Shear-wave splitting and implications for mantle flow beneath the MELT region of the East Pacific Rise

Shear-wave splitting across the fast-spreading East Pacific Rise has been measured from records of SKS and SKKS phases on the ocean-bottom seismometers of the Mantle Electromagnetic and Tomography (MELT) Experiment. The direction of fast shear-wave polarization is aligned parallel to the spreading direction. Delay times between fast and slow shear waves are asymmetric across the rise, and off-axis values on the Pacific Plate are twice those on the Nazca Plate. Splitting on the Pacific Plate may reflect anisotropy associated with spreading-induced flow above a depth of about 100 km, as well as a deeper contribution from warm asthenospheric return flow from the Pacific Superswell region. Observations of shear-wave splitting by the ocean-bottom seismometer (OBS) network of the MELT Experiment (1) describe the seismic anisotropy of the upper mantle be-neath the southern East Pacific Rise. A shear wave passing through a homogeneou