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Soil moisture is a key metric to assess soil health. Water held in the shallow subsurface between soil particles enables various biogeochemical and hydrological processes indispensable to soil functions. Potential soil moisture deficit may raise the irrigation demands, which further exacerbates the stress on the water supply. The changes in soil moisture can impact climate, further amplifying the climatic anomalies and intensifying extreme weather events. Thus, understanding soil moisture and its dynamics over time are of broad scientifical interest and practical implications. Despite the vital importance of soil moisture, it still lacks sufficient means to properly assess the parameter at a regional scale, which is an essential research dimension for addressing practical issues in the agricultural and environmental sectors. Ambient noise seismology provides new possibilities to infer subsurface changes in a real-time, non-intrusive, and costless manner. In this study, we map the temporal variations in soil moisture for the great Alpine region and the Italy peninsular with ambient seismic noise. It is the first time that the seismic method has been applied to map water resources at a regional scale using an ordinary seismic network setup. The seismic method helps in bridging the resolution gap between current pointwise (e.g., tensio-, electrical- and neutron-meter) and global (e.g., satellite-based remote sensing) investigations, providing complementary information for both scientific research and public decision-making.

Major faults such as the Periadriatic Fault and the Giudicarie Fault have been active in the past, and they have even been central features of the larger-scale deformation in the Alps. It seems that these faults are not active anymore though and we investigate why this is so by inspecting the orientation of the regional stress field which loads the faults mechanically. The orientation of maximum horizontal compressive stress (SHmax) is commonly estimated from in-situ borehole breakouts and earthquake focal mechanisms. Borehole measurements are expensive, and therefore sparse, and earthquake measurements can only be made in regions with many well-characterized earthquakes. Here we derive the stress-field orientation using stress-induced anisotropy in nonlinear elasticity. In this method, we measure the strain derivative of velocity as a function of azimuth. We use a natural pump-probe approach which consists of measuring elastic wave speed using empirical Green’s functions (probe) at different points of the earth tidal strain cycle (pump). The approach is validated using a larger data set in the Northern Alpine Foreland region where the orientation of SHmax is known from borehole breakouts and drilling-induced fractures. The technique resolves NNW-SSW to N-S directed SHmax which is in good agreement with conventional methods and the recent crustal stress model. The technique is then applied to the Southern Alps to understand the contemporary stress pattern associated with the ongoing deformation due to the counterclockwise rotation of the Adriatic plate with respect to the European plate. Our results explain why the two major faults in Northeastern Italy, the Giudicarie Fault and the Periadriatic Line (Pustertal-Gailtal Fault) are currently inactive, while the currently acting stress field allows faults in Slovenia to deform actively. We have demonstrated that the pump-probe method has the potential to fill in the measurement gap left by conventional approaches, both in terms of regional coverage and depth.

In this study, we used coda wave interferometry to investigate four years of continuous data from AlpArray and other locations throughout Europe. We estimate the hourly Green’s function by cross-correlating ambient seismic noise recorded at pairs of stations. The results indicate short and long-term variations of the seismic velocities and show the feasibility of large-scale monitoring with ambient seismic noise at high temporal resolution. The relative seismic velocities (dv/v) show temporal variations on the order of 10-3 in a frequency band around 1 Hz. Spectra of the velocity time series contain strong daily and sub-daily behaviour, which are primarily caused by the coupling of atmospheric processes and solid Earth. The explanatory model focuses on depth variations of the groundwater table, linking atmospheric pressure (loading and unloading the Earth's surface) to variations in seismic velocity. This study aims to understand and explain differences in daily and sub-daily behaviour across Europe. This may contribute to the hydrological characterization of the near-subsurface in central Europe.

Variations in strain/stress and fluid content can change seismic velocities in the subsurface. Monitoring velocity changes, e.g., using ambient seismic noise, may thus constrain these variations as well as the material elastic properties and their non-linear behaviour. We can test this capability by inspecting velocity changes from known effects, such as tides, temperature or atmospheric pressure affecting the upper crust. Here we present a workflow to use ambient seismic noise to derive the non-linear behaviour of hard rocks during the influence of tides, temperature and atmospheric pressure. We study one year of data from the GERES array in south Germany, which provides data to the Comprehensive Nuclear Test Ban Treaty Organization (CTBTO). The seismological GERES array consists of 25 high-quality stations located in concrete vaults with depths between 3 and 5 meters. The aperture is 4 km. We estimate hourly Green’s function by cross-correlating ambient seismic noise recorded at pairs of stations. A Wiener filter increases the signal-to-noise ratio and stabilizes the hourly calculation of relative seismic velocity change in the 1-4 Hz frequency band. We compare different techniques to measure seismic velocity changes with high precision in time, frequency, and wavelet domain. The results indicate short and long term variations of the seismic velocities. This study aims to compare this non-linear behaviour of hard rocks with other geological settings used in earlier investigations.

The Mantle Transition Zone (MTZ), the region between 410 km and 660 km depth, is vital for the Earth’s thermal and chemical evolution. The thickness of the Mantle Transition Zone (MTZ), and seismic anomalies in this region could hold clues to its current thermal and chemical state. The seismic structure of MTZ is inferred from P-to-S (Ps) receiver function imaging, using data from 723 stations from 2016 – 2020, covering a much larger area of the Alpine region than previously imaged. A total of 1368 events with magnitude > 5.5 Mw, and a distance range of 35 to 80 degrees were used. Seismic records were pre-processed to remove instrument response, filtered using a zero-phase second order Butterworth filter with cut-off frequencies at 0.025 and 0.2 Hz, followed by rotation to ZRT and LQT system. Ps receiver functions were computed by deconvolving traces with clear P-wave phase from radial (R) and SV components (Q). Iterative time domain deconvolution and extended time multi-taper frequency domain deconvolution were used. Waves with clear P-, P410s, and P660s were selected and migrated to depth using a smoothed 1D velocity model for the AlpArray region derived from EU60 seismic model for the crust and upper mantle, and IASP91 for the deeper regions. Observations of the Moho from Ps receiver functions are consistent with global and regional crustal thickness estimates. The Alpine MTZ shows evidence for subduction of the European plate in the western Alps. Punctuated anomalies beneath the Alpine region, indicated by the deflection of the 410 km and 660 km discontinuities, show evidence of slab from past subduction events reaching the MTZ, consistent with previous regional studies. Further analysis could provide clues to the dipping orientation of the subducted slab.

