Constraints on the structure of the East Pacific Rise from seismic refraction data

University of Washington seismic data taken during ROSE phase 1 have been used to study the structure of the East Pacific Rise in the vicinity of 12°N. Off axis but within 50 km of the axis we find that the oceanic crust can be modeled with a rapid change in velocity in the top 2.5 km, a zone of low but uniform velocity gradient from 2.5 to about 5.5 km and a transitional zone about 1 km thick to mantle material. There is also evidence for substantial anisotropy in mantle velocity gradients, with a positive velocity gradient in the spreading direction and a negative velocity gradient parallel to the rise. Under the rise axis the data allow at most a very narrow zone of low velocity (less than about 2 km wide) in the crust.