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Keywords:

  • anisotropy;
  • D″;
  • lowermost mantle;
  • shear wave splitting

SUMMARY

Seismic anisotropy is an important tool for studying the nature, origin and dynamics of the lowermost mantle (D″). We introduce differential S–ScS splitting as a tool for removing the effect of near-source and near-receiver anisotropy to estimate splitting accrued in the D″ region. This is applicable to events recorded at epicentral distances between 60° and 85°. Near-source anisotropy has often been ignored in previous studies of lowermost mantle anisotropy. We apply differential S–ScS splitting to records from Canadian National Seismic Network stations of western Pacific earthquakes; these sample the lowermost mantle beneath the north Pacific. The residual splitting in ScS, which we attribute to D″, shows lag times between 1.0 and 3.9 s. Given the near horizontal ray path of ScS in D″, we interpret the recovered fast directions as the orientation of the fast shear wave in the plane defined by the vertical and transverse directions and observe a clearly non-VTI (transverse isotropy with a vertical axis of symmetry) style of anisotropy. The largest population of results shows an approximately southeasterly dipping symmetry axis which we speculate might be explained by descending palaeoslab material being swept horizontally across the core–mantle boundary towards an upwelling region beneath the central Pacific. Non-VTI symmetry and the many possible contributions to D″ anisotropy from lower-mantle minerals, melt and subducted materials suggest that our understanding of the lowermost mantle could be greatly improved by trying to resolve a more general style of anisotropy.