Analysis of a seismic-refraction profile on the crest of the East Pacific Rise at 21 °N reveals considerable lateral inhomogeneity in the velocity structure. A split profile, of a total length of 120 km, was shot to a tripartite array of ocean-bottom seismometer capsules on or close to the spreading axis. The recorded travel times and amplitudes indicate the presence of a low-velocity zone in the immediate vicinity of the spreading axis, at a depth of about 2.5 km below the sea floor. This low-velocity zone is not evident a few (∼10) kilometres from the axis.

Additional structural evidence was obtained from a deployment of the capsules at the intersection of the East Pacific Rise crest with the Rivera Fracture Zone, at 20 °N. A group of small earthquakes was recorded by the seismometers, and, for ray paths traversing or close to the rise crest axis, the shear waves appear to be anomalously attenuated, particularly at shorter periods. While the evidence is not conclusive, it suggests the presence of a shear-attenuative region beneath the rise crest, presumably identical with the low-velocity zone and consistent with the interpretation of this zone as a region of partial melting.

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