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We summarize ten years of the French seismicity recorded by the Geophysical and Detection Laboratory (LDG) of the French Alternative Energies and Atomic Energy Commission (CEA) network from 2010 to 2019. During this period, 25 265 natural earthquakes were detected by the LDG and located within metropolitan France and its immediate vicinity. This seismicity contributes to more than 47% of the natural earthquakes instrumentally recorded since 1962 (mainly due to the improvement of network capacity), and includes about 28% of the most significant earthquakes with a magnitude ML ≥ 4.0. Recent seismic events therefore significantly expand the available national catalogues. The spatial distribution of 2010–2019 earthquakes is broadly similar to the previously recorded instrumental pattern of seismicity, with most of the seismic activity concentrated in the French Alps, the Pyrenees, Brittany, the upper Rhine Graben and the Central Massif. A large part of the seismic activity is related to individual events. The largest earthquakes of the last ten years include the November 11, 2019 Le Teil earthquake with ML 5.4 and epicentral intensity VII–VIII, which occurred in the Rhone valley; the April 28, 2016 La Rochelle earthquake with ML 5.1 and epicentral intensity V, which occurred at the southernmost extremity of the Armorican Massif in the vicinity of the Oléron island; and the April 7, 2014 Barcelonnette earthquake with ML 5.1 and epicentral intensity V–VI, which occurred in the Ubaye valley in the Alps. In 2019, two other moderate earthquakes of ML 5.1 and ML 4.9 stroke the western part of France, in Charente-Maritime and Maine-et-Loire departments, respectively. The recent moderate earthquake occurrences and the large number of small earthquakes recorded give both the potential to revise some regional historical events and to determine more robust frequency-magnitude distributions, which are critical for seismic hazard assessment but complex due to low seismicity rates in France. The LDG seismic network installed since the early 1960s also allows a better characterization of the temporal structure of seismicity, partly diffused and in the form of mainshock-aftershocks sequences or transient swarms. These aspects are important in order to lower the uncertainties associated to seismogenic sources and improve the models in seismic hazard assessment for metropolitan France. Cette publication résume dix années de sismicité enregistrée par le réseau de stations du Laboratoire de Détection et de Géophysique (LDG) du Commissariat à l’énergie atomique et aux énergies alternatives (CEA) de 2010 à 2019. Au cours de cette période, 25 265 séismes naturels ont été détectés par le réseau et localisés en France métropolitaine ou jusqu’à 20 km des frontières et des côtes maritimes. Cette décade de sismicité correspond à plus de 47 % des séismes instrumentaux toutes magnitudes confondues enregistrés par le réseau depuis 1962 (en raison de l’amélioration des capacités du réseau), et comprend 28 % des séismes français de magnitude ML ≥ 4,0. Les événements sismiques récents élargissent donc considérablement les catalogues nationaux disponibles. La répartition spatiale des tremblements de terre naturels de 2010–2019 est globalement similaire à celles des observations instrumentales précédentes, la majeure partie de l’activité sismique étant concentrée dans les Alpes françaises, les Pyrénées, la Bretagne, la vallée du Rhin et le Massif central. Une grande partie de l’activité sismique est liée à des événements individuels. Les plus grands séismes des dix dernières années incluent le séisme du Teil du 11 novembre 2019 avec une ML 5,4 et une intensité épicentrale VII–VIII, qui s’est produit dans la vallée du Rhône ; le séisme de La Rochelle du 28 avril 2016 avec une ML 5,1 et une intensité épicentrale V, survenu à l’extrémité sud du Massif armoricain à proximité de l’île d’Oléron ; et le séisme de Barcelonnette du 7 avril 2014 avec une ML 5,1 et une intensité épicentrale V–VI, survenu dans la vallée de l’Ubaye dans les Alpes. En 2019, deux autres séismes modérés de ML 5,1 et ML 4,9 ont frappé l’ouest de la France, en Charente-Maritime et Maine-et-Loire respectivement. Les récentes réalisations de séismes modérés et le grand nombre de petits séismes enregistrés donnent à la fois le potentiel de réviser certains événements historiques régionaux et de déterminer des distributions fréquence-magnitude plus robustes, qui sont cruciales pour l’évaluation de l’aléa sismique mais complexes en raison des faibles taux de sismicité en France. Le réseau sismique du LDG étant installé depuis le début des années 1960, ces données permettent également une meilleure caractérisation de la structure temporelle de la sismicité, en partie diffuse, et sous forme de séquences de répliques ou d’essaims transitoires. Ces aspects sont importants pour diminuer les incertitudes liées aux sources sismogènes et pour in fine améliorer les modèles d’évaluation de l’aléa sismique pour la France métropolitaine.

