Description: One of the aims of Sonderforschungsbereich 574 is to develop and to apply a new technique for determining shear wave velocity of marine gas hydrate bearing sediments. Gas hydrates may be incorporated into sediments as part of the pore fluid, as part of the matrix or act as a cement in various ways. The possible fine structures may be discriminated by different changes in compressional and shear wave velocity caused by the presence of gas hydrates. Determining the lithology of gas hydrate bearing sediments is especially important for estimating the amount of bounded methane reliably and for understanding the role of gas hydrates in slope stability and slope failures, respectively. Since amplitude versus angle analysis may suffer from the interaction of effects caused by the presence of gas hydrates and effects due to the presence of free gas beneath the gas hydtrate stability zone in certain cases, we focus on evaluating absolute and relative traveltimes of compressional waves and converted shear waves recorded by ocean bottom seismometers. During RV Sonne cruise SO163-2 and RV Meteor cruise M54-1B, seismic surveys have been performed at the Cocos Subduction zone off Costa Rica. Among the sites, Nicoya Slide and its vicinity has been one of our major targets. We have observed approximately horizontally polarized phases with rather high amplitudes which may be explained by shear waves converted at the sea floor and the base of the gas hydrate stability zone, if there is a significant change in shear strength. Furthermore we have observed indications of anisotropy which might be related to the presence of small fractures and cracks through which fluids and volatiles may migrate preferentially. Since the subsurface structure at the studied sites is horizontally layered as a first approximation, we have fitted one-dimensional models which are capable to explain the main features of the data.