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A low velocity zone underlying a fast-spreading rise crest



WE present the results of an unreversed seismic refraction profile on the East Pacific Rise near the Siqueiros Fracture Zone recorded using a digital ocean bottom seismograph (OBS)1. An analysis of P wave arrival times and amplitudes indicates a velocity gradient in the top 2 km, with the velocity reaching 6.7 km s−1. This is underlain by a low velocity channel some 1.4 km thick in which the velocity decreases to around 4.8 km s−1. Below this low velocity region there is a velocity gradient from 6.2 to 6.8 km s−1 and mantle velocities of 7.7 km s−1 are reached at a depth of 6 km below the sea bed.

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