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Magnetic minerals form or alter in the presence of hydrocarbons, making them a potential magnetic proxy for identifying hydrocarbon migration pathways. In this paper, we test this idea by magnetically measuring core samples from the Tay Fan in the Western Central Graben in the Central North Sea. In a companion paper, 3-D petroleum systems modelling has been carried out to forward model migration pathways within the Tay Fan. Rock magnetic experiments identified a range of magnetite, maghemite, iron sulphides, siderite, goethite and titanohematite, some of which are part of the background signal, and some due to the presence of hydrocarbons. Typical concentrations of the magnetic minerals were ∼10–200 ppm. Importantly, we have identified an increasing presence of authigenic iron sulphides (likely pyrite and greigite) along the identified lateral hydrocarbon migration pathway (east to west). This is likely caused by biodegradation resulting in the precipitation of iron sulphides, however, though less likely, it could alternatively be caused by mature oil generation, which subsequently travelled with the migrating oil to the traps in the west. These observations suggest mineral magnetic techniques could be a rapid alternative method for identifying the severity of biodegradation or oil maturity in core sample, which can then be used to calibrate petroleum systems models. [ABSTRACT FROM AUTHOR]

In this paper, I analyse Vp, Vs and Vp / Vs ratio for the crust and uppermost mantle of Anatolia in $$\rm EU_{60}$$, a recently developed 3-D tomographic model for the European continent. Beneath volcanic fields in the Central and Eastern Anatolia, pronounced reductions in isotropic Vs are observed in the crust and uppermost mantle, reflecting high temperature associated with Neogene–Quaternary magmatism in the study region. These slow wave speed anomalies correlate well with high Vp / Vs ratio in model $$\rm EU_{60}$$. By analysing relative perturbations in Vp and Vs at different depths, I observe a distinct pattern for grid points associated with volcanic and non-volcanic regions. The linear regression results of volcanic grid points are characterized by high δlnVs/δlnVp slopes in comparison with non-volcanic grid points. Grid points with linear fitting slopes greater than 2.2 are observed around 50–60 km beneath volcanic provinces. These observations suggest the presence of partial melt in the crust and uppermost mantle beneath volcanoes in the Central and Eastern Anatolia. Comparisons between seismic observations with theoretical single phase partial melting calculations suggest the presence of cracks filled with a few volume per cent of melt in the upper and lower crust. Less than $$1\,{per\,cent}$$ melt-filled inclusions with aspect ratio smaller than 0.001 might exist in the uppermost mantle. [ABSTRACT FROM AUTHOR]

SUMMARY In this paper, we present a significant update of the Italian present-day stress data compilation not only to improve the knowledge on the tectonic setting of the region or to constrain future geodynamic models, but also to understand the mechanics of processes linked to faulting and earthquakes. In this paper, we have analysed, revised and collected new contemporary stress data from borehole breakouts and we have assembled earthquake and fault data. In total, 206 new quality-ranked entries complete the definition of the horizontal stress orientation and tectonic regime in some areas, and bring new information mainly in Sicily and along the Apenninic belt. Now the global Italian data set consists of 715 data points, including 499 of A-C quality, representing an increase of 37 per cent compared to the previous compilation. The alignment of horizontal stresses measured in some regions, closely matches the ∼N-S first-order stress field orientation of ongoing relative crustal motions between Eurasia and Africa plates. The Apenninic belt shows a diffuse extensional stress regime indicating a ∼NE-SW direction of extension, that we interpret as related to a second-order stress field. The horizontal stress rotations observed in peculiar areas reflect a complex interaction between first-order stress field and local effects revealing the importance of the tectonic structure orientations. In particular, in Sicily the new data delineate a more complete tectonic picture evidencing adjacent areas characterized by distinct stress regime: northern offshore of Sicily and in the Hyblean plateau the alignment of horizontal stresses is consistent with the crustal motions, whereas different directions have been observed along the belt and foredeep. [ABSTRACT FROM AUTHOR]

In this paper, we present results of the 3-D tomographic modelling of the crustal structure and Moho topography applied to data recorded in SE Poland during the CELEBRATION 2000 seismic experiment. The target area covers ca. 500 km × 500 km and represents a complex geological setting from old Precambrian platform (East European Craton, EEC), through the crustal blocks (terranes) that form the Trans-European Suture Zone, to the young Alpine orogen—the Carpathians. We test two different inversion strategies using two different algorithms: (i) coupled inversion of and arrivals to constrain both the crustal velocities and the Moho depths; (ii) decoupled inversion of arrivals only using previously obtained smooth 3-D crustal velocity model. The coupled inversion of and arrivals results in a much smoother crustal velocity field than the one previously obtained by inversion of first arrivals only. Also, the obtained Moho structure is much smoother than the Moho map compiled from the existing 2-D models. Decoupled inversion of the reflections provides Moho structure comparable in resolution to the compiled map. Synthetic tests indicate that with our data set we are able to resolve larger than 100-km-size Moho structures. The modelled Moho is shallowest in the area of the Upper Silesian Block ( ca. 32 km), then it is deepening by ca. 10 km over 100-km-wide zone along the margin of the EEC and finally it reaches up to 48 km depth in the area of the EEC. Our favoured strategy for modelling large refraction/wide-angle reflection data set consists of derivation of a smooth crustal velocity model by a first-arrival tomography and then complementing this model by a decoupled inversion of reflections in order to constrain minimum-structure Moho topography. [ABSTRACT FROM AUTHOR]

The capability to detect and locate seismicity in the North Sea is necessary to ensure the safety of future CO2 storage operations. Currently, the event location capabilities are hampered by a one-sided network configuration, as only sensors deployed on the Norwegian mainland are providing good signal-to-noise ratios. However, applying array processing techniques to groups of offshore sensors deployed at oil and gas reservoirs can improve the signal-to-noise ratio at such sites, which generally suffer from high noise contamination originating from platforms or seismic shooting. In this study we test the feasibility of array processing to sensors deployed at the Grane oil field to improve earthquake event locations. Out of the 3400 nodes installed, we have access to two data subsets: (i) continuous data from 10 sensors spread out over the whole field and (ii) short segments of data from 30 sensors selected with the purpose to apply array methods. As the average interstation distance between the 10 sensors is 6 km, the recorded wavefield is not coherent and traditional array processing methods are not applicable. To optimize the usage of the 10-sensor offshore data for event location purposes, we develop a new method for incoherent array processing. We compute a characteristic function based on the kurtosis of the seismic traces prior to a frequency–wavenumber analysis. This method successfully determines slowness and backazimuth for almost all the evaluated earthquakes in this study. Using common coherent array processing, the 30 sensors show superior detection capabilities over the 10 sensors and will therefore decrease the detection threshold for seismic events in the region significantly if continuous data are available. We conclude that offshore sensors at Grane can be included as a part of a system for passive seismic monitoring in the North Sea. We recommend using the 30-sensors data for this purpose. However, as we only have access to continuous data from the 10 sensors in this study, we found that they are a good substitute when the 30-sensor data are not available. [ABSTRACT FROM AUTHOR]

