A note on lateral variation in upper mantle shear-wave velocity across the Alpine front

We have detected what appear to be strong lateral variations in upper mantle shear-wave velocities under Europe and western Eurasia. The evidence suggests that the Alpine belt of southern Europe is underlain by a tectonic upper mantle, with a thin lid and thick low-velocity layer whereas the eastern Europe-western Russia Precambrian platform is underlain by shield-like upper mantle. The transition zone between the two appears to be sharply defined and coincident with the northern boundary of the Alpine belt. The techniques used in the analysis include the construction of synthetic seismograms for long-period S- and SS-waves in the distance range 30°–60°, where differences in upper mantle shear-wave velocity most strongly affect the differential times (SS-S) and the ratio of SS to S amplitudes as a function of epicentral distance. A comparison with upper mantle shear-wave structure in western North America as reported by Grand & Helmberger indicated that under the Alpine belt the upper mantle is considerably faster than under western North America, a situation that may be interpreted as due to the much larger and widespread thermal anomalies under western North America as a consequence of the existence of a subducted ridge.