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

Mise en place d'une méthode de détection, localisation et caractérisation rapide des évènements sismiques et détermination des incertitudes associées

International audienceLe CEA a une mission d'alerte séisme en France métropolitaine qui consiste à prévenir rapidement les autorités françaises de l'occurrence d'un séisme.La méthode GRiD MT se base sur un unique algorithme automatique d'inversion du tenseur des moments sur grille qui permet simultanément la détection, la localisation et la caractérisation rapides des évènements sismiques. Nous présentons les résultats de la méthode appliquée aux évènements de la région du Sud-Est de la France.De plus, grâce au nombre important de résultats obtenus sur la grille de sources virtuelles et dans le temps, nous montrons qu'il est possible de déterminer statistiquement les incertitudes sur les paramètres de la source.Des études sont en cours afin d'apporter des précisions sur le choix des stations sismiques RESIF à prendre en compte et sur l'estimation du niveau de confiance des solutions.A terme, cette étude vise à faciliter la mise en place de la méthode GRiD MT dans d'autre régions d'intérêt et pour d'autres sources sismiques, mais également d'obtenir un catalogue de mécanismes au foyer dans ces régions

A decade of seismicity in metropolitan France (2010–2019): the CEA/LDG methodologies and observations

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

Constraining the point source parameters of the 11 November 2019 Mw 4.9 Le Teil earthquake using multiple relocation approaches, first motion and full waveform inversions

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Tectonic significance of the 2021 Lamjung, Nepal, mid-crustal seismic cluster

International audienceSince the Mw 7.9 Gorkha earthquake of April 25, 2015, the seismicity of central and western Nepalese Himalaya has been monitored by an increasing number of permanent seismic stations. These instruments contribute to the location of thousands of aftershocks that occur at the western margin of the segment of the Main Himalayan Thrust (MHT) that ruptured in 2015. They also help to constrain the location of seismic clusters that originated at the periphery of the fault ruptured by the Gorkha earthquake, which may indicate a migration of seismicity along the fault system. We report here a seismic crisis that followed the Lamjung earthquake, a moderate Mw 4.7 event (ML 5.8, MLv 5.3) that occurred on May 18, 2021, about 30 km west of the Gorkha earthquake epicenter at the down-dip end of the locked fault zone. The study of the hypocentral location of the mainshock and its first 117 aftershocks confirms mid-crustal depths and supports the activation of a 30-40° dipping fault plane, possibly associated with the rupture of the updip end of the MHT mid-crustal ramp. The cluster of aftershocks occurs near the upper decollement of the thrust system, probably in its hanging wall, and falls on the immediate northern margin of a region of the fault that has not been ruptured since the 1344 or 1505 CE earthquake. The spatio-temporal distribution of the first 117 aftershocks shows a typical decrease in the associated seismicity rate and possible migration of seismic activity. Since then, the local seismicity has returned to the pre-earthquake rate and careful monitoring has not revealed any large-scale migration of seismicity towards the locked fault segments

FMHex20: An earthquake focal mechanism database for seismotectonic analyses in metropolitan France and bordering regions

We present a compilation of over 1700 focal mechanisms for nearly 1300 earthquakes in metropolitan France and bordering regions of Western Europe. It is based on both published and unpublished sources (articles, reports, observatory websites) for which the focal mechanism solutions have been verified for internal consistency, corrected in cases of minor errors and rejected in cases of major inconsistencies between the parameters. The database, labeled FMHex20, is a first version and should be regularly updated in the future as part of an ongoing effort within the Seismicity Transverse Action of the French Résif research infrastructure. We also present first-order seismotectonic analyses for the whole metropolitan France and for two regions (Western France and Northern Alps-Jura-Vosges) to illustrate how the FMHex20 database can serve as a basis for geodynamic or seismic hazard zonation studies. Combined with complementary datasets, it can improve our understanding of the kinematics of potentially active faults, including in very-low-strain-rate regions as is the case for most of France