The Pannonian basin is a continental back-arc basin in Central Europe, surrounded by the Alpine, Carpathian, and Dinaric mountain ranges. To better understand this area's tectonic affinity and evolution, a high-resolution model of the crust, the mantle lithosphere, and the asthenosphere is essential. The region's crustal structures are well documented, e.g., classical active seismic, receiver functions, and ambient noise surface wave studies, but consistent imaging of the entire lithosphere remains a challenge. Here we present a new high-resolution 3D shear wave velocity model of the crust and upper mantle of the broader Pannonian region using joint tomographic inversion of ambient noise and earthquake data.

For this purpose, we collected continuous waveform data from more than 1280 seismic stations for ambient noise cross-correlation measurements from a region centered to the Pannonian Basin and encompassing the rimming orogenic chains. This dataset embraces all the permanent and temporary stations operated in the time period from 2005 to 2018. We calculated Rayleigh wave ambient noise phase velocity dispersion curves using the phase of the noise cross-correlation functions of the vertical components in the period range from 5 to 80 s. Then we combined this dataset with existing measurements from earthquake data in the period range of 8-300 s.

At lower periods (< 50 s) and shorter interstation distances, there is a well-documented systematic discrepancy between the dispersion measurements collected by the two methods. The phase-velocity curves measured by the noise-based method are slower on average than the dispersion curves extracted by the earthquake-based method. A correction term is defined by comparing phase velocity curves from both data sets for the same station pairs. Phase velocity maps are then calculated from 5 s to 250 s periods using ambient noise and earthquake measurements.

Local dispersion curves extracted along each grid node of the 2D phase velocity maps are inverted for depth velocity models using a newly implemented Particle Swarm Optimization (PSO) algorithm to obtain the 3D distribution of the shear-wave velocities. The shear wave velocity structure reveals pronounced variations of the lithospheric thickness and physical properties related to deep tectonic mechanisms operated in the region.

How to cite: Timkó, M., El-Sharkawy, A., Wiesenberg, L., Fodor, L., Wéber, Z., Lebedev, S., and Meier, T. and the AlpArray Working Group: Crustal and upper mantle 3D Vs structure of the Pannonian Region from joint earthquake and ambient noise Rayleigh wave tomography, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7892, https://doi.org/10.5194/egusphere-egu22-7892, 2022.

The dense SWATH-D seismic network in the Central-Eastern Alps gives an unprecedented window into the collision of the Adriatic and European plates. We apply the receiver function method to the SWATH-D stations, covering approximately the area from 45-49°N and 10-15°E, supplemented by the AlpArray Seismic Network and the EASI data. A switch in the subduction polarity between the Central Alps (European subduction) and the Dinarides (Adriatic subduction) had been previously suggested to occur below the Eastern Alps but its location and nature are heavily debated. To probe this hypothesis we produce a high resolution Moho map of the Eastern Alps and derive Moho depths from joint analysis of receiver function images of direct conversions and multiple reflections, which enables us to map overlapping discontinuities. Contrary to the hypothesis suggesting the subduction of Adriatic lithosphere in the Eastern Alps, we observe the European Moho to be underlying the Adriatic Moho up to the eastern edge of the Tauern Window (~13.5°E). East of this longitude, a sharp transition from underthrusting European to a flat and thinned crust associated with Pannonian extension tectonics occurs, which is underthrust by both European crust in the north and by Adriatic crust in the south. The northeast-directed underthrusting of Adriatic lithosphere smoothly transitions to subduction below the northwestern Dinarides.

Teleseismic tomography and receiver functions show different aspects of the same system (velocity anomalies versus velocity gradients) making direct comparisons difficult. The common conversion point stacks and Moho picks show good agreement with the tomography however some key differences remain. In particular, teleseismic tomography indicates high velocity anomalies detached from the crust east of ~13°E while receiver functions, in particular the transverse component, show some evidence for connection with a continuous interface going to depth.

How to cite: Mroczek, S., Tilmann, F., Pleuger, J., Yuan, X., and Heit, B. and the SWATH-D and AlpArray Working Groups: Establishing the eastern alpine-dinaric transition with teleseismic receiver functions: Evidence for subducted European Crust, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8790, https://doi.org/10.5194/egusphere-egu22-8790, 2022.

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 v_SV 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 v_SV 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.

How to cite: Kvapil, J., Plomerová, J., Kampfová Exnerová, H., and Working Group, T. A.: Anisotropy of the Bohemian Massif lower crust from ANT - VTI model or additional azimuthal variations?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7455, https://doi.org/10.5194/egusphere-egu22-7455, 2022.

Active and past subduction systems influence the interpretation and understanding of current tectonics and velocity structures of the upper mantle of the Alps and Apennines. Computational advances over the years made it possible to identify remnant and active slabs up to great depths. SKS splitting measurements revealed mostly clockwise rotation in the Alpine region and mostly splitting parameters parallel to the Apennines (with new measurements in Central Italy). More than 700 stations were used in this study to calculate splitting intensities and with those similar but more stable fast polarization directions were recovered compared to SKS measurements. Splitting intensity measurements support a possible mantle material flowing through a tear in the Central Apennines. In the Po Plain region as well as east of the Apennine mountains anisotropy seems to be weaker. Moreover the complexity of layered anisotropy, upper mantle flow through possible slab detachments, and subduction related anisotropy with a dipping axis of symmetry are difficult to recover. Due to directional dependency of splitting intensity measurements, they can be used in tomographic inversions to get depth dependent horizontal anisotropy. So far we are able to recover the most prominent splitting patterns and see some changes with depth, especially for anisotropic strength. In this study we intend to use our results to improve tomographic images of the upper mantle by mapping and comparing existing and new anisotropy measurements (e.g., SKS, Pn anisotropy, azimuthal anisotropy from surface waves tomography, and splitting intensities).

