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

Abstract

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 5 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 Pacific 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.