On the anisotropy of seismic waves in the Carpathian region

¹ St. Petersburg State University, St. Petersburg, Russia

^* Corresponding author: e.lyskova@spbu.ru

Abstract

The anisotropy of seismic waves in the continental regions still belongs to the category of controversial issues, since its estimates in different areas show a different sign of the anisotropy coefficient. In contrast to studies of oceanic regions, where SH velocities always prevail over SV velocities, in the continental regions the relations between the velocities are very different. The explanation for this, first of all, is the difference in structure. The structure of the crust and upper mantle under the oceans is much more homogeneous in comparison with the structure of the continental regions. There are several approaches to the estimation of anisotropy. The most traditional method is to use the maximum amount of data separately for Love and Rayleigh waves to study the lateral distributions of SH- and SV-wave velocity, despite the fact that the density of the coverage by paths, and, consequently, the regions of best resolution can be of different shapes and sizes. It was decided to use this method as the first approximation in creating an anisotropic portrait of the Carpathian region. The Carpathian region was chosen as the object of study, since it contains interesting contrasting features: (1) the Pannonian Basin, which is characterized by a thin crust, a thinned lithosphere, and anomalously high values of the heat flux; (2) the Tornquist-Teisseyre zone, which is parallel to the strike of the Eastern Carpathians, and represents the contact zone of the Precambrian lithosphere of the EEP and the relatively young lithosphere of Western Europe. (3) The third feature is the Vrancea zone, one of the most active seismic zones in Europe. It is located in the junction of young tectonic structures: the Southern and Eastern Carpathians, the Transylvanian Depression and the Pre-Carpathian Depression. The results of the study confirm that the Tornquist-Teisseyre Zone divides the structures of the ancient East European Platform and orogenic zones of Western Europe: the upper mantle throughout EEP is characterized by high velocities, whereas velocities throughout WE are markedly lower. Low velocity anomalies prevail under Pannonian Basin which is characterized by anomalously high heat flow values. The distribution of the anisotropy coefficient demonstrates an extended layer of low values of the anisotropy coefficient at depths of 150-200 km. Above this layer, velocity distributions reveal the block structure of the lithosphere. The earthquake sources in the Vrancea zone fall into the transition zone from the highvelocity mantle under the EEP to the low-velocity mantle under the WE. Earthquakes do not occur below the revealed asthenospheric layer.