How to cite: Confal, J. M., Pondrelli, S., Baccheschi, P., Faccenda, M., Salimbeni, S., and AlpArray Working Group, T.: Identifying Seismic Anisotropy Patterns and Improving Tomographic Images in the Alps and Apennines Subduction Environments with Splitting Intensity , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7660, https://doi.org/10.5194/egusphere-egu22-7660, 2022.

Characterized by the coexistence of different compressional and extensional phases associated with episodes of orogenesis, slab rollback, slab tearing and oceanic spreading, the Central Mediterranean represents one of the most interesting convergent margin on Earth. Since the late 1990s, several seismologists have studied this region aiming at imagining the isotropic and anisotropic structures below its surface. Although numerous researchers have demonstrated that performing P-wave tomography neglecting seismic anisotropy can introduce significant imaging artefacts, prior tomographic studies have largely assumed an isotropic Earth. Using the method proposed by VanderBeek & Faccenda (2021), here we discard the isotropic approximation and invert for both P-wave isotropic velocity anomalies and seismic anisotropy and present the first 3D anisotropic P-wave tomography of the upper mantle covering the entire Central Mediterranean. Our results show that inverting for seismic anisotropy strongly reduces the magnitude of the isotropic P-wave anomalies. This suggests that lateral variations in temperature and/or composition are smaller that what can be inferred from purely isotropic tomographies. P-wave fast azimuths orient mostly parallel to the trend of the Balcanic and the Alpine orogens in Eastern and Central Europe, respectively. In the Central Mediterranean the P-wave fast azimuths are sub-parallel to the Oligocene/Miocene-to-present retreating direction of the Ionian trench which led to the opening of the Liguro-Provençal and Thyrrenian basins and rotation of the Corsica-Sardinia block. We find that the pattern of the P-wave fast azimuths is largely consistent with the S-wave fast azimuths determined from the splitting of SKS waves and from Rayleigh waves. This poses further constraints on the interpretation of the regional geodynamic evolution and on the accuracy of the employed inverse method.

References:

VanderBeek, B. P., & Faccenda, M. 2021. Imaging upper mantle anisotropy with teleseismic P-wave delays: insights from tomographic reconstructions of subduction simulations. Geophysical Journal International,225(3), 2097–2119.

How to cite: Rappisi, F., VanderBeek, B. P., Faccenda, M., Morelli, A., and Molinari, I.: 3D anisotropic P-wave tomography of the Central Mediterranean: new insights into slab geometry and upper mantle flow patterns, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8102, https://doi.org/10.5194/egusphere-egu22-8102, 2022.

The ICDP DIVE project (www.dive2ivrea.org) is aimed at addressing fundamental questions on the nature of the lower continental crust and its transition to the mantle, in a first phase through two drillings in the Ivrea Verbano zone (IVZ). The IVZ, considered the world's best outcrop of lower crustal continental rocks, is the exposed part of the Ivrea Geophysical Body (IGB), a major high gravity and high seismic velocity anomaly studied since the 1960s and strongly related to Western Alps structural and tectonic history. Beneath the IVZ the Moho possibly reaches very shallow depth (locally ~1±1 km b.s.l.), making this site unique all over the World.

The two proposed drillings will start in the 2022 in Val D’Ossola: the first in Ornavasso and the second in Megolo, 7 km apart from each other. The assemblage of the two will constitute the most complete record of lower continental crust. Physical and chemical data systematically collected downhole as well as along drill cores will be combined and compared with local/regional geophysical and geological surveys. Within this frame and scope, a dedicated seismographic network named DIVEnet has been planned to monitor local earthquakes and operation-related seismic activity.

Starting from summer 2021 the survey and seismic station deployment started to have all stations running by January 2022. So far 10 seismographic stations provided by INGV and University of Lausanne have been installed within a 15 km maximum distance from the mid-point between the two drilling sites and recording in continuous mode (100 sps). One of the seismometers will be housed in the first completed borehole while the second one is being drilled. Given that the area is characterized by low natural local seismicity and low seismic stations density, having a long time record of background activity and background noise, including the period before and after the drilling activities’ initiation, is of crucial importance. The acquisition and first elaboration of seismic data have been actively included in the routine work at INGV.

How to cite: Pondrelli, S., Hetényi, G., Salimbeni, S., Cavaliere, A., Danesi, S., Ercolani, E., Molinari, I., Giunchi, C., Michailos, K., Piromallo, C., Zaccarelli, L., Cultrera, G., Cogliano, R., Riccio, G., and Zanetti, A.: The DIVEnet: a local seismographic network monitoring the lower continental crust drillings for the ICDP-DIVE project, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8725, https://doi.org/10.5194/egusphere-egu22-8725, 2022.

The Ivrea-Verbano Zone (IVZ) located in the Italian Alps is known as one of most complete archetypes of continental crust–upper mantle section on Earth (e.g. Pistone et al., 2017). Because of its accessibility at the surface it can be used as natural laboratory to improve the understanding of the crust–mantle transition zone. Several geophysical observables indicate the presence of mantle rocks (high density, high seismic velocity) in the shallow sub-surface (~ 1 km), commonly known as the “Bird’s Head” or Ivrea body (Berckhemer, 1968; Diehl et al., 2009; Scarponi et al., 2021). 

