Robustness of kappa (κ) measurement in low-to-moderate seismicity 1 areas: insight from a site-specific study in Provence, France

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Modélisation 3D multi-échelle des structures géologiques de la région de la faille de la moyenne Durance (SE France)

La complexité des structures géologiques en Moyenne Durance rend difficile la caractérisation et l’évaluation du risque sismique dans cette région. La compréhension 3D des structures nécessite l’utilisation de techniques modernes de modélisation numérique pour réaliser des modèles 3D du substratum géologique qui soient cohérents à différentes échelles, pour effectuer des simulations des mouvements sismiques. Pour réaliser des modèles 3D géologiques, nous avons harmonisé des bases de données géologiques et géophysiques hétérogènes (forages pétroliers, sondages géotechniques, profils sismiques, profils géophysiques H/V, cartographie géologique haute et basse résolution, datations biostratigraphiques, ...) dans le géomodeleur 3D gOcad. La réalisation des modèles 3D permet de caractériser la géométrie 3D des principales structures tectoniques en Moyenne Durance, notamment la Zone de failles de la Moyenne Durance (ZFMD) ainsi que la géométrie haute résolution du substratum géologique de la vallée de Cadarache, notamment de son remplissage sédimentaire tertiaire. Notre étude permet de préciser la géométrie 3D de la ZFMD et son rôle sur les structures géologiques adjacentes. La ZFMD constitue une zone de transfert qui limite l’extension latérale et modifie la géométrie des plis et chevauchements. Les structures tectoniques en Moyenne Durance sont caractérisées par une déformation de type thin-skin. La tectonique salifère joue un rôle important dans cette déformation (niveau de décollement, formation et géométrie 3D des plis). A l’échelle de la vallée de Cadarache, l’étude géologique des affleurements couplée à l’étude géostatistique des sondages permet de réaliser une simulation en 3D de la répartition spatiale des faciès et de proposer un modèle sédimentaire 3D du remplissage tertiaire de la vallée. Il est caractérisé par une sédimentation en deux étapes : une première phase de remplissage par des apports détritiques importants en provenance du Sud-Est, liés au démantèlement du massif des Maures-Estérel, une deuxième phase de remplissage par des dépôts d’environnement fluviatile méandriforme à forte sinuosité et à dominante de sables. Enfin, l’imbrication multi-échelle des modèles 3D a permis d’expliquer la formation de la vallée de Cadarache dans le contexte géodynamique de la région de la Moyenne Durance au Tertiaire, et notamment sous l’influence de la compression pyrénéenne et de la ZFMD sur la région de la Moyenne Durance et sur la vallée de Cadarache.The complexity of geological structures in Middle Durance region makes difficult the characterization and evaluation of seismic risk in this region. Understanding these structures in 3D requires to use modern techniques of 3D digital modelling in order to achieve the 3D geological models of the bedrock with coherence on different scales, to perform ground motion simulations. Building 3D geological models need to normalize heterogeneous geophysical and geological databases (oil boreholes, geotechnical boreholes, seismic profiles, H/V geophysical profiles, high and low resolution geological mapping, biostratigraphic dating,...) in the 3D software gOcad. The realization of 3D geological models allows to characterize the 3D geometry of main the tectonic structures in Middle Durance region, including the Middle Durance Fault Zone (ZFMD) and the high resolution geometry of geological bedrock of the Cadarache Valley, and in particular its tertiary sedimentary fill. Our study allows to specify the ZFMD 3D geometry and its role on the adjacent geological structures. The ZFMD is a transfer zone that limits lateral extension and changes the geometry of folds and thrusts. The Middle Durance tectonic structures are characterized by a thin-skin deformation. Salt tectonics plays an important role in this deformation (detachment level, training and 3D geometry of folds). At the scale of the Valley of Cadarache, outcrop geological study coupled with boreholes geostatistical study allows to perform a 3D simulation of the spatial distribution of facies and to propose a 3D sedimentary model for the Tertiary filling of the Cadarache Valley. It is characterized by sedimentation in two steps: a first step of filling by important detrital inputs from the Southeast, related to the erosion of the Maures-Esterel massif, a second step of filling by deposits of fluvial meandering sand-dominant environment with high-sinuosity. Finally, building multi-scale 3D models allows explaining the formation of the Valley of Cadarache in the geodynamic context of the Middle Durance region at Tertiary times, and particularly under the influence of the Pyrenean compression and ZFMD on the Middle Durance region and the Valley of Cadarache

Utilité de l'instrumentation in situ : recommandations et exemples d'application pour l'évaluation des effets de site dans un contexte de sismicité faible à modérée

