Previous research of the Bohemian Massif (BM) crust with the use of ambient noise tomography (ANT) indicates a transversely isotropic structure of the lower crust (Kvapil et al. 2021). In this study, we have developed a new approach for evaluations of localised seismic anisotropy by travel time integration method. The method calculates synthetic Rayleigh (vertical ZZ correlation) and Love (transverse TT correlation) velocities and derives the vSH/vSV from the initial 3D isotropic vSV model. The higher ratio of measured vSH/vSV to synthetic vSH/vSV indicates the existence of velocity anisotropy in the lower crust of the BM in the reference ANT model (Kvapil et al., 2021). The new method evaluates azimuthal variations of the synthetic parameters due to heterogeneities reflecting local geology effects and corrects the observed velocity ratios. Then the 1D stochastic joint (ZZ, TT) inversion is applied to retrieve the depth dependence of the velocity ratio. We use cross-correlation of ambient noise and earthquake data from seismic stations included in the AlpArray, PACASE, and Adria Array passive seismic experiments and data from the PASSEQ experiment, which complement the sparse coverage in the northern part of the BM. Seismic anisotropy records the stress/strain conditions of each originally independent tectonic microplates during the formation of the BM crust. We demonstrate that synthetic modelling over the reference isotropic velocity model is an efficient tool for extracting radial and azimuthal shear velocity anisotropy in the lower crust directly from Rayleigh and Love wave dispersion curves. Regions with consistent parameters of seismic anisotropy correlate well with the major tectonic units of the BM. We show that variations in azimuthal and radial anisotropy of the lower crust on a regional scale can provide constraints for the reconstruction of geodynamic processes during the formation of the BM. We present complementary studies of the anisotropic structure of the mantle lithosphere in contributions by Zlebcikova et al. (GD7.1, EGU 2024), which shows an anisotropic model of the upper mantle derived from a 3D coupled anisotropic-isotropic teleseismic tomography (code anitomo), and in contributions by Vecsey et al. (GD7.1, EGU 2024), suggesting a new method for evaluation of anisotropy from shear waves.

The arc of the Western Alps is characterized by a complex crustal structure. Lower-to-middle crustal composition outcrops are exposed in the Ivrea-Verbano Zone (IVZ) and a major crustal anomaly, known as Ivrea Geophysical Body (IGB), presents dense and seismically fast rocks right below the surface. Understanding better their relation provides a key to refine our understanding of orogeny formation mechanisms. We performed seismic ambient noise tomography using data from the IvreaArray and the AlpArray Seismic Network, selected within a radius of ca. 100 km around the study area. Previous seismic investigations provided knowledge on the crustal structure in the Western Alps, by means of active refraction seismics and of more recent local earthquake and ambient noise tomography at regional scales (e.g. Solarino et al. 2018 Lithos, Lu et al. 2018 GJI). Recently, gravity data and receiver function analysis imaged the IGB as a dense and fast seismic anomaly, related to upper mantle material, reaching up to few km depth below sea level (Scarponi et al. 2021 Frontiers). However, local high-resolution constraints on the absolute vS distribution remain unknown. We used raw summer seismic data (June to September) across 3 years of recording, and computed daily ambient noise cross-correlation traces, for all the available station pairs (61 stations in total) in the 2-20s period range. Daily cross-correlations were stacked and processed to extract Green’s functions. Subsequently, we performed frequency-time analysis to get group velocity dispersions for the fundamental mode of surface Rayleigh waves. We computed 2D surface group velocity maps at each period, which clearly show the slow sediment area of the Po Plain, and the fast IGB structure within the crust. We are going to use the 2D group velocity maps to derive local dispersions curves and invert for 1D vS-depth profiles with the use of the Neighborhood Algorithm, to produce a 3D vS velocity model for the IVZ at high-resolution. This will also provide new geophysical constraints in the target area of the scientific drilling project DIVE (www.dive2ivrea.org) and reliable information for crustal corrections, which are necessary for upper mantle studies in such a complex area.

