SHEAR-WAVE VELOCITY IN MARINE-SEDIMENTS ON YOUNG OCEANIC-CRUST - CONSTRAINTS FROM DISPERSION ANALYSIS OF SCHOLTE WAVES

An experiment with a newly developed implosive source, located about 1 m above the seafloor at 3665 m depth, revealed a slow interface wave. This wave is attributed to S waves in a soft sediment layer covering the hard rock sub-bottom. Dispersion analysis by means of the multiple -filter technique yields the group velocity as a function of frequency . Model calculations showed that the observed dispersion curve can be reproduced by considering a steep gradient of S velocity within the se diment layer. Nearly perfect agreement of experimental and model data could be achieved with a sediment layer thickness of 21.5 m, an S spee d of 225 m s(-1) at the water-sediment interface and an increase by 23 s(-1) within the layer. These values are relatively high compared to data from the literature. However, previous estimates of in situ S-wav e velocity have been obtained on old oceanic crust in the vicinity of continents or islands, while our experiment was carried out on young P acific crust, Therefore, we suggest two mechanisms which could support a relatively high S speed in sediments: (1) the input of hydrothermal ly generated metalliferous sediments from the adjacent spreading axis; and (2) post-depositional diagenesis which has accelerated the indura tion of sediments.