International audienceABSTRACT. Implementing an instrumentation that allows to record real seismic events is an essential complement to numerical simulation approaches of site effect assessment. Such " empirical " approaches allow for example to apply the Standard Spectral Ratio (SSR) and to evaluate the high frequencies parameter " kappa " (κ). While these approaches seem readily available in areas of high seismicity (Japan, Greece ...), they are more difficult to implement in areas of low seismicity, as in metropolitan France. The first objective of this study is to provide some elements to evaluate the usefulness of such site instrumentation, especially in the aim of site effect evaluation. A second objective is to give some recommendations for the implementation of instrumentation. This study presents the databases acquired over a period of about two years at two sites in SouthEastern France. A very large number of seismic recordings was acquired. To obtain such results, we recommend the use of velocimeters recording in continuous mode. SSR method is robust even when a limited number of events are available. It is also possible to assess the κ 0 parameter, even though the conventional method requires a good signal to noise ratio up to high frequencies. The displacement method is very encouraging to evaluate κ 0 in less seismic zones. MOTS-CLÉS : Instrumentation, sismicité faible à modérée, effets de siteLa mise en place d'une instrumentation in situ permettant l'enregistrement d'événements sismiques est un complément essentiel aux approches d'évaluation des effets de site par simulation numérique. Une telle approche « empirique » permet, par exemple, l'emploi de la méthode site/référence (SSR) pour évaluer la fonction de transfert spécifique au site, ainsi que la mesure à haute fréquence du paramètre « kappa » (κ). Alors que ces approches semblent déjà utilisables dans les régions fortement sismiques (Japon, Grèce…), elles se heurtent souvent à un a priori défavorable pour les régions de sismicité faible à modérée (p. ex. la France métropolitaine) en raison du temps supposé qu'il faudrait pour obtenir un nombre suffisant d'enregistrements de qualité. Le premier objectif de cette étude est d'apporter quelques éléments permettant d'évaluer l'utilité de l'instrumentation des sites, en particulier pour ce qui concerne l'évaluation des effets de site. Un deuxième objectif est de fournir quelques recommandations pour leur mise en place. Cette étude présente les bases de données acquises sur une durée d'environ deux ans sur deux sites du Sud-Est de la France. Un nombre très important d'enregistrements sismiques a été acquis. Pour obtenir de tels résultats, nous recommandons l'emploi de vélocimètres enregistrant en continu. La méthode SSR se montre assez robuste même lorsque l'on sélectionne qu'un nombre limité d'événements. Il est également possible d'évaluer le paramètre κ 0 , alors même que la méthode classique nécessite un bon rapport signal sur bruit jusqu'à haute fréquence. La méthode en déplacement est très encourageante pour approcher la mesure de κ 0 dans les zones les moins sismiques

Utilité de l'instrumentation in situ : recommandations et exemples d'application pour l'évaluation des effets de site dans un contexte de sismicité faible à modérée

International audienceABSTRACT. Implementing an instrumentation that allows to record real seismic events is an essential complement to numerical simulation approaches of site effect assessment. Such " empirical " approaches allow for example to apply the Standard Spectral Ratio (SSR) and to evaluate the high frequencies parameter " kappa " (κ). While these approaches seem readily available in areas of high seismicity (Japan, Greece ...), they are more difficult to implement in areas of low seismicity, as in metropolitan France. The first objective of this study is to provide some elements to evaluate the usefulness of such site instrumentation, especially in the aim of site effect evaluation. A second objective is to give some recommendations for the implementation of instrumentation. This study presents the databases acquired over a period of about two years at two sites in SouthEastern France. A very large number of seismic recordings was acquired. To obtain such results, we recommend the use of velocimeters recording in continuous mode. SSR method is robust even when a limited number of events are available. It is also possible to assess the κ 0 parameter, even though the conventional method requires a good signal to noise ratio up to high frequencies. The displacement method is very encouraging to evaluate κ 0 in less seismic zones. MOTS-CLÉS : Instrumentation, sismicité faible à modérée, effets de siteLa mise en place d'une instrumentation in situ permettant l'enregistrement d'événements sismiques est un complément essentiel aux approches d'évaluation des effets de site par simulation numérique. Une telle approche « empirique » permet, par exemple, l'emploi de la méthode site/référence (SSR) pour évaluer la fonction de transfert spécifique au site, ainsi que la mesure à haute fréquence du paramètre « kappa » (κ). Alors que ces approches semblent déjà utilisables dans les régions fortement sismiques (Japon, Grèce…), elles se heurtent souvent à un a priori défavorable pour les régions de sismicité faible à modérée (p. ex. la France métropolitaine) en raison du temps supposé qu'il faudrait pour obtenir un nombre suffisant d'enregistrements de qualité. Le premier objectif de cette étude est d'apporter quelques éléments permettant d'évaluer l'utilité de l'instrumentation des sites, en particulier pour ce qui concerne l'évaluation des effets de site. Un deuxième objectif est de fournir quelques recommandations pour leur mise en place. Cette étude présente les bases de données acquises sur une durée d'environ deux ans sur deux sites du Sud-Est de la France. Un nombre très important d'enregistrements sismiques a été acquis. Pour obtenir de tels résultats, nous recommandons l'emploi de vélocimètres enregistrant en continu. La méthode SSR se montre assez robuste même lorsque l'on sélectionne qu'un nombre limité d'événements. Il est également possible d'évaluer le paramètre κ 0 , alors même que la méthode classique nécessite un bon rapport signal sur bruit jusqu'à haute fréquence. La méthode en déplacement est très encourageante pour approcher la mesure de κ 0 dans les zones les moins sismiques

