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Although the seismic activity at the eastern Pyrenees is nowadays moderate and sparse, with events usually not exceeding magnitudes 4.5, this area has been affected in the past by the most destructive event occurred in the Pyrenees, reaching an intensity of IX, whose seismogenic structure is not well understood and still under debate. In order to progress in the knowledge of these structures, we present here the results derived from a 14 months-long broad-band seismic deployment focused on the Cerdanya Basin, but encompassing the eastern termination of the Pyrenees. The dense station coverage has allowed us to obtain accurate hypocentral locations, as well as up to 23 focal mechanisms from local low-magnitude earthquakes. In addition to a relatively sparse seismicity, several clusters of seismic events located in well-defined, small areas and depth ranges have been identified. The results show a few low-magnitude seismic events located in the southern limit of the Cerdanya Basin that could be related to oblique secondary faults within the footwall of the Têt Fault, the major tectonic structure in the area. Our data shows also that the Capcir Fault has associated seismicity, with some of the events located out of the fault plane, perhaps on secondary fault branches. To the east, a cluster of low-magnitude events is detected in the epicentral area of two relatively large earthquakes occurred recently. Further west, the Maladeta Massif has a sustained seismic activity, although its origin does not seem to be related to the most relevant structure in the area, the North Maladeta Fault. Regarding focal mechanisms, most of them show normal fault solutions with nodal planes NW-SE oriented, which agree with the extensional regime perpendicular to the axis of the chain derived from the seismic and geodetic data

During the COVID-19 pandemic lockdown, the city of Barcelona was covered by a dense seismic network consisting of up to 19 seismic sensors. This network has provided an excellent tool to investigate in detail the background seismic-noise variations associated with the lockdown measures. Permanent stations facilitate comparing the seismic noise recorded during the lockdown quieting with long-term variations due to holiday periods. On the other hand, the data acquired by the dense network show the differences between sites located near industrial areas, transportation hubs or residential areas. The results confirm that the quieting of human activity during lockdown has resulted in a reduction in seismic vibrations in the 2–20 Hz band that is clearly higher than during holiday seasons. This effect is observed throughout the city, but only those stations not affected by very proximal sources of vibration (construction sites, industries) are clearly correlated with the level of activity denoted by other indicators. Our contribution demonstrates that seismic amplitude variations can be used as a proxy for human activity in urban environments, providing details similar to those offered by other mobility indicators.

An optimal strategy for building realistic geological models must combine different geophysical techniques, each with its advantages and limitations. However, dealing with multiple geophysical datasets to derive a geological interpretation is not straightforward since geophysical parameters are not always functionally related. In this work, we propose an innovative approach consisting of using machine learning techniques to jointly interpret three geophysical datasets (a pseudo-3D resistivity model, a 3D velocity model, and 4 well-logs). These datasets, among others, were acquired to characterize the suitability of an evaporitic sequence for hosting a temporary storage facility of hazardous radioactive waste, which was planned in Villar de Cañas (Spain). Our strategy consisted of integrating both models in a single 3D bi-parametric grid that nested the velocity and resistivity values in each node. We then used a supervised learning algorithm to lithologically classify each node according to a training set based on the borehole data, which acts as ground truth. The training set is composed of classifiers that lithologically label resistivity-velocity pairs. However, the very shallow nodes lack classifiers due to the poor well-log coverage at the top part of the evaporitic sequence. To fill this gap, we computed an unsupervised cluster analysis that provided new classes to complete the training set. Finally, the supervised classification was applied, providing a new 3D lithology model that is far more consistent with the geology than the models derived from each parameter independently. The 3D model also revealed geological features previously unknown, notably the existence of an inactive fault. The proposed method can be applied to integrate and jointly interpret any kind of multidisciplinary datasets in a wide range of geoscientific problems, including natural resource exploration, geological storage, environmental monitoring, civil engineering practice, and hazard

Seismic reflection data (normal incidence and wide angle) are unique assets for solid Earth sciences as they provide critical information about the physical properties and structure of the lithosphere as well as about the shallow subsurface for exploration purposes. The resolution of these seismic data is highly appreciated; however they are logistically complex and expensive to acquire, and their geographical coverage is limited. Therefore, it is essential to make the most of the data that have already been acquired. The collation and dissemination of seismic open-access data are then key to promote accurate and innovative research and to enhance new interpretations of legacy data. This work presents the Seismic DAta REpository (SeisDARE), which is, to our knowledge, one of the first comprehensive open-access online databases that stores seismic data registered with a permanent identifier (DOI). The datasets included here are openly accessible online and guarantee the FAIR (findable, accessible, interoperable, reusable) principles of data management, granting the inclusion of each dataset in a statistics referencing database so its impact can be measured. SeisDARE includes seismic data acquired in the last 4 decades in the Iberian Peninsula and Morocco. These areas have attracted the attention of international researchers in the fields of geology and geophysics due to the exceptional outcrops of the Variscan and Alpine orogens and wide foreland basins, the crustal structure of the offshore margins that resulted from a complex plate kinematic evolution, and the vast quantities of natural resources contained within. This database has been built thanks to a network of national and international institutions, promoting a multidisciplinary research and is open for international data exchange and collaborations. As part of this international collaboration, and as a model for inclusion of other global seismic datasets, SeisDARE also hosts seismic data acquired in Hardeman

The Iberian Central System, formed after the Alpine reactivation of the Variscan Iberian Massif, features maximum altitudes of 2500 m. It is surrounded by two foreland basins with contrasting elevation: the Duero Basin to the north, located at 750-800m, and the Tajo Basin to the south, lying at 450-500 m. The deep crustal structure of this mountain range seems to be characterized by the existence of a moderate crustal root that provides isostatic support for its topography. New seismic data are able to constrain the geometry of this crustal root, which appears to be defined by a northward lower-crustal imbrication of the southern Central Iberian crust underneath this range. Contrarily to what was expected, this imbrication also affects the upper crust, as the existing orogen-scale mid-crustal Variscan detachment was probably assimilated during the Carboniferous crustal melting that gave rise to the Central System batholith. In addition, the lower crust might have thinned, allowing coupled deformation at both crustal levels. This implies that the reactivated upper-crustal fractures can reach lower-crustal depths, thus allowing the entire crust to sink. This new model can explain the differences in topography between the Central System foreland basins. Also, it provides further constraints on the crustal geometry of this mountain range, as it seems to be that of an asymmetric Alpine-type orogen, thus hindering the existence of buckling processes as the sole origin of the deformation. The results presented here have been achieved after autocorrelation of seismic noise along the CIMDEF (Central Iberian Massif DEFormation Mechanisms) profile. Although the resolution of the dataset features limited resolution (0.5-4 Hz, stations placed at similar to 5 km), this methodology has allowed us to pinpoint some key structures that helped to constraint the deformation mechanisms that affected Central Iberia during the Alpine orogeny.