The orientation of SHmax is commonly estimated from in-situ borehole breakouts and earthquake focal mechanisms. Borehole measurements are expensive, and therefore sparse, and earthquake measurements can only be made in regions with many well characterized earthquakes. Here we derive the stress-field orientation using stress-induced anisotropy in nonlinear elasticity. In this method, we measure the strain derivative of velocity as a function of azimuth. We use a pump-probe method which consists of measuring elastic wave speed using empirical Green’s functions (probe) at different points of the tidal strain cycle (pump) as in Delorey et al. (2021). The approach is applied to data from the AlpArray in the Alpine foreland region, where the orientation of maximum horizontal compressive stress is well-known from borehole breakouts and drilling-induced fractures.Delorey, A., Bokelmann, G., Johnson, C., Johnson, P. Estimation of the orientation of stress in the Earth's crust without earthquake or borehole data. Nature Comm. Earth Environ. 2, 190 (2021). https://doi.org/10.1038/s43247-021-00244-1

The mantle transition zone is delineated by seismic discontinuities at approximately 410-km and 660-km depth. The lateral variations in reflectivity and depth of the two seismic discontinuities reflect changes in mineralogy composition, thermal state, and water content, that is key to understanding the Earth’s dynamics. Traditional imaging methods based on the analysis of earthquake signals, such as seismic tomography and receiver function analysis, are often limited by earthquake occurrence and uncertainties related to the earthquake source parameters. Recent studies demonstrated the feasibility of recovering body waves from noise correlations, providing new prospects for imaging deep Earth [e.g., Poli et al., 2012; Boué et al., 2013]. In this study, we map the 410-km and 660-km discontinuities beneath the European Alps using reflected body waves recovered from noise correlations. To that end, we compute noise correlations using four years of continuous recordings from ∼1200 broadband stations in the greater Alpine region. To enhance the signal-to-noise ratio of the body-wave reflection phases, for each station pair, we stack daily noise correlations in selected time spans with a high level of near vertical-incident body waves and less dominant surface waves [Lu et al., 2021]. We further stack noise correlations of station pairs with common/nearby reflection points to obtain local zero-offset reflection waveforms. The retrieved P410P and P660P reflection phases clearly reveal lateral variations of both reflectivity and depth of the two discontinuities in the studied region, providing new constraints in addition to existing results from earthquake tomography and receiver function analysis. Besides, this study also sheds light on the strategies to recover deep reflection phases from noise correlations. [1] Boué, P., Poli, P., Campillo, M., Pedersen, H., Briand, X., & Roux, P., 2013. Teleseismic correlations of ambient seismic noise for deep global imaging of the Earth, Geophys. J. Int., 194(2), 844-848. [2] Lu, Y., Pedersen, H.A., Stehly, L., and AlpArray Working Group, 2022. Mapping the seismic noise field in Europe: spatio-temporal variations in wavefield composition and noise source contributions, Geophys. J. Int., 228(1), 171-192. [3] Poli, P., Campillo, M., Pedersen, H., and L. W. Grp, 2012. Body-wave imaging of Earth’s mantle discontinuities from ambient seismic noise, Science, 338(6110), 1063-1065.

The ambient seismic field is now routinely used for imaging and monitoring purposes. Most commonly, applications aim at resolving crustal-scale features and utilize ocean-generated surface waves. At smaller scales and at frequencies above the microseismic peaks, local sources of seismic energy, often anthropogenic, are dominant, and understanding of their contributions to the ambient seismic field becomes important to apply ambient noise techniques. This study uses data of an industrial-scale seismic deployment covering ∼500 km2 with 10,532 stations, each equipped with several collocated 10 Hz geophones, to provide unique insight into anthropogenic sources of seismic energy in a suburban-to-rural area. We compute amplitude levels, their distance dependency, power spectral densities, and spectrograms to describe the source characteristics. The sources we observe in great detail include windmills, a railway track and trains, cars, oil pumpjacks, power lines, gas pipelines, and airplanes. These sources exhibit time-dependent behavior that is illustrated strikingly by videos of amplitude levels in certain frequency bands that we provide as supplemental material. The data described in this study are a potential resource for future studies, such as automatic signal classification, as well as underground imaging using microseismic noise or the sources presented here.

Seismic anisotropy is a unique observational tool for remotely studying deformation and stress within the Earth. Effects of anisotropy can be seen in seismic data; they are due to mineral alignment, fractures or layering. Seismic anisotropy is linked to local stress and strain, allowing modern geophysics to derive geomechanical properties from seismic data for supporting well planning and fracking. For unravelling anisotropic properties of the crust, the teleseismic receiver functions methodology has started to be widely applied recently due to its ability in retrieving the three‐dimensional characteristics of the media sampled by the waves. The applicability of this technique is tested here by a field test carried out around the Kontinental Tiefbohrung site in southeastern Germany. We compare our results to previous investigations of the metamorphic rock pile of the Zone Erbendorf‐Vohenstrauss, drilled down to 9 km depth, which sampled an alternating sequence of paragneiss and amphibolite, in which a strong foliation has been produced by ductile deformation. The application of the receiver functions reveals the presence of two distinct anisotropic layers within the metamorphic rock pile at 0–4 km and below 6 km depth, with up to 8% anisotropy; the depth of these two layers corresponds to the location of mica‐rich paragneiss which show intense foliation, and finally proves the relation between the signal in the receiver functions, rock texture and presence of cracks. We have now the capability of providing insights from passive seismic data on geomechanical properties of the rocks, useful for geological exploration and engineering purposes, which will help influencing expensive drilling decisions thanks to future application of this seismic technique.

Mechanical stress acting in the Earth`s crust is a fundamental property that has a wide range of geophysical applications, from tectonic movements to energy production. The orientation of maximum horizontal compressive stress, SHmax can be estimated by inverting earthquake source mechanisms and directly from borehole-based measurements, but large regions of the continents have few or no observations. Available observations often represent a variety of length scales and depths, and can be difficult to reconcile. Here we present a new approach to determine SHmax by measuring stress induced anisotropy of nonlinear susceptibility. We observe that nonlinear susceptibility is azimuthally dependent in the Earth and maximum when parallel to SHmax, as predicted by laboratory experiments. Our measurements use empirical Green’s functions that are applicable for different temporal and spatial scales. The method can quantify the orientation of SHmax in regions where no measurements exist today.

We study finite-frequency effects that arise in cavity detection. The task comes along with the Onsite-Inspection part for the Comprehensive Nuclear Test Ban Treaty (CTBT), where the remnants of a potential nuclear test need to be identified. In such nuclear tests, there is preexisting knowledge about the depths at which nuclear tests may take place, and also about sizes that such cavities can attain. The task of cavity detection has consistently been a difficult one in the past, which is surprising, since a cavity represents one of the strongest seismic anomalies one can ever have in the subsurface. A conclusion of this study is that considering finite-frequency effects are rather promising for cavity detection, and that it is worthwhile to take them into account. We utilize an analytical approach for the forward problem of the a seismic wave interacting with a underground cavity in order to develop an inversion routine that finds and detects an underground cavity utilizing the transmitted wave-field.

