Anisotropic 3‐D Ray Tracing and Its Application to Japan Subduction Zone

A new 3‐D ray tracing technique is developed to determine seismic raypaths and theoretical travel times in weak radial anisotropic media. Anisotropic schemes of the pseudo‐bending technique and Snell's law are derived and used iteratively to find the fastest ray trajectory. Many synthetic tests are performed to evaluate the accuracy of our algorithm and investigate the raypath differences between the 3‐D isotropic and anisotropic rays. The test results show that the travel time and raypath differences are smaller than 0.1 s and 10 km, respectively. Applying our technique to a model of radial anisotropy tomography of the Japan subduction zone, we find visible travel time and raypath differences due to variations of the 3‐D isotropic velocity and radial anisotropy, but the travel time difference caused by the radial anisotropy is generally <~0.1 s and the raypath difference is generally <20 km. Two models of Vp radial anisotropy tomography under Japan are obtained using the isotropic and anisotropic ray tracing codes. The two models are very similar to each other, and they have only small differences (<0.5%) in the amplitudes of isotropic velocity and radial anisotropy. Considering the limited resolution of the current anisotropic tomography models and the effects of damping and smoothing regularizations, the use of the conventional isotropic ray tracing techniques is acceptable at this stage. However, for high‐resolution models of anisotropic tomography in the near future, the 3‐D anisotropic ray tracing will be necessary.