Rupture complexity of a moderate intraplate earthquake in the Alps: the 1996M5 Epagny-Annecy earthquake

The magnitude 5 Epagny-Annecy earthquake of 1996 July 15 is the largest seismic event to have occurred in the Alps since the introduction of modern digital instrumentation. This strike-slip event was located on the Vuache Fault, near the town of Annecy, in the northern French Alps. The aim of our work was to retrieve the main parameters of the rupture process of this earthquake from seismograms recorded at local and regional distances (20-300 km). To eliminate path and site effects from the seismograms, we compared the main shock recordings at each station with those of the largest aftershocks nearby. We used a combination of techniques, including pulse-width measurements and cross-correlation of velocity traces, comparison of P-wave displacement pulses, and empirical Green's function deconvolution, to retrieve the apparent duration of the rupture process as seen at each station. Our results demonstrate that, in the absence of on-scale data, P-wave pulse-width measurements on clipped signals can be misleading if the rupture process is complex. In the case of the Annecy earthquake, comparisons of on-scale P-wave displacement seismograms and the empirical Green's function deconvolutions show that the rupture process consisted of at least two subevents separated by 0.2-0.3 s, and with a total duration of about 0.5 s. The systematic azimuthal dependence of both the shape and duration of the apparent source-time function is consistent with a nearly unilateral propagation of the main rupture phase in a southeast direction along the fault plane and parallel to the direction of slip. An isochron analysis reveals that the first subevent occurred slightly to the northwest of the nucleation point but that the second subevent was located further to the southeast, thus confirming the overall rupture directivity towards the southeast. An interpretation of our results in light of the previously documented aftershock distribution and of observations of ground cracks in the epicentral area suggests that the main shock occurred on the Vuache Fault, and that rupture in a northwest direction was inhibited by a right-lateral stepover in the fault. Accordingly, the vast majority of the subsequent aftershocks, which include several magnitude 3-4 events, occurred on a fault segment that is slightly offset from the inferred surface trace of the Vuache Fault and that was activated by the main shoc

Seismotectonics of southeast France: from the Jura mountains to Corsica

The analysis of the seismicity catalog (1996 to 2019) covering the region from the Jura mountains to Corsica provides a first-order image of the distribution of earthquakes, highlighting large structures such as the Briançonnais and Piedmontais seismic arcs, the eastward deepening of the focal depths through the Western Alps, several large active faults (e.g. Belledonne, Middle Durance, Ligure). Over this period the magnitudes are moderate and the focal mechanisms of the main events display a diversity of seismic behaviors that can be explained by the complexity of the different geological domains with a more or less strong structural inheritage, by variable rheological characteristics at the scale of the crust and by the joint action of different mechanisms of deformation. The distribution of the historical events is in fairly good agreement with the instrumental seismicity, but several earthquakes of $M >6$ are highlighted since the 14th century until the beginning of the 20th

Seismotectonics of southeast France: from the Jura mountains to Corsica

The analysis of the seismicity catalog (1996 to 2019) covering the region from the Jura mountains to Corsica provides a first-order image of the distribution of earthquakes, highlighting large structures such as the Briançonnais and Piedmontais seismic arcs, the eastward deepening of the focal depths through the Western Alps, several large active faults (e.g. Belledonne, Middle Durance, Ligure). Over this period the magnitudes are moderate and the focal mechanisms of the main events display a diversity of seismic behaviors that can be explained by the complexity of the different geological domains with a more or less strong structural inheritage, by variable rheological characteristics at the scale of the crust and by the joint action of different mechanisms of deformation. The distribution of the historical events is in fairly good agreement with the instrumental seismicity, but several earthquakes of $M >6$ are highlighted since the 14th century until the beginning of the 20th

Rapid response to the M_w 4.9 earthquake of November 11, 2019 in Le Teil, Lower Rhône Valley, France

