MOBNET pool of seismic stations of the Institute of Geophysics of the Czech Academy of Sciences has been created since nineties of the 20th century. It grew from a nucleus of 6 broad-band units (seismometer and data acqusition system) up to the current 80 portable stations for temporary installations. The need of having a dense network of transportable seismic stations to be installed temporarily in regions with different tectonics and to record data for detailed modelling of 3D anisotropic structure of the upper mantle, triggered formation of this pool. Since then, the pool equipment has been involved in several passive seismic experiments, both small- and international large-scale ones, which were in operation for only few months at the beginning up to several years nowadays. Data recorded during the experiments has contributed to different significant findings about the Earth, including those which revealed that the mantle lithosphere is formed by individual domains with their own tilted anisotropic fabrics. Changes of the fabric orientations mark boundaries of the domains amalgamated to cores of continental plates. Data from the passive experiments are essential for detailed research of the Earth structure and inferences about processes of plate formation.

We describe stations of the AdriaArray Seismic Network deployed on the territory of Bulgaria. The network consists of 50 stations, 25 of which are permanent stations of the Bulgarian National Seismic Network NOTSSI. Another 25 temporarily deployed stations belong to MOBNET (Institute of Geophysics) and DanSeis (Aarhus University) pools. The temporary stations of MOBNET began their operation in June 2022, and the DanSeis pool’s temporary stations in August and September 2022. For each of the stations, we document the site geology, station location, instrumental equipment, as well as data transmission, preprocessing and availability, which exceeds 95% at some stations. Noise at the stations stays between the limits of the New Low and New High Noise models. As an example of planned data exploiting, we present the P-to-S receiver functions for station BG11A in the Strandzha unit (southeast of Bulgaria) and respective Moho depth estimates from the P delay times, H-𝜅 stacking and from the 1D vS velocity-depth modeling at the station site. .

Seismic waves propagating through the Earth sample its structure, carry information about its fabrics and physical characteristics and record its present-day state and evolution. In the past, several velocity discontinuities within the radial Earth, which separate its fundamental regions, were retrieved. The lower mantle-core boundary was named as Gutenberg discontinuity in recognition of the Gutenberg’s discovery of the Earth’s core in 1913. This discontinuity relates to the abrupt decrease in P-velocity and diminishing of S-waves in the liquid core. In present-day terminology, the Gutenberg discontinuity is associated with the bottom of the D’’ layer. An area of low velocities in the Earth’s upper mantle denoted as G-discontinuity, has related to Gutenberg’s name until now. The low velocity zone exists just below the oceanic lithosphere, and its characteristics are often used globally in studies of lithosphere thickness in the view of modern plate tectonics. Gutenberg’s Seismicity of the Earth (1941) became a major influence in later scientists’ efforts to describe the theory of plate tectonics. The accuracy and validity of the Earth models depend on data quality and coverage, i.e., earthquake foci - seismic station ray distribution within the Earth volume studied. Small-sized to large-scale international passive seismic experiments, operated during several recent decades, recorded an unprecedented huge amount of high-quality data, which along with new techniques and computational facilities represent a big step forward in our knowledge of the Earth’s structure. However, many questions still remain unanswered and require further research. Current close international cooperation among seismologists involved in the experiments follow the spirit of Beno Gutenberg’s action as a driving force behind the acceptance of seismology as an international science of earthquake detection and the Earth studies. We present models of the European lithosphere derived from the propagation of body waves, shear-wave splitting and radial and azimuthal anisotropy of surface waves, including ambient noise. Data for individual studies has been collected from international seismological databases (ISC, EIDA) and from several passive experiments we have organized or participated in. Initial isotropic models are upgraded into anisotropic ones, following the fundamental condition that seismic anisotropy is a 3D phenomenon and thus it has to be evaluated in 3D to get more realistic images of the Earth. We invert/interpret jointly anisotropic parameters of independent observables (directional variations of P-wave travel times, shear-wave splitting parameters) which leads to 3D self-consistent anisotropic models of the continental lithosphere with tilted symmetry axes and characteristic domain-like structure. The individual domains at size from several tenths to several hundreds of kilometers are often sharply bounded and of different thicknesses. We interpret the often sharply bounded domains with systematically oriented dipping fabrics in the continental mantle lithosphere by successive subductions of ancient oceanic plates and their accretions enlarging primordial continent cores. Consequent continental break-ups and assemblages of wandering micro-plates preserve fossil anisotropic fabrics and create patchwork structures of the present-day continents. Supporting arguments for such model exist in petrological and geochemical studies (Babuska and Plomerova, 2020).

