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

Discussion on ‘Palaeoseismic structures in Quaternary sediments, related to an assumed fault zone north of the Permian Peissen-Gnutz salt structure (NW Germany) – Neotectonic activity and earthquakes from the Saalian to the Holocene’ (Grube, 2019)

International audienceWe discuss the significance of deformation structures in Quaternary sediments observed by Grube (2019) in the Peissen quarries (NW Germany) in light of the geological context. Evidence for polygonal patterns visible in aerial images in the study area shows that the wedge structures interpreted by Grube (2019) as earthquake-induced sand blows may rather correspond to thermal contraction cracks filled with aeolian sand in a permafrost environment. In the study sites, brittle deformations caused by (i) the rise of a salt diapir, (ii) salt dissolution, (iii) the development of Pleistocene permafrost and (iv) possibly, water circulation under pressure in the Scandinavian ice sheet margin may have coexisted. We support the idea that, while the morphology of deformation generally makes it possible to determine the stress state to which the sediments have been subjected and the quantity of water available in the system at the time of deformation, the nature of the factors causing the stresses remains difficult to identify. In the end, we highlight other useful criteria that should be privileged for palaeoseismic research in such complex geological settings

Impact du rebond post-glaciaire alpin : modélisation numérique et analyse GNSS

International audienceLe réseau permanent GNSS montre un soulèvement au niveau des Alpes. Le rebond post-glaciaire induit par la fonte du glacier alpin est envisagé comme processus majoritaire pour expliquer ce soulèvement. La modélisation numérique en éléments finis est choisi pour étudier l'impact des processus transitoires de rebond. Ces modèles sont comparés avec 1) les vitesses verticales GNSS affectées d'un lissage gaussien (Masson et al., 2019) et 2) les vitesses verticales brutes

Part du rebond post-glaciaire dans le soulèvement des Alpes occidentales

This poster presents the study, based on GPS data, of the share of post-glacial rebound in the uplift of the Western Alps. It was presented at the Résif Scientific and Technical Meetings held in Biarritz in November 2019. Résif is a national research infrastructure dedicated to the observation and understanding of the Earth's internal structure and dynamics. Résif is based on high technology observation networks, composed of seismological, geodetic and gravimetric instruments deployed in a dense manner throughout the French territory. The data collected allow the study of ground deformation, surface and deep structures, local and global seismicity and natural hazards, particularly seismic, on the French territory with a high spatio-temporal resolution. Résif is integrated into the European (EPOS - European Plate Observing System) and worldwide instruments that allow to image the Earth's interior in its entirety and to study many natural phenomena.Ce poster présente l'étude, à partir de données GPS, de la part du rebond post-glaciaire dans le soulèvement des Alpes occidentales. Il a été présenté aux Rencontres scientifiques et techniques Résif qui se sont déroulées à Biarritz en novembre 2019. Résif est une infrastructure de recherche nationale dédiée à l’observation et la compréhension de la structure et de la dynamique Terre interne. Résif se base sur des réseaux d’observation de haut niveau technologique, composés d’instruments sismologiques, géodésiques et gravimétriques déployés de manière dense sur tout le territoire français. Les données recueillies permettent d’étudier avec une haute résolution spatio-temporelle la déformation du sol, les structures superficielles et profondes, la sismicité à l’échelle locale et globale et les aléas naturels, et plus particulièrement sismiques, sur le territoire français. Résif s’intègre aux dispositifs européens (EPOS - European Plate Observing System) et mondiaux d’instruments permettant d’imager l’intérieur de la Terre dans sa globalité et d’étudier de nombreux phénomènes naturels

Experimental soft-sediment deformation caused by fluidization and intrusive ice melt in sand

(IF 3.41; Q1)International audienceIdentifying the driving mechanisms of soft‐sediment deformation in the geological record is the subject of debate. Thawing of ice‐rich clayey silt above permafrost was proved experimentally to be among the processes capable of triggering deformation. However, previous work failed so far to reproduce similar structures in sand. This study investigates fluidization and intrusive ice formation from soil models in the laboratory. Experimental conditions reproduce the growth of ice‐cored mounds caused by pore water pressure increase during freeze‐back of sand in permafrost context. Excess pore water pressure causes hydraulic fracturing and the development of water lenses beneath the freezing front. Later freezing of the water lenses generates intrusive ice. The main structures consist of sand dykes and sills formed when the increase in pore water pressure exceeds a critical threshold, and soft‐sediment deformations induced by subsidence during ice melt. The combination of processes has resulted in diapir‐like structures. The experimental structures are similar to those described in Pleistocene sites from France. These processes constitute a credible alternative to the seismic hypothesis evoked to explain soft‐sediment deformation structures in other European regions subjected to Pleistocene cold climates

Features caused by ground ice growth and decay in Late Pleistocene fluvial deposits

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Response to the discussion by Van Vliet-Lanoë et al. of the paper ‘Features caused by ground ice growth and decay in Late Pleistocene fluvial deposits, Paris Basin, France’ (Bertran et al., 2018, Geomorphology 310, 84–101)

(IF 3.82; Q1)International audienceThe response by Van Vliet-Lanoë et al. to our paper on potential thermokarst features in Pleistocene alluvial deposits from the Paris Basin (France) presents inconsistencies that we consider here successively. These are (1) the map of the maximum extent of Pleistocene permafrost in France, (2) the genesis of liquefaction and fluidization structures in periglacial environments, (3) the origin of thermokarst lakes and recumbent folds, (4) the depositional context of sandy units, and (5) the age of the studied deposits. All structures result from the interplay between (1) the growth and degradation of ice wedges, which are responsible for the development of a mound-like topography (badland thermokarst reliefs) on the edge of the alluvial terrace and from the initiation of ponding elsewhere on the terrace, (2) the degradation of lithalsa that developed later in the lacustrine deposits. The sequence was dated confidently to the Late Weichselian

Influences of the Climatic Cycles on the Crustal Deformation and Seismicity - Part 2

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