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- Estimation of the Crustal Bulk Properties Beneath Mainland Portugal from P-Wave Teleseismic Receiver FunctionsPublication . Dündar, Süleyman; Dias, Nuno; Silveira, Graça; Kind, Rainer; Vinnik, Lev; Matias, Luís; Bianchi, MarceloIn this work, we present results from teleseismic P-wave receiver functions (PRFs) obtained in Portugal, Western Iberia. A dense seismic station deployment conducted between 2010 and 2012, in the scope of the WILAS project and covering the entire country, allowed the most spatially extensive probing on the bulk crustal seismic properties of Portugal up to date. The application of the H-κ stacking algorithm to the PRFs enabled us to estimate the crustal thickness (H) and the average crustal ratio of the P- and S-waves velocities V p/V s (κ) for the region. Observations of Moho conversions indicate that this interface is relatively smooth with the crustal thickness ranging between 24 and 34 km, with an average of 30 km. The highest V p/V s values are found on the Mesozoic-Cenozoic crust beneath the western and southern coastal domain of Portugal, whereas the lowest values correspond to Palaeozoic crust underlying the remaining part of the subject area. An average V p/V s is found to be 1.72, ranging 1.63-1.86 across the study area, indicating a predominantly felsic composition. Overall, we systematically observe a decrease of V p/V s with increasing crustal thickness. Taken as a whole, our results indicate a clear distinction between the geological zones of the Variscan Iberian Massif in Portugal, the overall shape of the anomalies conditioned by the shape of the Ibero-Armorican Arc, and associated Late Paleozoic suture zones, and the Meso-Cenozoic basin associated with Atlantic rifting stages. Thickened crust (30-34 km) across the studied region may be inherited from continental collision during the Paleozoic Variscan orogeny. An anomalous crustal thinning to around 28 km is observed beneath the central part of the Central Iberian Zone and the eastern part of South Portuguese Zone.
- Crustal seismic structure beneath Portugal and southern Galicia (Western Iberia) and the role of Variscan inheritancePublication . Veludo, Idalina; Dias, Nuno; Fonseca, Paulo E.; Matias, Luís; Carrilho, Fernando; Haberland, Christian; Villasenor, AntónioThe crustal structure in Western Iberia, is the result of a complex geodynamic history. Most of the surface is covered by rocks dating to the Variscan orogeny, the coastal ranges dominated by Mesozoic structures and Mesocenozoic basins covering partially the mainland. In this study we present the results of a local earthquake tomographic study, performed to image in depth this complex crustal structure down to 24 km depth.& para;& para;The obtained tomograms present a good correlation with the surface geology, in general with higher Vp velocities in the north, and lower Vp velocities in the south. The heterogeneity observed on the surface geology of the Galicia-Tras-os-Montes Zone is well marked, being a relatively thin layer over the smoother structure of the Centro Iberia Zone CIZ. The CIZ crustal block confined between the Porto-Tomar-Ferreira do Alentejo and the Manteigas-Braganqa faults having higher Vp values, enhancing the contrast to the Lusitanian Basin to the west. The Ossa-Morena Zone corresponds to the unit presenting the greater heterogeneity in both Vp and Vp/Vs models, showing also with a complex transition to the South Portuguese Zone and a relatively smooth transition to the Centro Iberian Zone. Unexpectedly, the South Portuguese Zone present an east-west velocity variation with no apparent correspondence with the surface geology. The transition to the coastal Mesocenozoic basins is clearly marked, the Lower Tagus Valley corresponding to a similar to 4 km thick low velocity region.& para;& para;The relocation inland recorded seismicity in the period 2000-2014 allows cleansing some of the alignments and establish their correlation with some of the active structures in Portugal. The model features and seismicity pattern reveal the strong role played by the Variscan heritage.