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 AdriaArray initiative is a broad international collaborative effort to investigate the dynamic deformation of the Adriatic Plate and its interactions with surrounding regions. The main component of this effort is the AdriaArray Seismic Network, which provides an unprecedented dataset for studying the complex geodynamic processes that drive seismic activityby gathering recordings from both permanent and temporarybroadband seismic stations deployed across the Mediterranean area. The National Institute for Earth Physics (NIEP), Romania, contributes to AdriaArray through the deployment, operation, and maintenance of seismic stations, as well as data management, technical guidance, and research to better understand the geodynamics of this complex region. NIEP participated in the deployment of 44 temporary broadband stations, provided by several European institutions. The temporary stations complemented the permanent stations of the Romanian Seismic Network, enhancing station coverage and seismic monitoring resolution, particularly in notable tectonic areas such as Vrancea, one of Europe's most seismically active regions. Vrancea is well-known for its large-magnitude intermediate-depth earthquakes (M > 7.0), generated by the descent of a lithospheric slab beneath the southeastern Carpathians, making it a crucial area for many research topics. This study presents a comprehensive overview of AdriaArray activities in Romania, including details on sites and seismic equipment. We evaluate the quality of the recorded data to ensure the reliability of seismic observations. This includes near-real time data flow monitoring, data availability analysis, ambient noise characteristics, and sensor orientation assessment to confirm the accuracy of the recordings. High-quality seismic data are essential for refining seismic monitoring techniques and enhancing automated processing methods. The results of this study support future research aimed at improving our understanding of seismic activity in Romania, lithospheric structure, tectonic evolution, and earthquake dynamics, while also contributing to the broader goal of the AdriaArray initiative in advancing geophysical research and seismic hazard assessment across the Adriatic region and beyond.

Splitting of shear waves proves their propagation within an anisotropic medium. Frequently used methods of evaluating of upper mantle anisotropy search for two parameters - the delay time of the slow split shear wave and the polarization direction of the fast split shear wave. The parameters retrieved by the standard methods such as energy minimization on the transverse component of the shear waveforms or eigenvalue of cross-correlation matrix suffer from e.g., ubiquitous noise, errors in sensor orientation and numerous so-called ‘null splits’ or unrealistically large values. However, well-resolved splitting parameters from core-mantle refracted shear SK(K)S phases are limited to relatively narrow fans of back azimuths. Such incomplete back-azimuth coverage prevents modelling anisotropic structures with symmetry axes oriented generally in 3D, i.e., with tilted axes, to be compatible with 3D anisotropic models from independent observables. Generally used averages of time delays and polarization pairs lead to simplified models of the upper mantle, which concentrate on modelling the present-day flow in the sub-lithospheric mantle.Therefore, we propose a new method directly exploiting variations in width and orientation of particle motion (PM) of split shear waves, which allows measuring anisotropic characteristics for a larger amount of waveforms and improves azimuthal coverage in a region. We characterize the PM by two parameters, the PM width and the PM orientation. At each station, we plot the normalized width of the PM as a ratio of lengths of the minor to major axes in dependence on back-azimuths. Variations of the PM width with back-azimuth exhibit oscillations with several extremes of different amplitudes. Such behaviour results from wave propagation through the anisotropic upper mantle. One of the advantages of the method is that the width of the PM is invariant of potential mis-orientation of sensors.We test the PM method on a set of SKS waveforms recorded at a subset of stations included in several recent or running passive seismic experiments (EASI, AlpArray, PACASE, AdriaArray). The stations form a band of about 200km broad running from the western Bohemian Massif through the Eastern Alps to the Adriatic Sea. Stations characterized by similar variations of the PM parameters group into sub-regions, which are compatible with the main tectonic features of the whole region. The formation of such lithospheric blocks of similar anisotropic signals is in agreement with 3D self-compatible anisotropic models of the mantle lithosphere domains derived from independent observables. We present complementary studies of the anisotropic structure of the mantle lithosphere in contributions by Zlebcikova et al. (GD7.1, EGU 2024), which shows anisotropic model of the upper mantle derived from 3D coupled anisotropic-isotropic teleseismic tomography (code anitomo), 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 present the Moho depths in the Bohemian Massif and Western Carpathians derived from P-to-S receiver functions calculated from broad-band P-coda waveforms from teleseismic events recorded at temporary and permanent stations operated in a region within 10–23º E and 47.5–52º N during last two decades. By the Zhu and Kanamori method (2000) and the Ps time delays (Kvapil et al., 2021), we process data collected from running AdriaArray Seismic Network (since 2022), PACASE experiment (2019 – 2022), AlpArray Seismic Network (2015 – 2019) 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). By applying different methods, we aim at upgrading the current knowledge of the crust in the broader surroundings of the European Alps (Michailos et al., 2023), the Pannonian Basin (Kalmar et al., 2019), and the Carpathians. Locally, differences between Moho depth from individual methods could highly exceed 5 km, thus reflecting various sensitivities of individual methods to the local complex structure. An extended amount of data and regionally combined evaluation provide a homogeneous estimate of Moho depths, particularly for the usage in deep Earth studies, e.g., in applying crustal corrections in the upper mantle tomography of Central Europe.

