Surface Wave Tomography Across Europe-Mediterranean and Middle EastBased on Automated Inter-station Phase Velocity Measurements

Seismic tomography is an imaging tool which allows to construct 3-D models of the Earth’s internal structure from observables of seismic waves. Surface wave tomography can be performed using earthquakes and ambient noise data and is sensitive to isotropic as well as anisotropic 3-D shear-wave velocity structure in broad depth ranges sampling the crust and the lithosphere-asthenosphere structure. In this study, surface wave tomography is performed to characterize the structure of the lithosphere-asthenosphere underneath the Mediterranean and the adjacent regions. We utilize a large database consists of 3800 teleseismic earthquakes recorded by 4500 broadband stations provided by IRIS and EIDA in a combination, for the first time, with waveform data from the Egyptian National Seismological Network (ENSN). An automated algorithm for inter-station phase velocities is applied to obtain fundamental mode phase velocities from this database (3.5 millions of waveforms). Path average dispersion curves are obtained by averaging the smooth parts of single-event dispersion curves. We calculated new high resolution Rayleigh and Love wave phase velocity maps using an unprecedentedly large number (200.000) of measurements in the period range from 8s-350s. In order to relate the local dispersion curves to 1-D velocity models as function of depth, the Particle Swarm Optimization (PSO) algorithm has been developed and implemented. The 3-D model has been constructed based on the obtained 1-D shear velocity model

Surface Wavefield Tomography of the Alpine Region to Constrain Slab Geometries, Lithospheric Deformation and Asthenospheric Flow

Surface waves radiated by teleseismic earthquakes are ideally suited to constrain isotropic and anisotropic elastic properties of the upper mantle down to about 300 km depth beneath dense networks of broad-band stations. Rayleigh wave phase velocities were automatically determined in a broad period range from 8 s to 300 s and a very strict quality control was applied. This resulted in a data set of more than 200,000 inter-station phase velocity curves. Local dispersion curves, extracted from phase velocity maps were inverted for a 3D shear-wave velocity model (MeRE2020) using a newly developed stochastic inversion algorithm based on particle swarm optimization. It was shown that the presence of small and highly segmented slabs can be resolved by surface wave tomography in case of a high station density. In the western Alps, a short Eurasian slab was imaged down to about 150 km depth, whereas at larger depths a pronounced low velocity anomaly indicates slab break-off. In the northern Apennines, a nearly vertical south-dipping slab connected to the Adriatic mantle lithosphere beneath the Po Basin is observed. In the central Alps, the presence of Eurasian mantle lithosphere is found down to the bottom of the model at 300 km depth. Whereas in the eastern Alps, a short Eurasian nearly vertical dipping slab is found down to only 150 km depth. The presence of a short slab consisting of Adriatic mantle lithosphere is also indicated beneath the northern Dinarides extending towards the Alps east of the Giudicaria fault. Anisotropic phase velocity maps show at 25 s period (lower crustal depth) mostly fast orogen parallel directions, whereas in the western Alps azimuthal anisotropy is more inclined with respect to the Alpine arc. At 100 s period, azimuthal anisotropy beneath the western Alps indicates asthenospheric flow towards the Ligurian Sea and beneath the northern Dinarides towards the Pannonian Basin through slab gaps. Moreover, seismic wavefields were analysed using AlpArray and Swath-D data. Wavefield animations illustrate the considerable spatio-temporal variability of the wavefield's properties at a lateral resolution down to about 100km. Within denser station distributions like those provided by Swath-D, even shorter period body and surface wave features can be recovered. Considerable amplifications of the Rayleigh wave in the Alpine area are observed for several earthquakes. To analyse Rayleigh wave quantitatively, an algorithm has been developed to extract their phase and amplitude fields using cross correlation between synthetic waveforms and recordings of a dense array. Phase fields are unwrapped by solving a linear system of equations. Phase and amplitude fields are quality controlled and interpolated to determine structural phase velocity fields using Helmholtz tomography. It is shown that the observed amplitude fields depend heavily on lateral heterogeneity outside the array. Often, linear amplifications in the propagation direction are observed. In order to model the observed wavefields, the AxiSEM-SPECFEM Coupling algorithm has been improved and adapted concerning flexibility and efficiency, reducing the necessary wavefield interpolation significantly and allowing topography as well as existing 3D Models of the Alpine region to be easily implemented

