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 (S V) and horizontally (S H) polarized shear waves. By computing waveforms at a 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 S V and S H 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 s S. 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.
Highlights
The D” layer — the lowermost 150–250 km of the mantle [Bullen, 1950]— plays a key role in global dynamics
We find that models mod9–mod11 with strong velocity discontinuities lead to a clear apparent splitting while models mod4-mod6 and mod12-mod15, with low velocity in the lowermost mantle, cause a widening of the SH waveforms
Using 1-D and 3-D waveform simulations, we have demonstrated that phase interference can distort SH and shifts of vertically (SV) waveforms and cause apparent splitting between SH and SV
Summary
SH-SV splitting for 1-D and 3-D isotropic Earth models. The apparent splitting is due to phase interference between S and reflected waves. SH-SV splitting for 1-D and 3-D isotropic Earth models. The apparent splitting is due to phase interference between S and reflected waves. Anomalous Vs gradients, discontinuities, and heterogeneities may lead to misinterpretation of wave splitting. This is the author manuscript accepted for publication and has undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record.
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