Synthetic waveforms and polarizations at the free surface of an anisotropic half‐space

Abstract

The influence of the free surface on shear wavetrains in a homogeneous anisotropic medium is analysed. We use a numerical technique which allows one to compute full waveforms from a buried dislocation source in a model with arbitrary symmetry. Azimuthally isotropic (hexagonal) and orthorhombic media are considered. In a transversely isotropic model and in symmetry planes of an orthorhombic medium the SH‐wave preserves its polarization at the surface and orthogonal particle motions of the incident shear waves are clearly seen even beyond the critical angle. If sagittal symmetry is absent, particle motions in the horizontal and sagittal planes are seriously distorted beyond critical angles by head waves and inhomogeneous reflected SP phases. However, in spite of the transverse components acquired by head and reflected waves, shear wave polarizations in the transverse plane remain close to the polarizations of the incident waves up to incidence angles of 55°–60°. Some complications in the detection of shear wave splitting caused by source parameters and anisotropic structure are also investigated. Distinctive shear wave splitting in the transverse plane for angles of incidence 35°–60° indicates the possibility of extending the limits of the shear wave window. Investigation of particle motions beyond critical angles may help to avoid ambiguities in the interpretation of the limited data set obtained inside the ‘classical’ shear wave window. Copyright © 1990, Wiley Blackwell. All rights reserved