Constraining large-scale mantle heterogeneity using mantle and inner-core sensitive normal modes

Abstract

Attempts to resolve density heterogeneity within the mantle using normal-mode data revealed an unexpected feature near the core-mantle boundary: regions of strong sheer velocity reduction, known as superplumes, are characterized by heavier than average material [Science 285 (1999) 1231]. Thus far, free-oscillation studies of mantle structure have relied upon modes which sample only the mantle (e.g., [ J. Geophys. Res. 96 (1991) 551; Science 285 (1999) 1231; J. Geophys. Res. 104 (1999a) 993; Geophys. J. Int. 143 (2000b) 478; Geophys. J. Int. 150 (2002) 162]). In order to better constrain mantle heterogeneity, we add inner-core sensitive modes to our data set, and invert for mantle structure and inner-core anisotropy simultaneously. The additional modes do not alter the pattern of the density anomalies relative to model SPRD6 [Geophys. J. Int. 145 (2001) 77]. However, they help constrain the amplitude of lateral variations in density. In a previous study, obtaining a density model with reasonable amplitudes required supplemental gravity data, which are not included in the current study. Nonetheless, the new density model is generally weaker, especially at the bottom of the mantle. The root-mean square (RMS) amplitude of the model exhibits two maxima around 600 and 2300 km depth. Experiments indicate that these are robust components of the model and neither artifacts of the choice of radial basis functions nor the damping scheme. Comparisons of the density model with shear- and compressional-velocity models show negative or nearly zero correlation in the transition zone and near the base of the mantle, where the root-mean square density amplitude is high. Finally, the anomalous features near the core-mantle boundary are confirmed: strong anti-correlation between shear and bulk-sound velocities, and increased density at the locations of superplumes.