The dimensions, the geographical position and the complex geological history of the Iberian Peninsula makes it a unique and singular target to study its crustal and upper mantle structure and geodynamical evolution using geophysical methods. The lithospheric structure beneath Iberia has been investigated since the 1970’s using deep multichannel seismic reflection and refraction/wide-angle reflection profiling. Gravimetric and magnetic data were acquired following the deployment of permanent and temporary stations, mostly during the 1990’s. Beginning in the late 1990’s, the progressive use of Global Navigation Satellite Systems (GNSS) instruments contributed to monitor the present-day motions. During the last decades, numerous geological and geophysical surveys have investigated the Iberian lithosphere and upper mantle in the onshore and offshore regions, the most recent ones related to the TopoIberia project. The approach developed in this contribution is twofold. Firstly, we summarize the available geophysical information over Iberia, from focusing on the upper crust to the lithosphere-asthenosphere boundary and the transition zone marking the bottom of the upper mantle. Results of GNSS data, potential fields, controlled source seismic profiles, magnetotelluric data, body and surface wave tomography, receiver functions and 2D and 3D lithospheric modeling are reviewed and compared. Secondly, we focus on the areas of greater geodynamic interest and the regions where inconsistencies within the geophysical results, or contradictions in their tectonic interpretation exist, identifying the major questions that are still under debate. Besides shedding light to the state of knowledge and pointing out present-day research challenges, this review provides a tool for the integration of the diverse geophysical datasets with the surface geology and geodynamical processes that are interpreted to have built the complex geology of the Iberian Peninsula.

Intraplate ranges are topographic features that can occur far from plate boundaries, the expected position of orogens as described in the plate tectonics theory. To understand the lithospheric structure of intraplate ranges, we focused on the Spanish-Portuguese Central System (SPCS), the most outstanding topographic feature in the central Iberian Peninsula. The SPCS is an Alpine range that exhumes Precambrian-Paleozoic rocks and is located at >200 km from the northern border of the Iberian microplate. Here, we provide a P-wave velocity model based on wide-angle seismic reflection/refraction data of the central SPCS (Gredos sector). Our results show: (a) a layered lithosphere characterized by three major interfaces: Conrad, Mohorovicic, and Hales discontinuities, (b) an asymmetry of the crust-mantle boundary under the SPCS, (c) the extent of the Variscan batholith forming the main outcrops of Gredos, and (d) the thinning of the lower crust toward the south. This model suggests that the exhumation of the SPCS basement was driven by a south-vergent thick-skinned thrust system, developed in the southern part of the SPCS and that promoted crustal imbrication and a Mohorovicic discontinuity's offset under the SPCS. Thus, the deformation mechanisms of the crust seem to be controlled by the presence of the late- to post-Variscan granitoids that assimilated the Variscan mid-crustal detachment creating a new rheological boundary. This tectonic structure allowed the formation of Alpine crustal-scale thrust systems that eased coupled deformation of the upper and lower crust, leading to limited underthrusting of both crustal layers.

Wide-angle seismic reflection/refraction (WA) surveys provide data that can be modeled to obtain lithospheric-scale P-wave velocity (VP) models. The interpretation of these datasets is often performed as a laborious and time-consuming trial-and-error procedure, in which the relevant model parameters (layer thickness and VP) are manually adjusted until the forward modeling matches the observed travel-times. In this work, we present a fully automatic iterative nonlinear approach to invert WA datasets based on the simulated annealing technique. We test our proposed approach with data from the MARCONI-3 WA profile (southern Bay of Biscay) and compare the outcome with an existing detailed interpretation, discussing the similarities between the two models and the agreement between our model and the observed travel-times., Los estudios de reflexión/refracción sísmica de gran ángulo (WA) proporcionan datos que se pueden invertir para obtener modelos de velocidad de ondas P (VP) a escala litosférica. La interpretación de este tipo de datos se realiza a menudo a través de un laborioso procedimiento de prueba y error que puede requerir mucho tiempo, en el que los parámetros relevantes del modelo (espesor de las capas y VP) se ajustan manualmente hasta que la modelización directa se ajusta a los tiempos de llegada observados. En este trabajo, presentamos un enfoque no lineal iterativo totalmente automático para la inversión de datos de WA basado en la técnica de enfriamiento simulado. Probamos nuestro enfoque propuesto con datos del perfil MARCONI-3 WA (al sur del Golfo de Vizcaya) y comparamos el resultado con una interpretación detallada ya existente, discutiendo las similitudes entre los dos modelos y la concordancia entre nuestro modelo y los tiempos de llegada observados.