A full understanding of the dynamics of mountain ranges such as the Alps requires the integration of available geological and geophysical knowledge into a lithospheric-scale 3-D geological model. As a first stage in the construction of this geo-model, we derive a new 3-D shear wave velocity model of the Alpine region, with a spatial resolution of a few tens of kilometres, making it possible to compare with geological maps. We use four years of continuous vertical-component seismic noise records to compute noise correlations between more than 950 permanent broad-band stations complemented by ∼600 temporary stations from the AlpArray sea-land seismic network and the Cifalps and EASI linear arrays. A specific pre-processing is applied to records of ocean–bottom seismometers in the Liguro-Provençal basin to clean them from instrumental and oceanic noises. We first perform a 2-D transdimensional inversion of the traveltimes of Rayleigh waves to compute group-velocity maps from 4 to |$150\, \mathrm{ s}$|⁠. The data noise level treated as an unknown parameter is determined with a Hierarchical Bayes method. A Fast Marching Eikonal solver is used to update ray path geometries during the inversion. We use next the group-velocity maps and their uncertainties to derive a 3-D probabilistic Vs model. The probability distributions of Vs at depth and the probability of presence of an interface are estimated at each location by exploring a set of 130 million synthetic four-layer 1-D Vs models. The obtained probabilistic model is refined using a linearized inversion. Throughout the inversion for Vs , we include the water column where necessary. Our Vs model highlights strong along-strike changes of the lithospheric structure, particularly in the subduction complex between the European and Adriatic plates. In the South-Western Alps, our model confirms the existence of a low-velocity structure at |$50-80\, \mathrm{ km}$| depth in the continuation of the European continental crust beneath the subduction wedge. This deep low-velocity anomaly progressively disappears towards the North-Western and Central Alps. The European crust includes lower crustal low-velocity zones and a Moho jump of |$\sim \, 8-12$| km beneath the western boundary of the External Crystalline Massifs of the North-Western Alps. The striking fit between our Vs model and the receiver function migrated depth section along the Cifalps profile documents the reliability of the Vs model. In light of this reliability and with the aim to building a 3-D geological model, we re-examine the geological structures highlighted along the Cifalps profile. [ABSTRACT FROM AUTHOR]

Natural gas can be temporarily stored in a variety of underground facilities, such as depleted gas and oil fields, natural aquifers and caverns in salt rocks. Being extensively monitored during operations, these systems provide a favourable opportunity to investigate how pressure varies in time and space and possibly induces/triggers earthquakes on nearby faults. Elaborate and detailed numerical modelling techniques are often applied to study gas reservoirs. Here we show the possibilities and discuss the limitations of a flexible and easily formulated tool that can be straightforwardly applied to simulate temporal pore-pressure variations and study the relation with recorded microseismic events. We use the software POEL (POroELastic diffusion and deformation) which computes the poroelastic response to fluid injection/extraction in a horizontally layered poroelastic structure. We further develop its application to address the presence of vertical impermeable faults bounding the reservoir and of multiple injection/extraction sources. Exploiting available information on the reservoir geometry and physical parameters, and records of injection/extraction rates for a gas reservoir in southern Europe, we perform an extensive parametric study considering different model configurations. Comparing modelled spatiotemporal pore-pressure variations with in situ measurements, we show that the inclusion of vertical impermeable faults provides an improvement in reproducing the observations and results in pore-pressure accumulation near the faults and in a variation of the temporal pore-pressure diffusion pattern. To study the relation between gas storage activity and recorded local microseismicity, we applied different seismicity models based on the estimated pore-pressure distribution. This analysis helps to understand the spatial distribution of seismicity and its temporal modulation. The results show that the observed microseismicity could be partly linked to the storage activity, but the contribution of tectonic background seismicity cannot be excluded. [ABSTRACT FROM AUTHOR]

In this study, we focus on improved constraint of the glacial isostatic adjustment (GIA) signal at present-day, and its role as a contributor to present-day sea level budgets. The main study area extends from the coastal regions of northwestern Europe to northern Europe. Both Holocene relative sea level (RSL) data as well as vertical land motion (VLM) data are incorporated as constraints in a semi-empirical GIA model. 71 geological rates of GIA-driven RSL change are inferred from Holocene proxy data and 108 rates of vertical land motion from GNSS provide an additional measure of regional GIA deformation. Within the study area, the geological RSL data complement the spatial gaps of the VLM data and vice versa. Both data sets are inverted in a semi-empirical GIA model to yield updated estimates of regional present-day GIA deformations. A regional validation using tide gauges is presented for the North Sea, where the GIA signal may be complicated by lateral variations in Earth structure and existing predictions of regional and global GIA models show discrepancies. The model validation in the North Sea region suggests that geological data are needed to fit independent estimates of GIA-related RSL change inferred from tide gauge rates, indicating that geological rates from Holocene data do provide an important additional constraint for data-driven approaches to GIA estimation. [ABSTRACT FROM AUTHOR]

The Gravity Recovery and Climate Experiment (GRACE) and GRACE-FO missions have provided an unprecedented quantification of large-scale changes in the water cycle. However, it is still an open problem of how these missions' data can be referenced to a ground truth. Meanwhile, stationary optical clocks show fractional instabilities below 10−18 when averaged over an hour, and continue to be improved in terms of stability and accuracy, uptime and transportability. The frequency of a clock is affected by the gravitational redshift, and thus depends on the local geopotential; a relative frequency change of 10−18 corresponds to a geoid height change of about 1 cm. Here we suggest that this effect could be exploited for sensing large-scale temporal geopotential changes via a network of clocks distributed at the Earth's surface. In fact, several projects have already proposed to create an ensemble of optical clocks connected across Europe via optical fibre links. Our hypothesis is that a clock network with collocated GNSS receivers spread over Europe—for which the physical infrastructure is already partly in place—would enable us to determine temporal variations of the Earth's gravity field at timescales of days and beyond, and thus provide a new means for validating satellite missions such as GRACE-FO or a future gravity mission. Here, we show through simulations how glacial, hydrological and atmospheric variations over Europe could be observed with clock comparisons in a future network that follows current design concepts in the metrology community. We assume different scenarios for clock and GNSS uncertainties and find that even under conservative assumptions—a clock error of 10−18 and vertical height control error of 1.4 mm for daily measurements—hydrological signals at the annual timescale and atmospheric signals down to the weekly timescale could be observed. [ABSTRACT FROM AUTHOR]