Identifying Main Lithospheric Structures in the Eastern Alpine Domain by Joint Inversion of Receiver Function and Surface Wave Measurements for Seismic Anisotropy

Rayleigh-wave phase velocity measurements from both earthquakes and ambient noise were combined to image the 3-D shear-wave velocity structure beneath the eastern Alps and in the transitions towards the Pannonian Basin and the Dinarides. This allows us to resolve crust and upper mantle structures down to 300 km including the Moho topography. Continuous waveforms were collected from 1254 stations within a 9° radius for the time period from 2006 to 2018. More than 164,464 inter-station Rayleigh wave phase-velocity curves were automatically extracted after applying a strict quality control. Using the combined dataset, a period and distance dependence correction was applied to account for the bias observed between phase velocities from both datasets that amounts to ~1 % and increases towards longer periods. 2-D anisotropic phase velocity maps were then constructed spanning periods from 5 s to 250 s. 33,981 local dispersion curves were extracted and inverted for a 3-D shear-wave velocity model (PanREA2023) encompassing crust and mantle using a non-linear stochastic particle swarm optimization. At shallower crustal depths, the horst and graben structure of the Pannonian Basin is imaged, characterized by two NE-SW trending horsts and three graben systems. A pronounced crustal low-velocity anomaly extending to the Moho is found beneath the surrounding Carpathian orogen. A shallow south-dipping Eurasian slab was imaged beneath the eastern Alps down to only 150 km depth. Adriatic lithosphere is near-vertically dipping beneath the northern Apennines and northern Dinarides. The Adriatic slab is short reaching depths of around 150 km. Seismic discontinuities down to the mantle transition zone are analysed using S-to-P converted phases from teleseismic earthquakes. We stack broadband teleseismic S waveform data to retrieve S-to-P converted signals from below the seismic stations. In order to avoid processing artefacts, no deconvolution or filtering is applied. The Moho signals are always seen very clearly. In addition, a negative velocity gradient below the Moho depth is evident in many regions. A Moho depression is visible along larger parts of the Alpine chain reaching its largest depth of 60 km beneath the Tauern Window. The Moho depression ends abruptly near about 13°E below the eastern Tauern Window. East of 13°E the Moho shallows all the way to the Pannonian Basin. A prominent along-strike change was also detected in the upper mantle structure at about 14°E. There, the lateral disappearance of a zone of negative S-wave velocity gradient in the uppermost mantle is interpreted to indicate that the S-dipping European slab laterally terminates east of the Tauern Window. Joint inversion of surface wave dispersion curves and Moho travel times inferred from S-to-P converted phases allows to determine shear-wave velocity models consistent with both measurements. The uncertainty of the Moho depth estimates decreases from about 5 to 10 km considerably to 2 to 5 km depending on the depth of the Moho. The joint inversion further enables the determination of the sharpness of the negative discontinuity associated with the lithosphere-asthenosphere boundary. It appears to be rather sharp in the northern Alpine foreland and the Pannonian Basin

Surface-wave tomography of the central-western Mediterranean : new insights into the Liguro-Provençal and Tyrrhenian basins

The complex tectonic setting of the central-western Mediterranean has interested geoscientists for decades, but its geodynamic evolution remains a matter of debate. We rely on 807 seismometers from southern Europe and northern Africa to measure Rayleigh and Love phase velocities in the period range ∼5–200 s, based on teleseismic earthquakes and seismic ambient noise. By nonlinear joint inversion of the phase-velocity maps, we obtain a 3-D shear-wave velocity (VS) model of the study area. At shallow depths, our model correlates with surface geology and reveals the presence of a sedimentary cover in the Liguro-Provençal basin, as opposed to the Tyrrhenian basin where this is either very thin or absent. At ∼5-km depth, high velocities below the Magnaghi, Vavilov, and Marsili seamounts point to an exhumed, scarcely serpentinized mantle. These are replaced by lower velocities at larger depths, likely connected to the presence of partial melt. At 50–60-km depth, a very heterogeneous structure characterizes the Tyrrhenian basin, with low velocities pointing to the presence of fluids due to the lateral mantle inflow from the Ionian slab edges, and higher velocities associated with a relatively dry upper mantle. Such heterogeneity disappears at depths ≳75 km, replaced by more uniform velocities which are ∼2% lower than those found in the Liguro-Provençal basin. We infer that, at the same depths, the Tyrrhenian basin is characterized by a larger concentration of fluids and possibly higher temperatures

