ESTIMATION OF SEISMIC AZIMUTHAL ANISOTROPY PARAMETERS OF THE UPPER MANTLE OF THE CENTRAL PART OF THE EAST-EUROPEAN CRATON ACCORDING TO THE DATA OF THE “OBNINSK” AND “MIKHNEVO” SEISMIC STATIONS

The results of determining the parameters of seismic anisotropy of the upper mantle of the central part of the East-European craton are given. According to the tectonic concepts actively developing in recent decades, the East-European craton consists of megablocks (microplates) of different age: there are three main megablocks – Fennoscandia, Volga-Uralia and Sarmatia. The area of their triple junction is the subject of this article. The SKS/SKKS anisotropy method was applied. Its essence is to use SKS phases in order to obtain delay times for the arrival of two quasi-shear waves that are formed during the trace of a shear wave through an anisotropic medium and gets through it with different velocities. The result is to simulate the azimuth of the maximum velocity axis along which “fast” quasi-shear wave trace and to get the delay time between “fast” and “slow” waves. The technique does not allow to obtain accurate estimates of the depth of the anisotropic layer, however, taking into account the long periods of the SKS wave and the pronounced anisotropic properties of olivine, as the main material composing the upper mantle, it is assumed that these effects are related to the mantle processes. Authors used the data from “Obninsk” (OBN) and “Mikhnevo” (MHV) stations that are located in the zone of the triple junction of blocks. For Obninsk station – 1266 events were used, for Mikhnevo station – 472. For each station, we obtained estimates of the azimuth of the maximum speed axis and the delay time of arrival of quasi-shear waves. As a result of the study, the mantle anisotropy of the region is assessed as weak, which is an expected result for tectonically stable platform regions (delay time 0.4 s for the OBN station and 0.2 s for the MHV station). The direction of the maximum speed axis is strictly sub-latitude (90 for the OBN and 100 for MHV station), which is in accordance with the known the East-European lithospheric plate motion. For the OBN station, two groups of events were identified, with significantly different results, structured by azimuth. For events mainly with western azimuths of arrival of seismic waves, the solution is 90 and 0.4 s, while processing events from the eastern direction of arrival, along with a coordinated solution, a second pronounced local extremum is detected, corresponding to angles and delay time about 0.5–1.0 s. According to the results of MHV data processing, a second local extremum was also found, corresponding to similar angles of about 160, however, with a shorter delay time (about 0.1–0.2 s). It can be assumed that the presence of two extrema is associated with the anisotropic parameters of two different megablocks, near the junction boundary of which are located the OBN and MHV stations.