The case study presented in this article demonstrates the effectiveness of the Ambient Vibration Array (AVA) method for characterizing a 1D profile of S-wave velocities (Vs) in near surface. Ambient seismic noise was acquired simultaneously on three circular arrays with diameters of 30, 90, and 270 m, respectively, using 31 three-component nodes. To obtain dispersion curves over the widest possible frequency range, the circles were processed individually. The three-component recording allowed the use of the latest developments of the GEOPSY software, to jointly reconstruct and inverse the dispersion curves of the fundamental and first upper modes of Rayleigh waves and Love waves, as well as the ellipticity of the Rayleigh waves. The resulting 1D model enabled the characterization of Vs down to a depth of 150 m. Le cas d’étude présenté dans cet article montre l’efficacité de la méthode AVA (Ambient Vibration Array) pour la caractérisation d’un profil 1D des vitesses de propagation des ondes de cisaillement (Vs) en proche surface. L’acquisition du bruit sismique ambiant s’est effectuée simultanément sur 3 réseaux en cercle de respectivement 30, 90 et 270 m de diamètre avec 31 capteurs 3 composantes. Afin d’obtenir une caractérisation de la dispersion sur la plage fréquentielle la plus large possible, le traitement des cercles a été réalisé individuellement. L’enregistrement sur 3 composantes a permis d’utiliser les tous derniers développements du logiciel GEOPSY, pour reconstruire et inverser conjointement les courbes de dispersion du mode fondamental et du premier mode supérieur des ondes de Rayleigh, et des ondes de Love, ainsi que l’ellipticité des ondes de Rayleigh. Le modèle 1D ainsi obtenu a permis de caractériser les Vs jusqu’à 150 m de profondeur.

The b‐value from the Gutenberg‐Richter law is a crucial parameter in the assessment of seismic hazard. Its temporal variations may also bring useful insights on the processes driving seismicity at depth, even if not yet fully understood. In this paper, we focus on the temporal evolution of the b‐value in the Ubaye Region (French Western Alps) which was hit by seismic swarms (2003–2004) and complex sequences with several mainshocks (2012–2015). The swarm‐like sequences show a common temporal behavior of b‐value characterized by an increase and then a return to the initial level. The temporal b‐value pattern for the mainshock‐aftershock‐like sequences is quite different. After a drop in the b‐value that may follow the mainshock, the b‐value increases above the background level before going back to it. Moreover, no precursory pattern can be identified before the mainshock. Fluid processes are recognized to play a crucial role in the driving mechanisms of these seismic sequences. Drawing parallel between swarms and aftershock sequences suggests that the b‐value depicts fluid‐processes in the Ubaye Region seismicity. We propose that b‐value shows a complex behavior, with variations due to Coulomb stress‐transfer from the mainshock and fluid‐pressure processes. Therefore, even with a catalog made at the French national scale, the b‐value variations may help to monitor the on‐going processes at depth. Key Points We study b‐value evolution in the Ubaye Region (France), an area with a prolific and complex seismicity involving fluid‐driven processes Seismic swarms show a similar increase of b‐value b‐value evolution during mainshock‐aftershocks sequences is consistent with an interplay of fluid migration and stress transfer

