Hommage à Pierre Courtin (1921-2012)

Propos recueillis par Marie-Cécile Miessner. Emmanuel Pernoud « De pauvres choses minces ou épaisses trouées par endroits » Un pavillon de banlieue, le déjeuner s’éternise, on a bien mangé et bien bu, on a parlé de tout et de rien. Vers les 15 heures, il est temps de travailler, de préparer l’exposition. On vous emmène à la cave. On imaginait trouver des cartons de gravures, du papier imprimé ; on tombe sur du travail d’orfèvre, de grandes plaques de métal recouvertes de signes mystérieux qui..

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

Fiabilité de la détermination de l'âge osseux par la méthode de Sauvegrain et Nahum

The Sauvegrain and Nahum method can be used to assess bone age in addition to methods based on hand and wrist radiographs in children between 9 and 15 years of age. The aim of this study was to evaluate the reliability of this method in a contemporary population. MATERIAL AND METHODS: Elbow radiographs taken for acute trauma in 106 patients (42 girls, 64 boys) aged from 9 to 15 years old were analyzed by four radiologists. Inclusion in the study required the absence of chronic disease and previous trauma as well as parental consent. RESULTS: There was no significant difference between the four observers. The differences between bone age (BA) and chronological age (CA) were 5+/-1 months for the study group, 3.5+/-9 months for the female group, and 6+/-12 months for the male group. Bone age was overestimated compared with chronological age. There was a good correlation between CA and BA (r = 0.84). CONCLUSION: The Sauvegrain and Nahum method seems to overestimate CA in a contemporary population

Fiabilité de la détermination de l'âge osseux par la méthode de Sauvegrain et Nahum

The Sauvegrain and Nahum method can be used to assess bone age in addition to methods based on hand and wrist radiographs in children between 9 and 15 years of age. The aim of this study was to evaluate the reliability of this method in a contemporary population. MATERIAL AND METHODS: Elbow radiographs taken for acute trauma in 106 patients (42 girls, 64 boys) aged from 9 to 15 years old were analyzed by four radiologists. Inclusion in the study required the absence of chronic disease and previous trauma as well as parental consent. RESULTS: There was no significant difference between the four observers. The differences between bone age (BA) and chronological age (CA) were 5+/-1 months for the study group, 3.5+/-9 months for the female group, and 6+/-12 months for the male group. Bone age was overestimated compared with chronological age. There was a good correlation between CA and BA (r = 0.84). CONCLUSION: The Sauvegrain and Nahum method seems to overestimate CA in a contemporary population

Deciphering the dynamics of a heterogeneous sea cliff using ambient vibrations: case study of the Sutta-Rocca overhang (southern Corsica, France)

We report here an interesting case study of dynamic characterization of a limestone sea cliff area, named the Sutta-Rocca overhang, located near the medieval town of Bonifacio (southern Corsica, France). The site belongs to an UNESCO protected area with an important number of visitors per year. Therefore, it is of particular interest to evaluate the potential rockfall hazard along the cliff. With the objective of evaluating the feasibility of an operational technique to monitor the cliff using ambient vibrations, two seismic stations were installed during six months (from 2016 November to 2017 April) in order to identify and monitor the resonant frequencies of the a priori unstable rock mass. Several techniques of classical seismic monitoring are tested on the site: single-station spectral analysis for identification of frequency peaks related to the unstable mass, relative spectral amplifications between stations and the operational modal analysis of the cliff. In particular, data analysis from a temporary campaign of two days of continuous recordings by a linear array of seismometers perpendicular to the sea cliff highlights the main characteristics of the ground motion: the overall motion of the promontory mainly in the north–south direction (perpendicular to the coast) and bending vertical and parallel to the cliff (east–west direction) at higher frequencies. The strong vertical and east–west seismic amplifications are also confirmed by the analysis of five local seismic events recorded at both semi-permanent stations. Even if ambient vibrations and seismic data analysis allows to decipher the dynamics of the Sutta-Rocca overhang, we conclude that classical seismic monitoring of just one single frequency peak with a limited number of sensors is not recommended for this particular site

The Saint Paul en Forêt seismic swarm: an unusual activity in the seismically quiet Maures massif (South-Eastern France)

International audienceIn February 2018, the area around the locality of Saint Paul en Forêt (Eastern part of the hercynian Maures massif,south-eastern France) was struck by 3 earthquakes of ML 3, 3.5 and 2.9 within a few hours. Such seismic activityis quite unusual in this area that is considered to be stable and seismically quiet compared to the surroundingregions (Subalpine Thrust and Fold Belt of Castellane and Nice to the north and East, Ligurian Basin to theSouth-East, Provencal Thrust Belt to the West). These 3 earthquakes are the main shocks of a seismic swarm(72 events) located by the permanent French seismological network (RESIF) a few kilometers west of Saint Paulen Forêt and at around 15 km depth. A few hours after the beginning of the activity, we deployed a temporarynetwork of 5 seismometers around the epicenter in order to improve the location of the seismicity.Such unusual seismic swarm activity in this region questions about (1) the tectonic structure responsible for it, (2)the potential implication of aseismic deformation and fluids and (3) the assessment of the seismic hazard of thezone that is currently considered as low to moderate.To understand in detail the spatial and temporal activity of the swarm, we first perform a template matchinganalysis on the continuous recordings to detect additional earthquakes not detected by the usual routine processes(STA/LTA). We use as templates the 72 earthquakes of the swarm detected and located in routine. The templatematching process is applied on the four nearest permanent stations (distance 30-40 km) for the daily continuousrecordings between February 1st and June 8th in 2018. As a result, we detect more than 600 earthquakes (ML >-0.58) on the nearest permanent station (TRIGF station, ∼ 30 km). The seismic activity started on February 16thand the majority of the earthquakes occurred up to February 20th with a climax on February 17th and 18th (up to80 events/hour and the occurrence of the 3 main shocks). After February 20th, a small but regular activity (a fewearthquakes per week) lasted up to June.Second, we focus on the 282 earthquakes detected on the 4 nearest permanent stations. We locate these eventsby adding data of two nearby stations of the temporary campaign AlpArray (A205A and A206A), one nearbyCEA-LDG station (LMR) and data of our own temporary network. We use double-difference relocation method(hypoDD software) by inverting catalog times and delay times measured by cross-correlation. As a result, theearthquakes cluster in a 500 m x 500 m elongated NE-SW structures dipping toward the NW. This geometry is inagreement with the fault plane solution we otherwise computed for the 3 main shocks.Finally, we analyze the spatio-temporal dynamics of the swarm and discuss the possible causes of such seismicactivity

Hommage à Pierre Courtin (1921-2012) : témoignages réunis par Marie-Cécile Miessner

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Installation of a borehole vertical array in the Var valley, Nice, France

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