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.

The northern extension of the AdriaArray, a dense network of broadband seismic stations, covers the southeastern part of the Bohemian Massif, the Eastern Alps, the Western Carpathians, and the northernmost part of the Pannonian Basin. Considering also the previous passive experiments carried out since 2015, the existing 32 broadband permanent stations have been complemented by 89 temporary stations deployed in the collaborative effort of institutions from the Czech Republic, Poland, Austria, and Slovakia. We document the seismic station configuration, instrumental equipment, data transmission, preprocessing, and availability, as well as the general organization of the network. Since spring 2022, when the AdriaArray network started its operation, to January 2025, approximately 2.8 TB of data recorded by the temporary stations has been transmitted to the European Integrated Data Archive (EIDA), with an average completeness of 80% and real‑time operation for 91% of the stations. The network records valuable data for a wide range of Earth science studies, including earthquake location, seismic hazard assessment, and high-resolution images of the crust and upper mantle structure. As examples of data utilization, we show Moho depth variations from the Bohemian Massif to the West Carpathians and the northernmost part of the Pannonian Basin, as well as prevailing NW‑SE polarization azimuths of the fast shear waves from the splitting evaluations at stations in the broader surroundings of the Carpathians.

The high- to ultrahigh-pressure ((U)HP) metamorphic rocks are present within the European Variscan belt between the Bohemian and Iberian massifs (the Galicia-Moldanubian zone) and they are partly incorporated into the Alpine orogenic system. Due to their involvement in various allochthonous units, the affiliation of the (U)HP rocks to the suture zones that were the sites of their initial exhumation, is not always clear. The Bohemian Massif preserves the best evidence of Variscan sutures with clear relationships to the exposed (U)HP rocks. They are the Moldanubian and the Saxo-Thuringian sutures bounding the Tepla-Barrandian block from the SSE and NNE, respectively. The distribution of (U)HP rocks coincides with the boundaries of mantle lithosphere domains, delimited from large-scale seismic anisotropy, and reveals the NW-ward inclination of the Moldanubian mantle lithosphere domain beneath the Tepla-Barrandian block and thus a subduction polarity to the NW. The eastern margin of the Tepla-Barrandian block contains a magmatic arc, which is in direct contact with the Moldanubian orogenic wedge, and both are penetrated by lamprophyre dykes (similar to 340 Ma), which dates the cessation of the collision-related shortening and crustal consolidation. The overall crustal geometry of the Saxo-Thuringian suture implies the SE-ward polarity of subduction during its formation. However, based on seismic tomography and anisotropy model, the suture at mantle depths appears as a sub-vertical boundary between the Saxo-Thuringian and the Tepla-Barrandian lithosphere domains. The Saxo-Thuringian zone bears evidence of blueschist facies metamorphism in the (para)autochthonous units, which are strongly retrogressed. Compared to the Moldanubian zone, (U)HP rocks are less common in the Saxo-Thuringian zone and occur as nappes and klippes, some of which are exposed near the Moldanubian suture. The similarities of the Saxo-Thuringian (U)HP rocks to those in the Moldanubian zone and their allochthonous positions favour formation of some of the (U)HP rocks along the Moldanubian suture and their subsequent emplacement into the Saxothuringian zone. The Moldanubian suture appears to control the distribution of most of the (U)HP rocks exposed along the European Variscan Belt. They all show similarities regarding lithology, mainly fragments of mantle rocks included in felsic materials, and their granulite-amphibolite facies thermal overprint.