MARCONI (MARgen COntinental Nord Iberico - North Iberian Continental Margin) is a deep seismic reflection survey carried out in the southeastern part of the Bay of Biscay in September 2003. It included the acquisition of 11 multichannel (http://dx.doi.org/10.20350/digitalCSIC/8972) and wide-angle deep seismic reflection profiles covering a total length of 2000 km

The high degree of human activities in urban environments produces large background vibrations that makes it difficult to use data acquired in these areas for classical seismology. Seismometers installed within cities have been typically been used for the study of seismic hazard or for monitoring civil engineering problems. However, with the development of monitoring techniques based on the interpretation of the so-called seismic ambient noise, these data have gained scientific interest. Our objective is to discuss an additional utility of seismometers deployed within a city; its use as a tool to connect society with Earth sciences. Many citizen activities, from traffic to music concerts, produce vibrations that can be recorded seismically, and our experience shows that these records attract the attention of the media and social networks. With the emergence of low-cost and easy-to-use instruments in recent years, more citizens can now record ground motion and become interested in the interpretation of the recorded seismograms. The installation of permanent seismic networks in educational centers has proven to be a good approach to introduce students to Earth sciences at the national level and can also be developed at the urban scale using this new instrumentation. In this contribution we will first review the previous results related to the identification of the sources of vibration in urban areas and then present a new ongoing project based on the deployment of a seismic network in educational centers located in the city of Barcelona.

In this study we analyse three wide-angle seismic profiles in the Pyrenees to provide new insights into their crustal structure. Several earthquakes with magnitudes greater than 2.5 that occurred in the vicinity of the analysed profiles have been used as the seismic source. We demonstrate the feasibility of using natural seismicity as the seismic source, which would allow using deployments of seismic stations designed for other objectives. We have analysed a total of three profiles in this study, two of them being located in the eastern Pyrenees (Cataluña) with N-S and E-O directions, and the third in the western Pyrenees (Navarra) with NE-SO orientation. All of them were originally deployed to record teleseisms for receiver function analysis. Profiles in the eastern Pyrenean edge constrain the top of the lower crust at a depth of 20 km. In the easternmost sector of the E-W profile, Moho has been imaged as an eastward rising ramp, with a depth decreasing from 35- 40 km in the middle of the profile to a depth of 25 km in the eastern edge. The Navarra profile allowed us to constrain the crustal structure of the western Pyrenean edge in detail. The obtained model allowed the imaging of the mid-lower crustal indentation between the Iberian and European plates. The European Moho has been constrained at a depth of 30 km in a small region of the central part of the model, whereas the Iberian Moho has been imaged in its northernmost edge dipping northwards to a depth of 45-50 km., En este trabajo se analizan tres perfiles sísmicos de refracción y reflexión de gran ángulo registrados en los Pirineos con el fin de aportar nuevos conocimientos sobre su estructura cortical. Se emplearon como fuente sísmica varios terremotos de magnitud superior a 2.5 ocurridos en las proximidades de las líneas a estudiar. Se demuestra la viabilidad de utilizar sismicidad natural como fuente sísmica para la realización de perfiles de gran ángulo, lo que permite aprovechar despliegues dispuestos con otros objetivos. Se analizarán dos perfiles localizados en el Pirineo oriental (Cataluña) con direcciones N-S y E-O, y un tercero en el Pirineo occidental (Navarra) con dirección NE-SO, desplegados inicialmente para el registro de telesísmos y el análisis de funciones receptoras. Los perfiles adquiridos en el sector oriental del Pirineo permitieron acotar el techo de la corteza inferior a 20 km de profundidad. El perfil E-O muestra, en su extremo más oriental, el Moho como una rampa ascendente hacia el este, pasando de los 35-40 km de profundidad en la zona central del perfil a 25 km en su extremo oriental. El perfil de Navarra permitió acotar con gran detalle la estructura cortical del extremo occidental del Pirineo. El modelo obtenido muestra la indentación de la corteza inferior y media Europea en la corteza Ibérica. El Moho Europeo aparece a 30 km de profundidad en la zona central, mientras que el Moho Ibérico muestra una rampa buzante hacia el norte, llegando hasta a los 45-50 km de profundidad en su extremo más septentrional.

The Spanish Central System is an intraplate mountain range that divides the Iberian Inner Plateau in two sectors - the northern Duero Basin and the Tajo Basin to the south. The topography of the area is highly variable with the Tajo Basin having an average altitude of 450-500 m and the Duero Basin having a higher average altitude of 750-800 m. The Spanish Central System is characterized by a thick-skin pop-up and pop-down configuration formed by the reactivation of Variscan structures during the Alpine orogeny. The high topography is, most probably, the response of a tectonically thickened crust that should be the response to (1) the geometry of the Moho discontinuity, (2) an imbricated crustal architecture, and/or (3) the rheological properties of the lithosphere. Shedding some light on these features is the main target of the current investigation. In this work, we present the lithospheric-scale model across this part of the Iberian Massif. We have used data from the Central Iberian Massif Deformation (CIMDEF) project, which consists of recordings of an almost-linear array of 69 short-period seismic stations, which define a 320 km long transect. We have applied the so-called global-phase seismic interferometry. The technique uses continuous recordings of global earthquakes (ĝ' epicentral distance) to extract global phases and their reverberations within the lithosphere. The processing provides an approximation of the zero-offset reflection response of a single station to a vertical source, sending (near)-vertical seismic energy. Results indeed reveal a clear thickening of the crust below the Central System, resulting, most probably, from an imbrication of the lower crust. Accordingly, the crust-mantle boundary is mapped as a relatively flat interface at approximately 10 s two-way travel time except in the Central System, where this feature de

In recent years the analysis of the variations of seismic background signal recorded in temporal deployments of seismic stations near river channels has proved to be a useful tool to monitor river flow, even for modest discharges. The objective of this work is to apply seismic methods to the characterization of the snowmelt process in the Pyrenees, by developing an innovative approach based on the hierarchical classification of the daily spectrograms. The CANF seismic broad-band station, part of the Geodyn facility in the Laboratorio Subterráneo de Canfranc (LSC), is located in an underground tunnel in the Central Pyrenees, at about 400 m of the Aragón River channel, hence providing an excellent opportunity to explore the possibilities of the seismic monitoring of hydrological events at long term scale. We focus here on the identification and analysis of seismic signals generated by variations in river discharge due to snow melting during a period of six years (2011-2016). During snowmelt episodes, the temporal variations of the discharge at the drainage river result in seismic signals with specific characteristics allowing their discrimination from other sources of background vibrations. We have developed a methodology that use seismic data to monitor the time occurrence and properties of the thawing stages. The proposed method is based on the use of hierarchical clustering techniques to classify the daily seismic spectra according to their similarity. This allows us to discriminate up to four different types of episodes, evidencing changes in the duration and intensity of the melting process which in turn depends on variations in the meteorological and hydrological conditions. The analysis of six years of continuous seismic data from this innovative procedure shows that seismic data can be used to monitor snowmelt on long-term time scale and hence contribute to climate change studies.

