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

High resolution operational modal analysis of Sant’Agata del Mugello in light of its building history

International audienceThe building history of a cultural heritage building and the after-effects induced by extreme loads such as earthquakes have a durable impact on its modal parameters. This article aims to discuss the peculiarities of some modal parameters extracted from ambient vibration measurements in light of the complex history of Sant’Agata del Mugello. The Romanesque church located in the Mugello basin (Tuscany, Italy) suffered from many earthquakes, at least from 1542. In this context, we carried out two dense ambient vibration campaigns in March and June 2019. We extracted each mode’s natural frequency, mode shape, and damping using the Enhanced Frequency Domain Decomposition. We identified ten modes. Some modes highlight structural singularities related to the building history of the church: rocking of the base of the bell tower; phase opposition between the bell tower and the nave; bending of the walls of the nave. Since the instrumentation benefited from a vast number of recording positions (83), we tested the impact of the sensor placement strategy on the resolution of the identified modal parameters in the specific case of Sant’Agata del Mugello. The resolution of the higher-order modes decreases strongly in case of degraded sensor placement strategy impacting the dynamic analysis of the church

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

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

International audienc

METACity‐Quito: A Semi‐Dense Urban Seismic Network Deployed to Analyze the Concept of Metamaterial for the Future Design of Seismic‐Proof Cities

International audienceABSTRACT The presence of multiscale interactions means that the complex urban wavefield must be observed and analyzed in order to understand seismic ground motion in urban environments. Originally called site–city interaction, the interactions between soil and structures, structures and soil, and between structures contribute to the modification of urban seismic ground motion. This may affect the lateral variability of the ground motion observed in relation to earthquake damage, the characterization of site effects in urban areas, and also the response of civil engineering structures designed without consideration of the immediate urban environment. The METACity‐Quito experiment was designed to take accurate measurements of the effects of the resonator array formed by structures in the seismic wavefield, in the manner of an urban metamaterial. Current applications in the fields of physics, acoustics and, more recently, geophysics have shown the existence of forbidden bands, that is, frequencies at which seismic energy vanishes. This concept could help to understand the heterogeneous distribution of damage in urban areas as well as to imagine the future design of seismic‐proof cities

Installation d’un forage vertical dans la basse vallée du Var, Nice, France

National audienc

Caractéristiques des signaux d'explosions de contre-minage en mer, enregistrés sur les côtes et aux stations permanentes RESIF

RESIF 2019, Rencontres scientifiques et techniques, Biarritz, FRANCE, 12-/11/2019 - 14/11/201

Caractéristiques des signaux d'explosions de contre-minage en mer, enregistrés sur les côtes et aux stations permanentes RESIF

RESIF 2019, Rencontres scientifiques et techniques, Biarritz, FRANCE, 12-/11/2019 - 14/11/201

Accelerometer, Velocimeter Dense‐Array, and Rotation Sensor Datasets from the Sinaps@ Postseismic Survey (Cephalonia 2014–2015 Aftershock Sequence)

International audienc