- Imaging the crust and uppermost mantle structure of Portugal (West Iberia) with seismic ambient noisePublication . Silveira, Graça; Dias, Nuno; Kiselev, Sergey; Stutzmann, Eleonore; Custodio, Susana; Schimmel, MartinWe present a new high-resolution three-dimensional (3D) shear wave velocity (Vs) model of the crust and uppermost mantle beneath Portugal, inferred from ambient seismic noise tomography. We use broadband seismic data from a dense temporary deployment covering the entire Portuguese mainland between 2010 and 2012 in the scope of the WILAS project. Vertical component data are processed using phase correlation and phase weighted stack to obtain Empirical Green functions (EGF) for 3900 station pairs. Further, we use a random sampling and subset stacking strategy to measure robust Rayleigh wave group velocities in the period range 7-30 s and associated uncertainties. The tomographic inversion is performed in 2 steps: First, we determine group velocity lateral variations for each period. Next, we invert them at each grid point using a new trans-dimensional inversion scheme to obtain the 3D shear wave velocity model. The final 3D model extends from the upper crust (5 km) down to the uppermost mantle (60 km) and has a lateral resolution of similar to 50 km. In the upper and middle crust, the Vs anomaly pattern matches the tectonic units of the variscan massif and alpine basins. The transition between the Lusitanian Basin and the Ossa Morena Zone is marked by a contrast between moderate and high velocity anomalies, in addition to two arched earthquake lineations. Some faults, namely the Manteigas-Vilarica-Braganca fault and the Porto-Tomar-Ferreira do Alentejo fault, have a clear signature from the upper crust down to the uppermost mantle (60 km). Our 3D shear wave velocity model offers new insights into the continuation of the main tectonic units at depth and contributes to better understanding the seismicity of Portugal.
- Imaging exhumed lower continental crust in the distal Jequitinhonha basin, BrazilPublication . Loureiro, Afonso; Schnürle, P.; Klingelhoefer, Frauke; Afilhado, Alexandra; Pinheiro, J.; Evain, Mikael; Gallais, F.; Dias, Nuno; Rabineau, Marina; Baltzer, A.; Benabdellouahed, M.; Soares, J.; Fuck, R.; Cupertino, J. A.; Viana, Adriano Roessler; Matias, Luis; Moulin, Maryline; Aslanian, D.; Morvan, L.; Mazé, J. P.; Pierre, D.; Roudaut-Pitel, M.; Rio, I.; Alves, D.; Júnior, P. Barros; Biari, Youssef; Corela, C.; Crozon, J.; Duarte, J. L.; Ducatel, C.; Falcão, C.; Fernagu, P.; Lima, M. Vinicius Aparecido Gomes de; Piver, D. Le; Mokeddem, Z.; Pelleau, P.; Rigoti, C.; Roest, W.; Roudaut, M.Twelve combined wide-angle refraction and coincident multi-channel seismic profiles were acquired in the Jequitinhonha-Camamu-Almada, Jacuípe, and Sergipe-Alagoas basins, NE Brazil, during the SALSA experiment in 2014. Profiles SL11 and SL12 image the Jequitinhonha basin, perpendicularly to the coast, with 15 and 11 four-channel ocean-bottom seismometers, respectively. Profile SL10 runs parallel to the coast, crossing profiles SL11 and SL12, imaging the proximal Jequitinhonha and Almada basins with 17 ocean-bottom seismometers. Forward modelling, combined with pre-stack depth migration to increase the horizontal resolution of the velocity models, indicates that sediment thickness varies between 3.3 km and 6.2 km in the distal basin. Crustal thickness at the western edge of the profiles is of around 20 km, with velocity gradients indicating a continental origin. It decreases to less than 5 km in the distal basin, with high seismic velocities and gradients, not compatible with normal oceanic crust nor exhumed upper mantle. Typical oceanic crust is never imaged along these about 200 km-long profiles and we propose that the transitional crust in the Jequitinhonha basin is a made of exhumed lower continental crust.
