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Buildings that have existed for centuries undergo structural changes over time due to variations in use. In addition, many structures are severely damaged for example by moisture intrusion. To determine the distribution of moisture in the structure, they are often examined pointwise by core sampling. In addition to invasive methods, non-destructive methods may be applied to obtain three-dimensional hints on the moisture distribution with structures of interest. The purpose of this paper is to show that non-destructive determination of moisture distribution is possible by using and combining geophysical measurement methods such as infrared thermography (IR), ultrasound (US) and ground penetrating radar (GPR). There are examples for the combination of these methods for non-destructive examination, but it is not yet commonly applied in the field of restoration and conservation of historic buildings. We present results of geophysical investigations of medieval wall paintings in the cloister of the cathedral in Schleswig (Federal State Schleswig-Holstein, Northern Germany) in the framework of a project funded by the German Federal Foundation for the Environment (Deutsche Bundesstiftung Umwelt - DBU). In the cloister, large-scale alterations of the medieval red-line paintings occurred due to gypsum deposits and a shellac coating. In order to quantify the material properties of a vault section (yoke) in the cloister during the restoration ultrasound surface wave measurements, passive and active thermography and ground penetrating radar measurements were carried out. Repeating measurements at intervals of several months made it possible to evaluate the effectiveness of the test treatments by different solvents to remove the shellac as well as the gypsum deposits. In addition, our results from the passive thermography measurements show that in one section a defect in the horizontal barrier could be responsible for moisture ingress and associated damage. The radargrams recorded in this area confirm that a significant change in reflection amplitudes is present in the areas of increased moisture.

In the recent decade, the amount of available seismological broadband data has increased steeply. Picking later arriving phases such as S-phases is difficult, and the number of manual picks available for these phases is low compared to first arriving P-Phases. Moreover, data sets of manual picks can also be problematic, since phase arrival picks are sensitive to the parameters of the filtering, which are often unknown, and also depend on the individual picking behavior of the analysts. However, accurate arrival times, especially for these phases, could be used to improve the accuracy of velocity models obtained from seismic tomography. This necessitates the adoption of automatic techniques for determining teleseismic phase arrival times consistently over a large data set.In this work, a robust automatic picking algorithm based on multi-component, multiple-sample forward autoregressive prediction is examined with regards to its accuracy and applicability to massive data sets. The phase is identified using the Akaike criterion and the onset time is found evaluating discontinuities in the instantaneous period of the signal. This signal analytic approach is tested using synthetic waveforms as well as real data in conjunction with manual picks obtained from the reviewed ISC-catalog.Picking errors are estimated by comparing the automatic picks with manual picks, automatic picks at the neighboring stations as well as statistical methods. The quality evaluations suggest potential of using these automatically determined phase arrival times for SH and SV travel time tomography.

High-resolution Rayleigh phase velocity maps can help to improve our understanding of the 3-D vs anisotropic structure of the lithosphere of central Europe and its relationship to surface tectonics and volcanism.Using seismic broad-band recordings for the time period from 1990 to 2020, 600,000 inter-station fundamental mode Rayleigh wave phase velocities have been automatically determined by performing strict quality checks. Only smooth and reliable phase velocity curves with path-wise averaging and a standard error of 0.5% were chosen. For periods between 8 s and 350 s azimuthally anisotropic phase velocity maps were calculated for Central Europe. The phase velocity maps at short periods of up to 30 seconds show an NW-SE fast propagation direction along the East European Craton. At periods longer than 60 s, the anisotropic fast propagation direction shows slight variations from NE-SW to ∼N-S. At 150 s map, a NE-SW fast direction is observed. This might indicate a layering of anisotropy. Along the Tornquist-Teisseyre Zone, the fast propagation direction at all periods is NW-SE, except along its northern part as it shows slight variations. This might be due to the sharp change from the Precambrian continental mantle lithosphere to the younger Phanerozoic Europe. At short periods, central Europe anisotropy is following the Variscan front which changes abruptly to SE-NW near the Elbe line, whereas at longer periods the fast direction follows the Rheic suture and Saxothuringian suture. For the inversion of local azimuthally anisotropic phase velocity curves, we apply a newly elaborated stochastic inversion algorithm, the Particle Swarm Optimization algorithm (PSO). The result for extensive inversion and parameter tests of Rayleigh dispersion curves tests are shown. The lateral, as well as the vertical resolution of the resulting azimuthally anisotropic S-wave velocities and indications for layered anisotropy, are discussed.

For the understanding of the fate of the lithosphere when continents are colliding, it is necessary to image the structures of the lithosphere. In the case of the Alps, the structure of the Moho is very well known. This is, however, not yet the case for the lower boundary of the lithosphere, the lithosphere-asthenosphere boundary (LAB). We are using S-to-P converted seismic waves to study the structures of the Moho and the LAB beneath the greater Alpine Area with data from the Alparray project and the European networks of permanent seismic stations. Besides a new European Moho map, we present more detailed information about negative velocity gradients (NVG) below the Moho which may be interpreted as LAB. We found the European mantle lithosphere is deepening from about 50°N below the Alps to the Apennines and Dinarides along the entire east-west extension of the Alps. This area has also an east dipping component towards the Pannonian Basin and the Bohemian Massif. In the East and West of this area the European mantle lithosphere is dipping towards the North. We also discuss possible source locations of the volcanoes of the European Cenozoic Rift System in the light of our data.

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 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 990 permanent broad-band stations are operated by more than 40 institutions. Data of 97% of these stations are currently available via EIDA. In addition to the existing stations, 414 temporary stations from 24 mobile pools are deployed in the region achieving a coverage with an average station distance of 50 – 55 km. The experiment is based on intense cooperation between local network operators, mobile pool operators, field teams, ORFEUS, and interested research groups. Altogether, more than 50 institutions are participating in the AdriaArray experiment. We will report on the time schedule, participating institutions, mobile station pools, maps of 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.