The Upper Rhine Graben (URG), located in France and Germany, is bordered by north–south-trending faults, some of which are considered active, posing a potential threat to the dense population and infrastructures on the Alsace plain. The largest historical earthquake in the region was the M6.5±0.5 Basel earthquake in 1356. Current seismicity (M>2.5 since 1960) is mostly diffuse and located within the graben. We build upon previous seismic hazard studies of the URG by exploring uncertainties in greater detail and revisiting a number of assumptions. We first take into account the limited evidence of neotectonic activity and then explore tectonic scenarios that have not been taken into account previously, exploring uncertainties for Mmax, its recurrence time, the b value, and the moment released aseismically or through aftershocks. Uncertainties in faults' moment deficit rates, on the observed seismic events' magnitude–frequency distribution and on the moment–area scaling law of earthquakes, are also explored. Assuming a purely dip-slip normal faulting mechanism associated with a simplified model with three main faults, Mmax maximum probability is estimated at Mw 6.1. Considering this scenario, there would be a 99 % probability that Mmax is less than 7.3. In contrast, with a strike-slip assumption associated with a four-main-fault model, consistent with recent paleoseismological studies and the present-day stress field, Mmax is estimated at Mw 6.8. Based on this scenario, there would be a 99 % probability that Mmax is less than 7.6.

The b ‐value from the Gutenberg‐Richter law is a crucial parameter in the assessment of seismic hazard. Its temporal variations may also bring useful insights on the processes driving seismicity at depth, even if not yet fully understood. In this paper, we focus on the temporal evolution of the b ‐value in the Ubaye Region (French Western Alps) which was hit by seismic swarms (2003–2004) and complex sequences with several mainshocks (2012–2015). The swarm‐like sequences show a common temporal behavior of b ‐value characterized by an increase and then a return to the initial level. The temporal b ‐value pattern for the mainshock‐aftershock‐like sequences is quite different. After a drop in the b ‐value that may follow the mainshock, the b ‐value increases above the background level before going back to it. Moreover, no precursory pattern can be identified before the mainshock. Fluid processes are recognized to play a crucial role in the driving mechanisms of these seismic sequences. Drawing parallel between swarms and aftershock sequences suggests that the b ‐value depicts fluid‐processes in the Ubaye Region seismicity. We propose that b ‐value shows a complex behavior, with variations due to Coulomb stress‐transfer from the mainshock and fluid‐pressure processes. Therefore, even with a catalog made at the French national scale, the b ‐value variations may help to monitor the on‐going processes at depth.

The Ubaye Region is the most seismically active region in the Western Alps, with earthquakes that were commonly felt by the population and that even damaged local villages and cities. Since the first testimonies in 1844, this area has been regularly struck by seismic swarms with a high number of events, such as in 2003–2004 or 2012–2015, or by mainshock–aftershock sequences with a magnitude up to ML 5.3 in 1959. In this paper, we analysed both historical records and instrumental seismicity in the light of geological observations. Some earthquakes could be associated with known faults, even if most of them occurred on blind, unknown faults that reveal a highly fractured basement. The abnormal level of seismicity, together with its peculiar behaviour, suggests complex driving processes involving not only tectonic loading but also fluid pressure.

The Tricastin region in the lower Rhône Valley (France) is affected by an atypical seismic activity characterised by the development of long-lasting and recurrent seismic swarms. Indeed, since the 16th century, hundreds of seismic events sometime associated with underground noises of the explosion have been reported by local inhabitants. However, to date, none of the many scenarios of earthquake generation proposed for the area, involving either tectonics and/or hydrological forcings, appears consensual. To overcome that lack of comprehension, we compile and analyse an 880 seismic-events catalogue derived from both historical macroseismicity and instrumental records. The earthquakes appear to occur at shallow depths similar to those determined below a local network in 2002–2003. We confront to this catalogue models involving hydrological mechanisms, including aquifers elastic loading and karst-drains responses, as well as tectonic mechanisms, including transient aseismic processes and their related effects on the fold hinges or on the local fault planes. Most of the earthquakes are located at short distances from karst drains and fractured fold hinges, possibly affected by transient hydrological changes.