The three-dimensional shear-wave velocity structure of the Earth's lower-most mantle

We have measured travel times of various seismic phases, including S, SS, ScS, SKS and SKKS, by cross-correlating observed waveform with that predicted by a synthetic seismogram. We demonstrate that the lower-most mantle is dominated by large-scale structure by analyzing our raw measurements in both space and wavenumber domains. By including our travel time measurements with other seismic data, such as waveforms, we invert for a three-dimensional Earth model. The new model shows a correlation of fast velocity anomalies with slabs of major subduction zones. Most slabs subducted to the lower-mantle even though there is no constraint on the continuity of seismic velocity anomaly during the inversion. Our power spectrum of this model shows that the lower-most mantle, indeed, is dominated large-scale earth structure. The power spectrum of our model peaks at the Core-Mantle Boundary. We infer that the velocity anomalies in the Lower-most mantle result mainly from compositional variations and core-mantle reactions play an important role.; We present a uniformly valid ray theory for body-wave propagation in laterally heterogeneous Earth models. We apply the theory to our global earth model. Our experiments with a three-dimensional Earth model demonstrate that it is possible to produce the observed ScS precursor from the focusing of seismic signals due to the heterogeneity in the lower mantle. We argue that the so-called ScS precursor can be a result of triplications due to lower mantle heterogeneity, and the proposed velocity discontinuity in the lower-most mantle by Lay and Helmberger, may be an artifact of mapping 3-D heterogeneity into 1-D radial structure.