ALBERT

Description: Splitting of shear waves is considered to be evidence of their propagation through an anisotropic medium. We evaluated splitting parameters of core-mantle refracted shear waves (SKS) along with their particle motions (PM), recorded during the passive seismic experiments AlpArray-EASI (2014-2015) and the AlpArray Seismic Network (2016-2019). The study area covers the western part of the Bohemian Massif and the Eastern Alps in about 200 km broad NS-oriented transect. Careful signal preprocessing includes several steps: automated identification of SKS waveforms, filtering and quality check. Special attention is paid to checking the geographical orientation of seismometers at all stations. Parameters of anisotropy – shear-wave split delay time and direction of the fast split shear waves in the LQT coordinate system are evaluated in 3D by two modified splitting methods, the eigenvalue and the transverse energy methods. To improve the splitting analysis results, we also include so-called null splits, i.e., results for directions where SKS waves do not split or their splitting is close to null. In the case of waveforms with a low signal-to-noise ratio (SNR) we apply a more robust PM method. The modified version of splitting methods (Vecsey et al., 2008) allows us to retrieve the 3D orientation of large-scale anisotropic structures in domains of the mantle lithosphere and to detect abrupt changes of their fabrics as well as present-day deformations within the sub-lithospheric part of the upper mantle.

Description: 〉Body-wave recordings from passive seismic experiments AlpArray-EASI (2014-2015) and AlpArray Seismic Network (2016-2019) are evaluated to image the upper mantle large-scale anisotropy and to model the lithosphere-asthenosphere boundary (LAB) beneath the western part of the Bohemian Massif and the Eastern Alps. We analyze P-wave traveltime deviations and interpret them along with results of splitting parameters from core-mantle refracted shear waves at 240 broad-band stations in about 200 km broad and 540 km long band along 13.3° E longitude. The body-wave anisotropic parameters are inverted jointly for 3D self-consistent anisotropic-velocity models of individual mantle lithosphere domains, assuming hexagonal symmetry with inclined ‘slow’ or ‘fast’ axes. To control the depth extent of the fabric, we apply coupled 3D anisotropic-isotropic P-wave tomography (Munzarova et al., 2018). Anisotropic signals in body waves are consistent within sub-regions and often sharply change at tectonic boundaries. The coincidence of boundaries of the anisotropic models of the mantle lithosphere domains with main tectonic features, correlation of the anisotropy depth extent with the LAB models as well as a decrease of anisotropy strength in the sub-lithospheric mantle support fossil origin of the directionally varying component of detected anisotropic fabrics of the continental mantle lithosphere.