Detection of three-dimensional velocity anisotropy

Abstract

Procedures to detect three-dimensional (3-D) azimuthal variations of P-wave velocity in addition to lateral heterogeneities within the Earth are proposed. First, we present a new method of analysing the residuals that cannot be predicted from the model obtained by the 3-D isotropic velocity inversion. In this analysis, we use an algorithm originating from ART (algebraic reconstruction technique)-type methods to translate or back-project the residuals into the residual corrections of the 3-D isotropic velocity model. If there exist other structures than lateral heterogeneities that cause 3-D azimuthal velocity variations within a given block, the residual corrections of the isotropic velocity model for the block are expected to be dependent on the directions of the rays travelling through the block. Next, we discuss the 3-D anisotropic velocity inversion assuming a weakly anisotropic medium with a spheroidal velocity surface, which approximates the velocity in the medium with orthorhombic symmetry. This requires much more model parameters than the isotropic inversion, so that an ART-type solution technique is used. In this method, we can determine three principal axes of the velocity spheroid in each block, and the velocities and their directions. Applying these methods to the P-wave travel-time data in central Japan, we made an attempt to determine the anisotropic structure in the low-velocity bodies beneath active volcanoes in this region, which were found from the previous study of 3-D isotropic velocity inversion. These low velocity bodies were observed to be anisotropic from other studies of shear-wave splitting, but we could detect no definite anisotropic structures within these bodies from our P-wave travel-time analyses.