To constrain seismic anisotropy under and around the Alps in Europe, we study SKS shear-wave splitting from the region densely covered by the AlpArray seismic network. We apply a technique based on measuring the splitting intensity, constraining well both the fast orientation and the splitting delay. 4 years of teleseismic earthquake data were processed automatically (without human intervention), from 724 temporary and permanent broadband stations of the AlpArray deployment including ocean-bottom seismometers. We have obtained an objective image of anisotropic structure in and around the Alpine region, at a spatial resolution that is unprecedented. As in earlier studies, we observe a coherent rotation of fast axes in the western part of the Alpine chain, and a region of homogeneous fast orientation in the central Alps. The spatial variation of splitting delay times is particularly interesting. On one hand, there is a clear positive correlation with Alpine topography, suggesting that part of the seismic anisotropy (deformation) is caused by the Alpine orogeny. On the other hand, anisotropic strength around the mountain chain shows a distinct contrast between western and eastern Alps. This difference is best explained by the more active mantle flow around the Western Alps. We discuss earlier concepts of Alpine geodynamics in the light of these new observational constraints.

Consistent and monochromatic signals appear as sharp peaks in frequency spectra or as continuous lines in spectrograms on many permanent and temporary seismic stations in Central Europe, especially in South-Eastern Germany, Austria and the Czech Republic. Similar observations have already puzzled the seismic community more than 20 years ago. Here we report on new observations of such monochromatic seismic signals within a 1 – 10 Hz range across central Europe using the dense AlpArray network.We identify several monochromatic signals on both permanent and temporary stations. The respective frequencies of e.g. 1.72 Hz, 2.08 Hz, 2.77 Hz or 4.16 Hz are generally stable even over long time spans (months to years). Strikingly, all such signals at any given station show identical and simultaneous short-term (minutes to days) frequency variations of up to 0.4% of the central frequency. These variations precisely correspond to fluctuations of the frequency of the European electric power network, which is regulated to 50 Hz +/- 0.4%. In fact, all persistent seismic signals that follow this behavior have frequencies of 50 Hz / n with n being an integer number (50 Hz / 29 = 1.72 Hz, 50 Hz / 24 = 2.08 Hz, 50 Hz / 18 = 2.77 Hz, 50 Hz / 12 = 4.16 Hz). We show that if the frequency of an observed spectral line is an integer fraction of the power network frequency (and only in that case) it will perfectly follow the fluctuations of the power network. This obviously raises questions about the nature of the signal itself, in particular if it is of seismic or maybe electro-magnetic origin.We confirm that the signals are of seismic origin and we have identified water turbines inside river power plants as the source. The observed frequencies correspond well to reported rotation frequencies of water turbines at several different river power plants in Southern Germany and Austria. The seismic signals may propagate to almost 100 km from the corresponding plant. We analyze the spatial distribution of signal amplitudes for a selected river power plant in Austria, and show that it is similar to expected isolines of seismic shaking for an earthquake in the region.Knowing the source of those exotic signals potentially enables us to use them for seismo-tectonic purposes. The long-term (several years) stability and the permanent availability (24h operation of water turbines) render them very interesting sources e.g. for studying temporal seismic velocity variations in the shallow crust.

Shear-wave splitting observations of SKS and SKKS phases have been used widely to map azimuthal anisotropy, and to constrain the dominant mechanism of upper mantle deformation. As the interpretation is often ambiguous, it is useful to consider additional information, e.g. based on the non-vertical incidence of core-phases. Depending on the lattice-preferred orientation of anisotropic minerals, this condition leads to a variation of splitting parameters with azimuth and enables a differentiation between various types of olivine deformation. As the fabric of olivine-rich rocks in the upper mantle relates to certain ambient conditions, it is of key importance to further define it. In this study, we predict the azimuthal variation of splitting parameters for A-, C-, and E-type olivine, and match them with observations from the High Lava Plains, Northwestern Basin and Range, and Western Yellowstone Snake River Plain. This can help to constrain the amount of water in the upper mantle beneath an area, known for a consistent, mainly E-W fast orientation, and increased splitting delay in the back-arc of the Cascadia Subduction Zone. Comparing expected and observed variations renders a C-type olivine mechanism unlikely; a differentiation between A- and E-type olivine remains more difficult though. However, the agreement of the amplitude of azimuthal variation of the fast orientation, and the potential to explain larger splitting values, suggest the occurrence of E-type olivine and the presence of a hydrated upper mantle. Along with a discrepancy to predict delay times from azimuthal surface wave anisotropy, deeper sources could further affect shear-wave splitting observations.

Mechanical stress acting in the Earth’s crust is a fundamental property that is important for a wide range of scientific and engineering applications. The orientation of maximum horizontal compressive stress can be estimated by inverting earthquake source mechanisms and measured directly from borehole-based measurements, but large regions of the continents have few or no observations. Here we present an approach to determine the orientation of maximum horizontal compressive stress by measuring stress-induced anisotropy of nonlinear susceptibility, which is the derivative of elastic modulus with respect to strain. Laboratory and Earth experiments show that nonlinear susceptibility is azimuthally dependent in an anisotropic stress field and is maximum in the orientation of maximum horizontal compressive stress. We observe this behavior in the Earth—in Oklahoma and New Mexico, U.S.A, where maximum nonlinear susceptibility coincides with the orientation of maximum horizontal compressive stress measured using traditional methods. Our measurements use empirical Green’s functions and solid-earth tides and can be applied at different temporal and spatial scales. The orientation of maximum horizontal compressive stress in the crust can be estimated in regions lacking borehole or earthquake source data through the use of ambient Earth noise to measure stress-induced anisotropy of nonlinear anelastic behavior.

Separation of overlapping signals is an important task in signal processing, with application in music, speech, and seismic signal processing. We show that separation is possible also for seismic r...

