Rayleigh wave attenuation and its variation across the Atlantic Ocean

Abstract

Summary. Fundamental-mode Rayleigh walies generated by several earthquakes situated on great circle paths traversing pairs of seismograph stations in American and European coastal regions and on Atlantic islands have been analysed to obtain anelastic attenuation coefficients, group and phase velocities, and specific quality factors at periods between 15 and 100s. Inversions of the dispersion and attenuation data have yielded shear velocity and shear wave internal friction (Qj') models of the upper mantle for regions of varying age in the Atlantic Ocean. The attenuation coefficients and internal friction values can be related to the age of the Atlantic Ocean floor, older regions being characterized by lower internal friction (or higher Q) values. A lowQ zone is prominent in the upper mantle of all regions less than 65 Myr in age, but is poorly developed in older regions. In contrast to the attenuation results, no systematic variations of Rayleigh or shear velocities with age can be resolved using the dispersion data of this study. A comparison of internal friction models beneath the Atlantic with those beneath the Pacific indicates that for regions of similar age internal friction is smaller (or Q larger) beneath the Atlantic than beneath the Pacific, especially in the younger regions immediately contiguous with the ridges. In particular, the upper mantle low-velocity, low-Q zone is less prominent beneath the Atlantic. The fact that spreading rates of the lithospheric plates are also smaller in the Atlantic suggests that internal friction and the rate of long-term creep in the mantle are closely related.