9406146 Orcutt This experiment is designed to provide direct observational constraints on the distribution of melt and the pattern of upwelling beneath a mid-ocean ridge. Studies of electromagnetic and seismic wave propagation in the mantle with arrays of ocean- bottom instruments will furnish measurements that can be inverted to find horizontal and vertical variations in velocity and conductivity structure that can be used to test theoretical models of flow. An array of 47 ocean-bottom seismometers will be deployed for six months on the East Pacific Rise at about 17 degrees south to record teleseismic and regional earthquake sources. P and S waves, as well as other body wave phases and Love and Rayleigh surface waves recorded by the array will be used to probe lateral variations in mantle structure to a depth of 100 to 200 km and out to several hundred km from the ridge axis. The mail goal is to establish whether there is a narrow region of high melt concentration and low seismic velocities directly beneath the ridge axis and to map its vertical and horizontal extent. Another goal is to constrain the pattern of upwelling. ***
