A more complete version of this article appeared in web and print versions in the Lettre Résif n°17 of January 2020 (french)
On November 11, 2019 at 11:53 a.m., the Résif seismological stations recorded an earthquake of a magnitude greater than 5 near Montélimar. An analyst from the Eost in Strasbourg locates the earthquake manually less than an hour later. He confirms a magnitude of 5.2 Mlv. The characteristics of the earthquake immediately appear remarkable: high magnitude and very shallow depth (2 to 3 km). This information is confirmed at the same time by the relocation work carried out at ISTerre (Grenoble) and Geoazur (Nice), where the mechanism at the focus is validated from the Résif seismological data, confirming the probable NE-SW direction of the fault.
On the same day, the national post-seismic cell was activated and, from that moment on, many instruments and scientific teams were mobilized to monitor the evolution of the crisis and determine its characteristics with the greatest possible accuracy (magnitude, location, depth, type of associated fault movement, ground movements, etc.).
Another surprising feature of this earthquake was quickly observed: it was followed by very few aftershocks. Apart from an aftershock of magnitude 1.5 three minutes after the main shock, the first aftershock (2.2 MLv) felt occurred on 13 November.
A few days after the first tremor, the CNRS initiated a specific scientific mission in which specialists from the CNRS, universities and other organizations cooperated to study the event and its causes.
From the very first hours, mobile instruments were deployed to cover the area.
The area in which the earthquake occurred is poorly instrumented. However, it is crucial to have as much data as possible in order to be able to understand the phenomena in progress. This is why, just a few hours after the earthquake, an engineer prepares the instruments of the mobile park of the seismological observatory in Geoazur. Two seismologists leave Sophia-Antipolis at 5 p.m. with four seismological stations, as well as the batteries and all the equipment necessary for the mission.
The same evening they set up two stations: one in the north of Montélimar, in a building of the urban community (starting at 9:00 pm), the second on the commune of Le Teil in the cellar of the town hall (starting at 10:30 pm).
Geoazur’s two seismologists continue the operation the next day and install two other stations, in St-Thomé in a chapel (starting at 10:00 am) and in Alba-La-Romaine (starting at 11:15 am). They are joined during the day by a team from Grenoble who deploy five broadband seismological stations, three accelerometers and 24 autonomous short-period sensors (Fairfield nodes) from the Résif-SisMob park in two days. Some of these stations are telemetered, allowing the monitoring of aftershocks and strong movements in real time.
For its part, IRSN is installing three broadband stations around the epicentral zone: in Saint Thomé by co-locating a node installed by the Grenoble team, in Larnas near the town hall’s technical premises and in Chateauneuf-du-Rhône. IRSN also notes the recordings made by three broadband stations located about 20 km further south in the Tricastin area. These temporary stations had been installed since early November and therefore recorded the earthquake.
Cerema is installing five seismological stations with a velocimetric and accelerometric sensor around the epicentral zone and the village of Saint-Thomé where topographical site effects are suspected. It is also participating in the installation of 4 nodes with ISTerre in historic buildings affected by the earthquake (Château-Lafarge and Tour-Saint-Michel in Vivier) to better understand the dynamic behaviour of these structures. Measurements of the finite deformation recorded by the higher-than-average buildings (cracks and other damage) are carried out in parallel with the recording of aftershocks by these buildings to contribute to the studies of archaeoseismicity in the region.
In total, about 40 seismological stations are set up in less than five days in the area of interest to the scientists. In addition to the data thus collected, EDF is making available broadband and accelerometric data from one of its stations in the Cruas sector for the November 11 event, as well as the main replicas.
Geological surveys of surface fractures are carried out on the ground and from the air
A reflection group on the theme “Active Failures” was created within the “Aléa” axis of the Transversal Action on Résif Seismicity in January 2019. It brings together geologists and geophysicists from the academy and various organizations, some of which are external to Résif. Its objective is to contribute, through the study of active faults in France, to a better understanding of the processes that control active deformation and seismic hazard. To find out more.
As part of this “Active Faults” theme, a team of eight scientists from four structures (Géosciences Montpellier, IRSN, Géoazur, ISTerre) arrived in the area on November 13, before the potential surface traces of the rupture disappeared due to bad weather conditions and human activity. They concentrated their exploration along a discontinuity identified thanks to radar satellite images (InSAR), the analysis of which was made available the day after the earthquake (SENTINEL data from the European Copernicus programme) on Twitter.
In total, the team observed around fifteen point rupture indices distributed over a length of four kilometres, along a fault segment that had been mapped but whose quaternary activity had never been observed in the field. Some of these indices are scanned with a laser to accurately quantify ground deformation. In parallel with the ground analysis, a campaign of Lidar overflights by helicopter and UAV is carried out in an attempt to map the surface fault under the vegetation canopy that covers a large part of the area.
