anisotropy and its effect on tomography of Japan subduction zone
Keywords:Seismic anisotropy, Seismic discontinuity, Seismic tomography, Wave propagation, Subduction zone processes
Seismic tomography is one of the most powerful geophysical methods exploring the 3-D structure of the Earth’s interior (e.g., Zhao, 2015). Seismic anisotropy is a phenomenon characterized by seismic velocities depending on the direction of propagation and polarization of the waves, and it reflects the Earth’s interior structure, deformation, evolutional process, and dynamic process such as mantle convection in the past and present (e.g., Zhao et al., 2023). The elastic properties of the Earth’s interior are anisotropic (e.g., Karato et al., 2008), and as a consequence, the propagation of elastic waves is also anisotropic. However, to date, most researchers have used the 3-D ray-tracing method in isotropic media, and few studies have investigated the effect of anisotropy on seismic ray paths and tomography.
In this study, we develop a new 3-D azimuthal anisotropic (AAN) ray-tracing method to calculate 3-D ray paths and travel times in P-wave AAN media (Tsunashima et al, 2025). We assume anisotropic media with hexagonal symmetry and take advantage of the property that the AAN symmetry axis, the phase velocity vector, and the group velocity vector are located in the same plane. The 3-D ray tracing method combining the pseudo-bending technique and Snell’s law (Zhao et al., 1992; Gou et al., 2018) is improved for the AAN media. To evaluate the accuracy of our new ray-tracing code, isotropic and AAN rays are computed and compared in simple synthetic models and the actual 3-D P-wave AAN model of the East Japan subduction zone (Jia & Zhao, 2023). We also investigate its effect on AAN tomography (Wang & Zhao, 2008, 2013) of the Japan subduction zone. The data used for the analysis are P-wave arrival times of local earthquakes in and around East Japan, which are collected from the Japan Meteorological Agency (JMA) unified earthquake catalog. These events were recorded at seismic stations of the Hi-net and S-net. Geometries of the Conrad and Moho discontinuities and the upper boundary of the subducting Pacific slab (Zhao et al., 2022) are considered in the analysis.
From our new ray-tracing code tests in the synthetic and actual models, the AAN rays in each model bend in reasonable directions and are consistent with Fermat’s principle. It means that, for a reasonable assumption of AAN media, the theory and approximations adopted in the ray-tracing calculations are valid. Our tomographic results using the code show that in the mantle wedge (depth>40 km) low-velocity anomalies and trench-normal fast-velocity directions (FVDs) appear along the volcanic front and in the back-arc area, and fan-shaped FVDs appear in the Hokkaido and Kanto areas at ~80 km depth. Within the high-velocity Pacific slab, trench-parallel FVDs appear in areas where the slab is bent laterally. These features are generally consistent with previous results (e.g., Jia & Zhao, 2023). A comparison of two models that are obtained with the isotropic and AAN ray-tracing code, respectively, shows that the two models are quite similar to each other in both isotropic 3-D P-wave velocity (Vp) variation and FVD distribution. Their differences in Vp and FVDs are less than 1% and close to zero degree, respectively, indicating a small influence of the 3-D AAN ray tracing on AAN tomography for the current resolution scale (30-50 km).
Acknowledgment:
In this study, we use the data of the JMA unified earthquake catalog, which are based on the seismic datasets provided by Hokkaido Univ., Hirosaki Univ., Tohoku Univ., Univ. of Tokyo, Nagoya Univ., Kyoto Univ., Kochi Univ., Kyushu Univ., Kagoshima Univ., National Research Institute for Earth Science and Disaster Prevention, National Institute of Advanced Industrial Science and Technology, Geographical Survey Institute, Aomori Pref., Tokyo Metropolitan Government, Shizuoka Pref., Kanagawa Prefectural Hot Springs Research Institute, City of Yokohama, Japan Agency for Marine-Earth Science and Technology, and JMA.
In this study, we develop a new 3-D azimuthal anisotropic (AAN) ray-tracing method to calculate 3-D ray paths and travel times in P-wave AAN media (Tsunashima et al, 2025). We assume anisotropic media with hexagonal symmetry and take advantage of the property that the AAN symmetry axis, the phase velocity vector, and the group velocity vector are located in the same plane. The 3-D ray tracing method combining the pseudo-bending technique and Snell’s law (Zhao et al., 1992; Gou et al., 2018) is improved for the AAN media. To evaluate the accuracy of our new ray-tracing code, isotropic and AAN rays are computed and compared in simple synthetic models and the actual 3-D P-wave AAN model of the East Japan subduction zone (Jia & Zhao, 2023). We also investigate its effect on AAN tomography (Wang & Zhao, 2008, 2013) of the Japan subduction zone. The data used for the analysis are P-wave arrival times of local earthquakes in and around East Japan, which are collected from the Japan Meteorological Agency (JMA) unified earthquake catalog. These events were recorded at seismic stations of the Hi-net and S-net. Geometries of the Conrad and Moho discontinuities and the upper boundary of the subducting Pacific slab (Zhao et al., 2022) are considered in the analysis.
From our new ray-tracing code tests in the synthetic and actual models, the AAN rays in each model bend in reasonable directions and are consistent with Fermat’s principle. It means that, for a reasonable assumption of AAN media, the theory and approximations adopted in the ray-tracing calculations are valid. Our tomographic results using the code show that in the mantle wedge (depth>40 km) low-velocity anomalies and trench-normal fast-velocity directions (FVDs) appear along the volcanic front and in the back-arc area, and fan-shaped FVDs appear in the Hokkaido and Kanto areas at ~80 km depth. Within the high-velocity Pacific slab, trench-parallel FVDs appear in areas where the slab is bent laterally. These features are generally consistent with previous results (e.g., Jia & Zhao, 2023). A comparison of two models that are obtained with the isotropic and AAN ray-tracing code, respectively, shows that the two models are quite similar to each other in both isotropic 3-D P-wave velocity (Vp) variation and FVD distribution. Their differences in Vp and FVDs are less than 1% and close to zero degree, respectively, indicating a small influence of the 3-D AAN ray tracing on AAN tomography for the current resolution scale (30-50 km).
Acknowledgment:
In this study, we use the data of the JMA unified earthquake catalog, which are based on the seismic datasets provided by Hokkaido Univ., Hirosaki Univ., Tohoku Univ., Univ. of Tokyo, Nagoya Univ., Kyoto Univ., Kochi Univ., Kyushu Univ., Kagoshima Univ., National Research Institute for Earth Science and Disaster Prevention, National Institute of Advanced Industrial Science and Technology, Geographical Survey Institute, Aomori Pref., Tokyo Metropolitan Government, Shizuoka Pref., Kanagawa Prefectural Hot Springs Research Institute, City of Yokohama, Japan Agency for Marine-Earth Science and Technology, and JMA.