Search

Recording of a volcanic earthquake in Hawaii on 11/07/2018 at 15:45 UTC (Mw = 5.3) at the Résif-RLBP station in Manonville in a configuration with a near-surface sensor installed for the AlpArray project (high-station Z3.A210A) and a sensor in the shallow borehole of the permanent station (middle - station FR.MANO). For comparison the bottom trace present at the recording at the Geoscope station in Echery (G.ECH station). The Permanent Broadband Network (RLBP) was officially accredited as a CNRS-Insu National Observation Service in 2007. It involves eight Observatories of the Universe Sciences (OSU) as well as the Analysis, Monitoring and Environment Department (DASE) of the CEA, and is coordinated by the School and Earth Sciences Observatory in Strasbourg. The need to develop the RLBP was partly at the origin of the establishment of the Résif Research Infrastructure (French Seismological and Geodetic Network), of which it is now a specific action.Comprising 17 stations at its creation, the RLBP's ambition is to build a network of around 190 stations covering the entire country by 2022, but with increased density in the most seismic areas. This density of stations will significantly improve our knowledge of the deep structure, as well as our estimate of seismic hazard in France, by reducing the magnitude of completeness to less than 2 throughout the territory. The objective is also to be able to use this network as a real seismological antenna for long periods of time, in particular through its integration into the European virtual network of the EPOS infrastructure. In addition, the RLBP aims to provide very high quality data. This involves choosing isolated sites to limit seismic noise generated by human activity and careful installation of seismometers to mitigate the influence of environmental parameters (temperature, pressure, wind). It is also about putting in place different tools and procedures to provide the most complete possible time series and correct and continuously updated metadata.The French Seismological and Geodetic Network Résif is a national research infrastructure dedicated to the observation and understanding of the structure and dynamics of the Internal Earth. Résif is based on high-tech observation networks, composed of seismological, geodetic and gravimetric instruments deployed in a dense manner throughout France. The data collected make it possible to study with high spatial and temporal resolution the deformation of the ground, surface and deep structures, seismicity on a local and global scale and natural hazards, and more particularly seismic events, on French territory. Résif is integrated into European (EPOS - European Plate Observing System) and global systems of instruments for imaging the Earth's interior as a whole and studying many natural phenomena., Enregistrement d’un séisme volcanique à Hawaï le 11/07/2018 à 15h45 UTC (Mw = 5.3) à la station Résif-RLBP de Manonville dans une configuration avec un capteur en proche surface installé pour le projet AlpArray (haut– station Z3.A210A) et un capteur dans le forage peu profond de la station permanente (milieu – station FR.MANO). Pour comparaison la trace du bas présente à l’enregistrement à la station Geoscope d’Echery (station G.ECH). Le Réseau Large Bande Permanent (RLBP) a été officiellement labellisé comme Service National d’Observation du CNRS-Inwu en 2007. Il implique huit Observatoires des Sciences de l’Univers (OSU) ainsi que le Département Analyse, Surveillance, Environnement (DASE) du CEA, et sa coordination est assurée par L’Ecole et Observatoire des Sciences de la Terre à Strasbourg. La nécessité de développer le RLBP a été en partie à l’origine de la mise en place de l’Infrastructure de Recherche Résif (Réseau sismologique et géodésique français) dont il constitue aujourd’hui une action spécifique.Constitué de 17 stations à sa création, l’ambition du RLBP est de bâtir à l’horizon 2022 un réseau d'environ 190 stations couvrant l’ensemble du territoire, mais avec une densité accrue dans les zones les plus sismiques. Cette densité de stations permettra d’améliorer significativement notre connaissance de la structure profonde, ainsi que notre estimation de l’aléa sismique en France, en abaissant la magnitude de complétude à moins de 2 sur tout le territoire. L’objectif est également de pouvoir utiliser ce réseau comme une véritable antenne sismologique à longues périodes, en particulier via son intégration dans le réseau virtuel européen de l’infrastructure EPOS. De plus, le RLBP vise à proposer des données de très grande qualité. Cela implique de choisir des sites isolés, pour limiter le bruit sismique généré par l’activité humaine, et de soigner l’installation des sismomètres pour atténuer l’influence des paramètres environnementaux (température, pression, vent). Il s’agit également de mettre en place différents outils et procédures permettant de fournir des séries temporelles les plus complètes possibles et des métadonnées correctes et continuellement à jour.Le Réseau sismologique et géodésique français Résif est une infrastructure de recherche nationale dédiée à l’observation et la compréhension de la structure et de la dynamique Terre interne. Résif se base sur des réseaux d’observation de haut niveau technologique, composés d’instruments sismologiques, géodésiques et gravimétriques déployés de manière dense sur tout le territoire français. Les données recueillies permettent d’étudier avec une haute résolution spatio-temporelle la déformation du sol, les structures superficielles et profondes, la sismicité à l’échelle locale et globale et les aléas naturels, et plus particulièrement sismiques, sur le territoire français. Résif s’intègre aux dispositifs européens (EPOS - European Plate Observing System) et mondiaux d’instruments permettant d’imager l’intérieur de la Terre dans sa globalité et d’étudier de nombreux phénomènes naturels.