Slabs in the Alpine region: inferences down to 300 km depth from surface wave tomography and receiver functions

Mountain building in the Alps is driven by a complex interplay between (i) subduction of oceanic lithosphere and/or continental mantle lithosphere and (ii) exhumation of crustal material. A major challenge represents passive seismic imaging of the various slab segments crucial for shaping the Alpine orogen. AlpArray and Swath-D provide the necessary dense station distribution for high-resolution surface wave tomography using earthquake and ambient noise data as well as for detailed P-to-S and S-to-P receiver function analyses. Absolute shear-wave velocity models of the crust and upper mantle down to 300 km depth have been obtained from stochastic particle‐swarm‐optimization inversion of a large data set of more than 200,000 Rayleigh wave phase velocity curves (4 -300 s period). This allows for imaging the slabs and their connection to the forelands with a lateral resolution of about 50 km to 75 km in the Alpine area. Moreover, about 300,000 P-to-S and about 80.000 S-to-P receiver functions have been obtained for the wider Alpine area. The common conversion point stacks of the P-to-S and S-to-P waveforms, concentrated in the Eastern Alps, provide high resolution images of the crustal structure as well as velocity discontinuities in the mantle at the interface between the European, Adriatic, and Pannonian domains. Moho topography indicating the tops of slabs as well as negative velocity gradients in the mantle beneath the Moho have been imaged. Thermochemical modelling provides evidence that the bottom of the negative velocity gradient causing S-to-P conversions is located close to the lithosphere-asthenosphere boundary. These conversions are thus hinting at the geometry of the bottom of mantle lithosphere and slabs, respectively. Beneath the northern Apennines, Adriatic lithosphere is subducting nearly vertically southwards down to at least 200 km depth as supported by the spatial distribution of a few intermediate-depth earthquakes. A short Eurasian slab subducting eastwards down to about 150 km depth and a slab gap beneath are present beneath the western Alps. Interestingly, the Eurasian slab is almost colliding with the east-west oriented Adriatic slab beneath the southwestern Po Basin. An attached Eurasian slab subducting to at least 250 km depth is imaged beneath the central Alps, whereas beneath the eastern Alps a short Eurasian slab is found down to only about 150 km depth. A short slab of continental mantle lithosphere is also present beneath the northern Dinarides. It is extending towards the Alps east of the Guidicaria fault. Broken-off Eurasian or Adriatic lithosphere may be indicated by high-velocity anomalies at depth larger than 250 km beneath the south-eastern Alps and the Adriatic Sea. Next, digital slab interface models are to be set up accounting for the various geophysical observations in order to create realistic input models for numerical geodynamic forward modelling of observed deformation rates

Prospective study for commercial and low-cost hyperspectral imaging systems to evaluate thermal tissue effect on bovine liver samples

Thermal ablation modalities, for example radiofrequency ablation (RFA) and microwave ablation, are intended to prompt controlled tumour removal by raising tissue temperature. However, monitoring the size of the resulting tissue damage during the thermal removal procedures is a challenging task. The objective of this study was to evaluate the observation of RFA on an ex vivo liver sample with both a commercial and a low-cost system to distinguish between the normal and the ablated regions as well as the thermally affected regions. RFA trials were conducted on five different ex vivo normal bovine samples and monitored initially by a custom hyperspectral (HS) camera to measure the diffuse reflectance (Rd) utilising a polychromatic light source (tungsten halogen lamp) within the spectral range 348–950 nm. Next, the light source was replaced with monochromatic LEDs (415, 565 and 660 nm) and a commercial charge-coupled device (CCD) camera was used instead of the HS camera. The system algorithm comprises image enhancement (normalisation and moving average filter) and image segmentation with K-means clustering, combining spectral and spatial information to assess the variable responses to polychromatic light and monochromatic LEDs to highlight the differences in the Rd properties of thermally affected/normal tissue regions. The measured spectral signatures of the various regions, besides the calculation of the standard deviations (δ) between the generated six groups, guided us to select three optimal wavelengths (420, 540 and 660 nm) to discriminate between these various regions. Next, we selected six spectral images to apply the image processing to (at 450, 500, 550, 600, 650 and 700 nm). We noticed that the optimum image is the superimposed spectral images at 550, 600, 650 and 700 nm, which are capable of discriminating between the various regions. Later, we measured Rd with the CCD camera and commercially available monochromatic LED light sources at 415, 565 and 660 nm. Compared to the HS camera results, this system was more capable of identifying the ablated and the thermally affected regions of surface RFA than the side-penetration RFA of the investigated ex vivo liver samples. However, we succeeded in developing a low-cost system that provides satisfactory information to highlight the ablated and thermally affected region to improve the outcome of surgical tumour ablation with much shorter time for image capture and processing compared to the HS system