In November 2014, a temporary land and marine seismic network was deployed to monitor the drilling of an exploratory well in the Canary Channel (eastern Canary Islands). This region is characterized by low-seismic activity; however, because of the increased awareness of the potential seismic hazard caused by hydrocarbon exploitation activities, the drilling operations were monitored with an unprecedented level of detail for an activity of this kind. According to the reported earthquakes, there was not a measurable increase in seismicity in the vicinity of the well. Overall seismic activity was low, which is consistent with the historical seismicity records. Harmonic tremor, explained here as resonances of the instrument-seafloor system generated by bottom water currents in the area, was commonly detected on the ocean-bottom seismometer (OBS) recordings. The marine network data also revealed dozens of nonseismic short-duration signals per day that appear similar to other events on OBS recordings throughout the world. We suggest that they may be caused by direct perturbations on the OBS, mostly induced by ocean currents in the Canary Channel.

A high-resolution seismic tomography survey was acquired to obtain a full 3-D P-wave seismic velocity image in the Záncara river basin (eastern Spain). The study area consists of lutites and gypsum from a Neogene sedimentary sequence. A regular and dense grid of 676 shots and 1200 receivers was used to image a 500 m×500 m area of the shallow subsurface. A 240-channel system and a seismic source, consisting of an accelerated weight drop, were used in the acquisition. Half a million travel-time picks were inverted to provide the 3-D seismic velocity distribution up to 120 m depth. The project also targeted the geometry of the underground structure with emphasis on defining the lithological contacts but also the presence of cavities and fault or fractures. An extensive drilling campaign provided uniquely tight constraints on the lithology; these included core samples and wireline geophysical measurements. The analysis of the well log data enabled the accurate definition of the lithological boundaries and provided an estimate of the seismic velocity ranges associated with each lithology. The final joint interpreted image reveals a wedge-shaped structure consisting of four different lithological units. This study features the necessary key elements to test the travel time tomographic inversion approach for the high-resolution characterization of the shallow subsurface. In this methodological validation test, travel-time tomography demonstrated to be a powerful tool with a relatively high capacity for imaging in detail the lithological contrasts of evaporitic sequences located at very shallow depths, when integrated with additional geological and geophysical data.

The Iberian Central System represents an outstanding topographic feature in the central Iberian Peninsula. It is an intraplate mountain range formed by igneous and metasedimentary rocks of the Variscan Iberian Massif that has been exhumed since the Eocene in the context of the Alpine orogeny. The Iberian Central System has been conventionally interpreted as a thick-skinned popup mountain range thrust over the Duero and Tajo foreland basins. However, its lithospheric structure and the P-wave velocity distribution are not yet fully resolved. In order to place geophysical constraints on this relevant topographic feature, to identify lithospheric discontinuities, and to unravel the crustal deformation mechanisms, a wide-angle seismic reflection and refraction experiment, CIMDEF (Central Iberian Mechanism of DEFormation), was acquired in 2017 and 2019. It is a NNW-SSE oriented 360-km long profile that runs through the Duero basin, the Iberian Central System and the Tajo basin. First results based on forward modeling by raytracing show an irregularly layered lithosphere and allow to infer the depth extent of the northern Iberian Central System batholith. The crust is ~ 31 km thick under the Duero and Tajo basins and thickens to ~ 39 km under the Iberian Central System. A conspicuous thinning of the lower crust towards the south of the Iberian Central System is also modeled. Along this transect, a continuous and high amplitude upper mantle feature is observed and modeled as the reflection of an interface dipping from 58 to 62 km depth featuring a P-wave velocity contrast of 8.2 to 8.3 km/s. Our preliminary results complement previous models based on global-phase seismic and noise interferometry and gravity data, provide new constraints to validate the accuracy of passive seismic methods at lithospheric scale, and contribute with a resolute P-wave velocity model of the study area to unravel the effect of the Alpine reactivation on the central Iberian Massif.

The SANIMS project is focused into the development and application of methods based on seismic ambient noise to image and monitor natural and human-altered environments focusing in two test sites; the Cerdanya Basin in the eastern Pyrenees, and the city of Barcelona. Broad-band and short period seismometers and a high density node network have been used to acquire new data. Broad-band data has been processed using the frequency-dependent phase cross-correlation and time-scale phase-weighted stacking to extract Rayleigh and Love waves. We have obtained Rayleigh and Love group and phase velocities for periods in the 1.5 ¿ 4 s range, that will be inverted to velocity-depth models. The preliminary results show higher velocities to the North, with a well-defined zone with lower than average velocities around the Cerdanya Basin. The geometry of the basin basement has also been investigated using the amplitudes of ambient noise, HVSR methods and RFs, obtained consistent results. The recently acquired high density data has already been processed in terms of amplitude variations and will be integrated in the tomographic images. The data acquired in Barcelona has first been used to monitor the human activity during the COVID19 pandemic. Amplitude variations of seismic noise allow to delineate the main geological units of the Barcelona area. HVSR measures using the new data expand the already available results, hence improving the existing seismic hazard maps, and will allow to analyse eventual temporal variations in the measurements. As in the Cerdanya Basin, the data will be used to extract Rayleigh waves and invert for velocities. Both datasets will also be used to analyse the applicability of the methods based on Rayleigh wave ellipticity inversion of ambient noise and earthquake data to provide S-velocity depth profiles. We expect that the use of ambient noise methods will allow to map the basement and to obtain new higher resolution ambient noise tomographic images of th

