A new look at the seismic velocity structure of the oceanic crust

Abstract

Within the last decade a new picture of the oceanic crust has emerged from advances in seismic experimental design, instrumentation, and analysis techniques. In this new picture, layer 2 is a region in which velocity increases rapidly with depth. While there is evidence of finer structure within layer 2, the exact nature of this structure is still poorly resolved. Layer 3 is much more homogeneous vertically than layer 2 and appears to have gentle vertical velocity gradients and occasional low‐velocity zones in νp and νs. Although it has been observed at several sites, the widespread existence of a high‐velocity basal crustal layer is in doubt. The thickness of the crust‐mantle transition has been observed to vary between 0 and 2 km from site to site, and even at flat lying, uncomplicated sites, seismic evidence for lateral heterogeneities on a scale of a few kilometers can be found. This seismic picture of the crust is in good agreement with the seismic velocities of rocks from ophiolite complexes and is consistent with the theoretically expected behavior of seismic velocities in porous, water‐saturated rocks at elevated pressures. A review of velocity results obtained from use of synthetic seismogram modeling techniques is given, and the types of synthetic techniques suitable for marine work are described.