Seismic anisotropy, modelled from propagation of teleseismic longitudinal (P) and shear (S/SKS) waves, provides unique constraints on tectonic fabrics and character of past and present-day deformations of the continental lithosphere in different tectonic environments (e.g., Babuška and Plomerová, Solid Earth Sci. 2020). We evaluate body-wave anisotropic parameters (directional variations of velocities or shear-wave splitting) in 3D and invert for three-dimensional structure of the upper mantle (Munzarová et al., GJI 2018) with no limitation imposed on the symmetry axis orientation into the horizontal or vertical directions. Resulting models of the continental lithosphere are based on data from several passive seismic experiments in Archean, Proterozoic and a variety of Phanerozoic provinces of Europe. We emphasize the importance of the three-dimensional approach of modelling anisotropy to be able to detect tilts of symmetry axes in individual domains of the mantle lithosphere. The extent of the domains is delimited by changes in orientation and strength of anisotropy. Assuming only azimuthal anisotropy, similarly to only isotropy, may create artefacts and lead to spurious interpretations (e.g., VanderBeek and Faccenda, GJI 2021). Prevailingly sub-horizontal preferred orientation of olivine, the most abundant mantle mineral, arises from mantle convection in newly formed oceanic lithosphere on both sides of the mid-oceanic ridges. Systematically oriented dipping fabrics in domains of the continental mantle lithosphere reflect series of successive subductions of ancient oceanic plates and their accretions enlarging primordial continent cores. Consequent continental break-ups and assemblages of wandering micro-plates preserve “frozen” anisotropic fabrics and create patchwork structures of the present-day continents.

The Pannonian-Carpathian-Alpine Seismic Experiment (PACASE) is a collaborative project based on a large, passive seismic network comprising 214 temporary stations. Among the primary aims are the imaging of the Earth’s crustal, lithospheric and upper mantle structure, including joint inversions; monitoring and mapping of seismic activity; and interpretation of the data from seismotectonic and geodynamic perspectives. The base of the cooperation is a high-quality, broadband seismic network covering the very centre of Europe: the Eastern Alpine and Western Carpathian Mountain ranges, the Bohemian Massif, and the sedimentary Molasse and Pannonian Basins. In this overview, we focus on the implementation and achieved field goals of PACASE, such as seismic station configuration, general network organization, data availability and access to the dataset. With selected seismological examples, we demonstrate the good usability of the records of earthquake detection, and a first publication attests to the structural imaging capability of the PACASE data. We assess the background noise level at various stations and its variations in time and space. Our aim is to collect all practical information relevant to serve as a long-term reference for the PACASE.

