Estimating crustal thickness in Belgium using Moho-reflected waves

We present the results of the determination of the Moho depth underneath Belgium using reflected P and S-waves (PmP- and SmS-waves). Previous studies suggest differences of the Moho depth in the different parts of the region. In the lower Rhine Embayment in the northeast, the Moho depth is considered to be shallow (25 km). In the Brabant Massif in the west the crustal thickness is supposed to be larger (up to 38 km). The southeast of Belgium is characterised by the Variscan allochtone, where the Moho depth is around 30 km. In this study, PmP/SmS-waves of ~150 well-located local earthquakes and explosions in the North Sea registered by 37 stations of the permanent seismic network and by mobile stations installed by the Royal Observatory of Belgium were used. More than 750 PmP/SmS-waves were modelled to determine the Moho depth with the following procedure. PmP-arrivals are picked and the locations of the PmP-bounce points are determined and mapped. Over this map a 20 x 20 km grid is placed and for each grid cell an iteration is performed to determine the Moho depth. The thickness of the crust varies between 25 and 36 km and is slightly shallower in the northeast of Belgium (28–30–32 km) than to the southwest (33-34 km). Underneath the Brabant massif however Moho depths of 31 km are found, which is in contradiction with previous results

Local seismic tomography in Belgium: implications for the geological structure

We present the results of a local seismic tomography in Belgium using well-located local earthquakes registered by 37 stations of the permanent seismic network and by mobile stations installed by the Royal Observatory of Belgium. Previous studies did not offer a lot of information on the middle and lower crust. The seismic profiles shot in the region (Belcorp, Decorp, Ecors, . . . ) all show an unreflective middle and lower crust. The gravimetric and magnetic data show the presence of a sharp transition between the Brabant Massive and the Ardennes allochtone, furthermore, a broad positive gravimetric anomaly, is interpreted as a Moho uplift underneath the Campine region. Our results confirm the sharp transition between the Brabant Massif (higher than expected velocities) and the Ardennes allochtone (lower than expected velocities). At 27 km of depth lower crust – upper mantle velocities (7.50 km/s) are found underneath the Campine region and the Eifelplume region, confirming the Moho uplifts to 28 km underneath these regions. At 13 km similar velocities (7.50 km/s) are seen underneath the Eifelplume, they correspond to a lower crust-upper mantle that trusted in the crust during the Variscan orogeny

Local stress sources in Western Europe lithosphere from geoid anomalies

We propose a method to evaluate the stress generated at the local scale by the spatial variations of the gravitational potential energy (GPE), which is related to inhomogeneous topography and mass distribution in the lithosphere. We show that it is possible to infer these local stress sources from the second spatial derivatives of a geoid height grid, used as a proxy of the GPE. The coherence of the method is validated on a passive margin, the Bay of Biscay. The result is that expected in such a geological configuration, with extensional local stress sources with the maximum horizontal principal stress parallel to the margin and compressive sources with the maximum horizontal principal stress perpendicular to the margin in the continental and oceanic lithosphere, respectively. We apply the method to Western Europe in order to provide a better understanding of the complex spatial variation of the present-day tectonic activity. Our results indicate a stress pattern from the local sources dominated by short-space-wavelength (of the order of a few tens of kilometers) variations in the tectonic style and in the direction of the maximal horizontal principal stress sigma(H). A comparison of the sigma(H) orientations and tectonic style from the local sources with the ones of the World Stress Map (WSM) data set indicates that the local stress sources can be representative of the deviatoric stress state in some regions. Our results explain 71% of the faulting styles for the earthquake fault-plane solutions in the WSM, which is better than the classical compressive NW-SE stress field model. In the central part of the Pyrenees, the agreement between earthquake fault-slip directions and the direction of shear stress from the local sources acting on the associated fault planes is compatible with the extensional stress field evidenced by recent investigations

The 6 May 1976 Friuli earthquake: re-evaluating and consolidating

The aim of this paper is to propose the creation, in terms of European Macroseismic Scale (EMS-98), of the entire macroseismic fi eld of the 6 May 1976 Friuli earthquake. Only forty odd years have passed, and nothwithsatnding that there is a huge quantity of existing data, it was still disturbing to fi nd that much of the original data are missing and probably lost forever Efforts have therefore been made to fi nd additional and still unknown primary data. For the majority of the collected national data sets, a reevaluation was then possible. This study presents the comprehensive macroseismic data set for 14 European countries. It is, to our knowledge, one of the largest European data sets, consisting of 3423 intensity data points (IDPs). The earthquake was felt from Rome to the Baltic Sea, and from Belgium to Warsaw. The maximum intensity 10 EMS-98 was reached in eight localities in Friuli (Italy). Compared to previous studies, the Imax values have changed from country to country, in some cases being lowered due to methodological differences, but in the case of three among the most hit countries, Imax is now higher than in the previous studies, mainly due to the new data.Published417-4444T. Sismicità dell'ItaliaJCR Journa