The deep crustal structure beneath the North Sea is poorly understood since it is constrained by only a few seismic reflection and refraction profiles. However, it is widely acknowledged that the mid to lower crust plays important roles in rift initiation and evolution, particularly when large-scale sutures and/or terrane boundaries are present, since these inherited features can focus strain or act as inhibitors to extensional deformation. Ancient tectonic features are known to exist beneath the iconic failed rift system of the North Sea, making it an ideal location to investigate the complex interplay between pre-existing regional heterogeneity and rifting. To this end, we produce a 3-D shear wave velocity model from transdimensional ambient seismic noise tomography to constrain crustal properties to ∼30 km depth beneath the North Sea and its surrounding landmasses. Major North Sea sedimentary basins appear as low shear wave velocity zones that are a good match to published sediment thickness maps. We constrain relatively thin crust (13–18 km) beneath the Central Graben depocentres that contrasts with crust elsewhere at least 25–30 km thick. Significant variations in crustal structure and rift symmetry are identified along the failed rift system that appears to be related to the locations of Laurentia–Avalonia–Baltica palaeoplate boundaries. We constrain first-order differences in structure between palaeoplates; with strong lateral gradients in crustal velocity related to Laurentia–Avalonia–Baltica plate juxtaposition and reduced lower crustal velocities in the vicinity of the Thor suture, possibly representing the remnants of a Caledonian accretionary complex. Our results provide fresh insight into the pivotal roles that ancient terranes can play in the formation and failure of continental rifts and may help explain the characteristics of other similar continental rifts globally. [ABSTRACT FROM AUTHOR]

We provide a high-resolution image of the Ivrea Geophysical Body (IGB) in the Western Alps with new gravity data and 3-D density modelling, integrated with surface geological observations and laboratory analyses of rock properties. The IGB is a sliver of Adriatic lower lithosphere that is located at shallow depths along the inner arc of the Western Alps, and associated with dense rocks that are exposed in the Ivrea-Verbano Zone (IVZ). The IGB is known for its high seismic velocity anomaly at shallow crustal depths and a pronounced positive gravity anomaly. Here, we investigate the IGB at a finer spatial scale, merging geophysical and geological observations. We compile existing gravity data and we add 207 new relative gravity measurements, approaching an optimal spatial coverage of 1 data point per 4–9 km2 across the IVZ. A compilation of tectonic maps and rock laboratory analyses together with a mineral properties database is used to produce a novel surface rock-density map of the IVZ. The density map is incorporated into the gravity anomaly computation routine, from which we defined the Niggli gravity anomaly. This accounts for Bouguer Plate and terrain correction, both considering the in situ surface rock densities, deviating from the 2670 kg m–3 value commonly used in such computations. We then develop a 3-D single-interface crustal density model, which represents the density distribution of the IGB, including the above Niggli-correction. We retrieve an optimal fit to the observations by using a 400 kg m–3 density contrast across the model interface, which reaches as shallow as 1 km depth below sea level. The model sensitivity tests suggest that the ∼300–500 kg m–3 density contrast range is still plausible, and consequently locates the shallowest parts of the interface at 0 km and at 2 km depth below sea level, for the lowest and the highest density contrast, respectively. The former model requires a sharp density discontinuity, the latter may feature a vertical transition of densities on the order of few kilometres. Compared with previous studies, the model geometry reaches shallower depths and suggests that the width of the anomaly is larger, ∼20 km in west–east direction and steeply E–SE dipping. Regarding the possible rock types composing the IGB, both regional geology and standard background crustal structure considerations are taken into account. These exclude both felsic rocks and high-pressure metamorphic rocks as suitable candidates, and point towards ultramafic or mantle peridotite type rocks composing the bulk of the IGB. [ABSTRACT FROM AUTHOR]

The volcanism of the Eifel volcanic field (EVF), in west-central Germany, is often considered an example of hotspot volcanism given its geochemical signature and the putative mantle plume imaged underneath. EVF's setting in a stable continental area provides a rare natural laboratory to image surface deformation and test the hypothesis of there being a thermally buoyant plume. Here we use Global Positioning System (GPS) data to robustly image vertical land motion (VLM) and horizontal strain rates over most of intraplate Europe. We find a spatially coherent positive VLM anomaly over an area much larger than the EVF and with a maximum uplift of ∼1 mm yr−1 at the EVF (when corrected for glacial isostatic adjustment). This rate is considerably higher than averaged over the Late-Quaternary. Over the same area that uplifts, we find significant horizontal extension surrounded by a radial pattern of shortening, a superposition that strongly suggests a common dynamic cause. Besides the Eifel, no other area in NW Europe shows significant positive VLM coupled with extensional strain rates, except for the much broader region of glacial isostatic adjustment. We refer to this 3-D deformation anomaly as the Eifel Anomaly. We also find an extensional strain rate anomaly near the Massif Central volcanic field surrounded by radial shortening, but we do not detect a significant positive VLM signal there. The fact that the Eifel Anomaly is located above the Eifel plume suggests that the plume causes the anomaly. Indeed, we show that buoyancy forces induced by the plume at the bottom of the lithosphere can explain this remarkable surface deformation. Plume-induced deformation can also explain the relatively high rate of regional seismicity, particularly along the Lower Rhine Embayment. [ABSTRACT FROM AUTHOR]

The growing seismic networks and the increasing number of permanent seismic stations can help in improving the physical basis of seismic hazard assessment. For this purpose, the definition of reference site conditions is of great significance. If a reliable estimate of the reference ground motion is known, its modification at any given site can be modelled with respect to that reference site. Since the choice of a well-characterized reference site is not straightforward, mainly due to the high variability in the shallow layers, such choices prove to be affected by large uncertainties. While proxy parameters like the average S -wave velocity over the uppermost 30 m (v S30) might help in characterizing reference site conditions, such parameters are neither available at all sites nor do they allow concluding that the site is not affected by amplification and attenuation effects. In this study, we identify prospective reference sites across Europe in a harmonized and fully data-driven way. All analysis is based on freely available geological and geophysical data and no on-site measurements or site-specific proxies are required. The study accounts for both the influence of amplification and attenuation in a large frequency range. To address the key conceptual issues, we verify our classification based on machine learning techniques in which the influence of the individual site characterization parameters is investigated. Our study indicates that around 250 sites in Europe over more than 2000 investigated are not affected by local site effects and can de facto be considered as reference sites based on the criteria applied. [ABSTRACT FROM AUTHOR]

Gravitational compaction of thick (2–10 km) sediment accumulations in sedimentary basins is controlled by the interplay of mechanical and chemical processes that operate over many orders of magnitude in spatial scale. The compaction of sediments into rock typically involves a density increase of ≈500 to 1000 kg m−3, occurring over a depth-scale of several kilometres. The volume decrease in the compacting sediments releases vast volumes of water, which plays an important part in the global hydrological cycle and also in tectonic and geochemical processes; including the formation of hydrocarbon and mineral deposits. This study utilizes recently developed tomographic seismic images from the Porcupine Basin, which lies in the deep-water North Atlantic Ocean. A generic method for predicting fluid pressure variations that are driven by gravitational compaction is developed over the scale of the entire sedimentary basin. The methodology is grounded upon both observational evidence and empirically based theories, relying on geophysical measurements and relationships between sediment porosities and densities. The method is based upon physical concepts that are widely used in the petroleum industry and applied extensively in models of overpressure development in sedimentary basins. Geological and geophysical data from exploration wells are used to test the predictions of the method at two locations within the basin and are found to be in good agreement with the theory. [ABSTRACT FROM AUTHOR]

