Anisotropy in the oceanic lithosphere -- theory and observations from the Ngendei seismic refraction experiment in the south-west Pacific

Abstract

Summary. P-wave travel-time data from a seismic refraction experiment on the 1983 Ngendei expedition to the south Pacific indicate anisotropy at two levels in the oceanic lithosphere. In the upper mantle, P-wave velocities vary between 8.0 and 8.5 km s-1 with the fast direction at N30°E. Crustal anisotropy within layer 2 is characterized by azimuthal P-wave velocity differences of about 0.2–0.4 km s-1, with the fast direction at N120°E, orthogonal to the upper mantle anisotropy. The observed anisotropy is consistent with a model in which aligned olivine crystals cause anisotropy in the upper mantle and aligned cracks within layer 2 cause anisotropy in the crust. We examine a model of P-wave propagation from a point source in an isotropic layer above an anisotropic half-space and show that under these conditions anisotropy can cause seismic ray-paths to deviate from the vertical plane connecting source and receiver. This deviation has a negligible effect on seismic travel times but must be taken into account in modelling P-wave polarization anomalies. P-wave polarization anomalies within the anisotropic half-space are the sum of the particle motion deviation from the ray-path, and ray-path deviation from the source-receiver azimuth, and are typically range-dependent. P-wave polarization anomalies will also occur in the overlying isotropic layer, purely as a result of the ray-path deviation, with greater deviations at long ranges. A simple model of upper mantle anisotropy (6kms-1 crust above 8.0–8.6 km s-1 anisotropic mantle) produces surface polarization anomalies of 1.5–3.5°. In practice, P-wave polarization anomalies are difficult to observe because they are small compared to observed scatter in polarization data and are sensitive to differences in horizontal seismometer gain levels. We examine the implications of anisotropic ray-path deviations on travel times and P-wave polarizations for the Ngendei data as well as some previous marine seismic refraction experiments.