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A stacking technique is applied to measure phase velocities of the fundamental and several higher Rayleigh modes over an array of long period stations located in Western Europe. The higher mode dispersion has been measured for periods between 25 and 100 s and for phase velocities up to 7.5 km/s. Using Backus-Gilbert inversion, a detailed model for the shear wave velocity in the upper mantle under the array is obtained. The low velocity zone is located between 150 and 230 km depth and is not very pronounced, but it is preceded by a rise in S velocity around 120 km depth. Strong velocity gradients are found at depths of 360 and 520 km. A good fit to the data can only be obtained if a zone of low density is assumed at a depth of 220 km or there about. As yet little can be said about depth, shape and extent of this zone, but the magnitude of the density drop implies a chemical or mineralogical stratification. A mechanism based on eclogite fractionation (Press, 1969) appears to be a likely candidate as the cause for such a gravitationally unstable stratification.
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