The Alpine-Dinarides are a complex orogenic system, with its tectonic evolution controlled by the ongoing convergence between Eurasian and African plates with the Adriatic microplate wedged between them. Our study focuses on the upper mantle of the wider Alpine-Dinarides region, and we present surface-wave tomography of two overlapping subregions, interpreting the seismic velocity features in the context of regional geodynamics.In the first part, we use records of 151 teleseismic earthquakes (2010-2018) at 98 stations distributed across the wider Dinarides region. Surface-wave phase velocities are measured in the range of 30 – 160 s by the two-station method at pairs of stations aligned along the great circle paths with the epicenters. We apply several data-quality tests before the dispersion curves are measured. We use Rayleigh waves recorded on both radial and vertical components. Only the dispersions measured coherently at both components are used for the tomography. In total, we reach the number of 9000 phase velocity measurements for the period of 50 s. Tomographic results including resolution estimates are provided for various frequencies; the local dispersion curves are inverted for depths from the surface down to 300 km. Results are shown as maps for various depths and as cross-sections along several profiles of shear-wave velocities in the whole region.The other study focuses on the Alps. The AlpArray seismic network stretches hundreds of kilometers in width and more than thousand kilometers in length. It is distributed over the greater Alpine region (Europe) and consists of around 250 temporary and around 400 permanent broadband stations with interstation distances around 40 km. The earthquakes are selected between years 2016-2019. The methodology differs from the Dinarides case in a sense, that while before we used many earthquakes and less stations pairs (due to sparser station coverage), for the Alps, we use less earthquakes (32) and many more stations pairs (tens of thousands) making use of the dense station coverage of the AlpArray network.Results of the depth inversion of the local dispersion measurements for the Alps are compared with local surface-wave phase-velocity measurement obtained from the (sub)array approach.

Pressure perturbations such as e.g. impulsive acoustic waves can couple into solid earth through the long-known phenomenom of seismo-acoustic coupling. Yet, the associated mechanisms are not always clear. Most studies investigate seismo-acoustic through low-frequency and high amplitude signals generated by e.g. natural or man-made explosions.We conducted a small-scale field experiment with firecrackers as acoustic sources and hundred 3-component nodal geophones as receivers in a 20m diameter ring layout, some of them co-located with seismically decoupled Hyperion IFS-5111 infrasound sensors. This allowed us to investigate seismo-acousting coupling for higher frequencies and very small (meter scale) offsets.The large receiver density enabled us to observe and distinguish different wave types induced by acoustic sources, including direct air waves, air-coupled Rayleigh waves, and possibly slow Biot waves. Having co-located seismic and pressure sensors additionally allowed us to investigate the coupling efficiency, which is in the order of 10-7 and thus similar to many of the low-frequency and large-offset studies. Furthermore, we can deduct soil properties such as rigidity, bulk modulus, and density from the co-located sensors, and efficiently infer near-surface (soil) properties using cheap acoustic sources.

Gaining insight into the regional stress field and deformation in the crust is challenging. As we cannot measure these directly, we rely on proxy measurements and numerical modelling to infer their orientation. For the Alpine-Pannonian-Carpathian junction, only a limited number of studies exist that provide such insights. They are based on either the interpretation of sparse and point-wise measurements of local stress-field orientations or on numerical modelling that aims to satisfy tectonic and geological constraints. We infer seismic azimuthal anisotropy that relates to the orientation of the regional stress-field and crustal deformation from ambient-noise-derived Rayleigh waves in the region. This approach provides a spatially broad and independent measurement that complements previous studies. We use Rayleigh-wave group-velocity residuals after isotropic inversion at 5s and 20s center period, which are sensitive to crustal structure at different depths. They allow us to gain insight into two distinct mechanisms that result in fast orientations. At shallow crustal depths (5s), fast orientations in the region are N/S to NNE/SSW, roughly normal to the Alps. This effect is most likely due to the formation of cracks aligned with the present-day stress field. At greater depths (20s), fast orientations rotate towards NE, almost parallel to the major fault systems that accommodated the lateral extrusion of blocks in the Miocene. This is coherent with the expected direction of aligned crystal grains during crustal deformation occurring along the fault systems and the lateral extrusion of the central part of the Eastern Alps.

Detecting seismic signals and identifying their origin is more and more used for understanding environmental activity. This usually depends on a good signal/noise ratio (S/N), especially for the more distant sources.A test area for detection and identification is the urban setting of the University of Vienna, a challenging environment with more than 4000 strong-acceleration events per day. These repetitive noise events would normally classify the site as "too noisy" for any advanced earthquake research.With the real-time open database from Wiener Linien it is possible to attribute many of the repetitive seismic signals (e.g. on a Raspberry Shake Citizen Science Station) to the surrounding trams and train lines. The detection challenge was initiated in a Citizen Science Hackathon, where public interest sparked this research. The available train schedule and more than one year of continuous seismic records is sufficient to train and test a machine learning classifier which finds most characteristic features in the signals of commuter trains and trams, such as the energy in each frequency band.The labeled dataset can be used to train our detection algorithm to find similar signals and to help determine whether a certain signal is present or not. An additional second seismic Raspberry Shake sensor is installed in the vicinity, to further constrain the directionality of the trains.Studying the vibrations of train signals and solving the classification task of these repetitive patterns first can help develop robust methodsfor seismically loud environments, and might lead to the detection of lower magnitude events such as regional earthquakes or landslides.

The association of seismic anisotropy and deformation, as e.g. exploited by shear-wave splitting measurements, provides a unique opportunity to map the orientation of geodynamic processes in the upper mantle and to constraint their nature. However, due to the limited depth-resolution of steeply arriving core-phases, used for shear-wave splitting investigations, it appears difficult to differentiate between asthenospheric and lithospheric origins of observed seismic anisotropy. To change that, we take advantage of the different backazimuthal variations of fast orientation φ and delay time Δt, when considering the non-vertical incidence of phases passing through an olivine block with vertical b-axis as opposed to one with vertical c-axis. Both these alignments can occur depending on the type of deformation, e.g. a sub-horizontal foliation orientation in the case of a simple asthenospheric flow and a sub-vertical foliation when considering vertically-coherent deformation in the lithosphere. In this study we investigate the cause of seismic anisotropy in the Central Alps. Combining high-quality manual shear-wave splitting measurements of three datasets leads to a dense station coverage. Fast orientations φ show a spatially coherent and relatively simple mountain-chain-parallel pattern, likely related to a single-layer case of upper mantle anisotropy. Considering the measurements of the whole study area together, our non-vertical-ray shear-wave splitting procedure points towards a b-up olivine situation and thus favors an asthenospheric anisotropy source, with a horizontal flow plane of deformation. We also test the influence of position relative to the European slab, distinguishing a northern and southern subarea based on vertically-integrated travel times through a tomographic model. Differences in the statistical distribution of splitting parameters φ and Δt, and in the backazimuthal variation of δφ and δΔt, become apparent. While the observed seismic anisotropy in the northern subarea shows indications of asthenospheric flow, likely a depth-dependent plane Couette-Poiseuille flow around the Alps, the origin in the southern subarea remains more difficult to determine and may also contain effects from the slab itself.