The development of new automated techniques to measure earthquake-based interstation phase velocities allows for more detailed imaging of the South American Lithosphere. We calculated Rayleigh-wave phase velocities using the earthquake records from 1022 broadband seismic stations (South America, Antarctica and the Caribbean) operated between 1990 and 2020. A total of 1.069.259 earthquakes were selected with the following criteria: (1) events aligned within 10° of the 2-station great circle path; (2) a linearly increasing minimum magnitude between 4 and 6 Mw as a function of the epicentral distance; (3) maximum magnitude of 8 Mw; and (4) epicentral distances between 2.5° and 30°.We used surface wave fundamental mode dispersion curves calculated automatically using a new implementation of the 2-station cross-correlation method and a number of strict quality criteria, which include the selection of the phase velocity curves based on a 3D background model, curve smoothness and width of the considered frequency range. Following this process, we obtained 46.763 broad-band dispersion measurements between 4 and 315 s. Finally, the single-event dispersion curves were averaged for each interstation pair with associated error estimates and further quality control comparing results from both directions, evaluating smoothness and the standard deviation of the resulting dispersion curve.The dispersion curves were simultaneously inverted for isotropic and anisotropic (2ψ and 4ψ) phase-velocity maps parameterized on a triangular grid with a knot spacing of 30 km.The isotropic phase velocity maps at periods of 15 and 30 s indicate around 8% high-velocity perturbations in the regions of: (1) cratonic blocks of the South American platform (Brazilian Shield, São Francisco and Rio Apá cratons); and (2) the basement of the Pantanal basin (a 500 m thick sedimentary basin) possibly related to a high-velocity lower crust. For those periods, we also observed between -8 to -4% low-velocity perturbations associated with the Andean Mountain range root below the Altiplano Boliviano region (central Andes), the forelands of the Andes. The Paraná, Chaco and Parecis intracratonic basins also have lower velocities with relation to the neighboring cratonic areas.At periods of 60 and 100 s, we observed around 4% high-velocity perturbation associated with the deep roots of the oldest region of the Amazonian craton (eastern area of the Brazilian Shield) and the São Francisco craton. Azimuthal anisotropy is laterally and vertically variable within the South American lithosphere. At longer periods, fast directions are pointing to asthenospheric flow guided by LAB topography, for example below the Pantanal basin (central-west Brazil).For the next steps, we plan to jointly invert for a isotropic and anisotropic 3D model using earthquake and ambient noise dispersion curves with Rayleigh and Love waves.

The obduction of the Semail Ophiolite onto the Arabian continental margin during the convergence of the Arabian and Eurasian plates has left a significant impact on the lithospheric structure beneath the Oman Mountains. However, there remains a degree of uncertainty concerning the extent to which the inherited structures (pre-existing features of the lithosphere) contribute to the obduction of ophiolites. To gain a deeper understanding of the impact of the obduction process on the mantle structure beneath northern Oman, we analyze seismic anisotropy beneath this region using splitting analysis of teleseismic shear wave data collected from a dense network of 40 seismic stations that have been operational for approximately 3 years since 2013. Based on azimuthal distribution of the shear wave splitting (SWS) parameters, φ and δt, we are able to divide the study area into two subregions. The stations located to the west of the Semail gap exhibit relatively azimuthally invariant SWS parameters suggesting a single anisotropic layer. On the other hand, at most of the stations located in the central and eastern regions we observe a 90-degree periodicity versus back-azimuth, indicative of a depth-dependent anisotropic medium. In the western part, the fast axes are aligned with the strike of the collision between the continental and oceanic plates, where the oceanic lithosphere is believed to be obducted over the continental lithosphere. We also invert the azimuthal variation of the SWS parameters from the central and eastern stations for two layers of anisotropy. The fast axes of the upper layer exhibit a predominantly NW-SE trend, in good agreement with the anisotropy directions of the one-layer models obtained in the western region. The fast axes of the lower layer display a NE-SW trend, possibly representative of the large-scale mantle flow resulting from the present-day plate motion.

During the Cenozoic, the Circum-Mediterranean has experienced extensive and widespread igneous magmatism (i.e. intraplate, subduction-related and mixed-origin) that reflects the response of the upper mantle to the geodynamic evolution of this area. The exact origin of the volcanic activities and its relation to the underlying thin lithosphere especially in the continental regions have been long-lasting debated. We investigate the structure of the lithosphere and the sub-lithospheric mantle in the Circum-Mediterranean using regional high-resolution 3-D surface wave tomography and integrated geophysical-petrological thermochemical modelling of the temperature field and explore the relation to the occurrence intraplate and mixed-origin volcanic provinces (IMVPs).We define 9 shallow asthenospheric volumes (SAVs) across the Circum-Mediterranean upper mantle that form an almost interconnected belt of reduced shear wave velocities starting from the western Mediterranean to the Middle East and surrounding the Calabrian, Adriatic, Alpine slabs, however only interrupted by the eastern Mediterranean thick oceanic lithosphere. The SAVs are characterized by pronounced variations in shear-wave velocity not only laterally but also vertically between 70 and 300 km depths. Results from integrated geophysical-petrological thermochemical modelling show that the low velocities of the SAVs correspond to areas of thinned lithosphere (i.e., 1300 ºC at about 60-80 km depth) and anomalously warm asthenosphere (down to 300 km approximately) with respect to the average ambient mantle geotherm. A remarkable correlation between these areas and locations of IMVPs is observed with a mean lateral distance of < 100 km separating any SAV to the neighboring IMVP. The maximum separating distances are in order of ~ 350 km indicating a dense network of volcanic provinces above the shallow SAVs.The origin of the SAVs is related either to asthenospheric upwelling caused by slab rollback and decompressional melting during the formation of the back-arc basins (i.e., Agean-Anatolia, Pannonian, Moesian, Western Mediterranean) or to lithospheric thinning and rifting (Middle East and Rhone-Rheine areas). For the origin of the remaining SAVs (Adriatic, Central European, North Africa), other processes, i.e. thermal erosion feed by input from deep mantle sources, are suggested. According to the oldest ages of the IMVPs in the Circum-Mediterranean, the development of the SAVs started at least about ~ 60 - 70 Ma ago and accelerated in the Neogene.