The Variscan belt was formed as a consequence of the collision of two major continents, Laurasia and Gondwana, in the late Paleozoic. Nowadays, it constitutes the basement of the Iberian peninsula (Iberian Massif) and a large part of western and central Europe. In the NW of Spain, the convergence between Iberia and Europe in the Cenozoic originated the uplift of the Cantabrian mountains (CM). In its central sector, the erosion of the Mesozoic sedimentary cover during orogenesis led to the exhumation of the underlying Variscan basement in their western sector. The section of the Variscan belt that is currently exposed in the CM illustrates the transition from the internal zones of an orogen, in the west, to the external ones, to the east. In order to acquire new passive data from this region, a portable seismic network consisting of 13 three-component broadband stations was deployed (GEOCANTÁBRICA-COSTA, doi:10.7914/SN/YR_2019). The recorded ambient noise seismic signal was cross-correlated using the phase cross-correlation (PCC) processing technique and the resulting daily cross-correlograms were stacked to obtain the empirical Green¿s function of the medium between each station pair. Since the vertical and the rotated horizontal components were processed, Rayleigh- and Love-wave group velocity dispersion curves were extracted. From these measurements, group velocity tomographic maps at periods between 2 ¿ 14 s were calculated. Based on this set of tomographic maps, a final 3D S-wave velocity model (2 - 12 km) was derived from the joint inversion of the pseudo-dispersion curves created by extracting the Rayleigh and Love velocity values for each point of a dense grid. Both the surface-wave and the S-wave velocity maps highlight essential elements of the surface geology of the area. The velocity pattern shows the boundary between two main geological domains: The Cantabrian Zone (CZ), to the east, which constitutes the foreland fold and thrust belt of the Variscan oro

The objective of this contribution is to document the effects of magnetic field variations of natural and anthropogenic origin on broad-band seismic stations. Regarding natural sources of magnetic perturbations, we have shown that the Sudden Storm Commencements (SSC) catalogued during the 24th solar cycle (2008-2019) with a mean amplitude above 30 nT are systematically identified in broad-band seismic stations distributed worldwide using a simple low-pass filter. Although the initial impulse of those signals is usually stronger at stations located at high latitudes, major SSC are observed at seismic stations distributed worldwide. Regarding anthropogenic sources, we show that low period seismic signals recorded in urban environments are correlated with the activity of the railway transportation system. Collocated measurements of electric field and seismic signals within Barcelona prove that these signals are due to the electrical leakage currents transmitted to the soil by trains. During space weather events, electric currents in the magnetosphere and ionosphere experience large variations inducing telluric currents near the Earth surface, which in turn generate a secondary magnetic field. In the case of underground trains, leakage currents are transmitted to the soil, which in turn can result in local variations in the magnetic field. The observed signals in modern seismometers can be related to the reaction of the suspension springs to these magnetic field variations or to the effect of the magnetic field variations on the force transducers used to keep the mass fixed.

La estructura cortical del Sistema Central Ibérico se ha estudiado mediante técnicas de sísmicas pasivas. Hemos aplicado interferometría sísmica a un conjunto de datos pasivos a través de dos técnicas para generar una corte a escala litosférica del orógeno. Las imágenes resultantes constriñen la estructura cortical de la cordillera y la de sus cuencas sedimentarias adyacentes. Ambas técnicas presentan resultados similares. Las imágenes obtenidas revelan que el límite entre la corteza y el manto es relativamente plano, con tiempos de llegada de aproximadamente de 10 s en tiempo doble, excepto bajo el Sistema Central, donde los reflectores están ubicados a 12 s en tiempo doble. Esto define un claro engrosamiento de la corteza debajo del Sistema Central, como resultado, muy probablemente, de una imbricación de la misma. El límite entre la corteza superior e inferior se encuentra a 5 s en tiempo doble y es también ligeramente más profundo debajo de la cordillera., The crustal structure of the Iberian Central System has been studied using passive seismic techniques. We have applied seismic interferometry to a passive dataset through two techniques to generate a lithospheric-scale cross-section of the orogen. The resulting images constrain the structure of the crust below the mountain range and that of its surrounding sedimentary basins. Both techniques provide complementary results. The resulting images reveal that the crust–mantle boundary is relatively flat, with arrival times of approximately 10 s two-way travel time except in the Central System, where reflectors are located at 12 s two-way travel time. This indicates a clear thickening of the crust below the core of the orogen, resulting, most probably, from an imbrication of the crust. The boundary between the upper and lower crust is found at 5 s two-way travel and it is also slightly deeper below the mountain range.

La estación sísmica CANF, integrada en el proyecto Geodyn del Laboratorio Subterráneo Canfranc (LSC), se localiza en la zona central de los Pirineos, en un emplazamiento subterráneo cercano al lecho del río Aragón, por lo que ofrece la oportunidad de verificar la viabilidad del uso de datos sísmicos para monitorizar eventos hidrológicos. En esta contribución nos centramos en la identificación de señales sísmicas generadas por variaciones en la descarga del río debidas al deshielo, a lo largo de un período de seis años (2011-2016). Durante los episodios de deshielo, las variaciones en la descarga en el río de drenaje generan señales sísmicas con características específicas que permiten su discriminación de otras fuentes de vibraciones. Hemos desarrollado una metodología basada en la clasificación jerárquica de los espectros sísmicos para identificar los días en los que se produce el deshielo y agruparlos en función de la intensidad de la descarga, identificando hasta cuatro tipos diferentes de episodios. Estos resultados demuestran que los datos sísmicos pueden proporcionar una buena herramienta para monitorizar los procesos de deshielo a largo plazo, contribuyendo así a los estudios sobre el cambio climático., The CANF seismic broad-band station, part of the Geodyn facility in the Laboratorio Subterráneo de Canfranc (LSC), is located in an underground site in the Central Pyrenees, close to the Aragón River channel, and hence offers a good opportunity to study the long-term performance of the seismic monitoring of hydrological events. We focus here on the identification of seismic signals generated by variations in river discharge due to snow melting over six years (2011-2016). During snowmelt episodes, the temporal variations of the discharge in the drainage river result in characteristic seismic signals that can be discriminated from other sources of background vibrations. We have developed a methodology based on the analysis of the seismic spectra in order to identify days with active thaw, hence allowing to monitor the time occurrence and intensity of the snowmelting stages. A hierarchical classification of the seismic spectra has allowed us to group active thaw days according to the intensity of the discharge generated by the melting process, identifying up to four different types of episodes. These results prove that seismic data can provide a good tool to monitor snowmelt on a long-term scale, hence contributing to climate change studies.