The project SEIZE images and characterizes the shallow upper crust at the Balmuccia site (Italy) providing depth, extent and shape of the outcropping Ivrea body as well as its rock properties. Our tomographic study covers the crust down to about 3 km depth, while seismic reflection imaging is possible down to 6 km depth or deeper. With SEIZE we contribute to the comprehensive ICDP Drilling program in the Ivrea-Verbano ZonE (DIVE, www.dive2ivrea.org).

To tackle this task, a controlled source (vibroseis) seismic experiment was carried out in the region around Balmuccia in October 2020. The seismic survey comprised two crossing profiles with a total length of 28 km which ran along (NNE-SSW) and across (W-E) the Balmuccia peridotite. In total, 432 vibro points were acquired with a nominal distance of ~60 m which were recorded using a fix-spread (110 receivers, ~250 m spacing) and a roll-along setup (330 receivers, ~20 m spacing).

To obtain a structural image of the shallow upper crust various seismic techniques are applied: The fix-spread data set is used to recover the velocity structure down to 3 km depth. By using a 3D Markov chain Monte Carlo travel time tomography a shallow, distinct high velocity body is imaged in 3D near Balmuccia, at the proposed drill site. Reflection seismic processing is applied to the roll-along data set. However, the difficult terrain setting (deep mountain valleys) results in complex wave propagation that is challenging for conventional processing methods (e.g. static and dynamic corrections, CDP stacking). Therefore, pre-stack migration techniques are applied enabling the imaging of steeply dipping structures.

How to cite: Wawerzinek, B., Ryberg, T., Bauer, K., Stiller, M., Haberland, C., Zanetti, A., Ziberna, L., Hetényi, G., Weber, M., and Krawczyk, C. M.: SEismic imaging of the Ivrea ZonE (project SEIZE) reveals the 3D structure of the Ivrea body near Balmuccia, Italy, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12266, https://doi.org/10.5194/egusphere-egu22-12266, 2022.

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 v_S 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 v_S-depth profiles with the use of the Neighborhood Algorithm, to produce a 3D v_S 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.

How to cite: Scarponi, M., Kvapil, J., Vecsey, L., Plomerová, J., Working Group, I., and Working Group, A.: Towards a high-resolution vS crustal velocity model for the Ivrea Geophysical Body: constraints from seismic ambient noise tomography, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7333, https://doi.org/10.5194/egusphere-egu22-7333, 2022.

With the advent of plate tectonics in the last century, our understanding of the geological evolution of the Earth system improved essentially. The internal deformation and evolution of tectonic plates remain however poorly understood. This holds in particular for the Central Mediterranean: The formerly much larger Adriatic plate is recently consumed in tectonically active belts spanning at its western margin from Sicily, over the Apennines to the Alps and at its eastern margin from the Hellenides, Dinarides towards the Alps. High seismicity along these belts indicates ongoing lithospheric deformation. It has been shown that data acquired by dense, regional networks like AlpArray provide crucial information on seismically active faults as well as on the structure and deformation of the lithosphere. The Adriatic Plate and in particular its eastern margin have however not been covered by a homogeneous seismic network yet.

Here we report on the status and preparation of AdriaArray – a seismic experiment to cover the Adriatic Plate and its actively deforming margins by a dense broad-band seismic network. Within the AdriaArray region, currently about 950 permanent broad-band stations are operated by more than 40 institutions. Data of 90% of these stations are currently available via EIDA. In addition to the existing stations, 385 temporary stations from 18 mobile pools are to be deployed in the region to achieve a coverage with an average station distance of about 50 – 55 km. The experiment will be based on intense cooperation between network operators, ORFEUS, and interested research groups. Altogether, more than 50 institutions will participate in the AdriaArray experiment. We will introduce the time schedule, participating institutions, mobile station pools, maps of suggested temporary station distribution with station coverage and main points of the agreed Memorandum of Collaboration. The AdriaArray experiment will lead to a significant improvement of our understanding of the geodynamic causes of plate deformation and associated geohazards.

How to cite: Kolínský, P., Meier, T., and Seismic Network Working Group, T. A.: Status and Implementation of the AdriaArray Seismic Network, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7246, https://doi.org/10.5194/egusphere-egu22-7246, 2022.

Tectonic reconstructions of lithospheric plate motion can be approached by different geological methods. However hypotheses derived from these findings are often not validated in a physically consistent manner. Therefore we employ 3D geodynamic modelling in order to test geological reconstructions.

In this work, 3D thermomechanical forward simulations of the Alpine-Mediterranean area are conducted using the software LaMEM (Kaus et al. (2016)). A viscoelastoplastic rheology and an internal free surface are applied, which means that apart from the internal dynamics also the surface response can be investigated. Kinematic reconstructions of Le Breton et al. (2021) at 35 Ma serve as an initial setup for the simulations. The goal of these simulations is to determine the main driving forces of plate dynamics in this area. This is done by evaluating effects of different model parameters such as the thermal structure and the geometry of the slabs, the viscosity of the mantle and brittle parameters of the crust.

The geodynamic behaviour of the Alpine-Mediterranean area is dominated by various subducting plates which makes it particularly difficult to distinguish the unique influence of different geodynamic processes. The Adriatic microplate plays a key role in the development of the Alpine Orogeny and its plate motion and therefore serves as a marker as it is possible to compare the current position of this plate with the simulation itself. Even though these forward simulations are not capable of exactly reconstructing the current tectonic setting, they provide insights into parameters which influence the subduction dynamics.

First results suggest that the plate motion of Adria is primarily driven by the interaction of the Calabria slab and the Hellenic slab and that the propagation of these slabs strongly depends on the slab geometry and the initial trench location. Furthermore the spreading rate of rifting in the Liguro-Provençal Basin massively affects the timing of Adria’s plate motion.

