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Apparent Splitting of S Waves Propagating Through an Isotropic Lowermost Mantle
Author(s) -
Parisi Laura,
Ferreira Ana M. G.,
Ritsema Jeroen
Publication year - 2018
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.983
H-Index - 232
eISSN - 2169-9356
pISSN - 2169-9313
DOI - 10.1002/2017jb014394
Subject(s) - classification of discontinuities , anisotropy , geology , shear wave splitting , isotropy , geophysics , seismic anisotropy , core–mantle boundary , shear (geology) , mantle (geology) , seismic wave , surface wave , shear waves , seismology , physics , optics , petrology , mathematical analysis , mathematics
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.