On November 11, 2019, a Mw 4.9 earthquake hit the region close to Montelimar (lower Rhône Valley, France), on the eastern margin of the Massif Central close to the external part of the Alps. Occuring in a moderate seismicity area, this earthquake is remarkable for its very shallow focal depth (between 1 and 3 km), its magnitude, and the moderate to large damages it produced in several villages. InSAR interferograms indicated a shallow rupture about 4 km long reaching the surface and the reactivation of the ancient NE-SW La Rouviere normal fault in reverse faulting in agreement with the present-day E-W compressional tectonics. The peculiarity of this earthquake together with a poor coverage of the epicentral region by permanent seismological and geodetic stations triggered the mobilisation of the French post-seismic unit and the broad French scientific community from various institutions, with the deployment of geophysical instruments (seismological and geodesic stations), geological field surveys, and field evaluation of the intensity of the earthquake. Within 7 days after the mainshock, 47 seismological stations were deployed in the epicentral area to improve the Le Teil aftershocks locations relative to the French permanent seismological network (RESIF), monitor the temporal and spatial evolution of microearthquakes close to the fault plane and temporal evolution of the seismic response of 3 damaged historical buildings, and to study suspected site effects and their influence in the distribution of seismic damage. This seismological dataset, completed by data owned by different institutions, was integrated in a homogeneous archive and distributed through FDSN web services by the RESIF data center. This dataset, together with observations of surface rupture evidences, geologic, geodetic and satellite data, will help to unravel the causes and rupture mechanism of this earthquake, and contribute to account in seismic hazard assessment for earthquakes along the major regional Cévenne fault system in a context of present-day compressional tectonics

DES PETITS SEISMES POUR COMPRENDRE ET PREVOIR LES PLUS GROS

Small earthquakes are not destructive but can be very helpful. This HDR thesis describes threw different research studies how small earthquakes can be used for (1) understanding seismic source of larger ones by an inversion using empirical Green's functions (2) imaging active faults using location and relative relocation methods (3) predict the ground motions generated by major earthquake with a new method described in details. Earthquakes of different magnitude (3 to 8) and from different regions of the world are studied (Mexico, French Indies, Italy, Greece, France).Les petits séismes ne sont pas destructeurs, ils font peu parler d'eux, pourtant ils sont très utiles. Ce mémoire d'HDR décrit à travers plusieurs travaux de recherche comment les petits séismes peuvent être utilisés pour (1) comprendre la source des séismes plus importants par un processus d'inversion des fonctions de Green empiriques (2) imager les failles actives grâce à des méthodes de localisation et relocalisation relative (3) prédire les mouvements du sol engendrés par des séismes majeurs à partir d'une méthode de simulation originale décrite en détail. Les travaux portent sur des séismes de magnitude 3 à 8 dans différentes régions du monde (Mexique, Antilles, Italie, Grèce, France)

Génération d'accélérogrammes synthétiques large-bande (contribution à l'estimation de l'aléa sismique par validation d'approches en aveugle)

L une des problématique scientifique majeure en sismologie est de pouvoir estimer les mouvements du sol attendus en un site pour un futur séisme. L objectif de cette thèse est de tester et de valider deux méthodes de simulation des mouvements du sol basées sur l approche des fonctions de Green empiriques et d apporter des éléments pouvant aider au développement d une méthodologie de simulation en aveugle. Dans une première partie, une méthode de simulation basée sur une approche stochastique en point-source est validée sur les données réelles de séismes récents bien instrumentés : le séisme des Saintes Mw6.4 et le séisme de L Aquila Mw6.3. Nous avons développé une approche de simulation en aveugle en prenant en compte une incertitude sur le paramètre de rapport des chutes de contrainte C. Cette approche permet de générer un ensemble d accélérogrammes synthétiques d un séisme cible suffisamment variés pour être représentatifs d un grand nombre de scénarios de sources possibles et prenant en compte dans un sens statistique de potentiels effets de directivité. Cette approche a également été appliquée à la simulation d un séisme historique pyrénéen Mw6.1. Dans une seconde partie, nous nous appuyons sur un modèle de source étendue plus complexe, combinant des modèles cinématiques de sources composites fractales avec l approche des FGEs. Le potentiel de la méthode est testé sur une application au séisme de L Aquila. Cela a permis de produire des résultats très satisfaisants sur l ensemble des paramètres des mouvements du sol analysés. Cette méthode de simulation apparaît comme étant très prometteuse pour la mise en œuvre d une méthodologie de simulation en aveugle, même si la principale difficulté réside dans la nécessité de définir la variabilité de nombreux paramètres d entrée mal connus dans le cadre de la simulation d un futur séisme.One of the major scientific problems in seismology is to estimate the ground motions expected at a given site from a future earthquake. The aim of this thesis is to test and validate two different methods of ground motions simulation based on the empirical Green s function approach and to provide elements that can help to develop a blind simulation methodology. In a first part, a simulation method based on a stochastic point source approach is validated on the real data of recent earthquakes well instrumented : the Les Saintes earthquake Mw6.4 and the L Aquila earthquake Mw6.3. We have developed a blind simulation approach by taking into account an uncertainty on the parameter of stress drop ratio C. This approach allows to generate a set of synthetic accelerograms of a target earthquake varied enough to be representative of a large number of possible source scenario and taking into account in a statistical sense potential directivity effects. This approach is also applied to the simulation of an historical Pyrenean earthquake Mw6.1. In a second part, we use a more complex extended source model, combining kinematic models of fractal composite sources with EGF approach. The potential of the method is tested on an application to L Aquila earthquake. This has produced very satisfying results on all ground motion parameters analyzed. This simulation method appears to be very promising for the implementation of a blind simulation methodology, even if the main difficulty lies in the need to define the variability of many poorly known input parameters in the simulation of a future earthquake.NICE-Bibliotheque electronique (060889901) / SudocSudocFranceF

