Mantle anisotropy beneath the Earth's mid-ocean ridges

Observations of seismic anisotropy at oceanic spreading centres offer insights into mid-ocean ridge processesand the formation of new plates. Here, remote observations of seismic anisotropy beneath mid-ocean ridgesare made using measurements of source-side shear wave splitting. Over 100 high-quality measurements aremade using earthquakes that occur near mid-ocean ridges and transform faults, but are observed at teleseismic distances. In general, for off-axis ridge events, the polarisation of fast shear waves, phi'', is approximatelyparallel to the spreading direction. Nearer the ridge (e50 km), phi'' becomes more scattered and is often ridge-parallel. Delay times, dt, tend to increase from ~1 s near the ridge axis to ~3 s further away. Slow-spreadingregions (Gakkel and Southwest Indian Ridges) show smaller amounts of splitting than faster spreading centres. At transform zones, the pattern is more complex. Coverage beneath the East Pacific Rise is especiallygood, and we observe a systematic increase in delay times in S wave splitting measurements compared toprevious SKS splitting observations made at ocean-bottom seismometers. One compatible explanation isthe presence of horizontally-aligned, connected layers of melt at depth; this is also compatible with other ob-servations of the 'LAB' discontinuity and surface-wave derived measurements of radial anisotropy.