Organic matter preservation and associated conditions during deposition, important in the context of fossil fuel exploration, are commonly determined by advanced geochemical analyses. However, the relation between organic matter preservation and magnetic mineral composition remains poorly constrained. The aim of the studies was to check the potential of magnetic mineral differentiation between facies containing various amounts of organic matter as a factor to better understand the processes which influence water chemistry at the bottom of sedimentary basins, and thus to better understand factors controlling the preservation of organic matter. To determine the composition and the properties of magnetic minerals, detailed low-temperature measurements of Saturation Isothermal Remanent Magnetization and hysteresis loops were performed on two types of rocks, Silurian shales from the Baltic Basin (northern Poland). The analysed shale facies are characterized by similar thermal evolution, but different amounts of organic matter: the Pelplin Formation, containing a modest content of organic matter, in which we also examined early diagenetic carbon concretions; and the Jantar Formation, which represents an organic-rich 'sweet spot' layer. In both facies, the results indicate the presence of multi- or pseudo-single domain magnetite, which is interpreted as detrital in origin. However, the main observation gained from this study is the relation between magnetic mineral assemblage in the studied shales and the amount of organic matter: in the rocks with modest amounts of organic matter we observed hematite, while in organic-rich layers hematite was absent. Hematite (mostly single-domain grains) preserved in the Pelplin Formation suggests that stable oxygen-rich conditions were present at the bottom of the sedimentary basin continuously during deposition, concretion cementation and compaction. In turn, its absence in the Jantar Formation suggests that during sedimentation and early diagenesis more anoxic conditions appeared. Generally, findings show that the presence of hematite is related to the significantly lower amount of organic matter in sedimentary rocks. Thus, presence of this mineral may be a useful indicator of organic matter preservation. [ABSTRACT FROM AUTHOR]

Anisotropy of magnetic susceptibility (AMS) measurements, a commonly used method, has been applied in loess to determine the dominant palaeowind direction during the Pleistocene. During the last session of sampling and detailed magnetic investigation of the Early Middle Pleistocene loess/palaeosol succession at Paks (Hungary), 'irregular' (i.e. non-flow-aligned) magnetic fabrics (MFs) were revealed from sedimentary records having uniform sedimentary parameters. Knowledge about the origin of irregular loess fabrics is still limited, although they may carry additional palaeoenvironmental information. The appearance of irregular MF suggests that a constant set of palaeoclimatological conditions, as indicated by the layer characteristics, was interrupted by abrupt events. To characterize the unusual event horizons, the MF of the horizons and those of both the under- and overlying material were analysed and compared by rock magnetic methods, such as hysteresis, temperature dependence of magnetic susceptibility, and AMS. The analysis separated three types of irregular fabrics and revealed three scenarios that might have been responsible for the forming of the irregular MF: abrupt climate changes, including rapid (catastrophic) weather events (e.g. intensification of strong wind- or water-lain processes); short-term pedogenesis during glacials; and post-depositional deformation triggered by mass movements or seismic activity. [ABSTRACT FROM AUTHOR]

Western Slovenia is part of an actively deforming region accommodating anticlockwise rotation of Adria and its continuous collision with Eurasia. The geometry of the active faulting system in this plate boundary is not well defined. In this study, detailed analysis of earthquake activity was performed with relocation of earthquakes in the period between 2006 and 2017. With inspection of the waveform data, slight temporal clustering of activity was observed. To increase the detection rate of microearthquakes we used a matched filter detection algorithm method. Templates of earthquakes were created and a database of continuous waveform data within the period 2006–2017 was investigated. As a result, high temporal correlation allowed us to identify swarms and earthquake sequences that affected the active fault system in the study region. Relocated seismicity allowed us to constrain the geometry of 5 nearly parallel faults, namely: Ravne, Idrija, Predjama, Selce and Raša faults. All these faults do have an expression in the geomorphology and reach a seismogenic depth of up to 20 km. Vertical and along strike extents of these active faults can favour earthquakes of moment magnitude equal to 7 or larger. The most recent large earthquake that occurred in this region is the 1511 earthquake with a magnitude 6.8. The leading fault in the system being the Idrija right-lateral strike-slip fault, experiences earthquake activity from 5 to 20 km on its northern segment, while on its southern segment no earthquake activity is detected over the decade of observations. We show that the interseismic loading on the southern segment of Idrija fault is likely unclamping the locked adjacent faults promoting the observed bursts of seismicity. Moreover, in 2009 the Predjama fault accommodated a sudden increase of the surface deformation at the extensometer accompanied by a simultaneous swarm activity at its seismogenic depth. This behaviour might correspond to velocity strengthening and weakening processes taking place at both the surface and depth terminations of a locked vertical fault. These processes can be driven by a slow-slip event on the deeper part of Idrija fault that would generate a temporary acceleration of the interseismic loading rate along with a change within the fluid circulation. [ABSTRACT FROM AUTHOR]

The Western Alps, although being intensively investigated, remains elusive whenit comes to determining its lithospheric structure. New inferences on the latter are important for the understanding of processes and mechanisms of orogeny needed to unravel the dynamic evolution of the Alps. This situation led to the deployment of the CIFALPS temporary experiment, conducted to address the lack of seismological data amenable to high-resolution seismic imaging of the crust and the upper mantle. We perform a 3-D isotropic full-waveform inversion (FWI) of nine teleseismic events recorded by the CIFALPS experiment to infer 3-D models of both density and P- and S-wave velocities of the Alpine lithosphere. Here, by FWI is meant the inversion of the full seismograms including phase and amplitude effects within a time window following the first arrival up to a frequency of 0.2 Hz. We show that the application of the FWI at the lithospheric scale is able to generate images of the lithosphere with unprecedented resolution and can furnish a reliable density model of the upper lithosphere. In the shallowest part of the crust, we retrieve the shape of the fast/dense Ivrea body anomaly and detect the low velocities of the Po and SE France sedimentary basins. The geometry of the Ivrea body as revealed by our density model is consistent with the Bouguer anomaly. A sharp Moho transition is followed from the external part (30 km depth) to the internal part of the Alps (70-80 km depth), giving clear evidence of a continental subduction event during the formation of the Alpine Belt. A low-velocity zone in the lower lithosphere of the S-wave velocity model supports the hypothesis of a slab detachment in the western part of the Alps that is followed by asthenospheric upwelling. The application of FWI to teleseismic data helps to fill the gap of resolution between traditional imaging techniques, and enables integrated interpretations of both upper and lower lithospheric structures. [ABSTRACT FROM AUTHOR]

