Depth of the upper mantle discontinuities beneath the East Pacific Rise

We observe discontinuities at depths of 415 and 655 lan beneath the northern East Pacific Rise (EPR). The data consist of precursors to SS waves from 71 tangential component seismograms ranging in epicentral distance from 107 degrees to 167 degrees. The mid-points of the paths are beneath the northern EPR region with about half the data bouncing beneath ocean younger than 9 Ma. The SS phases were aligned and then time shifted to account for the ocean age at the bounce point. The data were then stacked to remove theoretical moveout for underside reflections from the transition zone. The stacked results show clear underside reflections. These values are slightly more shallow than recently derived average values for oceans but the depth difference between the discontinuities is almost identical to the average for all oceans (Shearer, 1996). Although the absolute depths are dependent on the velocity model used in the estimate, the depth difference in the discontinuities is not sensitive to velocity structure. Our results suggest there are not unusually hot temperatures within the transition zone beneath the EPR relative to average ocean. This implies that deep upwelling beneath the ridge, if it exists, is passive.