Anisotropic tomography of the upper mantle beneath the Eastern Alps and the Bohemian Massif

Teleseismic body waves recorded during passive seismic experiments allow us to investigate isotropic velocities of the Earth’s upper mantle in a great detail, on scales of tens of kilometres. However, most of the tomography studies neglect the body-wave anisotropy completely or limit it either to azimuthal or radial anisotropy. We have developed a code called AniTomo for coupled anisotropic-isotropic travel-time tomography of the upper mantle (Munzarová et al., Geophys. J. Int. 2018) which allows for inversion of relative travel-time residuals of teleseismic P waves simultaneously for 3D distribution of P-wave isotropic-velocity perturbations and anisotropy of the upper mantle. We assume weak anisotropy of hexagonal symmetry with either ‘high-velocity’ axis a (lineation) and low velocity (b,c) plane or ‘low-velocity’ axis b and high velocity plane (a,c) (foliation) that is oriented generally in 3D. Such an approach of searching for orientation of the symmetry axes freely in any direction is unique and more general in comparison with the published methods that usually assume only horizontal or vertical orientation of the high-velocity symmetry axis. The code represents a step further from modelling homogeneously anisotropic blocks of the mantle lithosphere (e.g., Vecsey et al., Tectonophysics 2007; Plomerová et al., Solid Earth 2011) towards modelling anisotropy arbitrarily varying in 3D. We present complementary studies of anisotropic structure of the mantle lithosphere in contributions by Vecsey et al. (GD7.1, EGU 2024), suggesting a new method for evaluation of anisotropy from shear waves and in contribution by Kvapil et al. (GD7.1, EGU 2024), in which anisotropic structure of the lower crust is modelled from ambient noise. 

We have applied the AniTomo code on P-wave travel time deviations recorded during passive seismic experiments AlpArray-EASI (2014-2015) and AlpArray Seismic Network (2016-2019) to image the upper mantle large-scale anisotropy beneath the western part of the Bohemian Massif and the Eastern Alps. We interpret the P-wave tomography results 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 code allows to control the depth variations and an extent of the fabric. The joint inversion/interpretation allows for distinguishing which type of the models (a-axes model or b-axis model) approximates better the anisotropic structure.

The derived anisotropic-velocity models of the mantle lithosphere cluster into domains with boundaries coinciding with boundaries of the main tectonic sub-regions. These domains are compatible with domains inferred from a joint interpretation of directional variations of P-wave travel-time residuals and SKS-wave splitting parameters. 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 the detected anisotropic fabrics of the continental mantle lithosphere.