We used 6 hr of continuous seismic noise records from 2320 four-component sensors of the Valhall ‘Life of Field Seismic’ network to compute cross-correlations (CCs) of ambient seismic noise. A beamforming analysis showed that at low frequencies (below 2 Hz) the seismic noise sources were spatially homogeneously distributed, whereas at higher frequencies (2–30 Hz), the dominant noise source was the oil platform at the centre of the network. Here, we performed an ambient noise surface wave tomography at frequencies below 2 Hz. We used vertical-component geophones CCs to extract and measure the Scholte waves group velocities dispersion curves that were then processed with a set of quality criteria and inverted to build group velocity maps of the Valhall area. Although Scholte wave group velocity depends on S wave, our group velocity maps show features similar to that was previously obtained from P-wave velocity full-waveform inversion of an active seismic data set. Since the dominant noise source at high frequency (above 3 Hz) was the oil platform, we determined a 2-D S-wave velocity model along a profile aligned with the platform by inverting group velocity dispersion curves of Love waves from transverse-component geophones CCs. We found that S-wave velocity down to 20 m was low and varied along the profile, and could be used to estimate S-wave static. [ABSTRACT FROM AUTHOR]

The backarc Black Sea (BS) basin was formed in the Late Cretaceous-Palaeocene at the hinterland of the Pontide magmatic arc due to subduction of the Neotethys ocean below the southern Eurasian continental margin. At present the BS consists of two large depressions—the West- and East-Black Sea basins (WBS and EBS) filled with thick (up to 12 km) Cretaceous and younger sediments and underlain by a crust of oceanic/suboceanic type. The sediments mask poorly investigated crystalline crust that is thought to comprise an accretional collage of microplates and terranes of different affinities. To investigate the lithospheric structure of the BS we performed a 3-D gravity analysis and local seismic tomography study. 3-D gravity backstripping analysis allowed us to separate the gravity signal from different parts of the crustal model and then, by subtracting the crustal effect from the observed field, to obtain gravity anomalies of presumed mantle origin only. The broad positive long wavelength component of this might be indicative of good isostatic equilibrium of the deep structure of the Black Sea, that is, that the negative gravity effect of sediments is almost totally compensated by the strong positive gravity impact of Moho shallowing. Velocity structure of the BS lithosphere has been studied by P-wave local seismic tomography. It uses the traveltimes of the earthquakes occurring inside the study region and recorded by permanent seismic stations around the BS. Initial data were corrected for the effect of the crust. The resulting model shows the BS lithosphere as being rather heterogeneous with two domains of increased velocity in its western and eastern parts. The gravity analysis and seismic tomography approaches were integrated by calculating the upper-mantle gravity effect of the tomography model and comparing this to the mantle gravity signature inferred from the gravity analysis itself. The integrated results suggest the presence of rheologically strong and cold continental lithosphere below the BS, similar to Precambrian lithosphere of the East European Platform. [ABSTRACT FROM AUTHOR]

Here we present first-order results detailing the Anatolian crustal from receiver function analysis of data from approximately 300 stations within Turkey. Seismic data from the Kandilli Observatory array (KOERI; KO), the National Seismic Network of Turkey (AFAD-DAD; TU) and available IRIS data from the Northern Anatolian Fault experiment (YL) for the period between 2005 and 2010 is analysed. We calculate receiver functions in the frequency domain using water-level deconvolution. The results are analysed using a combination of H–K stacking and depth stacking to determine robust Moho conversion depths and VP/Vs ratios across Anatolia. We detect a deep Moho in eastern Anatolia of up to ∼55 km, a generally normal Moho in Central Anatolia of ∼37–47 km and a thinned Moho in western Anatolia and Cyprus of ∼30 km. The VP/Vs ratio across the Anatolian Plate is generally slightly elevated; regions of extremely high VP/Vs ratio (>1.85) can be associated with recent volcanism in eastern and central Anatolia. High VP/Vs ratio measurements (>1.85) in western Anatolia may be indicative of partial melt in the lower crust associated with regional extension. [ABSTRACT FROM AUTHOR]

SUMMARY A large storm surge event occurred on 2007 November 2009 in the southern North Sea where strong winds caused the sea level to rise drastically by up to 3 m within several hours. Based on the Proudman Oceanographic Laboratory storm surge model, the predicted loading displacements at coastal stations can reach a few centimetres in the vertical and several millimetres in the horizontal directions. In this study, we used two-hourly global positioning system (GPS) positions at 26 stations around the southern North Sea to identify the loading displacements caused by this storm surge event. We find that the mean rms of the differences between the estimated and predicted displacements are 4.9, 1.3 and 1.4 mm, which are insignificant compared to the one-sigma GPS positioning errors of 5.1, 2.0 and 2.4 mm for the Up, East and North components, respectively. More interestingly, in both vertical and horizontal directions, the estimated displacements successfully tracked the temporal evolution of the storm surge loading effects. In addition, within the whole of 2007 November, we used the predicted displacements to correct the two-hourly GPS positions, and consequently reduced the rms of the estimated displacements on average from 9.3, 3.0 and 2.9 mm to 7.8, 2.8 and 2.8 mm for Up, East and North components, respectively. Therefore, subdaily loading effects due to storm surges should be paid attention to in the GPS positioning that contributes to crustal-motion studies around shallow seas such as the North Sea, the Baltic Sea and the Gulf of Mexico. [ABSTRACT FROM AUTHOR]

SUMMARY We present a new, S-velocity model of the European upper mantle, constrained by inversions of seismic waveforms from broad-band stations in Europe and surrounding regions. We collected seismograms for the years 1990-2007 from all permanent stations in Europe for which data were available. In addition, we incorporated data from temporary experiments. Automated multimode inversion of surface and S-wave forms was applied to extract structural information from the seismograms, in the form of linear equations with uncorrelated uncertainties. The equations were then solved for seismic velocity perturbations in the crust and mantle with respect to a 3-D reference model with a realistic crust. We present two versions of the model: one for the entire European upper mantle and another, with the highest resolution, focused on the upper 200 km of the mantle beneath western and central Europe and the circum Mediterranean. The mantle lithosphere and asthenosphere are well resolved by both models. Major features of the lithosphere-asthenosphere system in Europe and the Mediterranean are indentified. The highest velocities in the mantle lithosphere of the East European Craton (EEC) are found at about 150 km depth. There are no indications for a deep cratonic root below about 330 km depth. Lateral variations within the cratonic mantle lithosphere are resolved as well. The locations of kimberlites correlate with reduced S-wave velocities in the shallow cratonic mantle lithosphere. This anomaly is present in regions of both Proterozoic and Archean crust, pointing to an alteration of the mantle lithosphere after the formation of the craton. Strong lateral changes in S-wave velocity are found at the northwestern margin of the EEC and may indicate erosion of cratonic mantle lithosphere beneath the Scandes by hot asthenosphere. The mantle lithosphere beneath western Europe and between the Tornquist-Teisseyre Zone and the Elbe Line shows moderately high velocities and is of an intermediate character, between cratonic lithosphere and the thin lithosphere of central Europe. In central Europe, Caledonian and Variscian sutures are not associated with strong lateral changes in the lithosphere-asthenosphere system. Cenozoic anorogenic intraplate volcanism in central Europe and the circum Mediterranean is found in regions of shallow asthenosphere and close to changes in the depth of the lithosphere-asthenosphere boundary. Very low velocities at shallow upper-mantle depths are present from eastern Turkey towards the Dead Sea transform fault system and Sinai, beneath locations of recent volcanism. Low-velocity anomalies extending vertically from shallow upper mantle down to the transition zone are found beneath the Massif Central, Sinai and the Dead Sea, the Canary Islands and Iceland. [ABSTRACT FROM AUTHOR]