The "Ears on the Ground" series follows the progress of a major scientific project, Résif, which mobilizes a large number of skills and knowledge to improve our understanding of the soil. The current French seismological network gives only a partial picture of the seismic activity of our territory, both in terms of coverage and the type of detectable activity. To be able to image the earth and accurately measure ground deformations, a consortium of French research organizations and institutions implement the Résif Research Infrastructure, the French SIsmological and Geodetic Network. A team from the École et Observatoire des Sciences de la Terre in Strasbourg is strongly involved in the implementation of the core of this equipment: the permanent seismological antenna in metropolitan France, which will provide France with one of the most homogeneous and dense seismological networks in the world. The series "Ears on the Ground" explores the scientific issues and the technical dimension of this project. As examples, different aspects of network building are informed by the men and women directly involved. They share their approach and the choices they must make when it comes to combining the requirement for scientific precision with the constraints of the field. The series was produced as part of the Oscahr platform, a collaborative and experimental website of the Jardin des sciences of the University of Strasbourg., La série "Des oreilles au sol" suit le déroulement d’un projet scientifique d’envergure, Résif, qui mobilise un grand nombre de savoir-faire et de connaissances, pour améliorer notre compréhension du sol. Le réseau sismologique français actuel ne donne qu’une image partielle de l’activité sismique de notre territoire, tant au niveau de la couverture que du type d’activité détectable. Pour être capable d’imager la terre et de mesurer précisément les déformations du sol, un consortium d'organismes et d'établissements de recherche français mettent en œuvre l’Infrastructure de Recherche Résif, le REseau SIsmologique et géodésique Français. Une équipe de l’École et Observatoire des Sciences de la Terre, à Strasbourg, est fortement impliquée dans la mise en œuvre du cœur de cet équipement : l’antenne sismologique permanente métropolitaine, qui permettra à la France de se doter de l’un des plus homogène et dense réseau sismologique dans le monde. La série "Des oreilles au sol" explore les enjeux scientifiques et la dimension technique de ce projet. Exemples à l’appui, différents volets de la constitution du réseau sont éclairés par les hommes et les femmes directement impliqués. Ils partagent leur démarche ainsi que les choix qu’ils doivent opérer quand il s’agit d’allier l’exigence de précision scientifique aux contraintes du terrain. La série a été réalisée dans le cadre de la plateforme Oscahr, un site internet collaboratif et expérimental du Jardin des sciences de l'Université de Strasbourg.

