Upper mantle anisotropy beneath Sudetes from shear wave splitting - passive seismic experiment AniMaLS

The passive seismic experiment AniMaLS was organized in 2017 in the Sudetes in Poland. One of the objectives was to study the anisotropy of the sub-crustal lithosphere and asthenosphere beneath the NE termination of the Bohemian Massif. Temporary seismic network of 23 broadband stations was operating in the area of Sudetes mountains and Fore-Sudetic Block, covering a ~200 x 100 km large area, with ~30 km spacing between stations. Obtained recordings were supplemented with data from permanent stations of Czech and Polish seismological networks located in the study area.

The Sudetes belong to internal zone of Variscan Orogen and are located in the NE part of the Bohemian Massif, between the Elbe Fault in SW and the Odra Fault in NE. The sudetic lithosphere represents a complex mosaic of several units with distinct histories of tectonic evolution and with consolidation ages ranging from the upper Proterozoic to the Quaternary. The aim of the project is to study seismic structure and anisotropy of the lithosphere-asthenosphere system based on broadband seismograms of local, regional and teleseismic events. The obtained data will be analysed using several interpretation methods. The poster presents the results of analysis by shear wave splitting method.

The analysis was done based on SKS and SKKS phases recorded during a ~2 years observation period. For analysis, three single-station methods were used: cross-correlation, eigenvalue minimization and transverse energy minimization. The dependence of resulting splitting parameters on the backazimuth of the event was also analysed. The results show that time delays between slow and fast S-wave components are typically in the range of ~0.5-1.6 sec, with average 1.2 sec. The splitting is interpreted as a result of lattice-preferred orientation (LPO) of mantle olivine. The azimuths of fast velocity axis are mostly consistent and showed largely WNW-ESE direction. They correlate well with trends of tectonic units observed at the surface and with strike directions of major fault zones. This suggests vertically coherent deformation throughout the lithosphere and frozen-in LPO, reflecting last tectonic episode which shaped Sudetic area. Obtained results were also compared with previous seismic studies of the upper mantle anisotropy in the neighboring areas by various methods.