SUMMARY We investigate different aspects concerning the application of swarm intelligence optimization to the inversion of Scholte-wave phase-slowness frequency ( p- f) spectra with respect to shear wave velocity structure. Besides human influence due to the dependence on a priori information for starting models and interpretation of p- f spectra as well as noise, the model resolution of the inversion problem is strongly influenced by the multimodality of the misfit function. We thus tested the efficiency of global, stochastic optimization approaches with focus on swarm intelligence methods that can explore the multiple minima of the misfit function. A comparison among different PSO schemes by applying them to synthetic Scholte-wave spectra led to a hybrid of Particle Swarm Optimization and Downhill Simplex providing the best resolution of inverted shear wave velocity depth models. The results showed a very low spread of best fitting solutions of 7 per cent in shear wave velocity and an average of 9 per cent for noisy synthetic data and a very good fit to the true synthetic model used for computation of the input data. To classify this method we also compared the probability of finding a good fit in synthetic spectra inversion with that of Evolutionary Algorithm, Simulated Annealing, Neighbourhood Algorithm and Artificial Bee Colony algorithm. Again the hybrid optimization scheme showed its predominance. The usage of stochastic algorithms furthermore allowed a new way of misfit definition in terms of dispersion curve slowness residuals making the inversion scheme independent on Scholte-wave mode identification and allowing joint inversion of fundamental mode and higher mode information. Finally we used the hybrid optimization scheme and the misfit calculation for the inversion of 2-D shear wave velocity profiles from two locations in the North- and Baltic Sea. The models show acceptable resolution and a very good structural correlation to high resolution reflection seismic data. [ABSTRACT FROM AUTHOR]

SUMMARY The Solfatara is one of the major volcanoes of the Phlegrean Fields ( Campi Flegrei) volcanic complex, and it is located in a densely populated area a few kilometres west of the city of Naples. It is an active resurgent caldera that has been characterized by a rich history of surface-ground deformation and soil diffuse degassing and fumarolic emissions, which are indications of the top of a hydrothermal plume. A seismic survey was completed in May 2009 for the characterization of the main subsurface features of the Solfatara. Using the complete data set, we have carried out surface wave inversion with high spatial resolution. A classical minimization of a least-squares objective function was first computed to retrieve the dispersion curves of the surface waves. Then, the fitting procedure between the data and a three-sediment-layer forward model was carried out (to a depth of 7 m), using an improved version of the neighbourhood algorithm. The inversion results indicate a NE-SW fault, which is not visible at the surface. This was confirmed by a temperature survey conducted in 2010. A passive seismic experiment localized the ambient noise sources that correlate well with the areas of high CO2 flux and high soil temperatures. Finally, considering that the intrinsic attenuation is proportional to the frequency, a centroid analysis provides an overview of the attenuation of the seismic waves, which is closely linked to the petrophysical properties of the rock. These different approaches that merge complete active and passive seismic data with soil temperature and CO2 flux maps confirm the presence of the hydrothermal system plume. Some properties of the top of the plume are indicated and localized. [ABSTRACT FROM AUTHOR]

SUMMARY Post-seismic relaxation is known to occur after large or moderate earthquakes, on time scales ranging from days to years or even decades. In general, long-term deformation following seismic events has been detected by means of standard geodetic measurements, although seismic instruments are only used to estimate short timescale transient processes. Albeit inertial seismic sensors are also sensitive to rotation around their sensitive axes, the recording of very slow inclination of the ground surface at their standard output channels is practically impossible, because of their design characteristics. However, modern force-balance, broad-band seismometers provide the possibility to detect and measure slow surface inclination, through the analysis of the mass position signal. This output channel represents the integral of the broad-band velocity and is generally considered only for state-of-health diagnostics. In fact, the analysis of mass position data recorded at the time of the 2009 April 6, L'Aquila ( MW= 6.3) earthquake, by a closely located STS-2 seismometer, evidenced the occurrence of a very low frequency signal, starting right at the time of the seismic event. This waveform is only visible on the horizontal components and is not related to the usual drift coupled with the temperature changes. This analysis suggests that the observed signal is to be ascribed to slowly developing ground inclination at the station site, caused by post-seismic relaxation following the main shock. The observed tilt reached 1.7 × 10−5 rad in about 2 months. This estimate is in very good agreement with the geodetic observations, giving comparable tilt magnitude and direction at the same site. This study represents the first seismic analysis ever for the mass position signal, suggesting useful applications for usually neglected data. [ABSTRACT FROM AUTHOR]

Strain- and tiltmeter observations are composed of signals of different origin. Tides are a well-known phenomenon that can be extracted from the time series by tidal analysis. Depending on the location of the station, residuals, however, contain contributions of different magnitudes from ocean loading, changes in atmospheric or hydrological loading, anthropogenic induced signals and tectonics. Reductions of the residual variations are indispensable for extraction of, for example the pure tectonical signal. One of the challenges in view of the reductions is to understand transfer mechanisms between loading and local conditions, such as topography, cavity and geological features, which lead to additional or modified deformations. In this study, systematic numerical modelling using the Finite Element method with elastic rheology is carried out to estimate these effects. Barometric pressure changes are used as example for the load to determine the additional deformations. Strain- and tiltmeters are considered as instrumentation, having nominal resolutions of 0.2 nstrain and 1 nrad, respectively. The cavity and the topographic effect are systematically investigated with sophisticated models incorporating typical topographic features, such as a plain, a hill flank and a valley. Representative for stations worldwide, four observatories in Central Europe are modelled: the Geodetic Observatory Wettzell (Germany), the Sopron Observatory (Hungary), the Geodynamic Observatory Moxa and the Black Forest Observatory (BFO) (both Germany). The modelling shows that cavity and topographic effects occur in the same order of magnitude and that both effects interact leading to deformations partly intertwined in a complex way. This notwithstanding, the results explain observed deformation signals at the observatories. The systematic modelling helps to better understand the basic processes, from which criteria for the selection of future observation sites are inferred. [ABSTRACT FROM AUTHOR]