Seismic anisotropy is an important tool for studying geodynamic processes in the Earth, and a common way of constraining it is to analyse shear-wave splitting of seismological body-wave phases,i.p. SKS. Different techniques exist to quantify shear-wave splitting, but they do not always give the same result, raising the question of how stable they are, and whether there are systematic biases. Furthermore, the strength of the splitting ("splitting delay") has generally been more difficult to determine than the other (the "fast orientation").A robust technique for determining shear-wave splitting can be set upbased on the splitting intensity method. That technique can in particular also constrain the splitting delay well. Ambient noise can however lead to an underestimation of splitting delay, and it needs to be accounted for, e.g. by a least-squares Wiener filter.We apply that modified splitting intensity method to data from the AlpArray. We have processed 3 years of teleseismic earthquake data for 336 stations of the AlpArray deployment and additional 315 stations of the Italian network to get a potentially broad and more complete image of anisotropic structures in and outside the Alpine region.The technique makes restrictive assumptions, e.g. assuming single-layer anisotropy. Yet, the new constraints, especially the one of the splitting delay are rather useful for understanding the deformation under the mountain belt and around it.

The association of seismic anisotropy and deformation, as e.g. exploited by shear-wave splitting measurements, provides a unique opportunity to map the orientation of geodynamic processes in the upper mantle and to constraint their nature. However, due to the limited depth-resolution of steeply arriving core-phases, used for shear-wave splitting investigations, it appears difficult to differentiate between asthenospheric and lithospheric origins of observed seismic anisotropy. To change that, we take advantage of the different backazimuthal variations of fast orientation ϕ and delay time Δt, when considering the non-vertical incidence of phases passing through an olivine block with vertical b-axis as opposed to one with vertical c-axis. Both these alignments can occur depending on the type of deformation, e.g. a sub-horizontal foliation orientation in the case of a simple asthenospheric flow and a sub-vertical foliation when considering vertically-coherent deformation in the lithosphere. In this study we investigate the cause of seismic anisotropy in the Central Alps. Combining high-quality shear-wave splitting measurements of three datasets leads to a dense station coverage. Fast orientations ϕ show a spatially coherent and relatively simple mountain-chain-parallel pattern, likely related to a single-layer case of upper mantle anisotropy. Considering the measurements of the whole study area together, our non-vertical-ray shear-wave splitting procedure points towards a b-up olivine situation and thus favors an asthenospheric anisotropy source, with a horizontal flow plane of deformation. We also test the influence of position relative to the European slab, distinguishing a northern and southern subarea based on vertically-integrated travel times through a tomographic model. Differences in the statistical distribution of splitting parameters ϕ and Δt, and in the backazimuthal variation of δϕ and δΔt, become apparent. While the observed seismic anisotropy in the northern subarea shows indications of asthenospheric flow, likely a depth-dependent plane Couette-Poiseuille flow around the Alps, the origin in the southern subarea remains more difficult to determine and may also contain effects from the slab itself.

SUMMARYTo calculate phase-velocity dispersion curves, we introduce a method which reflects both structural and dynamic effects of wave propagation and interference. Rayleigh-wave fundamental-mode surface waves from the South Atlantic Ocean earthquake of 19 August 2016, M = 7.4, observed at the AlpArray network in Europe are strongly influenced by the upper-mantle low-velocity zone under the Cameroon Volcanic Line in Central Africa. Predicting phase-delay times affected by diffraction from this heterogeneity for each station gives phase velocities as they would be determined using the classical two-station method as well as the advanced array-beamforming method. Synthetics from these two methods are thus compared with measurements. We show how the dynamic phase velocity differs from the structural phase velocity, how these differences evolve in space and how two-station and array measurements are affected. In principle, arrays are affected with the same uncertainty as the two-station measurements. The dynamic effects can be several times larger than the error caused by the unknown arrival angle in case of the two-station method. The non-planarity of the waves and its relation to the arrival angle and dynamic phase-velocity deviations is discussed. Our study is complemented by extensive review of literature related to the surface wave phase-velocity measurement of the last 120 years.

A new method is proposed for the study of crustal azimuthal anisotropy, and the lateral variation of other crustal aerogeophysical parameters, using a quantitative approach. The processing of a large, newly acquired, high-resolution set of aerogeophysical data is considered using the existing Radon transform and the geostatistical analysis approaches. The data set includes an area of the Guyana shield, in northern Brazil, which was not included in previous surveys. The area is covered by dense rain forest vegetation and thick soil layers. Parameterization was performed considering the possible anisotropic character of the geophysical 2D data. Application of the newly proposed geostatistical data processing yielded high-resolution images of the lateral variation of quantitative geophysical parameters, which indicate good correlation with previously determined seismic anisotropy in the area. Average anisotropy as measured by the Radon transform and variogram analysis is scale dependent. At scales greater than [Formula: see text], the results seem to match those of the previous seismic studies. Images of the derived quantitative parameters from magnetic and radiometric data in the shield area indicate sharp changes that follow the known geologic changes observable in the survey area. Observed large-scale anisotropy in the area seems to be autocorrelated with two different geochronological provinces of the Amazonian craton. Application of the new technique to two other lower resolution aeromagnetic surveys located on the Amazon basin has produced two different results, including one successful data treatment of the aeromagnetic survey. The results obtained for this area indicate that the sedimentary cover seems to play almost no role in the observed magnetic anomaly anisotropy in the middle Amazon basin area. Most of the observed magnetic anomalies appear to be controlled by the continuing geologic features of the cratonic basement.

We study the interaction of a seismic wave-field with a spherical acoustic gas or fluid-filled cavity. The intention of this study is to clarify whether seismic resonances can be expected, a characteristic feature, which may help detecting cavities in the subsurface. This is important for many applications, as in particular the detection of underground nuclear explosions which are to be prohibited by the Comprehensive-Test-Ban-Treaty. In order to calculate the full seismic wave-field from an incident plane wave that interacts with the cavity, we considered an analytic formulation of the problem. The wave-field interaction consists of elastic scattering and the wave-field interaction between the acoustic and elastic media. Acoustic resonant modes, caused by internal reections in the acoustic cavity, show up as spectral peaks in the frequency domain. The resonant peaks coincide with the eigenfrequencies of the undamped system described by the particular acoustic medium bounded in a sphere with stiff walls. The filling of the cavity could thus be determined by the observation of spectral peaks from acoustic resonances. By energy transmission from the internal oscillations back into the elastic domain, the oscillations experience damping, resulting in a frequency shift and a limitation of the resonance amplitudes. In case of a gas-filled cavity the impedance contrast is still high, which means low damping of the internal oscillations resulting in very narrow resonances of high amplitude. In synthetic seismograms calculated in the surrounding elastic domain, the acoustic resonances of gas-filled cavities show up as persisting oscillations. However, due to the weak acoustic-elastic coupling in this case the amplitudes of the oscillations are very low. Due to a lower impedance contrast, a fluid-filled cavity has a stronger acoustic-elastic coupling, which results in wide spectral peaks of lower amplitudes. In the synthetic seismograms derived in the surrounding medium of fluid-filled cavities, acoustic resonances show up as strong but fast decaying reverberations. This article is protected by copyright. All rights reserved