Three-dimensional structural modeling of an active fault zone based on complex outcrop and subsurface data: The Middle Durance Fault Zone inherited from polyphase Meso-Cenozoic tectonics (southeastern France)

International audienceThe objective of this study was to realize a three-dimensional (3-D) geological model of the deep basin structure of the Middle Durance region (of folds and faults) by integration of geological and geophysical data, and to evaluate its fault geometry and tectonic history. All of the available geophysical and geological data were compiled in three dimensions using the gOcad geomodeler. The geological and geophysical data were used to build a 3-D geological model of the Middle Durance region. The data on the 3-D geometry of fault surfaces and stratigraphic horizons and the thickness maps of the main stratigraphic units are supported by the 3-D geological model. We show that the Middle Durance Fault cannot be interpreted as a single fault plane that affected the entire Meso-Cenozoic sedimentary layers and the Paleozoic basement but as a listric segmented faulting system in sedimentary layers, rooted in Triassic evaporites and a normal block faulting system in the basement. This decoupling level in the Triassic layers reveals thin-skin deformation, formed by strong mechanical decoupling between the Mesozoic sedimentary cover and the Paleozoic basement. This study also confirms that the Provence geological structure has resulted mainly from Pyrenean deformation, which was partly reactivated by Alpine deformation. We demonstrate that the Middle Durance Fault Zone is a transfer fault that accommodates deformation of the sedimentary filling of the South-East Basin through modified fold geometry over a zone of 7 km to 8 km around the main segment of the fault zone

Source‐Related Variability of Site Response in the Mygdonian Basin (Greece) from Accelerometric Recordings and 3D Numerical Simulations

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Building a three dimensional model of the active Plio-Quaternary basin of Argostoli (Cephalonia Island, Greece): an integrated geophysical and geological approach

International audienceThis work is a multidisciplinary approach from geological and geophysical surveys to build a 3D geological model of Argostoli Basin (Cephalonia Island, Greece) aiming to be used for computational 3D simulation of seismic motion. Cephalonia Island is located at the north-western end of the Aegean subduction frontal thrust that is linked to the dextral Cephalonia Transform Fault (west of Cephalonia) where the seismic hazard is high in terms of earthquake frequency and magnitude. The Plio-Quaternary Koutavos-Argostoli basin site was selected within the French Research Agency PIA SINAPS@ project (www.institut-seism.fr/projets/sinaps/ - last accessed on July 14th 2019) to host a vertical accelerometer array. The long-term goal is to validate three-dimensional nonlinear numerical simulation codes to assess the site-specific amplification and nonlinearity. Herein the geological and geophysical surveys carried out from 2011 to 2017 are presented and in particular the complementary investigations that led to the identification of the main stratigraphic units and their structures. In addition, coral debris sampled from the vertical array deep borehole cores were used for 230Th/234U measurements, which confirmed the Pleistocene age of the Koutavos basin. The characterization of the three-dimensional structure of the stratigraphic units was achieved by coupling geological cross-sections (i.e., depth geometry) and geophysical surveys based of surface wave analysis

Building a Three Dimensional Model of Plio-Quaternary Basin of Argostoli (Cephalonia Island, Greece) From an Integrated Geophysical and Geological Approach to Perform Numerical Simulations of Seismic Motion

