Travel time inversion for three-dimensional P-wave velocity anisotropy.

Abstract

Inversion method for estimating the three-dimensional isotropic seismic velocity structure developed by Aki and his colleagues is extended to include the effect of velocity anisotropy. Assuming weakly inhomogeneous anisotropic media with some symmetry in anisotropy, we reduce the number of unknown parameters. We present two sets of linearized equations of quasi P-wave travel time residuals suitable for inversion. One set is for a weakly anisotropic medium with a hexagonal symmetry about a horizontal axis, and the other for a medium with a spheroidal P-wave velocity surface. Using these equations, we invert P-wave travel time residual data to estimate the isotropic and anisotropic velocity perturbations in the subdivided three-dimensional blocks within the modeling space. For local earthquakes, the hypocentral parameter perturbations should also be included in the inversion. This three-dimensional anisotropic velocity inversion is a natural extension of the previous isotropic velocity inversion. The three-dimensional anisotropic inversion method is applied to seismic data in Southwest Japan to determine velocity perturbations in the blocks which were well-determined in the previous isotropic study. The isotropic velocity perturbations are well determined, but the anisotropic velocity perturbations are less resolved. In spite of the rather poor resolution, our preliminary analysis shows some interesting features. Within the descending Pacific and Philippine Sea plates, the fast directions of P-wave velocity seem to be perpendicular to the magnetic lineations in the sea-floors near the trenches, suggesting that the descending lithosphere maintains the anisotropic structures which were formed before subducting.