The geology of eastern Arabia is dominated by a vast cover of mostly Phanerozoic sedimentary rocks and little was known about the architecture of the middle and lower crust. On the easternmost margin, obduction of the Semail Ophiolite during late Cretaceous times is the youngest first-order tectonic process that shapes the present-day geology across the Oman Mountains in northern Oman and the eastern United Arab Emirates. Within the obducted units, Neoproterozoic to Cretaceous autochthonous rocks of the Arabian shelf are exposed in two tectonic windows and provide a detailed view of the geodynamic evolution of the shallow Arabian continental crust during and after obduction. A new, unprecedented 3-D anisotropic shear-wave velocity (Vs) model reveals that - prior to obduction - the assembly of the eastern Arabian lithosphere in Neoproterozoic times and its modification during the Permian breakup of Pangea strongly control the present-day lithospheric architecture. Building upon previous geodynamic models that were restricted to the upper crust, reconstruction of the entire lithospheric evolution resolves some key unknowns in eastern Arabia’s geodynamics: The NNE-striking Semail Gap Fault (SGF) is primarily an upper crustal feature but another NE-striking deep crustal boundary zone west of the Jabal Akhdar Dome segments the Arabian continental crust in two structurally different units. While Permian Pangea rifting occurred on both eastern and northern margins of eastern Arabia, large-scale mafic intrusions occurred mostly east of the SGF. Eastward crustal thinning localizes at the eastern limit of obducted units, east of which the lower crust is strongly intruded and likely underplated. Late Cretaceous exhumation and overthrusting at the end of ophiolite obduction is the likely cause for crustal thickening below today‘s topography of the Oman Mountains. Lithospheric thickness is ~200-250 km in central Arabia but only ~100 km below the Oman Mountains. Thinning of the continental lithosphere is attributed to late Eocene times, which explains contemporaneous basanite intrusions into the continental crust and provides a plausible mechanism for observed crustal-scale extension and the broad, margin-wide emergence of the Oman Mountains. Thus, uplift of the mountain range might be unrelated to Arabia-Eurasia convergence.

During the Cenozoic, the Circum-Mediterranean and its periphery have experienced extensive and widespread anorogenic igneous magmatism that reflects the response of the upper mantle to the geodynamic evolution of this area. The exact origin of the volcanic activities and its relation to the underlying thin lithosphere especially in the continental areas have been long-lasting debated. We investigate the structure of the Mediterranean lithosphere and the sub-lithospheric mantle by surface waves that are mainly sensitive to the 3-D S-wave velocity structure at those depths. A high-resolution tomographic study based on automated broad-band measurements of inter-station Rayleigh wave phase velocities down to about 300 km depth is presented. We identify shallow asthenospheric volumes, characterized by low S-wave velocities between about 70 km and 250 km depth, and distinguish between five major shallow asthenospheric volumes in the Circum-Mediterranean: the Middle East, the Anatolian-Aegean, the Pannonian, the Central European, and the Western Mediterranean Asthenosphere volumes. Remarkably, they form an almost continuous circular belt of asthenospheric areas interrupted only by the thick Permo-Carboniferous oceanic lithosphere in the eastern Mediterranean.Integrated thermochemical modelling using surface wave phase velocities, topography, and heat flow as constraints indicates a remarkable variability of the lithospheric thickness across the area. Thick lithosphere is found in the Paris Basin, the East European Craton, and the eastern Mediterranean whereas thin lithosphere is found in areas of pronounced negative shear-wave anomalies at depth between 70 km and 200 km. Cenozoic intraplate volcanic fields are located in areas with thin lithosphere underlain by shallow asthenosphere. Thus, anorogenic intraplate volcanism in the Circum-Mediterranean appears to be associated with thin and hot lithospheric regions and low S-wave sublithospheric velocities. The distribution and properties of the shallow asthenosphere volumes in the region are discussed and related to the spatial-temporal occurrence of intraplate as well as subduction related volcanism in the western Mediterranean, central Europe, the Pannonian Basin, the Anatolian region and the Middle East.

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.

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.

In the recent decade, the amount of available seismological broadband data has increased steeply. Picking later arriving phases such as S-phases is difficult, and there are few manual picks available for these phases. Data sets of manual picks can also be problematic, since phase arrival picks are sensitive to the parameters of the filtering, which are often unknown, and the individual picking behavior of the analysts. However, accurate arrival times, especially for these phases, could be used to improve the accuracy of velocity models obtained from seismic tomography. This necessitates the adoption of automatic techniques for determining teleseismic phase arrival times consistently over a large data set. In this work, a robust automatic picking algorithm based on autoregressive, multi-component, multiple-sample forward prediction is examined with regards to its accuracy. The phase is identified using the Akaike criterion and the onset time is found by evaluating discontinuities in the instantenous period of the signal. This signal analytic approach is tested using synthetic waveforms as well as real data in conjunction with manual picks obtained from the reviewed ISC-catalog. Picking errors are estimated by comparing the automatic picks with manual picks, automatic picks at the neighboring stations as well as statistical meth- ods. The quality evaluations suggest potential of using these automatically determined phase arrival times for a travel time tomography.

Surface waves propagating from earthquakes, active sources or within the ambient noise wavefield are widely used to image Earth structure at various scales, from centimetres to hundreds of kilometres. The accuracy of surface-wave, phase-velocity measurements is essential for the accuracy of the Earth models they constrain. Here, we identify a finite-frequency phase shift in the phase travel time that causes systematic errors in time-domain, phase-velocity measurements. The phase shift arises from the approximation of monochromatic surface waves with narrow-band filtered surface waves. We derive an explicit formula of the finite-frequency phase shift and present a numerical method for its evaluation and for the correction of the measurements. Applications to high-frequency and long-period examples show that the phase shift is typically around π/60-π/16 for the common settings of ambient-noise imaging studies, which translates to 0.2-0.8% phase-velocity measurement errors. The finite-frequency phase shift depends on the (1) second derivative of the wavenumber with respect to frequency; (2) width of the narrow-band filter; (3) epicentral or interstation distance; (4) centre frequency of the filter. In conversion to phase velocity, the last two factors cancel out. Frequency-domain methods for phase-velocity measurements have the advantage of not producing the finite-frequency phase shift. Both time- and frequency-domain measurements, however, can be impacted by a break-down of the far-field approximation (near-field phase shift), which our calculations also show. Our method offers an effective means of improving the accuracy of the widely used time-domain, phase-velocity measurements via the evaluation of and corrections for the finite-frequency phase shift.