The Slab Puzzle of the Alpine‐Mediterranean Region: Insights from a new, High‐Resolution, Shear‐Wave Velocity Model of the Upper Mantle

Mediterranean tectonics since the Lower Cretaceous has been characterized by a multi‐phase subduction and collision history with temporally and spatially‐variable, small‐scale plate configurations. A new shear‐wave velocity model of the Mediterranean upper mantle (MeRE2020), constrained by a very large set of over 200,000 broadband (8‐350 s), inter‐station, Rayleigh‐wave, phase‐velocity curves, illuminates the complex structure and fragmentation of the subducting slabs. Phase‐velocity maps computed using these measurements were inverted for depth‐dependent, shear‐wave velocities using a stochastic particle‐swarm‐optimization algorithm (PSO). The resulting three‐dimensional (3‐D) model makes possible an inventory of slab segments across the Mediterranean. Fourteen slab segments of 200‐800 km length along‐strike are identified. We distinguish three categories of subducted slabs: attached slabs reaching down to the bottom of the model; shallow slabs of shorter length in down‐dip direction, terminating shallower than 300 km depth; and detached slab segments. The location of slab segments are consistent with and validated by the intermediate‐depth seismicity, where it is present. The new high‐resolution tomography demonstrates the intricate relationships between slab fragmentation and the evolution of the relatively small and highly curved subduction zones and collisional orogens characteristic of the Mediterranean realm

Gravity effect of Alpine slab segments based on geophysical and petrological modelling

In this study, we present an estimate of the gravity signal of the slabs beneath the Alpine mountain belt. Estimates of the gravity effect of the subducting slabs are often omitted or simplified in crustal-scale models. The related signal is calculated here for alternative slab configurations at near-surface height and at a satellite altitude of 225 km. We apply three different modelling approaches in order to estimate the gravity signal from the subducting slab segments: (i) direct conversion of upper mantle seismic velocities to density distribution, which are then forward calculated to obtain the gravity signal; (ii) definition of slab geometries based on seismic crustal thickness and high-resolution upper mantle tomography for two competing slab configurations – the geometries are then forward calculated by assigning a constant density contrast and slab thickness; (iii) accounting for compositional and thermal variations with depth within the predefined slab geometry. Forward calculations predict a gravity signal of up to 40 mGal for the Alpine slab configuration. Significant differences in the gravity anomaly patterns are visible for different slab geometries in the near-surface gravity field. However, different contributing slab segments are not easily separated, especially at satellite altitude. Our results demonstrate that future studies addressing the lithospheric structure of the Alps should have to account for the subducting slabs in order to provide a meaningful representation of the geodynamic complex Alpine area

Slab break-offs in the Alpine subduction zone

After the onset of plate collision in the Alps, at 32–34 Ma, the deep structure of the orogen is inferred to have changed dramatically: European plate break-offs in various places of the Alpine arc, as well as a possible reversal of subduction polarity in the eastern Alps have been proposed. We review different high-resolution tomographic studies of the upper mantle and combine shear- and body-wave models to assess the most reliable geometries of the slabs. Several hypotheses for the tectonic evolution are presented and tested against the tomographic model interpretations and constraints from geologic and geodetic observations. We favor the interpretation of a recent European slab break-off under the western Alps. In the eastern Alps, we review three published scenarios for the subduction structure and propose a fourth one to reconcile the results from tomography and geology. We suggest that the fast slab anomalies are mainly due to European subduction; Adriatic subduction plays no or only a minor role along the Tauern window sections, possibly increasing towards the Dinarides. The apparent northward dip of the slab under the eastern Alps may be caused by imaging a combination of Adriatic slab, from the Dinaric subduction system, and a deeper lying European one, as well as by an overturned, retreating European slab.GRNE graduate schoolDeutsche Forschungsgemeinschaft (DE)H2020 Marie Skłodowska-Curie Actions ()http://www.orfeus-eu.org/eid