Kaus, B. J. P., A. A. Popov, T. S. Baumann, A. E. Pusok, A. Bauville, N. Fernandez, and M. Collignon, 2016: Forward and inverse modelling of lithospheric deformationon geological timescales. Proceedings of NIC Symposium.

Le Breton, E., S. Brune, K. Ustaszewski, S. Zahirovic, M. Seton, R. D. Müller, 2021: Kinematics and extent of the Piemont–Liguria Basin–implications for subduction processes in the Alps. Solid Earth, 12(4), 885-913.

How to cite: Schuler, C., Kaus, B., Le Breton, E., and Riel, N.: Initial results of modelling 3D plate dynamics in the Alpine-Mediterranean area, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3979, https://doi.org/10.5194/egusphere-egu22-3979, 2022.

Present-day surface deformation in the Central Alps, that is, uplift and upper-crustal level seismicity in contrast to its northern and southern forelands, has been attributed to surface (i.e., climatic) and tectonic processes (i.e., subduction, slab detachment/break-off, mantle flow). Understanding the relative contribution of these processes is fundamental to understanding their coupling and role in mountain building. The present-day 3D architecture of the lithosphere (i.e., lateral variations of crustal layers and lithospheric mantle thickness) and asthenosphere (i.e., subducted slabs, attached or detached to the orogenic lithosphere) resulting from tectonic processes operating at geologic time scale serve as a boundary condition to test the contribution of surface processes. While the crustal structure in the Alps is well constrained by seismic and gravity data, the upper mantle (i.e., lithospheric mantle and asthenosphere) structure differs from that due to the diversity and subjective interpretation of seismic tomography models. We convert the results of regional shear-wave seismic tomography models to temperature models using the Gibbs-free energy minimization algorithm to define the base of the lithosphere and the position of slabs in the asthenosphere. Our results show that the shallow/attached slab in the Northern Apennines is a common feature in different tomography models, but there are differences in the Alps area. We statistically cluster tomography models into three end-members corresponding to the mean and 67% confidence intervals to address these differences objectively. These end-members represent scenarios ranging from shallow/attached slabs to almost no slabs in the Northern Apennines and Alps. The three end-member scenarios are then used as an input to model the topography and velocities by solving the buoyancy-forces driven instantaneous flow, subject to the first-order rheological structure of the lithosphere-asthenosphere system. Modelled topography and velocities are compared to the first-order patterns of observed topography and GPS derived vertical velocities to discern among the end-member scenarios. Our preliminary results suggest that the lithospheric slab subducting beneath the Northern Apennines should be connected to the overlying lithosphere, whereas it appears to be detached along most of the Alps. The sensitivity of results to the viscosity structure of the crust, lithosphere, and asthenosphere will be discussed.  

How to cite: Kumar, A., Cacace, M., Scheck-Wenderoth, M., Bott, J., Götze, H.-J., and Kaus, B.: Present-day upper mantle structure of the Alps: insights from data-driven dynamic modelling , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9206, https://doi.org/10.5194/egusphere-egu22-9206, 2022.

How to cite: Sonnet, M., Labrousse, L., Bascou, J., Plunder, A., Nouibat, A., Stehly, L., and Paul, A.: Seismic properties profiles of the alpine slab predicted by petrophysics versus ambient noise tomography lithospheric model, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7932, https://doi.org/10.5194/egusphere-egu22-7932, 2022.

Exploiting the new large seismic data set provided by the AlpArray Seismic Network (AASN) as part of the AlpArray research initiative (www.alparray.ethz.ch), we provide a highly consistent seismicity catalog with precise hypocenter locations and uniform magnitude calculations across the greater Alpine region (GAR) covering the period from 1st January 2016 to 31st December 2019.

With a backbone of 715 broadband seismic stations (415 permanent, 300 temporary) and a uniform interstation distance of ~50 km, the AASN provides a unique opportunity to assess the laterally heterogeneous GAR seismicity distribution. Regularly, the GAR seismicity is monitored and reported by a dozen national and international observatories, requiring a challenging effort to create a uniform and reliable catalog to document and investigate the complex seismicity and tectonics of the GAR.

To establish the highly consistent AlpArray Seismicity Catalog (AASC), we developed a new multi-step, semi-automated method. We applied the SeisComP3 (SC3) seismic-monitoring software and run it in playback mode to analyze the ~50 Tb of continuous data collected in 4 years for initial events detection and to calculate their hypocenter locations. We cleaned this preliminary, automatic seismic catalog from fake events and from events with an initial magnitude less than 2.0 MLv. We then made use of two additional software packages to refine phase picks and locations: the new ADAptive Picking Toolbox (ADAPT) Python library and the VELEST algorithm. The former was used to develop a new multi-picking algorithm for phase identification and precise arrival time determination. The latter was used to obtain the most reliable earthquakes locations, their quantitative error estimation and to reliably predict phase arrivals by solving the coupled hypocenter-velocity problem using the powerful joint-hypocenter determination technique (JHD). The JHD approach was also implemented as a filtering tool for outlier observations and to detect problematic events.

We eventually recalculate the local magnitude (MLv) in a consistent and uniform way, obtaining a statistical magnitude of completeness of 2.4 MLv with different catalog-based techniques. The AASC is also regionally consistent up to 3.0 M+  with seismic bulletins provided by national and international agencies.