On September 1st, 2018 a devastating explosion occurred on the facility of an oil refinery near Ingolstadt, Germany. We analyzed data of 400 permanent and temporary seismic stations and find strong seismo-acoustic signals on more than 80 seismic stations. The infrasound signal is detectable on seismic stations within 10–350 km from the source, with 40 km spatial resolution. We confirm the explosion site both by the seismic and seismo-acoustic arrivals. Apart from seismic P- and S-waves, we identified three separate acoustic phases with celerities of 332, 292, and 250 m/s, respectively, each of which has a particular spatial pattern of positive detections at the ground. Seismo-acoustic amplitudes are strongly affected by the type of seismic installation but still allow insight into regional infrasound attenuation. Our observations likely represent tropospheric, stratospheric, and thermospheric phases. We performed 3D acoustic ray tracing to validate our findings. Tropospheric and thermospheric arrivals are to some extent reproduced by the atmospheric model. However, ray tracing does not predict the observed acoustic stratospheric ducts. Our findings suggest that small-scale variations had considerable impact on the propagation of infrasound generated by the explosion.

SUMMARYWe infer seismic azimuthal anisotropy from ambient-noise-derived Rayleigh waves in the wider Vienna Basin region. Cross-correlations of the ambient seismic field are computed for 1953 station pairs and periods from 5 to 25 s to measure the directional dependence of interstation Rayleigh-wave group velocities. We perform the analysis for each period on the whole data set, as well as in overlapping 2°-cells to regionalize the measurements, to study expected effects from isotropic structure, and isotropic–anisotropic trade-offs. To extract azimuthal anisotropy that relates to the anisotropic structure of the Earth, we analyse the group velocity residuals after isotropic inversion. The periods discussed in this study (5–20 s) are sensitive to crustal structure, and they allow us to gain insight into two distinct mechanisms that result in fast orientations. At shallow crustal depths, fast orientations in the Eastern Alps are S/N to SSW/NNE, roughly normal to the Alps. This effect is most likely due to the formation of cracks aligned with the present-day stress-field. At greater depths, fast orientations rotate towards NE, almost parallel to the major fault systems that accommodated the lateral extrusion of blocks in the Miocene. This is coherent with the alignment of crystal grains during crustal deformation occurring along the fault systems and the lateral extrusion of the central part of the Eastern Alps.

Hydrocarbon extraction from unconventional reservoirs demands ever-increasing technological effort, for better understanding phenomena occurring within the reservoir. We review currently available geophysical techniques for reservoir monitoring. First, we describe basic characteristics of geophysical monitoring, identifying properties and their associated monitored quantities, according to the different fields of analysis in reservoir. Second, we present an overview of current monitoring techniques associating them to monitored quantities. Monitoring is extremely important in understanding how the reservoir reacts to external or internal perturbation of its state; secondly, monitoring is one of the first steps in preventing and addressing key environmental issues.

To better understand the evolution and present-day tectonics of the Carpathian-Pannonian Region (CPR), we characterize the upper mantle anisotropic structure. We present a shear wave splitting analysis from teleseismic events recorded at the Carpathian Basin Project and permanent stations located in the CPR. The results show a large-scale uniform NW-SE fast orientation under the entire CPR. Compared with the complexity of geologic structures, the anisotropy expresses a relatively simple pattern of deformation. We attribute this anisotropy to an asthenospheric origin and interpret it as flow-induced alignments within the upper mantle. We also observe a few measurements depicting NE-SW fast orientation in line with the Mid-Hungarian Shear Zone. This suggests the likely contribution of either lithosphere or northeastward flow into a slab gap under the northern Dinarides. We observe splitting delay times on average of 1 s, showing noticeable change (60%) in the middle Pannonian basin. This change correlates well with the variation in the thickness of low-velocity zones that were previously imaged between a depth of 75 and 400 km by velocity tomography. In order to evaluate the relation between anisotropy and tectonics, we compare our data with the tectonic models that have so far been suggested to explain the evolution and current-stage tectonics of the region. We present here a plausible tectonic model responsible for the NW-SE anisotropy within the asthenospheric mantle. In this model, NW-SE deformation is mainly generated in a northeastward compressional tectonic regime acting in a wide region between the Adriatic microplate and the East European platform.

We present results from ambient noise tomography applied to temporary seismological stations in the easternmost part of the Alps and their transition to the adjacent tectonic provinces (Vienna Basin, Bohemian Massif, Southern Alps, Dinarides). By turning each station into a virtual source, we recover surface waves in the frequency range between 0.1 and 0.6 Hz, which are sensitive to depths of approximately 2–15 km. The utilization of horizontal components allows for the analysis of both Rayleigh and Love waves with comparable signal-to-noise ratio. Measured group wave dispersion curves between stations are mapped to local cells by means of a simultaneous inverse reconstruction technique. The spatial reconstruction for Love-wave velocities fails in the central part of the investigated area, and we speculate that a heterogeneous noise source distribution is the cause for the failure. Otherwise, the obtained group velocity maps correlate well with surface geology. Inversion of Rayleigh-wave velocities for shear-wave velocities along a vertical N-S section stretching from the Bohemian Massif through the Central Alps to the Southern Alps and Dinarides reveals a mid-crustal low-velocity anomaly at the contact between the Bohemian Massif and the Alps, which shows a spatial correlation with the P-wave velocity structure and the low-frequency component of the magnetic anomaly map. Our study is validated by the analysis of resolution and accuracy, and we further compare the result to shear-wave velocity models estimated from other active and passive experiments in the area.

A notable sequence of calls was encountered, spanning several days in January 2003, in the central part of the Indian Ocean on a hydrophone triplet recording acoustic data at a 250 Hz sampling rate. This paper presents signal processing methods applied to the waveform data to detect, group, extract amplitude and bearing estimates for the recorded signals. An approximate location for the source of the sequence of calls is inferred from extracting the features from the waveform. As the source approaches the hydrophone triplet, the source level (SL) of the calls is estimated at 187 ± 6 dB re: 1 μPa-1 m in the 15-60 Hz frequency range. The calls are attributed to a subgroup of blue whales, Balaenoptera musculus, with a characteristic acoustic signature. A Bayesian location method using probabilistic models for bearing and amplitude is demonstrated on the calls sequence. The method is applied to the case of detection at a single triad of hydrophones and results in a probability distribution map for the origin of the calls. It can be extended to detections at multiple triads and because of the Bayesian formulation, additional modeling complexity can be built-in as needed.