Modélisation de la source sismique et sommation de petits séismes pour l'évaluation des mouvements forts (application à une meilleure estimation de l'aléa sismique dans le sud-est de la France)

L objectif de cette étude est de fournir un code efficace pour simuler les mouvements du sol. Pour s affranchir de la connaissance du milieu de propagation, nous utilisons la technique des fonctions de Green empiriques. Les mouvements du sols sont alors reconstruits par la sommation de plusieurs petits séismes décalés dans le temps de manière à reconstituer le processus de rupture du gros séisme. La principale problématique de cette étude est la mise en place d un schéma de sommation efficace permettant de générer des accélérogrammes réalistes dans le domaine temporel et en bon accord avec les lois d échelles (loi de similitude et modèles en Oméga carré) dans le domaine fréquentiel. Grâce à l intégration de méthodes statistiques, nous évitons les problèmes majeurs rencontrés classiquement dans ce type d étude. Nous about à deux nouvelles méthodes de simulations : une méthode purement stochastique nécessitant la connaissance de seulement deux paramètres concernant le séisme cible (moment sismique et chute de contrainte) et une méthode stochasto-cinématique permettant de reproduire certaines propriétés cinématiques de la rupture sismique sur le plan de faille. Ces deux méthodes sont testées avec succès sur des données réelles (Mexique et Italie). La méthode stochastique est ensuite appliquée pour simuler les mouvements du sol susceptibles d être produits par deux séismes de magnitude modérée dans la région niçoise : un séisme de magnitude 5.7 à terre (10 km au Nord de Nice) et un séisme de magnitude 6.3 en mer (25 km au Sud de Nice). Ces simulations apportent des informations permettant de mieux caractériser l aléa sismique dans la région niçoise.The aim of this study is to provide an efficient tool for strong ground motions simulation. To avoid the problem of unknown subsoil medium, we use the empirical green s function technique. The ground motions are then obtained by summing the recordings of small earthquakes delayed between each other so as to reproduce the rupture propagation effects. The main problem of this study is to establish an appropriate summation scheme, which is able to generate realistic accelerogramms in the time domain, and in agreement with scaling relationships (similarity relation and Omega square model) in the frequency domain. Using statistical methods, we avoid the main problems encountered classically in this kind of study. Finally, we establish two new methods of simulations : a purely stochastic method which requires the specification of only two parameters for the target event (seismic moment and stress drop) and a stochasto-kinematic method which is able to reproduce some of the kinematic rupture properties over the fault plane. These two methods are successfully tested on observed data (Mexico and Italy). Then, the stochastic method is applied to simulate the ground motion which could be produced by two moderate sized eartquakes in the French Riviera : a magnitude 5.7 onshore earthquake (10 km to the North of Nice) and a magnitude 6.3 offshore earthquake (25 km to the Soutg of Nice). These simulations give informations to better characterize the seismic hazard in the French Riviera.NICE-BU Sciences (060882101) / SudocSudocFranceF

Recurrence time of large earthquakes at the western Alps-Mediteranean sea junction : from geological observations and modelling of the seismicity rate

International audienceThe junction between the Western Alps and the Ligurian basin in the Mediterranean Sea is one of the intraplate areas of Western Europe where strong earthquakes occur. The last of these earthquakes occurred in 1887 activating the Ligurian thrust, the major structure which has controlled most of the deformation in the area for about 5 Ma. The available data (GNSS, geological, seismological) allow us to hypothesize the recurrence time of M w 6.8 earthquake, equivalent to the 1887 earthquake, and lead to a range of 6500-3000 years