El Macizo Ibérico, en la parte occidental de la Península Ibérica, constituye una sección prácticamente completa del orogéno Varisco. Desde los años 90 este macizo ha sido objeto de diferentes estudios geofísicos para conocer la estructura a escala cortical. No obstante, en su parte central ¿ la Zona Centro Ibérica ¿ todavía no existían datos de sísmica profunda. Con el fin de aportar información geofísica en esta zona, en 2017 y 2019 se llevó a cabo el experimento de sísmica de reflexión y refracción de gran ángulo, CIMDEF (Central Iberian Mechanism of DEFormation). Este proyecto consiste en tres perfiles de orientación NNO-SSE y E-O de entre 130 y 330 km de longitud que atraviesan la cuenca del Duero, el Sistema Central y la cuenca del Tajo, lo que nos ha permitido construir un nuevo modelo de velocidades de ondas P en este sector de la Zona Centro Ibérica. Los resultados complementarán los modelos previos basados en interferometría sísmica de fases globales y sísmica de ruido ambiente (Andrés et al., 2019, 2020), aportando nuevos datos para validar la precisión de métodos sísmicos pasivos a escala litosférica en el orógeno Varisco., The Iberian Massif, in the western part of the Iberian Peninsula, constitutes a nearly complete section of the Variscan orogen. Since the 1990s it has been target of different geophysical studies to unravel the crustal structure. However, there was a remarkable lack of deep seismic sounding data in its central part – the Central Iberian Zone. To fill this gap, a wide-angle seismic reflection and refraction experiment, CIMDEF (Central Iberian Mechanism of DEFormation), was acquired in 2017 and 2019. CIMDEF consists of three NNW-SSE and E-W oriented profiles ranging between 130 and 330 km in length. These profiles run through the Duero basin, the Central System and the Tajo basin, allowing us to construct a new P-wave velocity model in this sector of the Central Iberian Zone. Furthermore, it will help to complement previous models in this area based on Global-Phase Seismic Interferometry and ambient seismic noise (Andrés et al., 2019, 2020), thus providing new constraints to validate the accuracy of passive seismic methods at lithospheric scale in the Variscan orogen.

The crustal structure of the Iberian Central System has been studied using passive seismic techniques. We have applied seismic interferometry to a passive dataset through two techniques to generate a lithospheric-scale cross-section of the orogen. The resulting images constrain the structure of the crust below the mountain range and that of its surrounding sedimentary basins. Both techniques provide complementary results. The resulting images reveal that the crust¿mantle boundary is relatively flat, with arrival times of approximately 10 s two-way travel time except in the Central System, where reflectors are located at 12 s two-way travel time. This indicates a clear thickening of the crust below the core of the orogen, resulting, most probably, from an imbrication of the crust. The boundary between the upper and lower crust is found at 5 s two-way travel and it is also slightly deeper below the mountain range.

El proyecto SANIMS se centra en el desarrollo y la aplicación de métodos basados en el análisis del ruido sísmico ambiente para monitorizar entornos naturales y zonas fuertemente alteradas por actividad humana. Con respecto al entorno natural, se está investigando la cuenca de la Cerdanya, situada en la zona oriental de los Pirineo, mediante el uso de una red de estaciones sísmicas de banda ancha, operativa desde septiembre de 2019. En el otoño de 2020, el despliegue de una red de nodos de alta densidad durante dos meses completará el conjunto de datos. Por lo que se refiere al estudio de áreas fuertemente alteradas por la actividad humana, hemos desplegado una red de 15 estaciones sísmicas de período corto dentro de la ciudad de Barcelona, en la mayoría de los casos en institutos y escuelas de enseñanza secundaria. El objetivo de este despliegue es doble; adquirir nuevos datos de interés científico e introducir a los estudiantes en un proyecto de investigación de Ciencias de la Tierra., The SANIMS project is focused into the development and application of methods based on seismic ambient noise to image and monitor natural and human-altered environments. Regarding natural environment, the Cerdanya basin in the eastern Pyrenees is being explored using a broad-band seismic station network, operative since September 2019. A two months long deployment of a high density node network in Fall 2020 will complete the dataset. Regarding areas strongly altered by human activity, we have deployed a network of 15 short period stations within the city of Barcelona, in most of the cases installed in secondary schools. The objective of this deployment is twofold; acquire new valuable scientific data and introduce the students in a Earth Science research project.

Two passive seismic profiles have been acquired in the eastern Pyrenees as part of the Pyrope and Orogen projects to investigate the crustal structure differences between this area and the central and western Pyrenees. Up to 28 broad-band stations were deployed along two orthogonal lines, with an interstation spacing close to 10 km. High frequency receiver functions allowed us to obtain the main lithospheric interfaces along those lines. The NNE-SSW profile shows a well-defined Moho beneath Iberia, slightly deepening northwards. Beneath the Axial zone the Moho appears to be segmented but does not show evidence of crustal imbrication. Further North, the Moho appears again as a continuous interface located around 30 km depth. This image clearly differs from the conspicuous imbrication between the Iberian and Eurasian crusts observed westward. The E-W profile shows a smooth Moho thinning from a 40 km depth to the west of the profile to 23 km close to the coastline, evidencing the crustal thinning related to the Neogene extensional processes. Additional constraints on the geometry of the crust/mantle boundary in the Eastern Pyrenees are obtained from local moderate magnitude earthquakes recorded along the seismic lines during the experiment. In particular, a fan profile built from an event located near the Mediterranean coast suggests that crustal thickness differences between Iberian and Eurasian crusts can still be recognized 30 km westward of the Mediterranean coast, in an area that seems to mark the limit between regions dominated by compressive and extensive processes. We propose a model in which the observed dissimilar Moho structure beneath the Eastern Pyrenees is interpreted as the result of a different Mesozoic pre-shortening margin configuration. Seismic results are consistent with recently proposed tectonic models including an intermediate continental block separating Iberia and Europe by two basins with extremely thin crust or exhumed mantle.