SUMMARY We performed seismic ambient noise tomography to investigate the shallow crustal structure around the Ivrea geophysical body (IGB) in the Ivrea-Verbano Zone (IVZ). We achieved higher resolution with respect to previous tomographic works covering the Western Alps, by processing seismic data collected by both permanent and temporary seismic networks (61 broad-band seismic stations in total). This included IvreaArray, a temporary, passive seismic experiment designed to investigate the IVZ crustal structure. Starting from continuous seismic ambient noise recordings, we measured and inverted the dispersion of the group velocity of surface Rayleigh waves (fundamental mode) in the period range 4–25 s. We obtained a new, 3-D vS model of the IVZ crust via the stochastic neighbourhood algorithm (NA), with the highest resolution between 3 to 40 km depth. The fast and shallow shear wave velocity anomaly associated with the IGB presents velocities of 3.6 km s−1 directly at the surface, in remarkable agreement with the location of the exposed lower-to-middle crustal and mantle outcrops. This suggests a continuity between the surface geological observations and the subsurface geophysical anomalies. The fast IGB structure reaches vS of 4 km s−1 at 20–25 km depth, at the boundary between the European and Adriatic tectonic plates, and in correspondence with the earlier identified Moho jump in the same area. The interpretation of a very shallow reaching IGB is further supported by the comparison of our new results with recent geophysical investigations, based on receiver functions and gravity anomaly data. By combining the new geophysical constraints and the geological observations at the surface, we provide a new structural interpretation of the IGB, which features lower crustal and mantle rocks at upper crustal depths. The comparison of the obtained vS values with the physical properties from laboratory analysis of local rock samples suggests that the bulk of the IGB consists of a combination of mantle peridotite, ultramafic and lower crustal rocks, bound in a heterogeneous structure. These new findings, based on vS tomography, corroborate the recent interpretation for which the Balmuccia peridotite outcrops are continuously linked to the IGB structure beneath. The new outcomes contribute to a multidisciplinary framework for the interpretation of the forthcoming results of the scientific drilling project DIVE. DIVE aims at probing the lower continental crust and its transition to the mantle, with two ongoing and one future boreholes (down to 4 km depth) in the IVZ area, providing new, complementary information on rock structure and composition across scales. In this framework, we constrain the upper crustal IGB geometries and lithology based on new evidence for vS, connecting prior crustal knowledge to recent active seismic investigations.

First images of structure and dynamics of the Alpine orogeny came mostly from recordings of permanent observatories. Though density of permanent observatories has increased substantially since mid of the 20th century, yet it was not enough for detailed structural studies of the lithosphere-asthenosphere system in the complex Alpine-Mediterranean mountain belts. The tomographic images have changed especially during the last three decades, when several both small- and large-scale passive seismic experiments recorded huge amount of high-quality data at dense arrays, composed from both permanent and hundreds of temporarily installed stations. Thus the former monotonous eastward striking bend of the Alpine orogeny split into separated subductions with opposite polarity, one in the Western Alps and one in the Eastern Alps (Babuška et al., Tectonophysics 1990), later confirmed in more detailed tomography by Lippitsch et al. (2003), which included data from the TRANSALP experiment (TRANSALP Working Group, EOS 2001), the first research transect oriented on orogenic processes in the Eastern Alps. Data recorded during international AlpArray experiment, series of its complementary projects (e.g., EASI, SWATH-D, PACASE) as well as several other previous small-scale experiments (e.g., ALPASS, BOHEMA, CIFALPS, CPB) allowed to unravel details of the Alpine structure and to search geodynamic models of the Alpine subductions. However, new questions arise with the new more precise images of the Alps. Following questions belong among them: 1) what is the origin of the E. Alpine subduction (Adriatic or European, or both); 2) if the E. Alpine slab is attached or detached, or, at which depth it resides; 3) how different methods, particularly crustal models incorporated into the body-wave tomography, disturb the real visualization of the E. Alpine slab. In this contribution we image the E. Alpine slab, evaluate effects of the crustal models on perturbations in the upper 100 km of the mantle and aim at answering the basic questions on the subduction beneath the Eastern Alps.

We use seismic waveform data from the AlpArray Seismic Network and three other temporary seismic networks, to perform receiver function (RF) calculations and time-to-depth migration to update the knowledge of the Moho discontinuity beneath the broader European Alps. In particular, we set up a homogeneous processing scheme to compute RFs using the time-domain iterative deconvolution method and apply consistent quality control to yield 112 205 high-quality RFs. We then perform time-to-depth migration in a newly implemented 3D spherical coordinate system using a European-scale reference P and S wave velocity model. This approach, together with the dense data coverage, provide us with a 3D migrated volume, from which we present migrated profiles that reflect the first-order crustal thickness structure. We create a detailed Moho map by manually picking the discontinuity in a set of orthogonal profiles covering the entire area. We make the RF dataset, the software for the entire processing workflow, as well as the Moho map, openly available; these open-access datasets and results will allow other researchers to build on the current study.

