Boundary velocities of mohorovicic discontinuity and upper mantle structure of Pacific Ocean bed

Published data from deep seismic soundings of the Pacific Ocean bed were reviewed. The values studied were primarily obtained on 40 to 60 km travel-time curves with an average of 7 to 10 points on each travel-time curve and a spacing of 3 to 5 km between points. The results were used to construct a diagram of the distribution of boundary velocities along the M discontinuity and a graph of the distribution of V(m)sub h for various structures as well as the entire ocean bed. Lateral changes in the velocity characteristic of the M discontinuity are found to be significantly greater than changes which might be caused by velocity anisotropies and thus apparently result from variations in tectonic activity over the ocean bed. Lateral inhomogeneities in the upper mantle revealed by the studies are the main indications of differences in the earth down to a depth of 1 or 200 km. The trend toward slight increases in P wave velocity with depth immediately over the M discontinuity in the Pacific Ocean bed is firmly established. The lateral differences in V(m) sub h thus must be considered indications of the depth of mantle complexes which extend to the mantle surface.