We use teleseismic data from all available broadband stations, permanent and mobile, in the entire area. Our processing method applies distance moveout correction, amplitude normalization, sign equalization and summation of traces with piercing points in 1° latitude times 1° longitude cells. The traces are stacked along the picked SV onset times. We obtain very clear signals from the Moho, less strong signals from velocity reductions below the Moho and again clear signals from the 410 km discontinuity. We also see locally velocity reductions just above the 410 km discontinuity. We show a number of profiles through the study area and hope to show maps of all seismic discontinuities. We compare our results with earlier observation.

The maximum achievable resolution of a tomographic model varies spatially and depends on the data sampling and errors in the data. The significant and continual measurement-error decreases in seismology and data-redundancy increases have reduced the impact of random errors on tomographic models. Systematic errors, however, are resistant to data redundancy and their effect on the model is difficult to predict; often this results in models dominated by noise if the target resolution is too high. Here, we develop a method for finding the optimal resolving length at every point, implementing it for surface-wave tomography. As in the Backus-Gilbert method, every solution at a point results from an entire-system inversion, and the model error is reduced by increasing the model-parameter averaging. The key advantage of our method consists in its direct, empirical evaluation of the posterior model error at a point.We first measure interstation phase velocities at simultaneously recording station pairs and compute phase-velocity maps at densely, logarithmically spaced periods. Numerous versions of the maps with varying smoothness are then computed, ranging from very rough to very smooth. Phase-velocity curves extracted from the maps at every point can be inverted for shear-velocity (VS) profiles. As we show, errors in these phase-velocity curves increase nearly monotonically with the map roughness. We evaluate the error by isolating the roughness of the phase-velocity curve that cannot be explained by any Earth structure and determine the optimal resolving length at a point such that the error of the local phase-velocity curve is below a threshold.A 3-D VS model is then computed by the inversion of the composite phase-velocity maps with an optimal resolution at every point. Importantly, the optimal resolving length does not scale with the density of the data coverage: some of the best-sampled locations display relatively low lateral resolution, due to systematic errors in the data.We apply this method to image the lithosphere and underlying mantle beneath Ireland and Britain. Our very large data produces a total of 11238 inter-station dispersion curves, spanning a very broad total period range (4–500 s), yielding unprecedented data coverage of the area and providing state-of-the-art regional resolution from the crust to the deep asthenosphere. Our tomography reveals pronounced, previously unknown variations in the lithospheric thickness beneath Ireland and Britain, with implications for their Caledonian assembly and for the mechanisms of the British Tertiary Igneous Province magmatism.

The preservation of culturally significant buildings is challenging due to the variety of historical building materials, the often complex building history and damage patterns. It is usually associated with high financial costs. Non-destructive testing may help to plan, optimize, and monitor conservation measures. Here, we report on non-destructive testing of moisture distribution at the Cathedral St. Petri in Schleswig (Germany) using thermography and georadar measurements. These methods are standard methods in engineering geology and construction. In the field of heritage conservation, however, the application and especially the combination of several of these methods is not yet established.The walls of the ‘Schwahl’ (a three-sided cloister) show medieval paintings from the 14th century. In the Schwahl, large-scale alterations occur due to gypsum deposits and a shellac coating. Active thermography measurements were taken before and after test treatments to evaluate the effectiveness of the use of different solvents to remove the shellac and the gypsum deposits. Passive thermography and georadar measurements indicate increased moisture content in the area of the gypsum deposits likely caused by a permeable horizontal sealing barrier below the paintings. Examples of the measurements are shown and the processing of the thermography and georadar measurements including the attenuation analysis are discussed.

The Pannonian Basin is located in Central-Europe surrounded by the Alpine, Carpathian, and Dinarides mountain ranges. This is a back-arc basin characterized by shallow Moho depth, updoming mantle and high heat flow. In this study, we present the results of the Rayleigh wave based ambient noise tomography to investigate the velocity structure of the Carpathian-Pannonian region. For the ambient noise measurements, we collected the continuous waveform data from more than 1280 seismological stations from the broader Central-Eastern European region. This dataset embraces all the permanent and the temporary (AlpArray, PASSEQ, CBP, SCP) stations from the 9-degree radius of the Pannonian Basin which were operating between the time period between 2005 and 2018. All the possible vertical component noise cross-correlation functions were calculated and all phase velocity curves were determined in the 5-80 s period range using an automated measuring algorithm. The collected dispersion measurements were then used to create tomographic images that are characterized by similar velocity anomalies in amplitude, pattern and location that are consistent with the well-known tectonic and geologic structure of the research area and are comparable to previous tomographic models published in the literature.

The Semail Ophiolite is the world‘s largest and best exposed oceanic lithosphere on land and a primary reference site for studies of creation of oceanic lithosphere, initiation of subduction, geodynamic models of obduction, subduction and exhumation of continental rocks during obduction. Five decades of geological mapping, structural, petrological and geochronological research provide a robust understanding of the geodynamic evolution of the shallow continental crust in northern Oman and how the late Cretaceous obduction process largely shaped the present-day landscape. Yet, prior to obduction, other first-order tectonic processes have left their imprint in the lithosphere, in particular the Neoproterozoic accretion of Arabia and Permian breakup of Pangea. Due to the scarcity of deep structure imaging below the ophiolite, the presence and significance of inherited structures for the obduction process remain unclear.We discuss a new 3-D anisotropic shear wave velocity model of the crust below northern Oman derived from ambient noise tomography and Receiver Function analysis which allows to resolve some key unknowns in geodynamics of eastern Arabia: (1) Several NE-trending structural boundaries in the middle and lower crust are attributed to the Pan-African orogeny and align with first-order lateral changes in surface geology and topography. (2) The well-known Semail Gap Fault Zone is an upper crustal feature whereas two other deep crustal faults are newly identified. (3) Permian rifting occurred on both eastern and northern margins but large-scale mafic intrusions and/or underplating occurred only in the east. (4) While obduction is inherently lithospheric by nature, its effects are mostly observed at shallow crustal depths, and lateral variations in its geometry and dynamics can be explained by effects on pre-existing Pan-African and Permian structures. (5) Continental subduction and exhumation during late Cretaceous obduction may be the cause for crustal thickening below today‘s topography. (6) Thinning of the continental lithosphere below northern Oman in late Eocene times – possibly related to thermal effects of the incipient Afar mantle plume - provides a plausible mechanism for the broad emergence of the Oman Mountains and in particular the Jabal Akhdar Dome. Uplift might thus be unrelated to compressional tectonics during Arabia-Eurasia convergence as previously believed.

