Rayleigh Wave Phase Velocities in the Atlantic Ocean

Abstract

Summary The phase velocities of Rayleigh waves are found for many paths in the north Atlantic. The single station technique is used with earthquakes in the Mid-Atlantic Ridge. The major contributor to the error in phase velocity is the uncertainty in epicentral location and origin time. Focal depth and mechanism are sufficiently well controlled for these events that they do not significantly add to the phase velocity error. The resulting uncertainty is ± 0.02 km s−1 for the period range 20-100 s. The ocean is regionalized into ridge and basin. Both ray theory and Rytov's method are used to extract the dispersion characteristics of each region from the observations. Large lateral variations of the basin dispersion suggest a further subdivision. These variations can be explained entirely by variations in sediment thickness. The deduced upper mantle model of the ocean basin is characterized by a shear velocity reversal at 40 km depth with a lid shear velocity of 4.7 km s−1. The lowest shear velocities are centered at a depth of 125 km. The lid shear velocity can be reduced to 4.6 km s−1 only if both the thickness of the lid increases and the density increases. Phase velocities for the Mid-Atlantic Ridge imply that the upper mantle shear velocity under the ridge is significantly lower than that under the ocean basin at depths greater than 20 km.