Our final 4-year catalog contains 3293 precisely located earthquakes with magnitudes ranging between 0.4 - 4.9 MLv and it clearly delineates the major seismically active fault systems within the GAR. We additionally provide a new minimum 1D P-velocity model for the GAR and appropriate station delays, for both temporary and all permanent stations. These station delays for the permanent seismic station arrays, together with the velocity model, are key to consistently link the GAR past and future seismicity with our current catalog. This would allow the compilation of a broader consistent seismic catalog suitable for other seismological studies including, but not limited to, seismic hazard and a regional 3D local earthquake tomography.

How to cite: Bagagli, M., Molinari, I., Diehl, T., Kissling, E., and Giardini, D.: The AlpArray Seismicity Catalog, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6582, https://doi.org/10.5194/egusphere-egu22-6582, 2022.

Studies on moment tensors (MT) and focal mechanisms are of great importance for assessing regional and local seismotectonic processes, especially when a high-quality, dense network is in operation. However, common MT inversion methods are largely restricted to magnitudes > 3.5. In order to lower the completeness of MT catalogs, improved Green’s functions and/or hybrid inversion techniques are needed. In this study, we revisit small-to-moderate earthquakes, which occurred in Switzerland and surrounding regions by means of various MT inversion methods and assess the potential to improve completeness of MT catalogs in Central Alps region. To accomplish this, we implement state-of the art methods for MT inversion using either full waveform data or combinations of first-motion polarities with amplitudes and amplitude ratios. Methods based on full waveform inversion considered in this study are ISOLA (Sokos & Zahradnik 2013) and Grond (Heimann et al. 2018), as well as techniques based on amplitudes and/or polarities (HybridMT (Kwiatek et al. 2016), MTfit (Pugh & White 2018)), which can solve MTs for smaller magnitude earthquakes. Hence, the combination of multiple techniques allows to compute full or deviatoric MTs for a broader range of magnitudes and enrich the existing catalogs.

We first apply these methods to recent earthquake sequences occurred in the Central Alps between 2019 and 2021. During that period, several earthquake sequences, like the one associated with the 2021 M4.1 Arolla earthquake, occurred and show complexity on the waveforms, due to their shallow focal depths. In addition, several of the standard MT solutions calculated by the Swiss Seismological Service (SED) for these earthquakes indicate complex moment tensors with unusually high percentage of the CLVD component. To check whether such CLVD component is real and not an artifact caused, for instance, by unmodeled heterogeneities, we invert for full and deviatoric MTs using multiple 1D velocity models and algorithms. Additionally, we perform MT inversions for several earthquakes either within selected earthquake sequences or regional background seismicity. The resulting MT solutions are compared to existing high-quality focal mechanisms computed using first motion polarities as well as to high-precision double difference locations. Uncertainties of MT solutions are estimated using bootstrap-based methods. This work contributes towards an enriched high-quality focal mechanisms database for Switzerland, which could be used to revisit the regional to local stress field at unprecedented resolution and provides new insights into the complexities of active fault systems in the Central Alps region.

References:

Heimann, S., Isken, M., Kühnn, D., Sudhaus, H., Steinberg, A., Vasyura-Bathke, H., Daout, S., et al. (2018) Grond - A probabilistic earthquake source inversion framework., GFZ Data Services. doi:10.5880/GFZ.2.1.2018.003

Kwiatek, G., Martínez-Garzón, P. & Bohnhoff, M. (2016) HybridMT: A MATLAB/Shell Environment Package for Seismic Moment Tensor Inversion and Refinement. Seismol. Res. Lett., 87, 964–976. doi:10.1785/0220150251

Pugh, D.J. & White, R.S. (2018) MTfit: A Bayesian Approach to Seismic Moment Tensor Inversion. Seismol. Res. Lett., 89, 1507–1513. doi:10.1785/0220170273

Sokos, E.N. & Zahradnik, J. (2013) Evaluating Centroid-Moment-Tensor Uncertainty in the New Version of ISOLA Software. Seismol. Res. Lett., 84, 656–665. doi:10.1785/0220130002

How to cite: Mesimeri, M., Diehl, T., Clinton, J., Herwegh, M., and Wiemer, S.: Revisiting moment tensors in Switzerland: Unraveling source characteristics in Central Alps and their foreland, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11256, https://doi.org/10.5194/egusphere-egu22-11256, 2022.

At present time, the formerly much larger Adriatic Microplate is still actively being subducted beneath the Apennines and the Dinarides-Hellenides zone with continental collision and related processes occurring under the Alps and the Dinarides. These tectonic processes along with the large-scale component of the northward moving African Plate resulted in a complex 3D stress field.

In the light of the complex tectonic processes accompanying the movement of the Adriatic Plate, we aim to investigate the three-dimensional stress field in that area by stress inversion using focal mechanism data from the available CMT and RCMT earthquake catalogues. The focal mechanisms are inverted to better understand the stress regime in that region and how the stress pattern is depending on the current tectonic setting. A staggered grid algorithm was used for binning the focal mechanisms before the inversion.

The calculated 3D stress field indicates that the direction of the large-scale convergence of Africa and Eurasia is similar to the dominating direction of the maximum horizontal stress axis in the western central Mediterranean, with the exception of the Apennines, where the subduction of the Adriatic Plate beneath the northern Apennines is the primary source of stress. On the eastern margin of the Adriatic Plate the lack of deeper seismicity and a back arc basin, as well as the orogen normal orientation of the maximum horizontal stress axis in the Dinarides is pointing towards a continental subduction zone with an aseismic delaminating slab of lower lithosphere without a significant slab pull component.
Changes of the stress pattern within the Adriatic Plate may result from intraplate deformation, which points towards a fragmentation of Adria along the Mid Adriatic Ridge into two subplates, Adria Sensu Strictu in the north and Apulia in the south. While Adria Sensu Strictu is moving independently from Africa, Apulia is depending on the larger plates movement.
The inversion of the focal mechanisms from the Hellenic Subduction Zone yields results about the rotation of the stress field with depth, as the maximum horizontal stress rotates from trench normal at shallow depths to trench parallel deeper down.