Transversely isotropic lower crust of the Bohemian Massif (BM) has been revealed by an ambient noise tomography (ANT) of the BM (Kvapil et al., Solid Earth 2021). The significant feature of this 3D vSV model is the low velocity layer in the lower part of the crust at depth between 18-30 km and the Moho. The upper interface is characterized by a velocity drop in the 1D velocity models retrieved by the ANT. The interface is interrupted around boundaries of major tectonic units of the BM. The lower interface (Moho) exhibits a sharp velocity increase at 26-40km depths through the massif. In this work we test whether we are able to detect azimuthal anisotropy in the lower crust, approximated up to now by anisotropic VTI model. We use Rayleigh wave dispersion curves evaluated from station pairs sampling the BM in the period range sensitive to the lower crust. First, we analyze seasonal variations of noise sources and their effect on quality and repeatability of dispersion curve measurements. Then we remove the effect of local heterogeneities by subtraction of synthetic dispersion curves calculated for the 3D vSV model along each station-pair raypath. Retrieved variations of azimuthal anisotropy are period-dependent with the fast velocity directions around NE-SW. We interpret the lower crust anisotropy layer as an imprint of the Variscan orogenic processes such as the NW-SE shortening of the crust and the late-Variscan strike-slip movements along boundaries of the crustal unit recorded in the interruptions of velocity drop interface in zones where anisotropic fabric of the lower crust was modified or erased.

The capability of the ambient noise tomography (ANT) to image subtle regional-scale velocity variations in the lower crust is limited by strong directionality of ambient noise sources in central Europe, which affects the quality of dispersion curves. Significant decrease of sensitivity kernels and sparse coverage of long interstation ray-pathes result in lower resolution at longer periods and thus increase uncertainty of the inversion solution in depth. If these well-known ANT limitations are properly addressed, the ANT is able to retrieve reliable high-resolution 3‑D shear velocities of the lower crust. In this study we focus on seasonal variations of ambient noise sources in selected sites in different tectonic settings. We analyse ambient noise sources on continusly recorded wavefields from permanent observatories and temporary stations of AlpArray passive experiment with its complementary experiment and PACASE. These seismic networks with densely-spaced stations are well-suited for detailed analysis of period-dependent directionality of ambient noise sources and their effects on FTAN appearance and consequently on the quality of dispersion curves. In the second part of this study, we advocate a concept of layer-stripping during the stochastic inversion (enhanced ANT). It proved to be an efficient technique to explore the model space, particularly in the lower part of the crust. We discuss the sensitivity of the enhanced ANT to the imaged small-scale velocity features in the lower part of the crust, as well as the sensitivity to the sharp or gradational Moho in the models.

The recent development of ambient noise tomography, in combination with the increasing number of permanent seismic stations and dense networks of temporary stations operated during passive seismic experiments, provides a unique opportunity to build the first high-resolution 3-D shear wave velocity (vS) model of the entire crust of the Bohemian Massif (BM). This paper provides a regional-scale model of velocity distribution in the BM crust. The velocity model with a cell size of 22 km is built using a conventional two-step inversion approach from Rayleigh wave group velocity dispersion curves measured at more than 400 stations. The shear velocities within the upper crust of the BM are ∼0.2 km s−1 higher than those in its surroundings. The highest crustal velocities appear in its southern part, the Moldanubian unit. The Cadomian part of the region has a thinner crust, whereas the crust assembled, or tectonically transformed in the Variscan period, is thicker. The sharp Moho discontinuity preserves traces of its dynamic development expressed in remnants of Variscan subductions imprinted in bands of crustal thickening. A significant feature of the presented model is the velocity-drop interface (VDI) modelled in the lower part of the crust. We explain this feature by the anisotropic fabric of the lower crust, which is characterised as vertical transverse isotropy with the low velocity being the symmetry axis. The VDI is often interrupted around the boundaries of the crustal units, usually above locally increased velocities in the lowermost crust. Due to the north-west–south-east shortening of the crust and the late-Variscan strike-slip movements along the north-east–south-west oriented sutures preserved in the BM lithosphere, the anisotropic fabric of the lower crust was partly or fully erased along the boundaries of original microplates. These weakened zones accompanied by a velocity increase above the Moho (which indicate an emplacement of mantle rocks into the lower crust) can represent channels through which portions of subducted and later molten rocks have percolated upwards providing magma to subsequently form granitoid plutons.

