Compressional and shear wave anisotropy in the oceanic lithosphere ‐ the Ngendei seismic refraction experiment

Abstract

Synthetic seismogram modelling of seismic refraction data from the 1983 Ngendei expedition to the south Pacific indicate seismic anisotropy both within the top kilometre of oceanic crust and in the uppermost mantle. We calculate average P‐ and S‐wave velocity versus depth functions for two orthogonal azimuths: N30d̀E, approximately aligned with the fast mantle direction, and N120d̀E, close to the fast crustal direction. Probable lateral heterogeneities at the Ngendei site prevent perfect matching of data and synthetic waveforms. Crustal anisotropy is indicated by a 2θ pattern of both P‐ and S‐wave travel times as a function of azimuth, and is consistent with an approximate 0.2 km s‐1 difference in P‐wave velocities and 0.1 km s‐1 difference in S‐wave velocities. Upper mantle anisotropy is characterized by Pn velocity differences of 7.95‐8.4 km s‐1 and a nearly uniform Sn velocity of 4.65 km s‐1. We use these velocity profiles and traveltime data to calculate bounds on the properties of the elastic constants of the crust and upper mantle. The crustal anisotropy can be explained by a model involving aligned cracks parallel to the original spreading ridge, resulting in a fast direction perpendicular to the fossil spreading direction. The upper mantle anisotropy is consistent with aligned olivine crystal models, in which the fast direction is parallel to the fossil spreading direction. For both of these models, we use bounds from the Ngendei data to place constraints on the physical properties of the lithosphere. Copyright © 1986, Wiley Blackwell. All rights reserved