A reliable estimate of completeness magnitudes is vital for many seismicity- and hazard-related studies. Here we adopted and further developed the Probability-based Magnitude of Completeness (PMC) method. This method determines network detection completeness ( MP) using only empirical data: earthquake catalogue, phase picks and station information. To evaluate the applicability to low- or moderate-seismicity regions, we performed a case study in Switzerland. The Swiss Seismic Network (SSN) at present is recording seismicity with one of the densest networks of broad-band sensors in Europe. Based on data from 1983 January 1 to 2008 March 31, we found strong spatio-temporal variability of network completeness: the highest value of MP in Switzerland at present is 2.5 in the far southwest, close to the national boundary, whereas MP is lower than 1.6 in high-seismicity areas. Thus, events of magnitude 2.5 can be detected in all of Switzerland. We evaluated the temporal evolution of MP for the last 20 yr, showing the successful improvement of the SSN. We next introduced the calculation of uncertainties to the probabilistic method using a bootstrap approach. The results show that the uncertainties in completeness magnitudes are generally less than 0.1 magnitude units, implying that the method generates stable estimates of completeness magnitudes. We explored the possible use of PMC: (1) as a tool to estimate the number of missing earthquakes in moderate-seismicity regions and (2) as a network planning tool with simulation computations of installations of one or more virtual stations to assess the completeness and identify appropriate locations for new station installations. We compared our results with an existing study of the completeness based on detecting the point of deviation from a power law in the earthquake-size distribution. In general, the new approach provides higher estimates of the completeness magnitude than the traditional one. We associate this observation with the difference in the sensitivity of the two approaches in periods where the event detectability of the seismic networks is low. Our results allow us to move towards a full description of completeness as a function of space and time, which can be used for hazard-model development and forecast-model testing, showing an illustrative example of the applicability of the PMC method to regions with low to moderate seismicity. [ABSTRACT FROM AUTHOR]

Supplemented by 32 new directions, the Hungarian archaeomagnetic data set now consists of 217 archaeologically dated directions ranging in age from 300 BC to 1800 AD. From this data set, reference curves of secular variation of the geomagnetic field direction were computed using hierarchical modelling and curve estimation by moving average technique. Thanks to some of the new data, the gap in the earlier reference curves around 500 AD has now been filled up. For comparison's sake, directional records of comparable length from central Europe were also processed by the same curve building method. For this purpose, all dated directional data (declination and inclination with statistics) were drawn from the GEOMAGIA50 database for France, Germany, the Ukraine and Moldavia, Bulgaria and Italy and transferred via their virtual geomagnetic poles to a reference point of their respective countries. The resulting reference curves, including those for Hungary, show more or less similar temporal behaviour to the corresponding CALS7K.2 model curves (also available from the GEOMAGIA50 database), but significant deviations from the low-order CALS7K.2 predictions are also discernible owing to the likely presence of additional higher-order complications in the regional field. Therefore, the regional field and its secular variation are suggested to be approximated by the reference rather than the predicted curves. At any other location within the study area, the direction of the regional field can be obtained by spatial interpolation from the reference curves as illustrated by isogonic and isoclinic maps shown for selected times. Local time-series of interpolated directions for other central European countries lacking reference curves might serve as master curves for magnetic dating. [ABSTRACT FROM AUTHOR]

The 3-D P-wave velocity structure of the Alpine crust has been determined from local earthquake tomography using a set of high-quality traveltime data. The application of an algorithm combining accurate phase picking with an automated quality assessment allowed the repicking of first arriving P-phases from the original seismograms. The quality and quantity of the repicked phase data used in this study allows the 3-D imaging of large parts of the Alpine lithosphere between 0 and 60 km depth. Our model represents a major improvement in terms of reliability and resolution compared to any previous regional tomographic studies of the Alpine crust. First-order anomalies like crust–mantle boundary (Moho) and the Ivrea body in the Western Alps are well resolved and in good agreement with previous studies. In addition, several (consistent) small-scale anomalies are visible in the tomographic image. A clear continuation of the lower European crust beneath the Adriatic Moho in the Central Alps is not observed in our results. The absence of such a signature may indicate the eclogitization of the subducted European lower crust in the Central Alps. In agreement with previous results, the additional analysis of focal depths in our new 3-D P-wave model shows that all studied earthquakes in the northern foreland have occurred within the European crust. Waveforms and focal depths suggest that at least one of the analysed events south of the Alps is located in the Adriatic mantle. [ABSTRACT FROM AUTHOR]

Current constraints on the process of glacial-isostatic adjustment in Northern Europe are mainly provided by relative sea level data and GPS measurements. Due to a lack of resolving power in the shallow Earth (down to ∼200 km), these data sets only provide weak constraints on the shallow viscosity structure and the thickness of the lithosphere. Future high-resolution gravity data, as expected from ESA's Gravity field and steady-state Ocean Circulation Explorer (GOCE) to be launched 2009 March 16, are predicted to provide additional information on the shallow Earth, especially the viscosity structure. However, mass inhomogeneities due to chemical and thermal anomalies are expected to interfere with the gravity signals induced by shallow low-viscosity structures. We test therefore if heatflow data and laboratory-derived creep laws for the crust (plagioclase feldspars) and shallow upper mantle (olivine) can provide additional information on the shallow Earth. We show estimates of lithospheric thickness and viscosity that can be expected in the shallow Earth. Using a mechanical model based on the commercially available finite-element package ABAQUS and representative creep laws, we generate predictions of deformation-induced geoid height variations for Northern Europe. There, lateral heterogeneities in the shallow Earth are induced based on heatflow data. We use the RSES ice-load history to force our mechanical model, and we test the sensitivity of our predictions using the ICE-5G ice-load history. We show that perturbations, that is, differences to a background model, due to shallow low-viscosity structures are one to two orders of magnitude larger than the predicted accuracy of GOCE, which is at the cm-level for a resolution of about 100 km. Moreover, some features in geoid height perturbations are robust to changes in composition and creep regime, and have therefore a spatial signature that is representative for low-viscosity structures, without detailed a priori knowledge on these structures. We argue that these signatures are therefore more likely to be detectable by GOCE. Finally, we show, using normalized prediction errors, that GOCE is sensitive to the creep regime in the lower crust but not to the composition, at least not for the plagioclase feldspars used here. These conclusions are, in general, independent of assumptions (creep regime in the shallow upper mantle, ice-load history) on the background model. However, if the wrong background model is assumed, we can no longer predict the correct properties of the lower crust, because prediction errors are larger than 60 per cent. [ABSTRACT FROM AUTHOR]

We present a new surface-wave tomographic study of the broad European and Mediterranean region. Our goal is to enhance the resolution of previously published group velocity models using new data from European permanent seismic networks and a dense broad-band array in Northern Apennines (RETREAT). We measure fundamental mode Rayleigh and Love wave group velocities from long-period seismograms recorded at regional distance (between 600 and 7000 km). Our measurement technique is based on iterative application of multiple filters and phase-matched filters; we accurately estimate dispersion curves for more than 1500 Rayleigh wave and about 850 Love wave paths in the period range 35–170 s. Consistency of measurements is evaluated by comparing ray clusters from sample earthquakes to closely spaced RETREAT stations. In the whole data set, measurement errors in group velocity decrease with increasing distance and show to be caused by inaccuracy in the estimate of group arrival time. We calculate maps of Love and Rayleigh group velocity at selected periods by linear tomographic inversion, accounting for group arrival time errors and evaluating a posteriori group slowness errors. Data coverage in this region is not uniform, and it is highly influenced by the uneven distribution of earthquakes and seismic stations. We therefore build a laterally heterogeneous reference model by inverting a global data set of group velocity derived from the phase velocity library of Ekström et al. (1997) . Use of this reference as an a priori model during inversion improves preliminary data coverage at the borders of our study region and warrants consistency with global models. The implications of different regularization constraints (mathematically equivalent to norm damping or smoothing with different criteria) are analysed and compared. Group velocity maps confirm the large-scale geological lineaments known for the region: short-periods maps differentiate well among thinner oceanic and thicker continental crust; the most dominant feature in long-period maps is the difference between the fast Precambrian East European Platform and the low velocity signature of central Europe and western Mediterranean, separated by a sharp gradient in correspondence of the Tornquist–Tesseyre Zone. The seismically active Tethyan Belt is clearly marked by a continuous slow anomaly. Smaller scale, possibly thermally related, low velocity anomalies are found under Iceland and Mid-Atlantic Ridge, Rhine Graben and Tyrrhenian back-arc basin, whereas the Hellenic Arc is characterized by fast velocity. [ABSTRACT FROM AUTHOR]

