Large-Scale Heterogeneities in the Lower Mantle.

摘要

Coefficients of a spherical harmonic expansion, up to angular order 3, of velocity anomalies in five shells within the earth's mantle were obtained from an analysis of nearly 700,000 P wave travel time residuals. The results for depths less than 1100 km are unreliable; on the basis of tests and numerical experiments we infer that lateral heterogeneities in this depth interval are dominated by velocity perturbations of lateral dimensions less than 5000 km. Relatively large wavelength features were resolved below 1500-km depth; the average perturbation level increases in the lowermost mantle.