The crustal structure of the Eastern Alps and adjacent tectonic units investigated in this work sheds new light on the relationship of surface geology to geodynamic processes operating at depth. Of particular interest are the nature of a previously proposed Moho gap south and east of the Tauern Window, the plate tectonic affinity of the steeply dipping Eastern Alpine slab, and the relationship of the Alps to the Neogene sedimentary basins and the Bohemian Massif. To address these questions, we use various seismological approaches based on converted waves from the temporary passive experiment EASI (Eastern Alpine Seismic Investigation), a complementary experiment of the AlpArray project. The EASI is a densely spaced, 540 km long seismic network along 13.3°E we operated for more than a year. The uppermost-crustal structures in and near the Alps exhibit dipping layers and/or tilted anisotropy that correlate well with surface geology observations. The Moho, despite its variable appearance, is clearly identified along most of the swath. The Variscan lithospheric blocks beneath the Bohemian Massif are imaged with sub-vertical boundaries. Beneath the Eastern Alps, the shape of the Moho is consistent with bi-vergent orogenic thickening, with a steeper and deeper-reaching Adriatic plate plunging northwards beneath the European plate in the north. At the junction of these plates at depth, around the previously proposed Moho gap, the root of the Eastern Alps is a broad trough characterized by a zone of low velocity-gradient that is up to 20 km thick, transitioning between crust and mantle. Our receiver-function results corroborate earlier lithosphere-upper mantle seismic tomography images, and highlight the Adriatic affinity of the Eastern Alpine slab. The zigzag deployment pattern of stations in the EASI experiment also allows distinction of short-wavelength variations perpendicular to the profile, both within the shallow and the deep crust. This underlines the importance of applying 3D imaging in complex geodynamic systems. ISSN:0040-1951 ISSN:1879-3266

The Vienna Basin in Eastern Austria is a region of low to moderate seismicity, and hence the seismological network coverage is relatively sparse. Nevertheless, the area is one of the most densely populated and most developed areas in Austria, so accurate earthquake location, including depth estimation and relation to faults is not only important for understanding tectonic processes, but also for estimating seismic hazard. Particularly depth estimation needs a dense seismic network around the anticipated epicenter. If the station coverage is not sufficient, the depth can only be estimated roughly. Regional Depth Phases (RDP) like sPg, sPmP and sPn have been already used successfully for calculating depth even if only observable from one station. However, especially in regions with sedimentary basins these phases prove difficult or impossible to recover from the seismic records. For this study we use seismic array data from GERES. It is 220 km to the North West of the Vienna Basin, which – according to literature – is a suitable distance to recover PmP and sPmP phases. We use array processing on recent earthquake data from the Vienna Basin with local magnitudes from 2.1 to 4.2 to reduce the SNR and to search for RDP. At the same time, we do similar processing on synthetic data specially modeled for this application. We compare real and synthetic results to assess which phases can be identified and to what extent depth estimation can be improved. Additionally, we calculate a map of lateral propagation behavior of RDP for a typical strike-slip earthquake in our region of interest up to 400 km distance. For our study case RDP propagation is strongly azimuthally dependent. Also, distance ranges differ from literature sources. Comparing with synthetic seismograms we identify PmP and PbP phases with array processing as strongest arrivals. Although the associated depth phases cannot be identified at this distance and azimuth, identification of the PbP phases limits possible depth to less than 20 km. Polarization analysis adds information on the first arriving Pn wave for local magnitudes above 2.5.

Eastern Austria is a region of low to moderate seismicity, and hence the seismological network coverage is relatively sparse. Nevertheless accurate earthquake location is very important, as the area is one of the most densely populated and most developed areas in Austria. In 2013 a series of earthquakes with magnitudes up to 4.2 was recorded in the Southern Vienna Basin. With portable broadband, semi-permanent, and permanent installed seismic sensors from different institutions it was possible to record the main- and aftershocks with an unusual multitude of close-by seismic stations. In this study we combine records from all available stations up to 240 km distance in one dataset. First, we stabilize the location with three stations deployed in the epicentral area. The higher network density moves the location of smaller magnitude events closer to the main shocks, with respect to preliminary locations achieved by permanent and semi-permanent networks. Then we locate with NonLinLoc using consistent picks, a 3-D velocity model and apply station corrections. This second approach results in stable epicenters, for limited and even changing station availability. This dataset can then be inspected more closely for the presence of regional phases, which then can be used for more accurate localizations and especially depth estimation. Further research will address directivity effects and the asymmetry in earthquake intensity observed throughout the area, using double differences and cross-correlations.

On December 12, 2017 a devastating release and combustion of gas occurred at the Baumgarten gas hub in Eastern Austria, which is a major European distribution node for natural gas. We have detected the resulting seismo-acoustic signal on permanent and temporary broadband seismic stations at distances between 30 and 175 km from the gas hub, most prominently in the 2–4 Hz range. Two distinct phase arrivals correspond to acoustic waves traveling through the troposphere and stratosphere. The passing of a cold front shortly before the explosion led to several temperature inversions at low altitude, and acoustic waveguides within the troposphere that facilitated our infrasound detections at distances as close as 50 km from the source, in addition to the commonly observed stratospheric reflections. 3D acoustic raytracing using temperature and wind velocities from the HRES (high-resolution) forecast model of the European Center for Medium Range Weather Forecast (ECMWF) has allowed to precisely relate the spatial distribution of our detections with calculated surface bounce points of infrasound rays. This has provided a precise and independent estimate of the time of the accident, to be used in forensic investigations. In addition to the acoustic signal we find evidence for weak seismic phases on the stations closest to the gas hub, yet the sudden release of gas above the surface generated acoustic waves more effectively than seismic waves. After the first explosion signal, we also detect a prolonged coda of elevated noise, which is probably due to ongoing gas release and/or the fire from the escaping gas. Systematic analyses like the one conducted here are of great value to detect, locate, and characterize anthropogenic sources at a regional scale.

Recently, it has been argued that natural, intact stalagmites in caves give important constraints on seismic hazard since they have survived all earthquakes over their (rather long) life span. This suggests that the pattern of oscillation should be fully understood, including the splitting of eigenfrequencies that has occurred in recent cave observations. In the present study, we simulate the oscillation of a given stalagmite by setting up four simplified models of the stalagmite. The dimensions of the intact stalagmite were measured in situ, and the geo-mechanical and elastic parameters of broken stalagmite samples, determined in geo-mechanical laboratory, have been taken into account. The eigenfrequencies of the stalagmite are then calculated numerically, by the finite element method, and compared with the measured in situ values. The latter have shown splitting of eigenfrequencies, which we were able to reproduce by the numerical model calculations taking into account the asymmetric shape of the stalagmite.

