Academic literature on the topic 'Ubaye swarms'

Abstract We study two earthquake swarms that occurred in the Ubaye Valley, French Alps within the past decade: the 2003–2004 earthquake swarm with the strongest shock of magnitude ML = 2.7, and the 2012–2015 earthquake swarm with the strongest shock of magnitude ML = 4.8. The 2003–2004 seismic activity clustered along a 9-km-long rupture zone at depth between 3 and 8 km. The 2012–2015 activity occurred a few kilometres to the northwest from the previous one. We applied the iterative joint inversion for stress and fault orientations developed by Vavryčuk (2014) to focal mechanisms of 74 events of the 2003–2004 swarm and of 13 strongest events of the 2012–2015 swarm. The retrieved stress regime is consistent for both seismic activities. The σ 3 principal axis is nearly horizontal with azimuth of ~ 103°. The σ 1 and σ 2 principal axes are inclined and their stress magnitudes are similar. The active faults are optimally oriented for shear faulting with respect to tectonic stress and differ from major fault systems known from geological mapping in the region. The estimated low value of friction coefficient at the faults 0.2–0.3 supports an idea of seismic activity triggered or strongly affected by presence of fluids.

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.

Summary The Ubaye Region (French Western Alps) is one of the most seismically active regions in France. It is regularly struck by mainshock-aftershocks sequences like in 1959 (ML5.3), seismic swarms (2003–2004), and complex sequences (2012–2015) characterized by successive mainshocks clustered in time and space. This diversity of seismic behaviour highlights the complex processes at play in this area. To improve our understanding of these processes, in this study, we compile a regional catalogue of existing focal mechanisms, completed by 100 new calculated focal mechanisms of aftershocks following the 07/04/2014 mainshock (ML5.1). The oriented stress-state we reconstruct for different periods and sub-areas are similar to each other and to previous published values focusing on swarm periods. We then calculate fluid-pressure required to trigger the earthquakes. Most of the events (65 per cent) need fluid-overpressure between 15 and 40 MPa (17-to-40 per cent of the hydrostatic pressure) with a median value of 24 per cent. Moreover, even the largest events, like the mainshocks in the 2012–2015 sequence, appear to be triggered by fluid-pressure, similarly as events within swarm sequences. However, while fluid-overpressure decreases with time in an aftershock sequence, it varies randomly at high levels during a swarm sequence. Therefore, based on a fault-valve model, we propose that: 1) the fluids trapped in the fault plane tend toward lithostatic pressure and trigger the mainshock rupture and 2) part of the aftershocks are induced by the diffusing fluid-pressure. On the contrary, swarms need external, likely deep, fluid-pressure feedings. Fluid-pressure is likely to be a common triggering mechanism of the seismicity in the Ubaye Region, even if the involved processes should differ to explain the different types of seismic sequences.

Ce travail s'intéresse à l'étude du comportement en conditions statiques des failles dans la croûte supérieure continentale et plus particulièrement à l'effet des surpressions de fluides sur la réactivation des failles et le déclenchement des séismes. Pour ce faire, une analyse associant sismologie, géologie structurale, pétrophysique, géochimie et modélisation hydromécanique a été menée dans la région de l'Ubaye (Alpes du sud) où a eu lieu, en 2003-2004, un essaim sismique en relation avec des failles régionales affleurant plus au sud dans le massif cristallin de l'Argentera. Les mécanismes au foyer de 74 événements de cet essaim sismique ont été déterminés. À partir de ces mécanismes et d'autres données sismologiques ainsi que de modèles mécaniques basés sur la théorie de Mohr-Coulomb, cette étude a permis de confirmer que l'activité sismique de l'essaim était liée à la présence de surpressions de fluides et d'expliquer l'évolution spatio-temporelle des surpressions. Un modèle hydromécanique est proposé afin de concilier les évolutions spatio-temporelles de la sismicité et des surpressions de fluides. L'étude d'un affleurement d'une faille régionale de l'Argentera combinant analyse structurale, mesures pétrophysiques et modélisation hydromécanique a ensuite permis de préciser le comportement hydromécanique des failles aux profondeurs hypocentrales et plus particulièrement leur capacité à être compactées et à développer des surpressions de fluides. Finalement, l'initiation des séismes à la base de la zone sismogénique est explorée à partir d'analyses géochimiques et mécaniques menées sur de veines à paragénèse quartz-chlorite formées à la base de la zone sismogène. Ces résultats sont comparés avec ceux déduits de l'analyse de l'essaim sismique de l'Ubaye. Ce travail a permis d'étudier le comportement sismomécanique des failles et les interactions entre failles, fluides et séismes à travers la zone sismogène. Il met l'accent sur l'importance de coupler les approches sismologiques, hydrauliques et mécaniques dans l'étude des failles actives
This work adresses the behavior of faults in the upper continental crust under static conditions and moreparticularly the effect of fluid overpressures on fault reactivation and earthquake triggering. In order toreach this goal, an analysis combining seismology, structural geology, petrophysics, geochemistry andhydromechanical modeling has been carried out in the Ubaye region (southern French-Italian Alps) wherea seismic swarm related to regional faults exposed in the Argentera basement massif (located furthersouth) occurred in 2003-2004. Focal mechanisms of 74 events from this seismic swarm have beendetermined. Based on these mechanisms and other seismological data and on mechanical modeling basedon the Mohr-Coulomb theory, this study allows to confirm that the seismic activity of the swarm waslinked to the presence of overpressurized fluids and to explain the spatio-temporal evolution ofoverpressures. A hydromechanical model is proposed in order to account for the spatio-temporalevolutions of both seismicity and pore fluid overpressures. The study of an exposure of an Argenteraregional fault combining a structural analysis, petrophysical measurements and a hydromechanicalmodeling has allowed to decipher the hydromechanical behavior of faults at hypocentral depths, and moreparticularly to determine the ability of faults to be compacted and to develop fluid overpressures. Lastly,the initiation of earthquakes at or near the base of the seismogenic zone is explored through geochemicaland mechanical analyses of quartz-chlorite veins formed at the base of the seismogenic zone. Theseresults are then compared with those deduced from the analysis of the Ubaye seismic swarm. This workallows to study the seismo-mechanical behavior of faults and the interactions between faults, fluids andearthquakes across the seismogenic zone. It emphasizes the importance of associating seismological,hydraulic et mechanical analyses in the study of active faults