Apparent splitting of S waves propagating through an isotropic lowermost mantle

Abstract

Observations of shear-wave anisotropy are key for understanding the mineralogical structure and flow in the mantle. Several researchers have reported the presence of seismic anisotropy in the lowermost 150–250 km of the mantle (i.e., D” layer), based on differences in the arrival times of vertically (SV) and horizontally (SH) polarized shear waves. By computing waveforms at period > 6 s for a wide range of 1-D and 3-D Earth structures we illustrate that a time shift (i.e., apparent splitting) between SV and SH may appear in purely isotropic simulations. This may be misinterpreted as shear wave anisotropy. For near-surface earthquakes, apparent shear wave splitting can result from the interference of S with the surface reflection sS. For deep earthquakes, apparent splitting can be due to the S-wave triplication in D”, reflections off discontinuities in the upper mantle and 3-D heterogeneity. The wave effects due to anomalous isotropic structure may not be easily distinguished from purely anisotropic effects if the analysis does not involve full waveform simulations.