International audience; In most geologic environments, accounting for anisotropy is necessary to perform acoustic full waveform inversion (FWI) of wide-azimuth and wide-aperture seismic data because of the potential dependence of wave speeds on the direction of the wave propagation. In the framework of multiparameter FWI, the subsurface parameterization controls the influence of the different parameter classes on the modeled seismic data as a function of the scattering angle and hence the resolution with which the parameters can be reconstructed and the potential trade-off between different parameters. We have evaluated a numerical procedure based on computation of the scattering patterns of the different parameters to assess the sensitivity of the seismic data to different parameterizations of vertical transverse isotropic media in the acoustic approximation. Among the different categories we have tested, a monoparametric FWI was found for imaging one wave speed with a broad wavenumber content, keeping the Thomsen parameters fixed, which have a small influence on the data relative to the wave speed. This raises the question of the initial information required in the background models of the Thomsen parameters to perform reliable monoparameter FWI. Alternatively, simultaneously inverting the horizontal and vertical wave speeds introduces limited trade-off effects because these wave speeds have significant influence on the data for distinct ranges of scattering angles, while the influence of the Thomsen parameter δ remains weak. With such parameterization, the short-to-intermediate wavelengths of the vertical velocity are updated from the short-to-intermediate scattering angles, while the long-to-intermediate wavelengths of the horizontal velocity are updated from the wide-to-intermediate scattering angles. We concluded that the choice of the subsurface parameterization can be driven by the acquisition geometry, which controls the scattering-angle coverage and hence the resolving power of FWI, and by the accuracy of the available initial FWI models. Read More: http://library.seg.org/doi/abs/10.1190/geo2012-0204.1

As part of the modernization of the OSUG/ISTerre seismological instrumental network, we have evaluated the benefit of the TANK by comparing the data recorded before and after the installation of this barometric/thermal insulation. This comparison has been achieved for 5 different stations, over at least one year of data. The period of observation without and with the insulation are different for each site. This poster presents some relevant results., Dans le cadre de la modernisation du réseau d'instruments sismologiques OSUG/ISTerre , nous avons évalué l'intérêt du TANK en comparant les données enregistrées avant et après la pose de cette isolation barométrique et thermique. Cette comparaison a été réalisée pour 5 stations différentes, sur au moins une année de données. La période d'observation sans et avec l'isolation est différente pour chaque site. Ce poster présente quelques résultats pertinents.

International audience; Here we assess the potential of the visco-acoustic frequency domain full-waveform inversion (FWI) to reconstruct P-wave velocity (VP) and P-wave attenuation factor (Q) from surface onshore seismic data. First, we perform a sensitivity analysis of the FWI based upon a grid search analysis of the misfit function and several synthetic FWI examples using velocity and Q models of increasing complexity. Subsequently, we applied both the acoustic and visco-acoustic FWI to real surface wide-aperture onshore seismic data from the Polish Basin, where a strong attenuation of the seismic data is observed. The sensitivity analysis of the visco-acoustic FWI suggests that the FWI can jointly reconstruct the velocity and the attenuation factor if the signature of the attenuation is sufficiently strong in the data. A synthetic example corresponding to a homogeneous background model with an inclusion shows a reliable reconstruction of VP and Q in the inclusion, when Q is as small as 90 and 50 in the background model and in the inclusion, respectively. A first application of acoustic FWI to real data shows that a heuristic normalization of the data with offset allows us to compensate for the effect of the attenuation in the data and reconstruct a reliable velocity model. Alternatively, we show that visco-acoustic FWI allows us to reconstruct jointly both a reliable velocity model and a Q model from the true-amplitude data. We propose a pragmatical approach based upon seismic modelling and source wavelet estimation to infer the best starting homogeneous Q model for visco-acoustic FWI. We find the source wavelet estimation quite sensitive to the quality of the velocity and attenuation models used for the estimation. For example, source-to-source wavelets are significantly more consistent when computed in the final FWI model than in the initial one. A good kinematic and amplitude match between the early-arriving phases of the real and time-domain synthetic seismograms computed in the final FWI model provides an additional evidence of the reliability of the final FWI model. We find the recovered velocity and attenuation models consistent with the expected lithology and stratigraphy in the study area. We link high-attenuation zones with the increased clay content and the presence of the mineralized fluids.

