SHEAR-WAVE SPLITTING IN THE MANTLE OF THE PACIFIC

The bulk of previously known data on seismic anisotropy in the suboceanic mantle was obtained from observations of azimuthal variations of the surface-wave and P(n) velocities. Our study of azimuthal anisotropy in the mantle of the Pacific is based on observations of shear-wave splitting. The idea of our method of measurement (Vinnik & Farra 1992) is to analyse evolution of the waveform of the long-period multiple reflected ScS and sScS phases as a function of the order of reflection. The estimates of anisotropy correspond to circular regions with a radius of several hundred kilometres around the surface bounce points. The technique was applied to the digital recordings of deep and intermediate events from the circum-Pacific seismic belt at seismograph stations in and around the Pacific. The estimates of polarization direction of the fast split wave and time lag of the slow wave are obtained for a few regions of the Pacific basin, marginal basins and the Andes. In the back-arc extensional basins whose spreading has continued to the present time or ended later than about 15 Ma, the inferred direction of extension in the mantle is close to that imprinted in the crust (Lau basin, Japan Sea). In the back-arc basins whose active development ended 15-20 Ma or earlier (South Fiji and Parece Vela basins), the inferred directions of extension in the mantle are in strong disagreement with the crustal data. In the Central Andes, the region of plate collision and low-angle subduction, the inferred direction of flow in the mantle is perpendicular to the plate boundary, contrary to the regularity previously found in several other collisional belts. The fast direction of anisotropy in the mantle beneath the southwestern part of the 100 Ma old Pacific plate is very close to the absolute plate motion direction; the magnitude of anisotropy corresponds to the lowest values found in the continental mantle and is much lower than in most of the marginal basins and the Andes. The weakness of this, presumably asthenospheric, anisotropy in comparison with asthenospheric anisotropy under some old continental plates is probably related to a difference in upper mantle viscosities.