Observation and Modeling of the SVdiff-SHdiff splitting induced by elastic anisotropy and finite-frequency effects in D”

Abstract:

The existence of seismic anisotropy in D” has been diagnosed by the evident arrival-time shifts between SVdiff and SHdiff phases and discrepancies in SKS and SKKS splitting. Previous numerical modeling of seismic wave propagation demonstrates that the SVdiff and SHdiff arrivals are not synchronized in time even in an isotropic Earth model due to intrinsically different finite-frequency sensitivity of non-geometrical diffracted S waves between radial and transverse components. Therefore, prior to using diffracted shear wave phases to constrain seismic anisotropy in the lowermost mantle, we need to first clarify the finite-frequency effects of diffracted shear waves propagating through D” velocity heterogeneity.

In this study, we collect broadband waveforms from intermediate and deep-focus earthquakes (> 150 km) with epicentral distances of 90-145^o and magnitudes greater than 5.8 during 1997-2012. The splitting between the vertically (SV) and transversely (SH) polarized arrivals of S(diff) phases after correcting for upper mantle anisotropy are analyzed to investigate seismic anisotropy in D” induced by anisotropic material elasticity and finite-frequency wave propagation effects. Both positive and negative SVdiff-SHdiff split times are observed in seismically slow regions; however, the proposed viable scenario in which partial melts produced at the base of the hot mantle plumes have been aligned laterally in D” by the basal boundary flow can only explain relatively earlier SHdiff arrivals. Positive SV-SH times (i.e., SH traveling faster) found in high-velocity regions are consistent with the consequences caused by intrinsic LPO of pPv-MgSiO₃ in horizontally-lying slabs and different finite-frequency sensitivity between SVdiff and SHdiff in the heterogeneous lowermost mantle. We will differentiate and quantify these two effects on the observed Sdiff splitting through simulating ground-truth finite-frequency shear wave diffraction in the 3-D tomography Earth model and comparing the observed SVdiff-SHdiff split times with those predicted by their distinct finite-frequency sensitivity kernels constructed with the interactions of forward and adjoint wavefields computed by a spectral element method.