We propose a procedure for the detection of converted phases in seismograms between the P- and S-arrivals. This procedure has been successfully applied to waveforms of different stations from the 1991 passive experiment around the Aigion area (Greece). Three major converted phases have been detected at two stations on the northern Gulf of Corinth. The first phase has been interpreted as a P-to-S transmitted wave occurring at an interface located at a depth of 3 km in the northern part of the Psaromita Peninsula. The two later phases that have a more complex polarisation might come from deeper interfaces. To cite this article: D. Latorre et al., C. R. Geoscience 336 (2004).

A 250 km long NE-SW lithospheric transect spanning the 40 km wide island of La Réunion and its submarine edifice is derived from lines of air gun shots at sea on either side, along the assumed hotspot trace. Seismic records were obtained from an array spanning the whole transect and including sea bottom and land receivers, providing a system of reversed and overlapping observations. Low seismic velocity, and hence density, is found on average for the whole edifice above the oceanic plate. We attribute high-velocity anomalies within the edifice to an intrusive core confined under the central northern quarter of the island-crossing segment. Unexpectedly, the main seismic interfaces, top and bottom of the prevolcanic crust, do not show significant flexural downwarping under the island. In addition, clear multipathing in the recorded wave field requires the presence of a body with a seismic velocity intermediate between the prevolcanic crustal material and the normal mantle. This lithospheric structure provides the first example where underplating occurs beneath an active volcanic island, suggesting a genetic relationship. The underplated body could represent residues of the evolution of primary picritic melts that yield erupted basalts. Evidence for reflectors deeper in the lithosphere may indicate further related heterogeneity. In the plate/hotspot model commonly assumed, the structural variation along the transect could be interpreted as a variation with time of the amount and physical state of underplated material. Copyright 1999 by the American Geophysical Union., Support by the authors’ institutions and by the European Community Volcanic Risk programme under contract EV5V-CT92-0188 is acknowledged. IFRTP and IFREMER are thanked for the technical and financial support to the cruise MD-76 REUSIS. UMR 6526 (Ge´osciences Azur) contribution 214.

A network of 60 seismographs was deployed across the Andes at approximately 23.5°S. The array was centered in the backarc; atop the Puna high plateau in NW Argentina. P and S arrival times of 426 intermediate depth earthquakes were inverted for 1-D velocity structure and hypocentral coordinates. Average velocities and v sub p/v sub s in the crust are low. Average mantle velocities are high but difficult to interpret because of the presence of a fast velocity slab at depth. Although the hypocenters sharply define a 35° dipping Benioff zone, seismicity in the slab is not continuous. The spatial clustering of earthquakes is thought to reflect inherited heterogeneties of the subducted oceanic lithosphere. Additionally, 57 crustal earthquakes were located. Seismicity concentrates in the fold and thrust belt of the foreland and Eastern Cordillera, and along and south of the El Toro-Olacapato-Calama Lineament (TOCL). Focal mechanisms of two earthquakes at this structure exhibit left lateral strike-slip mechanisms similar to the suggested kinematics of the TOCL. We believe that the Puna north of the TOCL behaves like a rigid block with little internal deformation, whereas the area south of the TOCL is weaker and currently deforming. (Résumé d'auteur)

Wide-angle seismic lines recorded by ocean bottom and land seismometers provide a pseudo three-dimensional investigation of the crust and upper mantle structure around the volcanically active hotspot island of La Réunion. The submarine part of the edifice has fairly low seismic velocities, without evidence for intrusives. An upper unit with a velocity-depth gradient is interpreted as made of material erupted subaerially then transported and compacted downslope. Between this unit and the top of the oceanic plate, imaged by normal incidence seismic reflection, a more homogeneous unit indicated by shadow zones on several wide-angle sections may correspond to lavas of a different nature, extruded underwater in the earlier phase of volcanism. Coincident wide angle and normal incidence reflections document that the oceanic plate is not generally downwarping toward the island but doming instead toward its southeastern part, with limited evidence for some intracrustal intrusion. Deeper in the lithosphere, the presence of a layer of intermediate velocity between the crust and mantle is firmly established. It is interpreted as resulting from the advection of hotspot magmatic products, possibly partially molten, and of a composition for which the crust is a density barrier. The extensive wide-angle coverage constrains the extent of this body. It does not show the elongated shape expected from plate drift above a steady hotspot supply. Alternative propositions can hence be considered, for example, that La Réunion is caused by a solitary wave of hotspot material or by a young hotspot. The size of the underplate, 140 km wide and up to 3 km thick, corresponds to less than half the volume of the edifice on top of the plate. Copyright 1999 by the American Geophysical Union.

