This "undershooting" experiment, on the East Pacific Rise between 8 and 10 degrees north, will use seismic velocity and attenuation and anomalies of Pn arrivals, and Sn waves if they are observed, to map km-scale variations in the vertical distribution on melt near the base of the crust along the ridge axis. The magnitude and orientation of anisotropy in Pn velocities will be used to constrain the direction of mantle flow near the Moho. The goals of the project are to: (1) map along-axis variations in the seismic velocity and attenuation structure of the uppermost mantle along he entire segment, (2) map the orientation and magnitude of anisotropy in the uppermost mantle, (3) determine the vertical structure of the lowermost crust, Moho transition zone, and uppermost mantle directly beneath the axis of accretion at three contrasting sites, (4) measure crustal thickness variations along an entire transform-bounded ridge segment, and (5) compare the results with existing observations and models of segmentation.
