Seismic Anisotropy in the Lower Oceanic Crust

Abstract

HESS¹ suggested that upper mantle compressional wave velocities near the Mendocino and Molokai fracture zones of the Pacific vary with azimuth and this anisotropy is the result of preferred mineral orientation. Recent seismic refraction evidence indicates that in many regions the Earth's upper mantle is anisotropic^2–4 and the observed Anisotropy in the upper mantle correlates well with laboratory studies of velocity Anisotropy in dunites and peridotites^5,6. Although anisotropy complicates seismic refraction investigations its importance cannot be overestimated; it places severe limitations on probable upper mantle compositions and provides a means of estimating the stress fields which existed during the formation of preferred mineral orientation in the upper mantle.