Seismic reflection data (normal incidence and wide-angle) are unique assets for Solid Earth Science. They provide critical information about the physical properties and structure of the lithosphere, and are useful to understand the geometry and evolution of the tectonic plates, and for exploration of natural resources, civil engineering, characterization of seismogenic zones and hazard assessment. The resolution of seismic reflection data is highly appreciated for basic and applied Earth Sciences. However, these datasets are logistically complex and expensive to acquire, and their geographical coverage is limited. In addition, legacy seismic reflection data also have an added value as new information can be retrieved by applying new processing approaches. The preservation and dissemination of seismic open access data is an asset to promote accurate and innovative research. Here, we present the Seismic DAta REpository (SeisDARE), which is, to our knowledge, one of the first comprehensive open access online databases that stores seismic data registered with a permanent digital object identifier (DOI). The datasets included here are accessible online and guarantee the FAIR (Findable, Accessible, Interoperable, Reusable) principles of data management, so that each dataset enters into a statistics referencing database and its impact can be measured. This database has been built thanks to a network of several institutions, promoting a multidisciplinary research, and is open for international data exchange and collaborations. SeisDARE includes deep seismic sounding and high resolution data acquired in the last three decades in the Iberian Peninsula and Morocco. In addition, as result of fruitful collaborations, we are starting to host data acquired worldwide. Our first incorporation of this kind are seismic profiles recorded in Hardeman County, Texas, within the COCORP project. SeisDARE aims to make easily accessible legacy and recently acquired seismic data and establish

The Iberian Massif is one of the most complete sections of the Variscan orogen and crops out in the western part of the Iberian Peninsula. Since the early 90s it has been the target of a number of geophysical studies to characterize its lithospheric structure. The internal architecture of the Iberian microplate presents critical constraints on the way the Iberian lithosphere responds to the imposed stress regime. This is a compressive tectonic setting resulted from the collision between the Iberian and the European plates. This stress regime is most probably responsible for the deformation patterns that is reflected by the topographic relief of the Iberian Peninsula with few intraplate orogens. The Iberian Central System constitutes a major intraplate orogenic belt conventionally interpreted as a pop-up structure. In order to place geophysical constraints on this relevant topographic feature, crustal structure and deformation style a wide-angle seismic reflection and refraction experiment, CIMDEF (Central Iberian Mechanism of DEFormation), was acquired in 2017 and 2019. It consisted of three NNW-SSE and E-W oriented profiles ranging between 130 and 330 km in length. These profiles run through the Duero basin, the Iberian Central System and the Tajo basin, allowing us to construct a P-wave velocity model in this sector of the Central Iberian Zone. First results based on forward modeling by raytracing show a crust of ~ 32 km thick under the Duero and Tajo basins that thickens to ~ 39 km under the Iberian Central System. A reduction of the lower crust thickness towards the south of the Iberian Central System is also modeled. At far offsets (> 190 km), an upper mantle reflection is observed, modeled as an interface at 58 to 62 km depth with a P-wave velocity contrast of 8.2 to 8.3 km/s. Our preliminary results complement previous models in this area based on Global-Phase Seismic Interferometry and gravity data; provide new constraints to validate the accuracy of passive

This work explores the first results of the seismic data acquired in the Sotiel-Coronada mine as part of the SIT4ME (Seismic Imaging Techniques for Mineral Exploration) project. In this experiment, a multi-source seismic data-set was acquired at the end of 2018 in the Sotiel- Coronada area of the Iberian Pyrite Belt (IPB) (southwest Spain). In the seismic experiment presented, 653 seismic receivers were deployed distributed in a pseudo 3D grid and six crooked lines across the study area. The sensors recorded c. 900 vibration points produced by a 32 Tn vibrotruck. Here, we present the stack results of the 2D seismic sections. Data from over 100 wells have been incorporating to aid in the structural interpretation. The preliminary interpretation reveals the complexity of this highly faulted ore-bearing area. Correlations between well log data information and 2D seismic profiles, suggest the location of a potentially mineralized area. The SIT4ME project has been funded by EIT Raw Materials (17024).

Seismicity in the Eastern Pyrenees has shown moderate and diffuse over time. Although, there is evidence of catastrophic historical earthquakes and some of the events in the last decade that have caused considerable damage. This work shows the feasibility to obtain seismic data of best quality in the temporary seismic network installed on Cerdanya Neogene basin. The network has recorded local events that are added to the permanent seismic networks of the ICGC, IGN and RéNaSS distributed region, for the period between December of 2019 and June 2020. Seismic data processing has been performed using the Seiscomp3 seismological software package. A cortical velocity model has been created to improve the accuracy of the location, adjusted to the geology of the Cerdanya basin and the region around it. The catalogue contains 641 seismic events and 251 quarry blasts. The results of the study show that the locations of earthquakes have higher resolution (with vertical errors as low 1 km) in the Cerdanya when including the temporary network as a result of a denser network in this area. Also, it shows that seismicity of Eastern Pyrenees is concentrated in the north and easternmore areas. The recorded data has allowed to discuss the seismotectonic of the Cerdanya, Selva and Fenollet areas, where 202, 158 and 54 earthquakes have been identified, respectively. The hypocentres are distributed in depth from to surface to a maximum of 16 km. In particular, two areas show lineaments that could be related to individual structures: At Meranges (Cerdanya), the events could be grouped in two to three fault planes (dipping NW and SE) perpendicular to Cerdanya fault. In Santa Coloma, (Selva), the network has recorded a seismic cluster (main event 4,2 magnitude) that seems to follow a SE dipping structure. As showed in this study, seismic data in larger periods of time using temporary network could be key for improving the seismotectonic knowledge in some target seismic regions, benefiting t