Teleseismic body-wave tomography represents a powerful tool to study regional velocity structure of the upper mantle. Particularly, a need of retrieving anisotropic signal calls for processing of a huge amount of P-wave travel times (Munzarová et al., GJI 2018). Therefore, automatic picking procedures are needed to supply tomography codes with a large amount of highly accurate absolute arrival times and/or travel-time residuals of body-wave propagation. We present and test a fully automated tool - TimePicker 2017 (Vecsey et al., 2021) for measuring P-wave arrival times on array recordings of passive experiments. The TimePicker 2017 is developed in the ObsPy/Python platform (Krischer et al., 2015) which combines picking, waveform cross-correlation and beamforming. The picker is based on two-step signal cross-correlations and allows us to measure absolute arrival times. Instead of a subjective selection of a reference trace, it cross-correlates all pairs of traces and forms a reference low-noise beam trace as a stack of the shifted traces at all stations. The picker cross-correlates all signals to the reference beam, automatically identifies outliers, and complements all picked absolute arrival times by their error estimates. We applied the TimePicker 2017 on a set of seismograms from 1920 earthquakes from epicentral distances greater than 30° recorded at 240 temporary and permanent stations involved in the AlpArray experiments. We show uncertainties of measured P-wave arrivals, means and medians of uncertainties for both the complete dataset as well as for subset selected for tomography, and test effects of the standard selection of a reference trace vs. the low-noise beam trace as the reference trace in the TimePicker 2017.

The European Alps, formed by the interactions between the European and Adriatic plates, is a unique geological structure that has been extensively studied over the past decades. Despite numerous active and passive seismic investigations in the past, the crustal structure across the whole Alpine domain is somehow limited - mainly due to the limited number of seismometers available. The deployment of the AlpArray Seismic Network provides, which consisted of around 600 broadband seismometers and was operational from early 2016 till mid-2019, offers a unique opportunity to further update the current knowledge of the crustal structure beneath the European Alps by employing Receiver function (RF) analysis. RF method can provide an efficient way to image the structures and the discontinuities within the uppermost part of the Earth. We use teleseismic earthquakes with M≥5.5 and M<8.5 and epicentral distances ranging between 30 and 90 degrees that occurred during the operational time of the AlpArray Seismic Network. We compute RFs using a time-domain iterative deconvolution method. We apply quality control steps to both the original three-component waveforms and the calculated RFs to ensure that we only use high-quality signals. As of abstract submission, we are in the process of calculating the RFs. We also intend to perform a time to depth migration, in a 3D spherical coordinate system, to the RFs. This methodology, together with unprecedented data coverage, will provide us with migrated profiles that will image the structure of the crust and map the Moho depths at a great level of detail.

Teleseismic body-wave tomography represents a powerful tool to study regional velocity structure of the upper mantle. Particularly, a need of retrieving anisotropic signal calls for processing of a huge amount of P-wave travel times (Munzarová et al., GJI 2018). Therefore, automatic picking procedures are needed to supply tomography codes with a large amount of highly accurate absolute arrival times and/or travel-time residuals of body-wave propagation. We present and test a fully automated tool - TimePicker 2017 (Vecsey et al., 2021) for measuring P-wave arrival times on array recordings of passive experiments. The TimePicker 2017 is developed in the ObsPy/Python platform (Krischer et al., 2015) which combines picking, waveform cross-correlation and beamforming. The picker is based on two-step signal cross-correlations and allows us to measure absolute arrival times. Instead of a subjective selection of a reference trace, it cross-correlates all pairs of traces and forms a reference low-noise beam trace as a stack of the shifted traces at all stations. The picker cross-correlates all signals to the reference beam, automatically identifies outliers, and complements all picked absolute arrival times by their error estimates. We applied the TimePicker 2017 on a set of seismograms from 1920 earthquakes from epicentral distances greater than 30° recorded at 240 temporary and permanent stations involved in the AlpArray experiments. We show uncertainties of measured P-wave arrivals, means and medians of uncertainties for both the complete dataset as well as for subset selected for tomography, and test effects of the standard selection of a reference trace vs. the low-noise beam trace as the reference trace in the TimePicker 2017.

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.