Spatial resolution, as the ability to distinguish different features that are close together, is a fundamental concept in seismic tomography and other imaging fields. In contrast with microscopy or telescopy, seismic tomography’s images are computed, and their resolution has a complex, non-linear dependence on the data sampling and errors. Linear inverse theory provides a useful resolution-analysis approach, defining resolution in terms of the closeness of the resolution matrix to the identity matrix. This definition is similar to the universal, multi-disciplinary one in some contexts but diverges from it markedly in others. In this work, we adopt the universal definition of resolution (the minimum distance at which two spike anomalies can be resolved). The highest achievable resolution of a tomographic model then varies spatially and depends on the data sampling and errors in the data. We show that the propagation of systematic errors is resistant to data redundancy and results in models dominated by noise if the target resolution is too high. This forces one to look for smoother models and effectively limits the resolution. Here, we develop a surface-wave tomography method that finds optimal lateral resolution at every point by means of error tracking.We first measure interstation phase velocities at simultaneously recording station pairs and compute phase-velocity maps at densely, logarithmically spaced periods. Multiple versions of the maps with varying smoothness are computed, ranging from very rough to very smooth. Phase-velocity curves extracted from the maps at every point are then inverted for shear-velocity (VS) profiles. As we show, errors in these phase-velocity curves increase nearly monotonically with the map roughness. Very smooth VS models computed from very smooth phase-velocity maps will be the most accurate, but at a cost of a loss of most structural information. At the other extreme, models that are too rough will be dominated by noise. We define the optimal resolution at a point such that the error of the local phase-velocity curve is below an empirical threshold. The error is estimated by isolating the roughness of the phase-velocity curve that cannot be explained by any Earth structure.A 3D VS model is then computed by the inversion of the phase-velocity maps with the optimal resolution at every point. The estimated optimal resolution shows smooth lateral variations, confirming the robustness of the procedure. Importantly, optimal resolution does not scale with the density of the data coverage: some of the best-sampled locations require relatively low lateral resolution, probably due to systematic errors in the data.We apply the method to image the lithosphere and underlying mantle beneath Ireland and Britain, using 11238 newly measured, broadband, inter-station dispersion curves. The lateral resolution of the 3D model is computed explicitly and varies from 39 km in central Ireland to over 800 km at the region boundaries, where the data coverage declines. Our tomography reveals pronounced, previously unknown variations in the lithospheric thickness beneath the region, with implications for the Caledonian assembly of the islands’ landmass and the mechanism of the magmatism of the British Tertiary Igneous Province.

The Earth’s oldest oceanic lithosphere preserved in-situ is in the eastern Mediterranean Sea. It can offer essential information on the oceanic plate evolution. Yet, its thickness and other properties have been difficult to determine by means of seismic imaging due to the high heterogeneity of the region. Here, we combine a large, new surface wave dataset with published wide-angle data in order to map the properties and lateral variability of the oceanic lithosphere, as well as the ocean-continent transition in the easternmost Mediterranean beneath the Levant Basin. We use stochastic joint inversion of broad band, phase-velocity dispersion measurements and seismic refraction P-wave velocity models to obtain 1-D, shear wave velocity models down to 300 km depth and compare the structure beneath the Ionian Sea and the Levant Basin. The thickness of the crust is about 16.4 ± 3 km and 22.3 ± 2 km beneath the chosen locations within the Ionian Sea and the Levant Basin, respectively. The Poisson’s ratio of about 0.32 and Vp/Vs of about 1.93 in the crystalline crust, yielded by the inversion, confirm the presence of oceanic crust beneath the Ionian Sea. The thickness of the Ionian oceanic lithosphere is around 180 km, whereas the continental lithosphere beneath the eastern Levant Basin is ~70 km thick, with low crustal Vp/Vs (~1.7) and Poisson’s (~0.24) ratios. According to 3-D shear wave velocity tomography using the surface wave data, the thickness of the oceanic lithosphere increases from the Triassic Ionian Sea towards the Permian-Carboniferous Libyan Sea and Herodotus Basin. Thicknesses of the Permo-Triassic oceanic lithosphere considerably larger than 100 km indicate that oceanic lithosphere can thicken by cooling substantially beyond the limits suggested by the plate cooling model. The transition from oceanic to continental lithosphere occurs at about 31°E in the crust, as indicated by magnetic and gravity measurements. The continental mantle lithosphere further to the east of this boundary is ~150 km thick beneath the westernmost Levant Basin, as indicated by shear wave velocity tomography and long wavelength gravity anomalies, and strongly thins eastward towards the area of the Levantine Coast and the Dead Sea Fault. The localization of the lithospheric deformation and crustal seismicity along the Dead Sea Fault correlates spatially with the thinning of the underlying continental lithosphere.Key words: surface wave tomography, wide angle seismic imaging, joint inversion, Vp/Vs and Poisson’s ratios, eastern Mediterranean, Oceanic Lithosphere, Continental Lithosphere, Dead Sea Fault.