How to cite: Glück, E., Meier, T., and Stipcevic, J.: The three-dimensional stress field around the margins of the Adriatic Plate derived from source mechanisms, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9691, https://doi.org/10.5194/egusphere-egu22-9691, 2022.

The Northern Alpine Foreland Basin developed in response to the collision between the European and Adriatic plates. During the Oligocene-Early Miocene coeval along-strike deposition of terrestrial and deep marine conditions are recorded in the western and eastern parts of the basin respectively. However, the mechanisms driving the observed variability in along-strike development of the basin are still poorly understood.

To study the causes of the observed along-strike variability we review published geological data and (re)interpret available 2D and 3D seismic data, constrained by well data. We interpret (1) seismic facies, (2) stratigraphic surfaces and (3) tectonic structures. Our current focus area covers the transitional zone between the western and eastern parts of the basin.

In our study we distinguish 6 stratigraphic surfaces from the Base Tertiary to the Top Aquitanian. From Upper Swabia to the German-Austrian border (along the basin strike) we observe that the top of the crystalline basement is tilted towards the east with an angle of 2-3°. Furthermore, the base of the Tertiary deposits is also tilted towards the east with an angle of 0.3°. The main structural features are E-W and NW-SE striking normal faults. In the western part of our study area the normal faults cut the crystalline basement, Mesozoic and Oligocene deposits. The faults are sealed by Rupelian deposits. Thickness changes (~20 m) occur in Rupelian and overlying Chattian deposits. Maximum offsets of up to 60 m are observed for Mesozoic reflectors. In the eastern part of our study area the normal faults cut the crystalline basement, Mesozoic, Oligocene and Early Miocene deposits. Thickness changes across these faults indicate fault activity during the Rupelian, Chattian and Aquitanian. Maximum offsets (>150 m) are observed for Chattian reflectors. Upper Aquitanian deposits seal these faults, which is younger than observed in the western part of the study area. The NW-SE striking faults confine Paleozoic grabens within the crystalline basement.

We relate the observed normal faulting of the Oligocene and Early Miocene deposits to flexural downbending of the European plate, assumed to have been caused by tectonic loading of the Alps and/or European slab pull. Furthermore, we suggest that the observed temporal variation in termination of fault activity is related to temporal and spatial variations in tectonic loading of the Alps and/or European slab pull. Finally, based on the observed eastward tilt of the top crystalline basement and Base Tertiary along the basin strike, variations in pre-existing crustal architecture must be considered.

How to cite: Eskens, L., Andrić-Tomašević, N., Süss, P. M., Ehlers, T. A., Herrmann, R., and Müller, M.: Controls on along-strike variations of basin development: a case study of the Northern Alpine Foreland Basin, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4500, https://doi.org/10.5194/egusphere-egu22-4500, 2022.

Owing to still ongoing convergence within the Europe-Adria collision zone, Switzerland is affected by heterogeneously distributed moderate seismic activity. The project SeismoTeCH aims to improve the understanding of the links between the seismic activity, existing fault structures and geodynamics in Switzerland and its close vicinity. We started with compiling existing databases on faults (fault densities, lengths and orientations), seismic activity (spatial hypocenter and magnitude distributions, detection of seismic lineaments, focal mechanisms), orientations of mean principal stress axes and recent crustal movements (GNSS, high precision levelling) in order to establish potential correspondences as well as regional variations.

Due to the long-lasting Alpine deformation, fault-orientation patterns as well as fault-densities vary between specific tectonic domains (Jura/North-Alpine foreland, Alpine frontal sediment nappe systems, External Crystalline Massifs, inner-Alpine domains and Southern Alps). Despite this variability, the fault patterns show first order correlations with the spatial arrangement of newly mapped seismic lineaments, earthquake focal planes and associated focal mechanisms. This correlation indicates a regional geodynamics-controlled reactivation of the specific fault networks during current crustal movements. In terms of recent surface movements, variations in (i) horizontal GNSS movements with respect to stable Europe and (ii) vertical uplift (from levelling and GNSS data) have to be discriminated. (i) From E to W in southern Switzerland (S-Grisons–Ticino–Valais, S of Rhone-Simplon line), horizontal movements change from NW to SW directions (velocities >0.5-0.8mm/yr). The southern Adria crustal block shows minimal to no lateral motions in the W-part and a clear NE-directed motion that is progressively increasing towards the E. This motion can be correlated with the so-called counter-clockwise rotation of the Adriatic plate. North of aforementioned domain, N- to NW-directed movements dominate but velocities decrease progressively from the central Alpine domains (<0.3-0.5mm/yr) towards southern Germany, where they are generally small (<0.3-0.4mm NE-CH). This variability between southern and central/northern Switzerland as well as that from E to W, respectively, is accommodated by NE-SW (Rhone-Simplon system) and N-S oriented strike-slip systems. (ii) Most substantial vertical uplift occurs in a WSW-ENE oriented central Alpine belt ranging from the Valais to the Grisons. Note that absolute values of this vertical uplift are 2-3 times larger compared to horizontal movements in the corresponding domains. Focal mechanisms in this high uplift belt indicate orogen-parallel NE-SW extension mainly in the S-Valais and Grisons accommodated by active normal faulting S of the Penninic front. Uplift rates gradually decrease towards the N- and S-Alpine foreland as well as towards Austria and France. Data even suggest tendencies of subsidence at very low rates in the Bresse graben, Upper Rhine graben as well as somewhat more pronounced ones in the eastern Po-plane but not in the CH-Molasse basin. Parts of the northern Alpine foreland exhibit upper to lower crustal seismic activity, while in the thick-crustal-root-enhanced high uplift domains upper crustal seismicity dominates and earthquakes below 20km depth do not occur.

