Synthesizing Shear Wave Splitting in the Lowermost Mantle With Low-Velocity Lamellae and Transverse Isotropy Models

Splitting between horizontally- and vertically-polarized components of shear waves (SH and SV, respectively) that propagate in the lowermost mantle has been documented in past studies for several regions, including beneath the northern Pacific, Alaska, central Pacific, Indian Ocean, Eurasia, and the Caribbean. SV lagging SH by up to 8 sec has been reported. The origin of observed splitting is not well constrained, but may be due to either shape or lattice preferred orientation (SPO or LPO, respectively) in the deepest mantle. In this work we explore systematically the SPO possibility of low velocity sheets, or lamellae, with properties approximating partial melt, and also the LPO possibility of transverse isotropy (TI). Reflectivity synthetic seismograms were made from lamellae models containing Vs to Vp reductions (dVs:dVp) of 3:1 relative to the PREM model. Parameter ranges tested included dVp=5-10%, lamellae zone thickness =100-300 km, and variations of lamellae spacing and thickness (0.5-20 km). Excessively complex waveforms are produced for thick melt lamellae (> 5 km) or for structures with > 10% of the D" thickness comprised of the lamellae. We tested models with thin lamellae concentrated near the top, bottom, or throughout D". Models with lamellae occupying 5-10% of D" by volume produce splitting that matches the average magnitude of the observed splits for diffracted shear wave data. The origin of low velocity lamellae may be somehow related to subducted paleoslabs (melting of former crustal component). Alternatively, iron-enrichment of melts produced near the core-mantle boundary could offset thermal or fusion-induced buoyancy effects, generating stable stratification of melts within the thermal boundary layer at the base of the mantle. The lamellae structures, however, do not produce significant splits at closer distances (e.g., 60-75 deg) where ScS splitting. Models with 0.5-3% TI were tested, where TI was imbedded in discontinuous D" shear velocity increases of 0-3%, over D" thicknesses of 100-300 km. TI is approximated with thin alternating layers of strongly contrasting properties. TI models resulted in splits for ScS and Sdiff, and easily reproduce the range of splitting seen in observations. In regards to producing an arrival observed between S and ScS due to a reflection of the top of a high velocity D" interface (the Scd phase), models having TI contained in a fast D" layer cause splitting as well as construction of the Scd arrival. The synthetic results will be compared to a summary of observed splitting from past studies.