There are few studies of internal deformation under northern Africa; here we present such a study. We analyze teleseismic shear-wave splitting for northeast Algeria, to improve our knowledge of lithospheric and asthenospheric deformation mechanisms in this region. We study waveform data generated by tens of teleseismic events recorded at five recently installed broadband (BB) stations in Algeria. These stations cover an area 2° across, extending from the Tellian geological units in the North to the Saharan Atlas units in the South. Analysis of SKS-wave splitting results insignificant spatial variations in fast polarization orientation, over a scale length of at most 100 km. The seismic anisotropy shows three clear spatial patterns. A general ENE–WSW orientation is observed under the stations in the north. This polarization orientation follows the direction of the Tell Atlas mountain chain, which is perpendicular to the convergence direction between Africa and Eurasia. Delay times vary significantly across the region, between 0.6 and 2.0 s. At several stations there is an indication of a WNW–ESE polarization orientation, which is apparently related to a later geodynamic evolutionary phase in this region. A third pattern of seismic anisotropy emerges in the South, with an orientation of roughly N–S. We discuss these observations in light of geodynamic models and present-day geodetic motion.

Summary Using data from the geothermal field in the Cooper Basin (South Australia), we study the relationship between the horizontal and vertical relative location of consecutive events (RLCE) and magnitude. The obtained spatial distribution of RLCE shows a striking shape, in that it suggests a magnitude dependence, with smaller event separations towards higher magnitudes. This effect is real, and apparently caused by the existence of aftershocks for low to moderate energy events. In order to understand the factors controlling induced seismicity, we investigate the relation between the injected volume and seismic moment. Seismicity appears predominantly controlled by the fault area rather than the injected volume. Finally, estimations of the seismic efficiency provide the basis for a distinction between “triggered” and “induced” events.

We have studied the scattering of P-waves from an acoustic inclusion in a 2D half-space with a free surface. The motivation for our study comes from detecting a cavity that might be caused by an underground nuclear explosion. This is relevant to on-site inspections, an element of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). The waveform modeling we address is implemented in the frequency domain; i.e., we consider the wavefield as well as the source to be time harmonic. We numerically investigate the cases in which the source of the scattered field is either a plane wave from the bottom or the side as from passive sources, such as teleseismic waves or ambient noise, or a spherical wave from the surface as from an active point source, such as a vibroseis or an explosion. To this end, we split the total field in an incident and an unknown scattered field to understand the effects more explicitly. Modeling the response of a void in a medium is not trivial, and many numerical algorithms commonly used for seismic propagation modeling will fail. Therefore, we want to highlight the advantage of high-order methods for this type of application in general and reveal the benefit of using the finite-element method code Ngsolve. This is in particular the case for the situation we have at hand, in which the ratio between the size and the depth of the cavity is notably high. We have addressed this scenario numerically for the first time because there are few field observations of the effects and the number of papers addressing the theoretical basis is sparse. Finally, we found that our splitting strategy together with the numerical scheme that we apply give rise to a constructive approach for studying this specific issue.

We present the results from receiver function analysis applied to a comprehensive data set in the Eastern Alps. Teleseismic events were recorded at 70 stations with an average deployment of 1 year. The investigated area includes the eastern part of the Eastern Alps and their transition to the Bohemian Massif, the Pannonian domain, and the Southern Alps. The crustal structure at each station is examined with the Zhu-Kanamori (ZK) method, which yields well-resolved interface depths in laterally homogeneous media with limited layering. The application of the ZK technique is challenged because of the complex tectonic setting; therefore, we include additional constraints from recent active-source seismic studies. In particular, the well-known crustal P-wave velocity and, where available, the V p/V s ratio are kept fixed, thus reducing the ambiguity in determining Moho depths. Individual depth values vary strongly between adjacent stations, showing that the employment of the ZK technique in tectonically complex settings is limited. We therefore avoid interpreting the results in detail, but rather compare them to existing crustal models of the Eastern Alps. We regard this receiver function study in the easternmost part of the Alps as a documentation of a data set that has potential to be exploited in the future.

The Vrancea region of the southeastern Carpathians is one of the most active seismic zones in Europe and it is well-known for its strong intermediate depth earthquakes. Seismic tomography had revealed a high-velocity body beneath Vrancea and the Moesian platform that extends to a depth of at least 350 km and can be interpreted as descending lithosphere. The strong earthquakes occur within the northeastern part of this high-velocity body, in a very limited seismogenic volume at intermediate depth (70–180 km). Several geodynamic models have been proposed for this area. They can be split into two main categories, in terms of the nature of the high-velocity anomaly, which may (a) be associated with descending relic oceanic lithosphere beneath the bending zone of the SE-Carpathians, either attached or already detached from the continental crust; or (b) it may represent continental lithosphere that has been delaminated, after continental collision and orogenic thickening. Based on currently available information, it appears difficult to distinguish between these two types of models. In this paper we attempt to shed more light on the nature of the seismic anomaly, as well as that of the origin of the intermediate depth seismicity in the Vrancea zone, by investigating the waveform character of P-waves excited by local earthquakes beneath this area, and in particular the dependence of group arrival times on frequency. We present observations of such a dispersion from stations situated at the bending zone of the SE-Carpathians. On the other hand, signals from the same earthquakes, but observed at reference stations outside of the anomalous zone do not show that frequency dependence. A natural explanation for these observations is that it is caused by the presence of a low-velocity channel at the top of the seismic anomaly, which is too thin to be resolved by classical seismic tomographic techniques. Similar observations of dispersed first-arriving P-waves have been made above subduction zones around the world, in which low-velocity layers with a thickness of several kilometers are known to exist. This suggests that a tabular slab of subducted oceanic crust is present within the seismic anomaly under the Vrancea region, and that the anomaly consists of subducted oceanic lithosphere rather than continental lithosphere, at least at depths shallower than the seismically active zone.

Mountain chains at the Earthʼs surface result from deformation processes within the Earth. Such deformation processes can be observed by seismic anisotropy, via the preferred alignment of elastically anisotropic minerals. The Alps show complex deformation at the Earthʼs surface. In contrast, we show here that observations of seismic anisotropy suggest a relatively simple pattern of internal deformation. Together with earlier observations from the Western Alps, the SKS shear-wave splitting observations presented here show one of the clearest examples yet of mountain chain-parallel fast orientations worldwide, with a simple pattern nearly parallel to the trend of the mountain chain. In the Eastern Alps, the fast orientations do not connect with neighboring mountain chains, neither the present-day Carpathians, nor the present-day Dinarides. In that region, the lithosphere is thin and the observed anisotropy thus resides within the asthenosphere. The deformation is consistent with the eastward extrusion toward the Pannonian basin that was previously suggested based on seismicity and surface geology.