Radial Anisotropy in the European Upper Mantle from Surface Waves

Аннотация

The group velocity dispersion curves are constructed for Rayleigh and Love waves along the paths intersecting the European continent in the interval of periods from 10 to 100 s from the records of earthquakes and seismic noise. The radial anisotropy of the European upper mantle is estimated based on these data. At first, the average velocity sections of the SV- and SH-waves are calculated along each trace from the Rayleigh and Love wave data, respectively. Based on these sections, the average anisotropy coefficient is determined for each path in four depth intervals (the crust + the three 30-km upper mantle layers). These results are used for identifying the variations in the lateral anisotropy in the studied region with the use of tomographic inversion. This approach makes it possible to exclude the different degrees of smoothness of the lateral variations of the SV- and SH-waves if these variations are determined separately from the Rayleigh and Love waves: in this case, large errors in the anisotropy coefficient are possible due to different sets of the paths. The resolution of the data used for tomography was estimated by a “checkerboard test,” which demonstrated the possibility to resolve structural features with a linear size of 1200 to 1300 km in the central part of the studied area, i.e., approximately 15°–50° in longitude and 40°–65° in latitude. The tomographic inversion of the lateral variations in the anisotropy coefficient shows that in the continental part of the studied region, the anisotropy coefficient at all depths in the upper mantle is zero within the error limits, whereas in the region of the Black and Baltic seas, it is positive and equal to 4–4.5% in the subcrustal mantle at the depths of 34 to 64 km. In the underlying layer in the Baltic Sea region, this parameter is close to zero, whereas beneath the Black Sea Basin it remains positive although it decreases to 2–3%. In the lowermost layer, anisotropy is not observed anywhere in the entire region; however, this can be due to the lack of the data for large periods. Positive anisotropy (\({{V}_{{{\text{SH}}}}} > {{V}_{{{\text{SV}}}}}\)) is typical of the oceanic areas which can testify in favor of the oceanic hypothesis of the origin of the Black Sea basin.