SUMMARY The Crozet Bank, the easternmost region of the Crozet Plateau (austral Indian Ocean), is capped by two groups of islands which form the Crozet Archipelago (Terres Australes and Antarctiques Francaises). A N-S-trending 2 km deep basin, the Indivat Basin, extends between the two groups of islands and bisects the Crozet Bank into two separate domains. The interpretation of the KeOBS8 seismic refraction profile shot during the KeOBS/MD66 cruise (January-February 1991) over the eastern Crozet Plateau was carried out by ray tracing and the computation of synthetic seismograms. This interpretation leads to a rather complex seismic structure and outlines a pro- nounced eastward crustal thinning from 16.5 to 10 km at the expense of layer 2. The thinning of the crust is abrupt east of the Indivat Basin. Unlike below the Hawaiian Islands and the Marquesas Islands, no underplated mantle material underlies the crust of the Crozet Bank. Moreover, this deep seismic sounding has further revealed that a high-velocity feature outcropping the seabed 30 km east of Ile aux Cochons could be a major structural feature, deeply rooted within the lower crust. The occurrence of this feature (a submarine volcano?) is associated with a mantle dyke causing a steep shallowing of the lower crustal interfaces. Gravity modelling was performed along line KeOBS8, with a density structure deduced from the seismic section, to model free-air anomalies derived from altimetry data. This modelling confirms that the Indivat Basin, underlined by a gravity low running roughly N-S between the two groups of islands, is a major structural boundary. As the model generates medium-wavelength anomalies of adequate amplitude, it also confirms that the volcano, located west of the Indivat Basin, is a deeply rooted feature. The Crozet Bank clearly appears as a plume-aVected structure, which may have originated from a deep thermal anomaly within the lithosphere. More recent volcanic episodes, related to a still active plume activity under the Crozet Bank, could have uplifted upper-mantle material and caused the emplacement of the newly discovered feature and of the western group of islands.

A detailed study of P and S seismic waves produced by hundreds of earthquakes located along the New Hebrides, Fiji plateau, Solomon, New Britain and New Guinea seismic zones and recorded at stations in Fiji, New Hebrides and New Caledonia shows significant variations in the amplitudes and frequencies of the waves. The data define zones of anomalously high seismic wave attenuation in the uppermost mantle beneath the Fiji plateau and the Woodlark basin. The active volcanoes of the New Hebrides arc seem to mark the western boundary of the high-attenuation zone that exists beneath the Fiji plateau. Our observations are in agreement with the hypothesis that the Fiji plateau and the Woodlark basin are young oceanic features formed by crustal extension. In addition, a zone of high attenuation exists in the uppermost mantle somewhere between the northeastern coast of Australia and the New Hebrides arc; the data further require that this zone be located not closer than about 400 km to the New Hebrides trench. Observations at New Caledonia stations of very low frequency shear waves from intermediate-depth earthquakes in the westward dipping Tonga—Kermadec seismic zone yield an estimate of Qs of about 10 in the zone of extremely high attenuation located beneath the Lau basin, the actively spreading inter-arc basin behind the Tonga arc.

Recordings of quarry blasts along a 300 km profile across the Adamawa Plateau at the eastern of the Cameroon Volcanic Line permitted the determination of the crustal structure of the region. To the north of the Plateau the crust is about 23 km thick. The southern part of the Plateau by comparison is associated with a more normal crustal thickness of about 33 km. Between these two areas the seismological experiment was not able to delineate how the croust changes. Howerer, if the topography and gravity field reflect crustal thickness then there is a rapid change in crustal thickness at the northern edge of Adamawa Plateau.