Radial anisotropy in the European upper mantle from surface waves

Abstract

The group velocity dispersion curves of the Rayleigh and Love waves are constructed 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 from these data. Primarily, along each path, the average velocity sections of the SV- and SH-waves were calculated from the Rayleigh and Love wave data, respectively. Based on these sections, the average anisotropy coefficient was determined for each path in four depth intervals (the crust + three 30-km upper mantle layers). These results were used for identifying variations in lateral anisotropy in the studied region based on tomographic inversion. This approach eliminates different degree of smoothness of lateral variations of the SV- and SH-waves when these variations are determined separately from the Rayleigh and Love waves: in this case, the anisotropy coefficient can have large errors due to the 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 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 coefficient is close to zero, whereas beneath the Black Sea Basin it remains positive albeit 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 the large periods. Positive anisotropy (VSH > VSV) is typical of the oceanic areas, which can testify in favor of the oceanic hypothesis of the origin of the Black Sea basin.