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Abstract
Observations of shear wave splitting provide unambiguous evidence of the presence of anisotropy in the Earth’s lowermost mantle, a region known as D00. Much recent work has attempted to use these observations to place constraints on strain above the core–mantle boundary (CMB), as this may help map flow throughout the mantle. Previously, this interpretation has relied on the assumption that waves can be modelled as infinitefrequency rays, or that the Earth is radially symmetric. Due to computational constraints it has not been possible to test these approximations until now. We use fully threedimensional, generallyanisotropic simulations of ScS waves at the frequencies of the observations to show that ray methods are sometimes inadequate to interpret the signals seen. We test simple, uniform models, and for a D00 layer
as thin as 50 km, significant splitting may be produced, and we find that recovered fast orientations usually reflect the imposed fast orientation above the CMB. Ray theory in these cases provides useful results, though there are occasional, notable differences between forward methods. Isotropic models do not generate apparent splitting. We also test more complex models,
including ones based on our current understanding of mineral plasticity and elasticity in D00. The results show that variations of anisotropy over even several hundred kilometres cause the raytheoretical and finitefrequency calculations to differ greatly. Importantly, models with extreme mineral alignment in D00 yield splitting times not dissimilar to observations (δt ≤ 3 s),
suggesting that anisotropy in the lowermost mantle is probably much stronger than previously thought—potentially ∼10 % shear wave anisotropy or more. We show that if the base of the mantle is as complicated as we believe, future studies of lowermost mantle anisotropy will have to incorporate finitefrequency effects to fully interpret observations of shear wave splitting.
as thin as 50 km, significant splitting may be produced, and we find that recovered fast orientations usually reflect the imposed fast orientation above the CMB. Ray theory in these cases provides useful results, though there are occasional, notable differences between forward methods. Isotropic models do not generate apparent splitting. We also test more complex models,
including ones based on our current understanding of mineral plasticity and elasticity in D00. The results show that variations of anisotropy over even several hundred kilometres cause the raytheoretical and finitefrequency calculations to differ greatly. Importantly, models with extreme mineral alignment in D00 yield splitting times not dissimilar to observations (δt ≤ 3 s),
suggesting that anisotropy in the lowermost mantle is probably much stronger than previously thought—potentially ∼10 % shear wave anisotropy or more. We show that if the base of the mantle is as complicated as we believe, future studies of lowermost mantle anisotropy will have to incorporate finitefrequency effects to fully interpret observations of shear wave splitting.
Original language  English 

Pages (fromto)  15731583 
Number of pages  11 
Journal  Geophysical Journal International 
Volume  207 
Issue number  3 
Early online date  23 Sep 2016 
DOIs  
Publication status  Published  Dec 2016 
Keywords
 Seismic anisotropy
 Dynamics of lithosphere and mantle
 Mantle processes
 Computational seismology
 Body waves
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Projects
 2 Finished

SP3: Superplumes, superpiles or superpuddings? Understanding the thermochemical dynamics of the mantle with waveform seismology.
Davies, J. & Wookey, J. M.
1/09/13 → 31/08/17
Project: Research
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CoMITAC: An Integrated Geoscientific Study of the Thermodynamics and Composition of the CoreMantle interface
1/09/09 → 1/09/15
Project: Research