Seismic Anisotropy Indicates Ridge-parallel Asthenospheric Flow Beneath the Eastern Lau Spreading Center

Seismic anisotropy beneath the Eastern Lau Spreading Center (ELSC) was investigated using both Rayleigh waves crossing the study area and the splitting of shear waves from intermediate and deep focus earthquakes occurring on the Tonga-Fiji subduction zone beneath it, using data from the 51 broadband seismometers of the 2009-2010 ELSC Ocean Bottom Seismogram experiment. Phase velocities in the 8-20 s range were determined for inter-station Rayleigh waves reconstructed using ambient noise cross-correlation, and were tomographically inverted for the three dimensional anisotropic shear velocity structure of the upper 50 km. Splitting of direct shear waves (S waves) from six intermediate and deep focus earthquake were determined by analysis of the waveforms of the long-period horizontal-component seismograms. A three-parameter grid search was used to determine the polarization direction, fast axis of anisotropy and splitting delay time that best fit each individual S wave. Delay time is used to infer the average anisotropy in the upper ~300 km. Taken together, Rayleigh wave and S wave data sets indicate that strong (>5%) anisotropy extends to at least 300 km depth. Both data sets indicate a fast direction aligned within a few degrees of the N10E striking ELSC and somewhat oblique to the N25E strike of the neighboring volcanic arc. We therefore describe the fast direction as ridge-parallel, not arc-parallel and interpret it as due to ridge-parallel flow of the asthenosphere. However, the region arc-ward (east) of the ELSC has the stronger anisotropy, suggesting that the strongest flow may occur in the region between the spreading center and the arc, as contrasted to being centered beneath the ELSC. The mantle beneath the arc is manifestly partially-molten and thus probably of lower viscosity than the mantle to the west, so the pattern may be due to the enhancement of the flow by low viscosity. Sea floor spreading diminishes south of the seismic array, ceasing altogether south of latitude 25S (500 km south of the array center), a region dominated by much-slower tectonic slow tectonic extension. While no Rayleigh wave data is available for this region, limited S wave data indicate a ridge-perpendicular fast direction there. This observation suggests that mantle deformation south of the ELSC is still dominated by east-west oriented corner flow driven by the extension.