High-frequency seismic surface waves sample the top few tens of meters to the top few kilometres of the subsurface. They can be used to determine three-dimensional distributions of shear-wave velocities and to map the depths of discontinuities (interfaces) within the crust. Passive seismic imaging, using ambient noise as the source of signal, can thus be an effective tool of exploration for mineral, geothermal and other resources, provided that sufficient high-frequency signal is available in the ambient noise wavefield and that accurate, high-frequency measurements can be performed on this signal. Ambient noise imaging using the ocean-generated noise at 5-30 s periods is now a standard method, but less signal is available at frequencies high enough for deposit-scale imaging (0.2-30 Hz), and few studies have reported successful measurements in broad frequency bands. Here, we develop a workflow for the measurement of high-frequency, surface-wave phase velocities in very broad frequency ranges. Our workflow comprises (1) a new noise cross-correlation procedure that accounts for the non-stationary properties of the high frequency noise sources, removes bandpass filtering, replaces temporal normalization with short time window stacking, and drops the explicit spectral normalization by adopting cross-coherence; (2) a new phase-velocity measurement method that extends the bandwidth of reliable measurements by exploiting the (resolved) 2π ambiguity of phase-velocity measurements; (3) interstation-distance-dependent quality control that uses the similarity of subgroups of dispersion curves to reject outliers and identify the frequency ranges with accurate measurements. The workflow is highly automated and applicable to large arrays. Applying our method to data from a large-N array that operated for one month near Marathon, Ontario, Canada, we use rectangular subarrays with 150-m station spacing and, typically, 1 hour of data and obtain Rayleigh-wave phase-velocity measurements in a 0.55-23.8 Hz frequency range, spanning over 5.4 octaves, nearly twice the typical frequency range of 1.5-3 octaves in previous studies. Phase-velocity maps and the subregion-average 1D velocity models they constrain show a high-velocity anomaly consistent with the known, west-dipping gabbro intrusions beneath the area. The new structural information can improve our understanding of the geometry of the gabbro intrusions, hosting the Cu-PGE Marathon deposit.

The fascinatingly complex tectonic make-up of the Mediterranean region comprises small, strongly-curved retreating subduction zones, associated back-arc basins, and the continental collisions along the northern and eastern margins of the Adriatic microplate. It remains a challenge to resolve the geometry of the subducted slabs in the Mediterranean upper mantle. Here, we present new evidence for the location and lateral and vertical extent of slab segments from a new, high-resolution, Rayleigh-wave tomography. The tomographic model spans the depth range from the crust down to 300 km and is complemented by intermediate-deep seismicity data in the circum-Mediterranean region.An automated procedure to measure inter-station Rayleigh wave phase velocities is applied to a large, heterogeneous dataset from all publically available stations around the Mediterranean in the time period from 1990 to 2015. Furthermore, for the first time, data from the Egyptian National Seismological Network (ENSN) are used regional seismic tomography. The resulting large set of about 200,000 inter-station phase velocity measurements is inverted for a set of phase-velocity maps spanning a very broad period range (8 - 350 s). The maps are then inverted, point by point, for a 3D, S-velocity model using a stochastic, particle-swarm-optimization inversion.We distinguish between attached slab segments reaching down to the bottom of the model and shallow slabs of shorter length or detached slab segments resulting both from horizontal tearing. We discuss evidence for continental subduction east of Cyprus, for continuous NE-dipping subduction in the Antalyan region and NW dipping subduction in the SE Aegean in the area of Rhodes. An attached slab is imaged beneath the Hellenides reaching down to at least 300 km depth whereas beneath the Dinarides a short slab is found down to about 150 km depth above a slab tear. The slab in the southern Carpathians seems to be partly detached. A south-dipping slab is imaged in the central Alps but shallow bivergent subduction is favoured in the eastern Alps. In the western Alps, a shallow slab east-dipping Eurasian slab segment is in close proximity to the nearly vertically dipping attached slab segment beneath the northern Apennines and the southern Po plain. In the central Apennines a slab gap is found whereas the NE-dipping Calabrian Slab seems to partly detached along the northern Sicilian coast. The Kabylides Slab that appears to be attached along the North African coast but detached along the margin of the shelf in the Sicily Channel, is clearly separated from the Calabrian Slab in the NE and the Alboran-Betics Slab in the west. According to our model, the latter slab consists of two segments: a shallow Alboran one and a detached Betics slab segment. We summarize our interpretations in a map of the Mediterranean slab segments and indicate open questions.

Collisional tectonics of the Alps is driven by several slab segments. A detailed imaging of the lithosphere-asthenosphere system beneath the Alps is, however, challenging due to the relatively small size of the slab segments and the highly curved geometry of the Alps. Surface waves, due to their high sensitivity to variations in seismic velocities at lower crustal and upper mantle depth, are well suited to study the Alpine deep structure. New azimuthally anisotropic Rayleigh wave phase velocity maps are calculated from automated inter-station phase velocity measurements in a very broad period range (8 – 350 s). The constructed local dispersion curves are then inverted individually for 1-D shear-wave velocity models using a new implementation of the stochastic Particle Swarm Optimization (PSO) inversion algorithm that enables the calculation of a high-resolution 3-D shear-wave velocity model from the crust down to 300 km beneath the Alps. In the Central Alps, a nearly vertical high velocity anomaly down to depth of 250 km is imaged and interpreted as subducting Eurasian mantle lithosphere. In contrast, low velocities in the Western Alps at depth of approximately 100 km and downwards are supporting the shallow slab break-off model. In the Eastern Alps, the presence of a vertically continuous high-velocity anomaly from 75 km to about 200 km depth beneath the northern Eurasian foreland and the almost continuous extension of a high-velocity anomaly from the Dinarides towards the Eastern Alps hint at a bivergent slab geometry beneath the Eastern Alps caused by subducting mantle lithosphere of both Eurasian and Adriatic origin. There is also evidence for subduction of Adriatic lithosphere to the east beneath the Pannonian Basin and the Dinarides down to a depth of about 150 km. Beneath the northern Apennines, the model indicates an attached Adriatic slab, whereas a slab window is found beneath the central Apennines. The results show that surface wave tomography can contribute to the imaging of complex slab geometries and slab segmentation in the Alpine region.

