Lower-mantle structure from ScS–S differential travel times

Abstract

KNOWLEDGE of the spectrum of structure in the lower mantle is crucial to our understanding of the dynamical evolution of the Earth and, in principle, can be inferred from the analysis of global seismic data sets. The long-wavelength compressional velocity structure has usually been constrained through tomographic inver-sion of the ISC catalogue of P-wave travel times^1–3, whereas shear velocity has been inferred from waveform modelling of long-period shear waves^4–6. The models that have been produced are similar in only the largest-scale features⁷ and explain little of the variance of the raw data used in their construction. Thus, there is a legitimate concern that the current generation of global-scale models give only a crude approximation to the largest-scale structure and that there may be significant aliasing of short-wavelength heterogeneity. Here we use ScS – S differential travel times to demonstrate that the three-dimensional structure of the lower mantle is indeed dominated by continental-scale features. Particularly convincing is the fact that the features are apparent in the raw data and are not the product of a complicated modelling procedure