The objective of this contribution is to get new insights into the effects of magnetic field variations of natural and anthropogenic origin on broad-band seismic stations. Regarding natural sources of magnetic perturbations, we have investigated if the Sudden Storm Commencements (SSC) cataloged during the 24th solar cycle (2008–2019) can be systematically identified in broad-band seismic stations distributed worldwide. The results show that the 23 SSC events with a mean amplitude above 30 nT and most of those with lower energy but still clearly identified in the magnetometer detection network can be observed at broad-band stations’ network using a simple low-pass filter. Although the preliminary impulse of those signals is usually stronger at stations located at high latitudes, major SSC are observed at seismic stations distributed worldwide. Regarding anthropogenic sources, we focus on the short period seismic signals recorded in urban environments which are correlated with the activity of the railway transportation system. We have analyzed collocated measurements of electric field and seismic signals within Barcelona, evidencing that significant changes in the electric field following the activity of the transportation systems can be attributed to leakage currents transmitted to the soil by trains. During space weather events, electric currents in the magnetosphere and ionosphere experience large variations inducing telluric currents near the Earth surface, which in turn generate a secondary magnetic field. In the case of underground trains, leakage currents are transmitted to the soil, which in turn can result in local variations in the magnetic field. The observed signals in modern seismometers can be related to the reaction of the suspension springs to these magnetic field variations or to the effect of the magnetic field variations on the force transducers used to keep the mass fixed.

In this work, we present lithospheric-scale model of across the Iberian Massif in western Iberia. The image comprises the Spanish Central System mountain range and, the Cenozoic Duero and Tajo basins which bound it to the N and S, respectively. The target area represents one of the most characteristics topographic features of central Iberia; the Tajo Basin has an average altitude of 450-500 m while, the Duero Basin presents higher average altitude 750-800 m. The Central System represents part of the Variscan Central Iberian Zone (southern margin of the Paleozoic Gondwana) which was reactivated and uplifted during the Alpine Orogeny. This intraplate orogen features a thick-skin pop-up and pop-down configuration formed by the reactivation of Variscan structures. Its high topography is the response of a tectonically thickened crust evidenced by 1) the geometry of the Moho discontinuity 2) an imbricated crustal architecture and/or 3) the rheological properties of the lithosphere. A research project (CIMDEF) was carried out in the area and, it includes an almost 330-km long multi-seismic profile acquired using natural and controlled source seismic data. The former consists of two months of continuous recording by an almost-linear array of 69 short-period seismic stations, which recorded earthquakes and, ambient seismic noise. The controlled source data included over 900 stations and a series of acoustic sources. The data places strong constraints on the geometry of the base of the crust. The research effort includes: seismic interferometry (using seismic noise recordings), wide-angle analysis of the controlled source data and, Global-Phase seismic imaging. The latter uses PKP, PKiKP, and PKIKP phases of earthquakes at epicentral distances > 120° and, their reverberations in the lithosphere. The selected phases are autocorrelated and stacked to construct a high-resolution pseudo zero-offset reflection image. Seismic noise interferometry uses the autocorrelation of seismic

The general objectives of the ¿Seismic Ambient Noise Imaging and Monitoring of Shallow Structures¿ (SANIMS) project, funded by the Spanish Ministry of Science, Research and Innovation (Ref.: RTI2018-095594-B-I00), are focused into the application and development of methods based on ambient noise seismic data recorded by dense networks to image and monitor natural and human-altered environments. To achieve this objective, temporal seismic networks have been installed since late 2019 in two very different settings; the Cerdanya Basin, a sedimentary basin located in the eastern Pyrenees and the city of Barcelona. Regarding the Cerdanya Basin, a relatively unaltered setting, a network of up to 25 broad-band stations has been installed for a period of one year. Additionally, a high resolution grid of seismic nodes will be deployed for 2 months in the central part of the basin, with interstation distances of 1.5 km. In order to constraint the uppermost crustal structure using ambient noise, vertical component recordings will be processed using the phase cross-correlation and time-frequency domain phase-weighted stacking to extract fundamental mode Rayleigh waves. The surface waves will then be used to measure inter-station group and phase velocity dispersion curves that will be inverted using the Fast Marching Surface Tomography method. Depending on data quality, we will also process the horizontal components to extract Love waves for joint inversions with Rayleigh waves to constrain radial anisotropy and/or the application of new strategies to perform attenuation tomography. Regarding areas strongly altered by human activity, we have deployed a network of 15 short-period stations within the city of Barcelona, in most of the cases installed in the basement of secondary schools, for a duration of 9-12 months. The objective of this deployment is twofold; acquire new valuable scientific data and introduce the students in an Earth Science research project. Although the Barcel

Horizontal-to-Vertical Spectral Ratios (HVSR) and Rayleigh group velocity dispersion curves (DC) can be used to estimate the shallow S-wave velocity (Vs) structure. Knowing the shallow Vs structure is important for geophysical data interpretation either in order to better constrain data inversions for P-wave velocity (Vp) structures such as travel time tomography or full waveform inversions, or to directly study the Vs structure for geo-engineering purposes (e.g. ground motion prediction). The purpose of this study is to appraise in particular how much information HVSR can add in a large N experiment and how different instrumentation types affect this. During the Maupasacq large-scale experiment, 197 three-component short-period stations, 190 geophone nodes and 54 broadband seismometers were continuously operated in Southern France for 6 months (April to October 2017) covering an area of approximately 1500 km2 with a site spacing of approximately 1 to 3 km. On the obtained HVSR and DC data, a statistical Joint inversion is performed for the shallow Vs structure. The results indicate that the addition of HVSR data to the DC inversion reduces the variance of the recovered shallow Vs model and improves the convergence to a smaller data misfit. While broadband and short period instruments delivered similar results, geophone nodes performed significantly worse due to their much higher cut off frequency.