- Seismic imaging of the western iberian crust using ambient noise: boundaries and internal structure of the Iberian MassifPublication . Silveira, Graça; Dias, Nuno; Villasenor, AntonioWe present new Rayleigh-wave dispersion maps of the western Iberian Peninsula for periods between 8 and 30 s, obtained from correlations of seismic ambient noise, following the recent increase in seismic broadband network density in Portugal and Spain. Group velocities have been computed for each station pair using the empirical Green's functions generated by cross-correlating one-day-length seismic ambient-noise records. The resulting high-path density allows us to obtain lateral variations of the group velocities as a function of period in cells of 0.5 degrees x 0.5 degrees with an unprecedented resolution. As a result we were able to address some of the unknowns regarding the lithospheric structure beneath SW Iberia. The dispersion maps allow the imaging of the major structural units, namely the Iberian Massif, and the Lusitanian and Algarve Meso-Cenozoic basins. The Cadiz Gulf/Gibraltar Strait area corresponds to a strong low-velocity anomaly, which can be followed to the largest period inverted, although slightly shifted to the east at longer periods. Within the Iberian Massif, second-order perturbations in the group velocities are consistent with the transitions between tectonic units composing the massif. (C) 2013 Elsevier B.V. All rights reserved.
- PRISM3D: a 3-D reference seismic model for Iberia and adjacent areasPublication . Arroucau, Pierre; Custódio, S.; Civiero, Chiara; Silveira, Graça; Dias, Nuno; Diaz, J.; Villasenor, A.; Bodin, ThomasWe present PRISM3D, a 3-D reference seismic model of P- and S-wave velocities for Iberia and adjacent areas. PRISM3D results from the combination of the most up-to-date earth models available for the region. It extends horizontally from 15 degrees W to 5 degrees E in longitude, 34 degrees N to 46 degrees N in latitude and vertically from 3.5 km above to 200 km below sea level, and is modelled on a regular grid with 10 and 0.5 km of grid node spacing in the horizontal and vertical directions, respectively. It was designed using models inferred from local and teleseismic body-wave tomography, earthquake and ambient noise surface wave tomography, receiver function analysis and active source experiments. It includes two interfaces, namely the topography/bathymetry and the Mohorovkie (Moho) discontinuity. The Moho was modelled from previously published receiver function analysis and deep seismic sounding results. To that end we used a probabilistic surface reconstruction algorithm that allowed to extract the mean of the Moho depth surface along with its associated standard deviation, which provides a depth uncertainty estimate. The Moho depth model is in good agreement with previously published models, although it presents slightly sharper gardients in orogenic areas such as the Pyrenees or the Betic-Rif system. Crustal and mantle P- and S-wave wave speed grids were built separately on each side ofthe Moho depth surface by weighted average of existing models, thus allowing to realistically render the speed gradients across that interface. The associated weighted standard deviation was also calculated, which provides an uncertainty estimation on the average wave speed values at any point of the grid. At shallow depths (<10 km), low P and S wave speeds and high V-P/V-S are observed in offshore basins, while the Iberian Massif, which covers a large part of western Iberia, appears characterized by a rather flat Moho, higher than average V-P and V-S and low V-P/V-S. Conversely, the Betic-Rif system seems to be associated with low V-P and V-S, combined with high V-P/V-S in comparison to the rest of the study area. The most prominent feature of the mantle is the well known high wave speed anomaly related to the Alboran slab imaged in various mantle tomography studies. The consistency of PRISM3D with previous work is verified by comparing it with two recent studies, with which it shows a good general agreement.The impact of the new 3-D model is illustrated through a simple synthetic experiment, which shows that the lateral variations of the wave speed can produce traveltime differences ranging from -1.5 and 1.5 s for P waves and from -2.5 and 2.5 s for S waves at local to regional distances. Such values are far larger than phase picking uncertainties and would likely affect earthquake hypocentral parameter estimations. The new 3-D model thus provides a basis for regional studies including earthquake source studies, Earth structure investigations and geodynamic modelling of Iberia and its surroundings.
