Diffraction Tomography of Shear Velocity Heterogeneity in the Lowermost Mantle

Lateral variation in shear velocity perturbations near the core-mantle boundary (CMB) has been unraveled exclusively by traveltimes of diffracted S waves. From an infinite-frequency ray-theoretical point of view, the S_d wave emerging in the core shadow uniquely samples the grazing ray segment along the CMB. Synthetic experiment suggests that the traveltime shift measured by an actual finite-frequency wave can differ substantially from the prediction of ray theory because of intrinsic wavefront healing effects. The Fréchet kernel theory based on the Born single scattering approximation is developed lately to correct such deficiency. A S traveltime residual yields unexpectedly zero sensitivity right at the geometrical ray; rather, it is most sensitive to the surrounding off-path heterogeneity. The SH_d traveltime kernel exhibits even paradoxical features on the CMB where the S_d ray glides; it has the opposite (or positive) sign suggesting that a S_d arrival could speed up by a slow anomaly at the CMB. Constrained mostly by compelling seismic evidence, the remote D'' zone is thought to be one of the most heterogeneous and dynamic region in the earth's interior. Recent ray-based tomographic inversion of long-period differential S(or S_d)-SKS traveltimes revealed a global distribution of anomalous D'' shear velocity on the scale length > ~1000 km. To assess the potential bias of the resolved D'' structure due to inadequate interpretation of finite-frequency arrivals, we will invert the S-SKS dataset using the 3D kernels. The model is parameterized by both the common block and multiscale scheme that invokes spherical wavelets. A computationally-efficient paraxial formulation based upon body-wave propagation together with full wave theory is implemented to construct the S_d and SKS kernels. This study will discuss the plausible difference between the kernel and ray-based results. Inconsistency among the models arising from imperfect parameterization for nonuniform data coverage will be also addressed.