Fault activity in the epicentral area of the 1580 Dover Strait (Pas-de-Calais) earthquake (northwestern Europe)

On 1580 April 6 one of the most destructive earthquakes of northwestern Europe took place in the Dover Strait (Pas de Calais). The epicentre of this seismic event, the magnitude of which is estimated to have been about 6.0, has been located in the offshore continuation of the North Artois shear zone, a major Variscan tectonic structure that traverses the Dover Strait. The location of this and two other moderate magnitude historical earthquakes in the Dover Strait suggests that the North Artois shear zone or some of its fault segments may be presently active. In order to investigate the possible fault activity in the epicentral area of the AD 1580 earthquake, we have gathered a large set of bathymetric and seismic-reflection data covering the almost-entire width of the Dover Strait. These data have revealed a broad structural zone comprising several subparallel WNW–ESE trending faults and folds, some of them significantly offsetting the Cretaceous bedrock. The geophysical investigation has also shown some indication of possible Quaternary fault activity. However, this activity only appears to have affected the lowermost layers of the sediment infilling Middle Pleistocene palaeobasins. This indicates that, if these faults have been active since Middle Pleistocene, their slip rates must have been very low. Hence, the AD 1580 earthquake appears to be a very infrequent event in the Dover Strait, representing a good example of the moderate magnitude earthquakes that sometimes occur in plate interiors on faults with unknown historical seismicity

Monitoraggio in area sismica di beni monumentali: tecniche NDT e procedure di verifica

Negli ultimi anni il concetto di vulnerabilità sismica è tristemente entrato a far parte delle conoscenze anche dei non addetti ai lavori. Infatti, gli eventi sismici che hanno interessato dagli inizi del ‘900 il territorio Italiano, hanno sistematicamente messo in risalto l’elevata vulnerabilità sismica del nostro patrimonio edilizio, ivi compresi i beni monumentali, nonché, l’inesistenza di qualsiasi attività di programmazione della manutenzione periodica ordinaria e straordinaria delle strutture sismo-resistenti, che garantiscono nel tempo la conservazione delle loro capacità di risposta alle perturbazioni esterne.Il progetto PON sul Monitoraggio in Area Sismica di SIstemi MOnumentali nasce con la prerogativa di produrre uno strumento dedicato alla tutela di strutture a valenza storico – artistica, attraverso un percorso di catalogazione, di analisi del bene inteso come elemento costituito da elementi resistenti e da materiali, di studio del sito dove la struttura è ubicata e di attività di monitoraggio

Comparison of ground motions estimated from prediction equations and from observed damage during the <i>M</i> = 4.6 1983 Liège earthquake (Belgium)

On 8 November 1983 an earthquake of magnitude 4.6 damaged more than 16 000 buildings in the region of Liège (Belgium). The extraordinary damage produced by this earthquake, considering its moderate magnitude, is extremely well documented, giving the opportunity to compare the consequences of a recent moderate earthquake in a typical old city of Western Europe with scenarios obtained by combining strong ground motions and vulnerability modelling. <br><br> The present study compares 0.3 s spectral accelerations estimated from ground motion prediction equations typically used in Western Europe with those obtained locally by applying the statistical distribution of damaged masonry buildings to two fragility curves, one derived from the HAZUS programme of FEMA (FEMA, 1999) and another developed for high-vulnerability buildings by Lang and Bachmann (2004), and to a method proposed by Faccioli et al. (1999) relating the seismic vulnerability of buildings to the damage and ground motions. The results of this comparison reveal good agreement between maxima spectral accelerations calculated from these vulnerability and fragility curves and those predicted from attenuation law equations, suggesting peak ground accelerations for the epicentral area of the 1983 earthquake of 0.13–0.20 <i>g</i> (<i>g</i>: gravitational acceleration)

Palaeozoic to recent tectonics in the NW European variscan front zone: Papers from the the Belgian Symposium on structural geology and tectonics, held in Leuven, Belgium

status: publishe