Overall recent surface movements and seismicity in and along Central Alpine crustal blocks are affected by buoyancy-driven vertical combined with transpressional/-tensional horizontal movements indicating a lithosphere-scale geodynamic forcing. 

How to cite: Herwegh, M., Mock, S., Diehl, T., Brockmann, E., Truttmann, S., Kissling, E., Kurmann-Matzenauer, E., Wiemer, S., and Möri, A.: Seismotectonics in the Central Alps: An attempt to link fault structures, seismic activity and recent crustal movements, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4420, https://doi.org/10.5194/egusphere-egu22-4420, 2022.

The Dolomites Indenter represents the front of the Neogene to ongoing N(W)-directed continental indentation of Adria into Europe. Deformation of the Dolomites Indenter is well studied along its rim, documented by important fault zones such as the Periadriatic fault system, the Giudicarie belt, and the Valsugana and Montello fault systems. With this study, we aim to investigate the internal deformation of the Dolomites Indenter, which has been much less studied so far but is important for understanding crustal-scale processes during the Alpine orogeny.

Our approach to unravel the indenters exhumation and deformation history comprises (i) the compilation and acquisition of detailed structural and sedimentological field data within the Dolomites Indenter, (ii) a collection of a new and comprehensive low-temperature thermochronological dataset (this contribution), and (iii) crustal- to lithospheric-scale physical analogue modelling experiments (see contribution of Sieberer et al. in session TS7.2 – Internal deformation of the Dolomites Indenter, eastern Southern Alps: Orthogonal to oblique basin inversion investigated in crustal scale analogue models).

New field data comprise evidence for four distinguishable shortening directions. Examined intersection criteria along N-S cross sections covering the indenters extend from Periadriatic to Bassano fault system support a succession of Top SW, Top (S)SE, Top S and Top E(SE) movement. However, preexisting geometry strongly seems to affect the regional expression of respective compression phases and along strike variation of lineation trends can be observed within coherent fault systems.

The limited amount of existing thermochronological data already indicates the presence of relative vertical displacements within the Dolomites Indenter after the onset of indentation, including mostly Miocene apatite fission track (AFT) cooling ages along the Periadriatic and the Valsugana fault and several age clusters of Triassic to Jurassic AFT data. In order to obtain a detailed picture of the indenters thermotectonic evolution, an extensive set of samples has been collected along three roughly N-S striking corridors between Bolzano in the west and Tolmezzo in the east. In this contribution we present the new apatite (U-Th)/He and fission track data along the westernmost corridor (Mauls - Brixen - Valsugana - Schio).

The results of field work, comprehensive modelling of time temperature paths, and physical analogue modelling substantially contribute to the understanding of internal deformation and thus enable conclusions to be drawn about the processes at lithospheric scale.

How to cite: Klotz, T., Pomella, H., Sieberer, A.-K., Ortner, H., and Dunkl, I.: Internal deformation of the Dolomites Indenter, eastern Southern Alps: structural field data and low-temperature thermochronology, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11266, https://doi.org/10.5194/egusphere-egu22-11266, 2022.

The interpretation of seismotectonic processes within the uppermost few kilometers of the Earth’s crust has proven challenging due to the often significant uncertainties in hypocenter locations and focal mechanisms of shallow seismicity. Here, we revisit the shallow seismic sequence of Saint-Ursanne of March and April 2000 and apply advanced seismological analyses to reduce these uncertainties. The sequence, consisting of five earthquakes of which the largest one reached a local magnitude (M_L) of 3.2, occurred in the vicinity of two critical sites, the Mont Terri rock laboratory and Haute-Sorne, which is currently evaluated as a possible site for the development of a deep geothermal project. Template matching analysis for the period 2000-2021, including data from mini arrays installed in the region since 2014, suggests that the source of the 2000 sequence has not been persistently active ever since. Forward modelling of synthetic waveforms points to a very shallow source, between 0 and 1 km depth, and the focal mechanism analysis indicates a low-angle, NNW-dipping, thrust mechanism. These results combined with geological data suggest that the sequence is likely related to a backthrust fault located within the sedimentary cover and shed new light on the hosting lithology and source kinematics of the Saint-Ursanne sequence. Together with two other more recent shallow thrust faulting earthquakes near Grenchen and Neuchâtel in the north-central portion of the Jura fold-and-thrust belt (FTB), these new findings provide new insights into the present-day seismotectonic processes of the Jura FTB of northern Switzerland and suggest that the Jura FTB is still undergoing seismically active contraction at rates likely <0.5 mm/yr. The shallow focal depths provide indications that this low-rate contraction in the NE portion of the Jura FTB is at least partly accommodated within the sedimentary cover and possibly decoupled from the basement. This trenspressive regime is confirmed by the ML4.1 Réclère earthquake of December 24. 2021, which occurred ~20 kilometres west of St. Ursanne in the uppermost crust.

How to cite: Lanza, F., Diehl, T., Deichmann, N., Kraft, T., Nussbaum, C., Schefer, S., and Wiemer, S.: The Saint-Ursanne earthquakes of 2000 revisited: Evidence for active shallow thrust-faulting in the Jura fold-and-thrust belt, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9314, https://doi.org/10.5194/egusphere-egu22-9314, 2022.

