PHASE VELOCITIES OF RAYLEIGH-WAVES IN THE MELT EXPERIMENT ON THE EASTPACIFIC RISE

The phase velocities of Rayleigh waves increase more rapidly with dist ance from the East Pacific Rise (EPR) axis than is predicted by models of conductive cooling of the lithosphere. Low velocities near the axi s are probably caused by partial melt at depths of 20 to 70 kilometers in a zone several hundred kilometers wide. The lowest velocities are offset to the west of the EPR, Wave propagation is anisotropic; the fa st direction is approximately perpendicular to the ridge, parallel to the spreading direction. Anisotropy increases from a minimum near the axis to 3 percent or more on the flanks.