Shear-wave velocity structure of the oceanic lithosphere from ocean bottom seismometer studies

Abstract

Summary. A comprehensive interpretation of two shear wave refraction profiles recorded on ocean bottom seismometers provides velocity-depth models for the oceanic crust and upper mantle. The data were obtained as part of an investigation of lithospheric structure carried out in the Nootka fault zone area off the west coast of Canada. One-dimensional S-wave velocity-depth models are derived from the data by trial and error comparisons with WKBJ synthetic seismograms. The results of this study confirm crustal characteristics interpreted previously from the compressional wave data. Layers 3A and 2B in the upper crust are characterized by rapid increases in seismic velocities; shear-wave velocity increases from 2.2 to 3.6 km s-l over an interval of 3.5 km, although the exact nature of the shearwave velocity gradient is uncertain. In contrast, the shear velocity in layer 3A remains fairly constant with depth at values near 3.6km s-'. No velocity discontinuity is observed at the Moho. Instead, the transition from crust to upper mantle is marked by the presence of a velocity gradient zone, layer 3B. The shear-wave velocities in the upper mantle are 4.5 and 4.6 km s-l in directions approximately parallel to the spreading ridge and parallel to the direction of spreading respectively. This low degree of velocity anisotropy contrasts sharply with the 10 per cent anisotropic effect observed for the Pn velocities. Combining the P-wave interpretation with this S-wave study enables evaluation of Poisson's ratio as a function of depth. Except for low values of Poisson's ratio found in the upper crust, the seismic properties of the oceanic crust and upper mantle interpreted in this study agree remarkably well with those of laboratory measurements on ophiolite samples.