Apparent Splitting of S Waves Propagating Through an Isotropic Lowermost Mantle

Laura Parisi, Ana M. G. Ferreira, Jeroen Ritsema

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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 urn:x-wiley:jgrb:media:jgrb52636:jgrb52636-math-0002 layer), based on differences in the arrival times of vertically (SV) and horizontally (SH) 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 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 urn:x-wiley:jgrb:media:jgrb52636:jgrb52636-math-0003, 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.
Original languageEnglish
Pages (from-to)3909-3922
JournalJournal of Geophysical Research: Solid Earth
Volume123
Issue number5
Early online date23 Mar 2018
DOIs
Publication statusPublished - May 2018

Keywords

  • shear wave splitting
  • D' layer
  • mantle anisotropy
  • core‐mantle boundary
  • lowermost mantle

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