The Carpathians belong to the European Alpine system, which was formed during the convergence and collision of the European and African plates. The study area, which comprises the Magura and Silesian nappes of the Outer Western Carpathians, were the locations of intensive folding and nappe transport during the Tertiary. There are a number of models depicting the changes in shapes and positions of the original sedimentary basins as a consequence of the above processes. However, it is this study that provides the first palaeomagnetic constraints for reconstructing the displacement history of the Silesian and Magura nappes during Tertiary. For the palaeomagnetic and anisotropy of the low-field magnetic susceptibility (AMS) studies 554 independently oriented cores were drilled, mostly from claystones, at 57 geographically distributed localities of Oligocene (and partly of Late Eocene in the Magura nappe) age. In the laboratory, the magnetic mineral identified was always magnetite, invariably accompanied by paramagnetic pyrite. The AMS measurements showed that the magnetic fabric was dominantly foliated. The foliation and bedding poles were very close at most of the localities, suggesting that the foliation was of sedimentary/compaction origin. Magnetic lineations, even if clustered at the locality level, did not exhibit a regional pattern for the Magura nappe. On the other hand, lineations related to folding were NE–SW oriented in the western and E–W oriented in the central and eastern segments of the Silesian nappe. Following the AMS measurements the samples were subjected to detailed stepwise demagnetization and analysis of the demagnetization curves that provided the input data for regional tilt tests. Both the tilt and inclination only tests suggested that the remanence is of post-folding age for the Magura nappe, showing a general 50° counterclockwise (CCW) rotation. Similar palaeomagnetic directions were observed for the central and eastern segments of the Silesian nappe, whereas a significantly larger CCW rotation for the western segment. The orientations of the AMS lineation directions averaged for the central, the eastern and the western segments of the Silesian nappe correspond to a ‘rotated’ palaeomagnetic pattern, as they are practically E–W oriented for the first two and NE–SW oriented for the third. The palaeomagnetic results of this study are interpreted in terms of a 50°‘en bloc’ rotation in the CCW sense of the Magura and Silesian nappes during their Miocene emplacement. This result has two important implications. The first is that the CCW rotation of the nappes that accompanied the displacement of the flysch basins from SW to NE is not of Palaeogene age, as earlier suggested, but younger. The second is that the change in the stress field orientation during the Miocene, observed for the Silesian nappe is not a CW far field stress rotation, but the consequence of CCW block rotations. The larger CCW rotation observed in both the palaeomagnetic and AMS data sets for the western segment of the Silesian nappe relative to the central and eastern segments is connected to local block rotations in a left-lateral wrench corridor. [ABSTRACT FROM AUTHOR]

In this work, we analyse continuous measurements of microseisms to assess the reliability of the fundamental resonance frequency estimated by means of the horizontal-to-vertical ( H/ V) spectral ratio within the 0.1–1 Hz frequency range, using short-period sensors (natural period of 1 s). We apply the H/ V technique to recordings of stations installed in two alluvial basins with different sedimentary cover thicknesses—the Lower Rhine Embayment (Germany) and the Gubbio Plain (Central Italy). The spectral ratios are estimated over the time–frequency domain, and we discuss the reliability of the results considering both the variability of the microseism activity and the amplitude of the instrumental noise. We show that microseisms measured by short period sensors allow the retrieval of fundamental resonance frequencies greater than about 0.1–0.2 Hz, with this lower frequency bound depending on the relative amplitude of the microseism signal and the self-noise of the instruments. In particular, we show an example where the considered short-period sensor is connected to instruments characterized by an instrumental noise level which allows detecting only fundamental frequencies greater than about 0.4 Hz. Since the frequency at which the peak of the H/ V spectral ratio is biased depends upon the seismic signal-to-instrument noise ratio, the power spectral amplitude of instrumental self-noise should be always considered when interpreting the frequency of the peak as the fundamental resonance frequency of the investigated site. [ABSTRACT FROM AUTHOR]

Data from 90 permanent broad-band stations spread over central and eastern Europe were analysed using Ps receiver functions to study the crustal and upper-mantle structure down to the mantle transition zone. Receiver functions provide valuable information on structural features, which are important for the resolution of European lithospheric dynamics. Moho depths vary from less than 25 km in extensional areas in central Europe to more than 50 km at stations in eastern Europe (Craton) and beneath the Alpine–Carpathian belt. A very shallow Moho depth can be observed at stations in the Upper Rhine Graben area ( ca. 25 km), whereas, for example, stations in the SW Bohemian Massif show a significantly deeper Moho interface at a depth of 38 km. Vp/ Vs ratios vary between 1.60 and 1.96, and show no clear correlation to the major tectonic units, thus probably representing local variations in crustal composition. Delayed arrivals of converted phases from the mantle transition zone are observed at many stations in central Europe, whereas stations in the cratonic area show earlier arrivals compared with those calculated from the IASP91 Earth reference model. Differential delay times between the P410s and P660s phases indicate a thickened mantle transition zone beneath the eastern Alps, the Carpathians and the northern Balkan peninsula, whereas the transition zone thickness in eastern and central Europe agrees with the IASP91 value. The thickening of the mantle transition zone beneath the eastern Alps and the Carpathians could be caused by cold, deeply subducted oceanic slabs. [ABSTRACT FROM AUTHOR]

A regional model of the 3-D variation in seismic P-wave velocity structure in the crust of NW Europe has been compiled from wide-angle reflection/refraction profiles. Along each 2-D profile a velocity–depth function has been digitised at 5 km intervals. These 1-D velocity functions were mapped into three dimensions using ordinary kriging with weights determined to minimise the difference between digitised and interpolated values. An analysis of variograms of the digitised data suggested a radial isotropic weighting scheme was most appropriate. Horizontal dimensions of the model cells are optimised at 40 × 40 km and the vertical dimension at 1 km. The resulting model provides a higher resolution image of the 3-D variation in seismic velocity structure of the UK, Ireland and surrounding areas than existing models. The construction of the model through kriging allows the uncertainty in the velocity structure to be assessed. This uncertainty indicates the high density of data required to confidently interpolate the crustal velocity structure, and shows that for this region the velocity is poorly constrained for large areas away from the input data. [ABSTRACT FROM AUTHOR]