- C3G - A portuguese collaboration for geology, geodesy and geophysicsPublication . Caldeira, Bruno; Caldeira, R.; Carrilho, F.; Carvalho, J.; Custódio, S.; Dias, Nuno; Fernandes, R.; Moreira, M.; Seixas, T.; Silva, M.; Quental, L.; Silveira, GraçaPortugal joined the effort to create the EPOS infrastructure in 2008, and it became immediately apparent that a national network of Earth Sciences infrastructures was required to participate in the initiative. At that time, FCT was promoting the creation of a national infrastructure called RNG - Rede Nacional de Geofísica (National Geophysics Network). A memorandum of understanding had been agreed upon, and it seemed therefore straightforward to use RNG (enlarged to include relevant participants that were not RNG members) as the Portuguese partner to EPOS-PP. However, at the time of signature of the EPOS-PP contract with the European Commission (November 2010), RNG had not gained formal identity yet, and IST (one of the participants) signed the grant agreement on behalf of the Portuguese consortium. During 2011 no progress was made towards the formal creation of RNG, and the composition of the network – based on proposals submitted to a call issued in 2002 – had by then become obsolete. On February 2012, the EPOS national contact point was mandated by the representatives of the participating national infrastructures to request from FCT the recognition of a new consortium - C3G, Collaboratory for Geology, Geodesy and Geophysics - as the Portuguese partner to EPOS-PP. This request was supported by formal letters from the following institutions: ‐ LNEG. Laboratório Nacional de Energia e Geologia (National Geological Survey); ‐ IGP ‐ Instituto Geográfico Português (National Geographic Institute); ‐ IDL, Instituto Dom Luiz – Laboratório Associado ‐ CGE, Centro de Geofísica de Évora; ‐ FCTUC, Faculdade de Ciências e Tecnologia da Universidade de Coimbra; ‐ Instituto Superior de Engenharia de Lisboa; ‐ Instituto Superior Técnico; ‐ Universidade da Beira Interior. While Instituto de Meteorologia (Meteorological Institute, in charge of the national seismographic network) actively supports the national participation in EPOS, a letter of support was not feasible in view of the organic changes underway at the time. C3G aims at the integration and coordination, at national level, of existing Earth Sciences infrastructures, namely: ‐ seismic and geodetic networks (IM, IST, IDL, CGE); ‐ rock physics laboratories (ISEL); ‐ geophysical laboratories dedicated to natural resources and environmental studies; ‐ geological and geophysical data repositories; ‐ facilities for data storage and computing resources. The C3G - Collaboratory for Geology, Geodesy and Geophysics will be coordinated by Universidade da Beira Interior, whose Department of Informatics will host the C3G infrastructure.
- Estimation of the Crustal Bulk Properties Beneath Mainland Portugal from P-Wave Teleseismic Receiver FunctionsPublication . Dündar, Süleyman; Dias, Nuno; Silveira, Graça; Kind, Rainer; Vinnik, Lev; Matias, Luís; Bianchi, MarceloIn this work, we present results from teleseismic P-wave receiver functions (PRFs) obtained in Portugal, Western Iberia. A dense seismic station deployment conducted between 2010 and 2012, in the scope of the WILAS project and covering the entire country, allowed the most spatially extensive probing on the bulk crustal seismic properties of Portugal up to date. The application of the H-kappa stacking algorithm to the PRFs enabled us to estimate the crustal thickness (H) and the average crustal ratio of the P- and S-waves velocities V (p)/V (s) (kappa) for the region. Observations of Moho conversions indicate that this interface is relatively smooth with the crustal thickness ranging between 24 and 34 km, with an average of 30 km. The highest V (p)/V (s) values are found on the Mesozoic-Cenozoic crust beneath the western and southern coastal domain of Portugal, whereas the lowest values correspond to Palaeozoic crust underlying the remaining part of the subject area. An average V (p)/V (s) is found to be 1.72, ranging 1.63-1.86 across the study area, indicating a predominantly felsic composition. Overall, we systematically observe a decrease of V (p)/V (s) with increasing crustal thickness. Taken as a whole, our results indicate a clear distinction between the geological zones of the Variscan Iberian Massif in Portugal, the overall shape of the anomalies conditioned by the shape of the Ibero-Armorican Arc, and associated Late Paleozoic suture zones, and the Meso-Cenozoic basin associated with Atlantic rifting stages. Thickened crust (30-34 km) across the studied region may be inherited from continental collision during the Paleozoic Variscan orogeny. An anomalous crustal thinning to around 28 km is observed beneath the central part of the Central Iberian Zone and the eastern part of South Portuguese Zone.