We present a new detailed map of the Moho in the Bohemian Massif (BM) derived from P-to-S conversions calculated from broad-band waveforms of teleseismic events recorded at 325 temporary and permanent stations operating in a region framed in 10–19º E and 48–52º N during last two decades. We processed data collected from running AlpArray Seismic Network (2015 – 2019) (http://www.alparray.ethz.ch/) and its complementary experiment AlpArray-EASI (2014 – 2015), as well as from previous passive seismic experiments in the region – BOHEMA I-IV (2001 – 2014), PASSEQ (2006 – 2008) and EgerRift (2007 – 2013). The study aims at upgrading the current knowledge of structure of the BM crust and providing a homogeneous estimate of Moho depths, particularly for the use in deep Earth studies, e.g., the upper mantle tomography. Different velocity models, including the new one retrieved from the ambient-noise study (see Kvapil et al., EGU2020_SM4.3), are tested in the time-depth migration procedures. Regional variations of the Moho depth correlate with main tectonic units of the BM. The crust thickens significantly in the Moldanubian part of the BM and thins along the Eger Rift in the western part of the massif. Detailed variations of the Moho depth from the receiver functions along several profiles are compared with crustal sections retrieved from the ambient noise tomography.

The current knowledge of the structure of the Bohemian Massif (BM) crust is mostly based on interpretation of refraction and reflection seismic experiments performed along 2D profiles. The recent development of ambient noise tomography, in combination with dense networks of permanent seismic stations and arrays of passive seismic experiments, provides unique opportunity to build the high-resolution 3D velocity model of the BM crust from long sequences of ambient seismic noise data. The new 3D shear-wave velocity model is built from surface-wave group-velocity dispersion measurements derived from ambient seismic noise cross-correlations by conventional two-step inversion approach. First, the 2D fast marching travel time tomography is applied to regularise velocity dispersions. Second, the stochastic inversion is applied to compute 1D shear-wave velocity profiles beneath each location of the processing grid. We processed continuous waveform data from 404 seismic stations (permanent and temporary stations of passive experiments BOHEMA I-IV, PASSEQ, EGER RIFT, ALPARRAY-EASI and ALPARRAY-AASN) in a broader region of the BM (in an area of 46-540 N 7-210 E). The overlapping period of each possible station-pair and cross-correlation quality review resulted in more than 21,000 dispersion curves, which further served as an input for surface-wave inversion at high-density grid with the cell size of 22 km. We present the new high-resolution 3D shear-wave velocity model of the BM crust and uppermost mantle with preliminary tectonic interpretations. We compare this model with a compiled P-wave velocity model from the 2D seismic refraction and wide-angle reflection experiments and with the crustal thickness (Moho depth) extracted from P-wave receiver functions (see Kampfová Exnerová et al., EGU2020_SM4.3). 1D velocity profiles resulting from the stochastic inversions exhibit regional variations, which are characteristic for individual units of the BM. Velocities within the upper crust of the BM are ~0.2 km/s higher than those in its surroundings. The highest crustal velocities occur in its southern part (Moldanubian unit). The velocity model confirms, in accord with results from receiver functions and other seismic studies, a relatively thin crust in the Saxothuringian unit, whilst thickness of the Moldanubian crust is at least 36 km in its central and southern parts. The most distinct interface with a velocity inversion at the depth of about 20 to 25 km occurs in the Moldanubian unit. The velocity decrease in the lower crust reflects probably its transversely isotropic structure.