Lateral heterogeneity of the upper mantle determined from the travel times of multipleScS

Abstract

We have extended our investigation of lateral variations in upper mantle shear velocity structure by employing multiple ScS phases. An analysis of 108 multiple ScS differential travel times indicates that to a first approximation, upper mantle heterogeneity as expressed in vertical, travel time differences can be correlated with surface tectonic features. Travel times for continental regions with Phanerozoic orogenic histories are significantly greater than those from Precambrian continental regions and correlate with heat flow. Oceanic regions have large delays relative to stable continental regions, and the travel time residuals show a consistent decay as a function of crustal age. The average one-way vertical shear wave travel time difference between oceans and stable continental regions is estimated to be +4.0±0.5 s, not significantly different from the value obtained in our previous study. These results support the hypothesis that shield ocean heterogeneity persists to great depths, probably exceeding 400 km. For shear waves with characteristic frequencies of 0.1 Hz, the two-way vertical travel time through the mantle of the spherically averaged earth is 937.3±1.0 s, about 3 s less than the value derived from eigenfrequency data and 1.6 s greater than the Jeffreys-Bullen value. This result is inconsistent with the hypothesis that the base line discrepancy between free oscillation models and classical body wave travel time models is because of continental bias in the travel times, but it is consistent with the hypothesis that the base line discrepancy is an expression of anelastic dispersion.