Shear-wave velocity in marine sediments on young oceanic crust: constraints from dispersion analysis of Scholte waves

SUMMARY An experiment with a newly developed implosive source, located about 1 m above the sea£oor 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-¢lter 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 sediment layer. Nearly perfect agreement of experimental and model data could be achieved with a sediment layer thickness of 21.5 m, an S speed 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-wave velocity have been obtained on old oceanic crust in the vicinity of continents or islands, while our experiment was carried out on young Paci¢c crust. Therefore, we suggest two mechanisms which could support a relatively high S speed in sediments: (1) the input of hydrothermally generated metalliferous sediments from the adjacent spreading axis; and (2) post-depositional diagenesis which has accelerated the induration of sediments.