Evidence from core-reflected shear waves for anisotropy in the Earth's mantle

Abstract

Seismologists have begun to recognize that the upper mantle at depth is anisotropic and that seismological data are not fully explicable if anisotropy is ignored^1–5. On entering an anisotropic zone a plane shear wave splits into two orthogonally polarized phases⁶; an effect observed by several researchers⁷, notably by Ando et al.^5,8,9. The near vertical ScS wave, an S wave reflected once at the core-mantle boundary, is one of the most suitable phases to study the phenomenon of shear wave birefringence because it is isolated from other phases and because its particle motion is nearly horizontal and thus free from S to P conversion. The split ScS shear waves travel with different velocities in the anisotropic zone and consequently there is a time delay between their arrival at stations. My aim was to determine this time delay and the azimuth of polarization using the ScS records of two horizontal components from the Japanese islands for a deep shock beneath the Sea of Okhotsk. From the total of 41 stations 34 show a polarization of the maximum velocity phase in the NNW–SSE direction with an average time advance of ∼0.8s from the minimum velocity phase. The relevant anisotropic zone seems to be deep-seated.