In the recent decade, the amount of available seismological broadband data has increased steeply. Picking later arriving phases such as S-phases is difficult, and there are few manual picks available for these phases. Data sets of manual picks can also be problematic, since phase arrival picks are sensitive to the parameters of the filtering, which are often unknown, and the individual picking behavior of the analysts. This neccesitates the adoption of automatic techniques for determining teleseismic phase arrival times consistently over a large data set. In this work, a robust automatic picking algorithm based on autoregressive prediction in a moving window is explained. In this algorithm, a characteristic function is calculated as the autoregressive prediction error in a moving window. This characteristic function is then transformed with the Akaike-Information Criterion to obtain the phase arrival time estimate. This estimate is further improved in a second iteration of a similiar scheme in a smaller time window. The algorithm is applied to a global data set including AlpArray stations, covering a time period from 1995 to present, to obtain arrival times for teleseis- mic P- and S-phases. Residuals to theoretical travel times and to local averages are shown. Different methods for automatically evaluating the quality of indi- vidual picks are used, based on signal to noise ratio of the seismic trace and impulsiveness of the arrival. The picking errors are estimated by comparision with manual picks and neighboring stations as well as statistical methods. The quality evaluations suggest potential of using these automatically determined phase arrival times for a travel time tomography.

Using AlpArray and European networks, it is now possible to resolve the shape and inherent distortion of teleseismic surface waves across the greater alpine area at an as of yet unprecedented resolution. With well over 1500 available broadband stations within a 20° radius around the central Alps we demonstrate our approach for measuring both phase and amplitude distributions of surface wave signals in the space-frequency domain, leading to structural phase velocity information corrected for possible dynamic effects. Knowledge of the amplitude fields is particularly important to understand wave front deformations and to correct dynamic phase velocity measuremnts.To diminsh the influence of noise, higher modes, coda waves, or adjacent events on our measurement, we analyse correlations with synthetic fundamental mode signals for a spherically symmetric earth model. The resulting wave field parameters are consequently expressed as amplitude and phase perturbations of the synthetic background wave fields. The measurements are explained and examples for phase and amplitude Rayleigh wave fields are shown and discussed.Examining the wave field properties it becomes apparent that the dynamic contributions to the eikonal phase velocity are indeed significant, caused by both heterogeneities inside and (far) outside the observed region. Smaller local anomalies are for instance frequently observed around Mount Etna and Vesuvius, with the active vulcanism causing noticable reverberations. Surprisingly, large wave field anomalies are often oriented almost parallel to the propagation direction and can potentially span the entire station distribution, manifesting themselves as contiguous stripes of elevated amplitudes from positive interference of off-axis scattered waves.

Cape Verde is an intraplate archipelago located in the Atlantic Ocean about 560 km west of Senegal, on top of a ~130 Ma sector of the African oceanic lithosphere. Until recently, due to the lack of broadband seismic stations, the upper-mantle structure beneath the islands was poorly known. In this study we used data from two temporary deployments across the archipelago, measuring the phase velocities of Rayleigh-waves fundamental-modes in a broad period range (8–250 s), by cross-correlating teleseismic earthquake data between pairs of stations. Deriving a robust average, phase-velocity curve for the Cape Verde region, we inverted it for a shear-wave velocity profile using non-linear gradient search.Our results show anomalously low velocities of ∼4.2 km/s in the asthenosphere, indicating the presence of high temperatures and, eventually, partial melting. This temperature anomaly is probably responsible for the thermal rejuvenation of the oceanic lithosphere to an age as young as about 30 Ma, which we inferred from the comparison of seismic velocities beneath Cape Verde and the ones representing different ages in the Central Atlantic.The present results, together with previously detected low-velocity anomalies in the lower mantle and relatively He-unradiogenic isotopic ratios, also suggest a hot plume deeply rooted in the lower mantle, as the origin of the Cape Verde hotspot.The authors would like to acknowledge the financial support FCT through project UIDB/50019/2020 – IDL and FIRE project Ref. PTDC/GEO- GEO/1123/2014.

The ongoing formation of the alpine mountain belt is a major indicator of the active geological processes in the Mediterranean that for example causes volcanism in Italy as well as considerable hazard. To understand this complex system the European project AlpArray, with the German contribution 4D-MB, was funded in order to investigate the structure, dynamics and geology of the alpine area in more detail and through all scales. We focus on the large scale geodynamic processes that drive this complex system of multiple subduction zones, ranging from the surface to the mantle.Here, preliminary geodynamic modeling results will be presented, which are based on recent seismic imaging of the AlpArray target area. The model has been extended to a larger depth and the geometries of the slabs have been adopted. Several instantaneous forward simulations will be presented that aim to reproduce the major present day GPS velocity patterns. Additionally, the fit to the data is refined using a steepest descent adjoint gradient based inverse technique. These gradients can also be used to highlight the pointwise sensitivity of the surface velocity to the material parameters at depth. In order to be able to compare the modeling results to additional observations a framework is presented on how seismic anisotropy can be calculated with the geodynamic modeling code LaMEM (Kaus et al., 2016). This abstract is in co-operation with the whole AlpArray working group.

The Oman ophiolite is one of the best preserved and studied ophiolites, where oceanic lithosphere was obducted on top of a continent. It covers an area of about 700 x 140 km² but its 3D geometry, as well as the properties of the underlying continental lithosphere are largely unknown. We operated a temporary broadband seismic network with 40 instruments for continuous, passive seismic registration for 27 months, complemented by 18 permanent stations in the study region. Ambient noise cross-correlation functions are calculated for vertical and transverse components for all station pairs. We derive azimuthally anisotropic phase velocity maps for Rayleigh- and Love waves in the period range 2 – 40s which show velocity anomalies that are very consistent with geological features at the shortest periods (15s) the velocity pattern subdivides the study region into a faster eastern and slower northwestern part below the Oman Mountains. We then invert local dispersion curves to shear wave velocity profiles using a novel implementation of a radially anisotropic, probabilistic inversion. Combination of the obtained 1D models to a 3D model provides the first three-dimensional view of shear wave velocity variations along the Eastern Arabian Plate margin. The model highlights at shallow levels strong lateral velocity contrasts between unconsolidated young sediments south of the Oman Mountains (slow) and areas covered by ophiolite and where autochtonous shelf sediments are exposed (fast).At middle to lower crustal levels, we image linearly northeast trending velocity contrasts that we attribute to assembly of the Arabian plate in late Proterozoic. These features are overprinted by obduction-related convergence in late Cretaceous with thickening of the middle to lower crust below the Oman mountains. Moho depth is around 40-45km northwest of Semail Gap but shallows significantly east of it to 20km at the eastern coast. This is largely in consistency with independent estimates from Receiver Functions calculated with the same data.

