First shear-mode Rayleigh waves from five shallow focus earthquakes with epicenters in the mid-Atlantic ridge have been recorded by a hydrophone located on the ocean bottom in deep water (5.7 km) 260 km northeast of Antigua, West Indies, and by short-period seismographs at Bermuda. The surface wave train, in most cases, is a sinusoidal signal of long duration and nearly constant period; only the steep part of the dispersion curve is recorded. The predominant surface wave period is between 7.1 and 7.8 seconds for propagation paths to Bermuda and between 5.7 and 6.5 seconds for paths to the hydrophone. The steep parts of the dispersion curves for Rayleigh waves of the first shear mode are controlled primarily by the properties of the unconsolidated sediment layer. Theoretical dispersion curves have been computed as a function of sediment thickness and shear velocity gradient, and the results have been compared with the observed dispersion data. These comparisons give estimates of average sediment thicknesses that range between 0.5 and 0.9 km for the parts of the North Atlantic considered in this study. For the assumed sediment shear velocity profile, average sediment thicknesses derived from profiler results in the same region are consistently smaller than thicknesses derived from dispersion results. The difference may be explained by assuming that dispersion takes place principally in the thickest pockets of sediments along the path. Alternatively, the dispersion results can be made to agree with profiler results by assuming extremely low sediment shear velocities. The required average shear velocity for a layer of sediment 150 meters thick is found to be approximately 75 m/sec.
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Anderson et al. (1969) studied this question.