Three‐dimensional structure of the upper mantle beneath the Atlantic Ocean inferred from long‐period Rayleigh waves: 1. Group and phase velocity distributions

Abstract

Group and phase velocity distributions of Rayleigh wave fundamental modes are determined in the Atlantic Ocean using a method of regionalization without a priori constraints, for a period range from 50 to 200 s. Eighty‐six direct source‐station paths are studied, and 38 out of the best data from Weidner's (1974) phase velocities are added to our data set for periods lower than 100 s. The importance of shallow layer corrections is described in detail. The general features of the distributions agree with global models, and smaller‐scale heterogeneities are revealed. A spatial resolution of 1500 km is needed to detect fine lateral heterogeneities in the Atlantic. The small lateral extent of young oceanic regions and strong lateral velocity gradients do not allow to quantify velocity versus age relationships using our data set. On the other hand, the regionalization without a priori constraints of phase velocities confirms the presence of a high‐velocity anomaly beneath the central part of the Atlantic for periods greater than 100 s.