On 26th of November 2019 an Mw 6.4 earthquake ruptured near the port town of Durrës, only 25 km from Tirana, the capital of Albania. It caused major damage and killed 51 people, making it the deadliest earthquake in 2019 worldwide.The earthquake occurred on the eastern Adriatic margin, where the Adriatic micro-plate collides with Eurasia causing widespread distributed deformation and crustal shortening that built the peri-Adriatic orogenic belts. Convergence is accommodated in the external Dinarides/Albanides by thrust faulting, mostlyalong E-dipping low-angle detachments with subordinate W-dipping back-thrusts in the most external thrust belt segment. The deformation front, particularly along the southeastern Adriatic coast, is seismically highly active, manifested not only by this most recent event, but also, e.g., by one of the largest instrumentally recorded earthquakes in Europe, the 1979 M7.1 Montenegro event slightly further north and a number of disastrous historic earthquakes.The 2019 Durrës mainshock was apparently relatively deep (~25 km) and of thrust type. It was preceded by significant foreshock activity starting in September 2019 with two Mw 5.6 and 5.1 earthquakes a few kilometres south of the mainshock that also had a thrust mechanism, however with nodal planes differing from the mainshock, indicating that these occurred on a different fault.Approximately two weeks after the mainshock, we installed a 30-station short-period seismic network to densely cover the epicentral area. We will present a preliminary analysis of the mainshock and its aftershock sequencehopefully elucidating the fault network responsible for the earthquake sequence.

Based on a high-resolution Rayleigh-wave tomography of the Mediterranean region, shallow asthenospheric volumes are identified characterized by low S-wave velocities between about 70 km and 250 km depth. We distinguish between five major shallow asthenospheric volumes in the Circum-Mediterranean: the Middle East, the Anatolian-Aegean, the Pannonian, the Central European, and the Western Mediterranean Asthenospheres. Remarkably, they form an almost closed circular belt of asthenospheres interrupted only by thick Triassic oceanic lithosphere in the eastern Mediterranean. Shallow asthenosphere beneath the Rhone and Rhine Grabens connects the Western Mediterranean with the Central European Asthenosphere. Beneath the Serbian and Rhodope Mountains shallow asthenosphere forms a link between the Pannonian and the Anatolian-Aegean Asthenosphere.Cenozoic intraplate volcanic fields are found above all areas underlain by shallow asthenosphere, and is absent in areas of thick lithosphere. Thus, anorogenic intraplate volcanism in the circum-Mediterranean appears to be associated with shallow asthenospheric volumes. Specifically, this applies to volcanic fields in the central Apennines and Sicily underlain by the Western Mediterranean Asthenosphere. Regions without significant tectonic extension above shallow asthenosphere are characterized by elevated topography. Examples are the Anatolian Plateau, the Western Carpathians, the Atlas Mountains and the low mountain ranges in Central Europe and Iberia. In back-arc regions like the Aegean and Pannonian Basins, and the western Mediterranean, strong tectonic extension leads to low topography above shallow asthenosphere. High continental shoulders are present in transition regions towards thinner lithosphere. Examples are the Levantine coast, the Moesian platform and the Bohemian Massif or the southern Atlas Mountains. We further note that in regions of past volcanism continental lithosphere is thickening by long-term cooling. The showcase for this is the North-German Basin where sedimentation and an about 100 km thick lithosphere developed after extensive Permian volcanism. These observations hint at considerable variability in time of the continental lithosphere-asthenosphere boundary: Thinning of continental lithosphere may be caused by extension and/or thermal erosion whereas cooling may lead to thickening of continental lithosphere as is evident from the Mesozoic evolution of continental lithosphere in central Europe.

Longwall mining activity in the Ruhr coal mining district leads to mining-induced seismicity. For detailed studies the seismicity of a single longwall panel beneath the town of Hamm-Herringen in the eastern Ruhr area was monitored between June 2006 and July 2007 with a dense temporary network of 15 seismic stations. More than 7000 seismic events with magnitudes between –1.7 ≤ ML ≤ 2.0 were detected and localized in this period. Most of the events occurred in the vicinity of the moving longwall face. In order to find possible differences in the brittle failure types of these events an association of the events to distinct clusters is performed based on their waveform characteristics. This task is carried out using a new clustering algorithm utilizing a network similarity matrix which is created by combining all available 3-component single station similarity matrices. The resultant network matrix is then sorted with respect to the similarity of its rows leading to a sorted matrix immediately indicating the clustering of the event catalogue. Finally, clusters of similar events are extracted by visual inspection. This approach results in the identification of several large clusters which are distinct with respect to their spatial and temporal characteristics as well as their frequency magnitude distributions. Comparable clusters are also found with a conventional single linkage approach, however, the new routine seems to be able to associate more events to specific clusters without merging the clusters. The nine largest observed clusters can be tentatively divided into three different groups that indicate different types of brittle failure. The first group consists of the two largest clusters which constitute more than half of all recorded events. Results of a relative relocation using cross-correlation data suggest that these events are confined to the extent of the mined out longwall and cluster close to the edges of the active longwall at the depth of active mining. These events occur in lockstep with the longwall advance and exhibit a high b value of the Gutenberg–Richter relation (GR) of about 1.5 to 2.5 and consist of small magnitude events. Thus, these events represent the immediate energy release adjacent to the mined out area. The second group consists of clusters located either slightly above or below the depth of active mining and occurring at the current position of the longwall face within the confines of the longwall. They consist of generally stronger events and do not follow GR. This activity might be linked to the failure of more competent layers above and below the mined out seam resulting in larger magnitude events. Finally, one cluster represents seismic activity with a rather low b value below 1 and events located partly towards the north of the longwall which are delayed with respect to the advance of the longwall face. These events are interpreted as brittle failure on pre-existing tectonic structures reactivated by the mining activity.