In January 1988 a seismic tomography experiment was carried out on the East Pacific Rise at 9 degrees 30'N to image in three dimensions the seismic velocity structure of the rise axis. This experiment yielded a seismic data set consisting of more that 8000 records from approximately 500 shots and 15 ocean bottom seismometers. Initial result from the tomographic inversion of P wave delay times indicate substantial heterogeneity in structure both across and along the rise axis; for instance, the axial low velocity anomaly at 2 km depth is 1.5km/s slower than off axis; the along-axis variations in this velocity anomaly exceed 20% This renewal will continue the investigation of seismic velocity models using improved inversion techniques, will explore methods of utilizing wave form amplitude information to constrain further the 3-D seismic structure, and will analyze and interpret the gravity, magnetic, and microearthquake data obtained during thecourse of the tomograph experiment.
