Lateral variations in mantle Pvelocity from tectonically regionalized tau estimates

Abstract

Estimates of tau functions for a tectonically regionalized earth are obtained from over 1.25 million seismic ray paths of ISC Bulletin data to study the correlation of potential lateral variations in mantle P-wave velocities with recognized surface heterogeneity. The use of regionalized travel-time data relies on statistical regularity criteria to check the consistency of the global regionalization. Estimates of tau perturbations attributed to velocity structure at the seismic source and receiver regions are derived by a simple algebraic formulation. Contributions from near-surface heterogeneity are thereby removed and permit the assessment of lateral velocity variations at depth. Estimates of ‘single region’ equivalent tau functions are constructed and inverted to obtain velocity-depth functions and extremal bounds at the 99.9 per cent confidence level for seven different tectonic regions. Deviations from a regionally weighted reference mean velocity function which agrees well with PREM (Dziewonski & Anderson) indicate significant differences, particularly between oceanic and continental tectonic regions, extending to a depth of 700 km. P-wave velocity anomalies are typically less than ± 1 per cent of the reference velocity. The age-dependence of the shallow oceanic mantle is observed by increased velocities from young (> 25 Myr) to old (< 100 Myr) regions, and similarly from orogenic zones and magmatic belts to stable continental regions. Unlike for young and old oceanic regions, stable continental regions do not show a pronounced gradient in the mean velocity residuals above 250 km. Continental platforms and particularly shields, however, show a compensation in the sign of the mean velocity residual at depths between about 350 and 700 km. Evidence for a velocity anomaly between 700 and 950 km is indicated. Significant negative residuals are observed centred at a depth of about 780 km below oceanic ridges and about 880 km below active continental regions. This anomaly may be related to a boundary layer between the upper and lower mantle. The level of velocity variations decreases below 950 km. Lateral variations are also suggested within 250 km of the core boundary.