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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 Groningen gas field in the Netherlands experienced an immediate reduction in seismic events in the year following a massive cut in production. This reduction is inconsistent with existingmodels of seismicity predictions adopting compaction strains as proxy, since reservoir creep would then result in a more gradual reduction of seismic events after a production stop. We argue that the discontinuity in seismic response relates to a physical discontinuity in stress loading rate on faults upon the arrest of pressure change. The stresses originate from a combination of the direct poroelastic effect through the pressure changes and the delayed effect of ongoing compaction after cessation of reservoir production. Both mechanisms need to be taken into account. To this end, we employed finite-element models in a workflow that couples Kelvin-Chain reservoir creep with a semi-analytical approach for the solution of slip and seismic moment from the predicted stress change. For ratios of final creep and elastic compaction up to 5, the model predicts that the cumulative seismic moment evolution after a production stop is subject to a very moderate increase, 2-10 times less than the values predicted by the alternative approaches using reservoir compaction strain as proxy. This is in agreement with the low seismicity in the central area of the Groningen field immediately after reduction in production. The geomechanical model findings support scope for mitigating induced seismicity through adjusting rates of pressure change by cutting down production. [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]

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]

An energy-flux model (EFM) and a teleseismic fluctuation wavefield method (TFWM) have been applied to interpret the teleseismic P coda observed at three temporary and two permanent networks in northern and central Europe. The aim is to determine the small-scale random structure of the lithosphere below the receivers. Various subsets of these data have been exploited previously with one of the two methods. The main objectives here are: With TFWM, the product aσ2 can be reliably determined if L is known. L can be roughly estimated with EFM. Although EFM can, in principle, resolve a and σ separately, a is recovered with a rather large uncertainty. TFWM does not distinguish much between the ACF type, whereas with EFM determination of the ACF type is sometimes possible. By combining the results of both methods we determine improved random medium parameters of the lithosphere for eleven subregions in northern and central Europe. In the Baltic shield, Rhenohercynian belt, Ardenne and Brabant mountains, eastern Rhenish massif, Eifel, Hunsrück mountains, Lorraine, Frankonian Jura and massif Central scattering predominantly occurs in the crust. For the frequency range from 0.5 to 5 Hz correlation lengths of 1–7 km and rms velocity fluctuations of 3–7 per cent are obtained. For the Rhenohercynian belt (RH) and the N-German basin RMS velocity fluctuations and correlation lengths could not be resolved. The data from the N-German basin (NB) cannot be explained by scattering within the crust only. Smallest scattering Q was found in the N German basin ( at 1 Hz) and largest scattering Q in the Baltic shield ( at 2–3 Hz). For the Frankonian Jura only a Gaussian ACF can fit the Q−1 s values. The data from the eastern Rhenish massif also indicate a random medium with Gaussian ACF. For all other subregions we cannot distinguish between exponential or Gaussian ACF for the random medium structure. [ABSTRACT FROM AUTHOR]