For thorough understanding of the dynamics of mountain building processes, it is crucial to reconstruct the youngest crustal deformation history over time. Low-angle normal faults are features caused by orogen-parallel extension, which occurs in the last stage of collision. Low-angle normal faults play a key role in the exhumation of the lower crust and they are the reason for most of the seismicity within the chain.

We carried out microstructural analyses on an outcrop in the footwall of one of the major normal faults of the Alpine chain, the Simplon Fault Zone. This low-angle normal fault extended the crust by tens kilometers and it caused exhumation of its footwall, the deeper lower crust of the Alps, i.e. the Penninic nappes. The Simplon Fault Zone itself consists of a thick mylonitic zone overprinted by a narrow cataclastic zone, with the same kinematics. Its timing evolution history from ductile to brittle deformation is still under discussion. This study shows a new microstructural analysis from a fault gouge within the footwall of the northern part of the Simplon Fault Zone, and how it can reconstruct the different stages of exhumation history of this shear zone.

Results from micro-structural analyses show grain boundary migration features on folded quartz veins. This suggests that the protolith of the fault zone was at high temperature conditions, T>600°C, during dynamic deformation. This folding belongs to extension-parallel folds that affect only the ductile shear zone. The presence of greenschist facies minerals suggests that the rock was exposed to low temperature and pressure conditions (T=300-400°C, P=0.2GPa). Pressure-solution mechanisms affect both quartz and greenschist paragenesis, indicating formation in a shallow position of the shear zone. The last deformation was purely brittle, as shown by vertical calcite veins or fractures in quartz. It suggests a near-surface position of the fault.

Altogether, these multiple deformation phases within the gouge samples indicate a continuous exhumation history from high to low temperatures, with clear cross-cutting relationships. However, the lack of cataclasite features can be related to an involvement of the rocks within the fault core in a subsequent stage of deformation. To explain this we suggest a model in which the footwall maintained a high temperature over a long time, which inhibited cataclastic processes.

How to cite: Argante, V., Tanner, D. C., Brandes, C., Von Hagke, C., and Tsukamoto, S.: Inside the fault core in the footwall of Simplon Fault Zone (Central Alps): ductile to brittle deformation history shown by fault gouge, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12668, https://doi.org/10.5194/egusphere-egu22-12668, 2022.

In the Alps, the Adriatic plate convergence provoked eastward lateral extrusion compensated by strike-slip faulting and N-directed thrusting. Since the Miocene, these complex processes have led to several paleostress phases. Since the Quaternary phase is the least recognized, we used karst cave passages as the geomorphic displacement indicators. This study presents an overview of 190 Quaternary fault ruptures in totally 27 caves in the Eastern Alps, some radiometrically dated, and the paleostress analysis based on cave passages offset. Reactivated faults have been registered with their orientation, slip vector and offset, in caves adjacent to major fault systems of the Eastern Alps. The paleostress was computed using the multiple inversion method for heterogeneous fault-slip data.

Most active faults in caves along the southern part of the sinistral Vienna Basin Transfer Fault were NW-SE, and NNE-SSW oriented and revealed mostly normal to sinistral kinematics and cumulative offsets of a few mm to a couple of cms. The associated extensional paleostress state comprised the E-W σ₃ in agreement with the opening mode of the Vienna Basin. At sinistral Mur-Mürz Fault, the active faults striking NNE-SSE and ENE-WSW operated under a strike-slip regime with σ₁ NE-SW. In the eastern segment of sinistral Salzach-Ennstal-Mariazell-Puchberg fault associated strike-slip paleostress regime with horizontal SE-NE σ₁, and subhorizontal SE-trending σ_3. This stress regime was computed from reverse, oblique reverse, oblique normal, and sinistral strike-slip reactivated faults documented in the Hochschwab massif. The central segment of Salzach-Ennstal-Mariazell-Puchberg fault is adjoin to Totes Gebirge and Dachstein massifs. In the western part of Totes Gebirge, three stress regimes were recorded. N-S and NW-SE striking oblique normal strike-slip faults revealed an extensional regime with NE σ₃. Two strike-slip regimes with NE-SW σ₁ and subhorizontal σ₃ gently inclined to SE and NW were calculated from mostly steep oblique reverse NNE to NW striking faults with offsets up to a few decimetres. In the Dachstein massif, two paleostress phases were identified: the extensional regime with σ₃ subhorizontally tilted to NE and the strike-slip regime with N-S σ₁. Tens of active, mostly oblique normal strike-slip faults were documented in massifs adjacent to sinistral Königsee-Lammertal-Traunsee Fault: Hoher Göll, Tennengebirge and Hagengebirge. The dominating associated paleostress is an extensional regime with NE-SW σ₃. The polyphase sinistral and reverse-dextral NE-SW faults with Late Pleistocene to Early Holocene reactivations and up to 40 cm offsets, identified at the sinistral Obir Fault attributed to the dextral Periadriatic Line. Neither the strike-slip regime with ENE-plunging σ₁ nor the other strike-slip regime with σ₁ WNW oriented to fit the regional stress setting. It probably resulted from large-scale complex Karawanken Mts. transpressive shear zone deformation.

In conclusion, the paleostress multiple inversions from the Quaternary cave passage ruptures kinematic data brought original information on the paleostress regime over a significant portion of the Eastern Alps in their latest deformational period.

How to cite: Baroň, I., Szczygieł, J., Melichar, R., Plan, L., Grasemann, B., Kaminsky, E., and Scholz, D.: Quaternary paleostress regimes in the Eastern Alps inferred from ruptures in karst caves, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3790, https://doi.org/10.5194/egusphere-egu22-3790, 2022.