- Upper crustal structure of Madeira Island revealed from ambient noise tomographyPublication . Matos, Catarina; Silveira, Graça; Matias, Luís; Caldeira, Rita; Ribeiro, M. Luísa; Dias, Nuno; Krüger, Frank; Santos, Telmo Bento dosWe present the first image of the Madeira upper crustal structure, using ambient seismic noise tomography. 16 months of ambient noise, recorded in a dense network of 26 seismometers deployed across Madeira, allowed reconstructing Rayleigh wave Green's functions between receivers. Dispersion analysis was performed in the short period band from 1.0 to 4.0 s. Group velocity measurements were regionalized to obtain 20 tomographic images, with a lateral resolution of 2.0 km in central Madeira. Afterwards, the dispersion curves, extracted from each cell of the 2D group velocity maps, were inverted as a function of depth to obtain a 3D shear wave velocity model of the upper crust, from the surface to a depth of 2.0 km. The obtained 3D velocity model reveals features throughout the island that correlates well with surface geology and island evolution. (C) 2015 Elsevier B.V. All rights reserved.
- Earthquakes in western Iberia: improving the understanding of lithospheric deformation in a slowly deforming regionPublication . Custódio, Susana; Dias, Nuno; Carrilho, F.; Góngora, E.; Rio, I.; Marreiros, Célia; Morais, Iolanda; Alves, P.; Matias, LuísMainland Portugal, on the southwestern edge of the European continent, is located directly north of the boundary between the Eurasian and Nubian plates. It lies in a region of slow lithospheric deformation (< 5 mm yr(-1)), which has generated some of the largest earthquakes in Europe, both intraplate (mainland) and interplate (offshore). Some offshore earthquakes are nucleated on old and cold lithospheric mantle, at depths down to 60 km. The seismicity of mainland Portugal and its adjacent offshore has been repeatedly classified as diffuse. In this paper, we analyse the instrumental earthquake catalogue for western Iberia, which covers the period between 1961 and 2013. Between 2010 and 2012, the catalogue was enriched with data from dense broad-band deployments. We show that although the plate boundary south of Portugal is diffuse, in that deformation is accommodated along several distributed faults rather than along one long linear plate boundary, the seismicity itself is not diffuse. Rather, when located using high-quality data, earthquakes collapse into well-defined clusters and lineations. We identify and characterize the most outstanding clusters and lineations of epicentres and correlate them with geophysical and tectonic features (historical seismicity, topography, geologically mapped faults, Moho depth, free-air gravity, magnetic anomalies and geotectonic units). Both onshore and offshore, clusters and lineations of earthquakes are aligned preferentially NNE-SSW and WNW-ESE. Cumulative seismic moment and epicentre density decrease from south to north, with increasing distance from the plate boundary. Only few earthquake lineations coincide with geologically mapped faults. Clusters and lineations that do not match geologically mapped faults may correspond to previously unmapped faults (e.g. blind faults), rheological boundaries or distributed fracturing inside blocks that are more brittle and therefore break more easily than neighbour blocks. The seismicity map of western Iberia presented in this article opens important questions concerning the regional seismotectonics. This work shows that the study of low-magnitude earthquakes using dense seismic deployments is a powerful tool to study lithospheric deformation in slowly deforming regions, such as western Iberia, where high-magnitude earthquakes occur with long recurrence intervals.
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