Shear-wave Splitting Induced by Seismic Anisotropy in the Earth

It is believed that seismic anisotropy represents dynamics of the Earth's interior, directly reflecting either instantaneous stress, cumulative strain, or deformation of in-situ rocks. A shear wave passing through an anisotropic elastic medium splits into two orthgonally polarized quasi-shear waves with different propagation speeds. This phenomenon is called shear wave splitting, and is useful to understand the Earth's anisotropic fabric, with potential advantages as high lateral resolving power and relatively low sensitivity to seismic wave velocity heterogeneities. During the past decade, a variety of anisotropy-induced shear wave splitting has been observed in many different fields of seismology, indicating that anisotropy is an ubiquitous feature in the Earth's crust and upper mantle. In this review I summarize recent observations of shear wave splitting, with special emphases on their geophysical implications. I also discuss several problems concerned with shear wave splitting analyses, which are expected to be solved in the near future.