A review of the current understanding of seismic shear-wave splitting in the Earth’s crust and common fallacies in interpretation

Azimuthally-aligned shear-wave splitting is widely observed in the Earth’s crust. The splitting is diagnostic of some form of seismic anisotropy, although the cause of this anisotropy has been sometimes disputed. The evidence in this review unquestionably indicates cracks, specifically stress-aligned fluid-saturated microcracks, as the predominant cause of the azimuthally-aligned shear-wave splitting in the crust. Although, in principle, shear-wave splitting is simple in concept and easy to interpret in terms of systems of anisotropic symmetry, in practice there are subtle differences from isotropic propagation that make it easy to make errors in interpretation. Unless authors are aware of these differences, misinterpretations are likely which has led to incorrect conclusions and charges of controversy where only misinterpretations exist. As a consequence, stress-aligned fluid-saturated microcracks as the cause of azimuthally-aligned shear-wave splitting in the crust is still not universally accepted despite there being distinguishing features that directly indicate crack-induced anisotropy. This paper reviews observations and interpretations of crack-induced shear-wave splitting and demonstrates that claims for aligned crystals and other sources of shear-wave splitting are due to fallacies in interpretation. This review shows how previous contrary interpretations are resolved and discusses common fallacies and misinterpretations. It is suggested that this new interpretation of shear-wave splitting has such fundamental implications for almost all solid-earth geoscience that it amounts to a New Geophysics with applications to particularly exploration and earthquake geoscience but also to almost to all other branches of solid Earth geoscience.