International audienceThe Cephalonia Island (Greece) area is located in the north-western end of the Aegean subduction frontal thrust that is linked to the dextral Cephalonia Transform Fault (west of Cephalonia). As the mean slip rate and the length of the Cephalonia Transform Fault are large, the seismic hazard is high in terms of earthquake frequency and magnitude. The Plio-Quaternary Koutavos-Argostoli basin site was selected within the French Research Agency PIA SINAPS@ project (www.institut-seism.fr/projets/sinaps/) to host a vertical accelerometer array. The long-term goal is to validate three-dimensional nonlinear numerical simulation codes to assess the site¬specific amplification and nonlinearity in the framework of seismic hazard assessment. Geological and geophysical surveys were carried out from 2011 to 2017. The aim of this work is to present i) the geological and structural context, ii) the complementary geological, geotechnical, and iii) the geophysical investigations that led to the identification of the main geotechnical bodies and their structures. The characterization of the three-dimensional structure of the stratigraphic units was achieved by coupling geological cross-sections (i.e., depth geometry) and HVSR/AVA geophysical surveys that were designed to identify the sedimentary body thickness. The data gathered allowed to build a three-dimensional geological model of the Argostoli basin. Starting from this point, a 3D numerical model on the same area was constructed using spectral element code known as SEM3D. Several simulations of seismic scenarios were performed, for the purpose of highlighting the importance of the use a representative model of the complexity of the wave path on the hazard assessment

Building a Three Dimensional Model of Plio-Quaternary Basin of Argostoli (Cephalonia Island, Greece) From an Integrated Geophysical and Geological Approach to Perform Numerical Simulations of Seismic Motion

International audienceThe Cephalonia Island (Greece) area is located in the north-western end of the Aegean subduction frontal thrust that is linked to the dextral Cephalonia Transform Fault (west of Cephalonia). As the mean slip rate and the length of the Cephalonia Transform Fault are large, the seismic hazard is high in terms of earthquake frequency and magnitude. The Plio-Quaternary Koutavos-Argostoli basin site was selected within the French Research Agency PIA SINAPS@ project (www.institut-seism.fr/projets/sinaps/) to host a vertical accelerometer array. The long-term goal is to validate three-dimensional nonlinear numerical simulation codes to assess the site¬specific amplification and nonlinearity in the framework of seismic hazard assessment. Geological and geophysical surveys were carried out from 2011 to 2017. The aim of this work is to present i) the geological and structural context, ii) the complementary geological, geotechnical, and iii) the geophysical investigations that led to the identification of the main geotechnical bodies and their structures. The characterization of the three-dimensional structure of the stratigraphic units was achieved by coupling geological cross-sections (i.e., depth geometry) and HVSR/AVA geophysical surveys that were designed to identify the sedimentary body thickness. The data gathered allowed to build a three-dimensional geological model of the Argostoli basin. Starting from this point, a 3D numerical model on the same area was constructed using spectral element code known as SEM3D. Several simulations of seismic scenarios were performed, for the purpose of highlighting the importance of the use a representative model of the complexity of the wave path on the hazard assessment

Building a Three Dimensional Model of Plio-Quaternary Basin of Argostoli (Cephalonia Island, Greece) From an Integrated Geophysical and Geological Approach to Perform Numerical Simulations of Seismic Motion

International audienceThe Cephalonia Island (Greece) area is located in the north-western end of the Aegean subduction frontal thrust that is linked to the dextral Cephalonia Transform Fault (west of Cephalonia). As the mean slip rate and the length of the Cephalonia Transform Fault are large, the seismic hazard is high in terms of earthquake frequency and magnitude. The Plio-Quaternary Koutavos-Argostoli basin site was selected within the French Research Agency PIA SINAPS@ project (www.institut-seism.fr/projets/sinaps/) to host a vertical accelerometer array. The long-term goal is to validate three-dimensional nonlinear numerical simulation codes to assess the site¬specific amplification and nonlinearity in the framework of seismic hazard assessment. Geological and geophysical surveys were carried out from 2011 to 2017. The aim of this work is to present i) the geological and structural context, ii) the complementary geological, geotechnical, and iii) the geophysical investigations that led to the identification of the main geotechnical bodies and their structures. The characterization of the three-dimensional structure of the stratigraphic units was achieved by coupling geological cross-sections (i.e., depth geometry) and HVSR/AVA geophysical surveys that were designed to identify the sedimentary body thickness. The data gathered allowed to build a three-dimensional geological model of the Argostoli basin. Starting from this point, a 3D numerical model on the same area was constructed using spectral element code known as SEM3D. Several simulations of seismic scenarios were performed, for the purpose of highlighting the importance of the use a representative model of the complexity of the wave path on the hazard assessment