Seismic anisotropy of oceanic upper mantle: Shear wave splitting methodologies and observations

Abstract

We develop methodologies to obtain accurate measurements of shear wave splitting and apply these techniques to examine the pattern of oceanic upper mantle anisotropy. To obtain high‐quality estimates of receiver splitting at island stations, we devise a stacking method that finds the optimum splitting parameters and 95% error region for teleseismic shear‐wave phases from a suite of earthquake events. To obtain additional measurements in oceanic regions, we develop techniques to measure splitting parameters and errors for SS phases that sample upper mantle anisotropy at their bounce points. However, we find the data are often of low resolution, and anomalous characteristics are sometimes found that make splitting difficult to interpret. Ten second splitting of SS is observed across the BANJO seismic array, but we cannot unambiguously attribute this signal to mantle anisotropy at the bounce. The receiver splitting methods are used to assess the adequacy of a two anisotropic layer model for the Pacific region, with fast polarization azimuth (ϕ) in the lithosphere oriented in the fossil spreading direction and ϕ in the asthenosphere oriented in the absolute plate motion direction. This model has been proposed to explain surface wave data in the Pacific Ocean, but our splitting results demonstrate that oceanic anisotropy patterns are more heterogeneous than would be predicted. While the island splitting measurements could reflect the influence of individual hotspot upwellings, hotspot effects do not appear to be universally dominant. We propose that splitting observations alternatively indicate broad‐scale differences in the underlying character of oceanic upper mantle anisotropy, associated with coherant patters of lithospheric structure and asthenospheric flow. In particular, splitting, surface wave models, and regional studies all support a model where lithospheric anisotropy throughout the South Pacific has been erased or reoriented toward the absolute plate motion direction, whereas more limited observations in the North Pacific indicate that the fossil lithospheric signature appears to be preserved.