Field observations are also carried out by geologists from Lyon in order to clarify the geological and structural context of the earthquake. They supplement this information with a UAV overflight to acquire optical images of the area that is likely to present surface breaks. At the same time, a rock sampling mission is also carried out by a Geoazur team in order to study the frictional and mechanical properties of the rocks on the surface.
Two GPS receivers are installed on November 15, recording every second. On that day, two other sites are identified for the installation of stations. The installation of the four GPS stations is finalized on December 3rd.
On Monday, November 18, a Geoazur team connects a DAS (Distributed Acoustic Sensing) measurement system to a 14 km section of the local fiber-optic telecom network (line provided by the Ardèche-Drôme-Numérique network and its operator AD-TIM), between Alba-la-Romaine, Saint-Thomé and Valvignières, in order to detect future seismic shocks. This is the first time in Europe (and the second time in the world) that measurements of this type are made in rapid response to an earthquake.
The Macro-seismic Intervention Group investigates the intensity of the tremor in the communes
In contrast to the magnitude, which is calculated from seismological records, the intensity of the tremor in each municipality is only known by analysing the observable effects on people, objects and buildings.
In the case of the Teil earthquake, more than 2,000 people who felt the tremor responded to the survey on the effects of the earthquake via the website www.franceseisme.fr, some of them within the first few minutes, thus allowing a preliminary and rapid estimate of the intensity of the tremor. The BCSF-RéNaSS is launching an online survey on 12 November with the town halls of the communes potentially affected. In view of the initial damage reported, the GIM (Macro-seismic Intervention Group) is activated to precisely assess the EMS98 intensities of the communes close to the epicentre, based on the effects of the earthquake on buildings, taking into account their vulnerability and the levels of damage observed.
Seven experts (IRSN, ISTerre/Résif-Rap, Cerema, Pacte, IPGS and Eost/BCSF-Rénass) responded to the call and worked in the area from November 18 to 22. Divided into teams of 2 or 3, they inspected a total of 24 municipalities, assisted by the mayors or municipal services and accompanied by the fire brigade, as in the Teil municipality. In the majority of cases, they observe cracks, sometimes large, open and numerous, but in the most damaged areas, such as in Le Teil and Viviers, some old buildings have largely collapsed. Sometimes, very little damage is observed on the outside of the building, whereas a visit to the inside reveals significant damage to walls, ceilings and floors.
The conclusions of the macro-seismic survey, EMS98 intensities, are one of the major elements on which the inter-ministerial commission for the classification of communes as natural disasters for the assumption of damages by insurance companies is based. In view of the major damage caused by this earthquake, an accelerated commission was held on 20 November to rule on 9 of the most affected communes, which were analysed by the GIM on 18 and 19 November, all of which were classified as natural disasters. The 15 other communes studied by the GIM and those being analysed by the BCSF-Rénass will be dealt with by the following commissions.
The strongest intensities close to the epicentre are VII at the Teil and Viviers, locally reaching VIII at La Rouvière and Mélas for two districts of the Teil closest to the fault. These are the highest intensities recorded in metropolitan France since the 1967 Arette earthquake.
Scientists study the data in the labs
In parallel with field observations, geophysicists are mobilized behind their screens to collect and analyze the available data: exploration of seismic data, decoding of radar interferometry satellite images, study of GPS data, identification of the mechanism at the focus, detection of the weakest aftershocks, etc. Knowledge of the event is gradually being refined. In Grenoble, a team organizes the distribution of seismological data from -and for- the whole community.
Colleagues geologists come as reinforcements to specify the structural context of the earthquake. The geology laboratory in Lyon provides a geological section of the zone close to the earthquake rupture (2.7 km to the south) and begins detailed geological mapping.
A multidisciplinary team from the University of Grenoble-Alpes is launching an online study to understand people’s reactions to the effects of the Teil earthquake on November 11, 2019. The aim of this anonymous study is to improve earthquake prevention.
At Geoazur and ISTerre, seismic movement simulations are being launched to better understand the origins of this highly unusual earthquake.
Researchers, engineers and technicians from a wide range of disciplines are providing assistance for an hour, a day or over the long term.
At the microphone of the media
On 12 November, the day after the earthquake, a large part of the Résif community gathered for three days in Biarritz for the Scientific and Technical Meetings that are held every two years. Although some scientists cancelled their visit to go to the field or coordinate operations, many participants were asked by the media to comment on the event and contribute their expertise.
In order to best respond to the many requests, the scientific expertise mission initiated by the CNRS took charge of communication with the media a few days after the earthquake.
More than a hundred articles and reports are devoted to these events in the regional and national press. The earthquake is also attracting the attention of the international media, such as the National Geographic.
The speed and efficiency of the field interventions were possible thanks to the involvement of engineers and researchers from observatories and other organizations, coordinated by the post-earthquake unit, but also thanks to the cooperative practices that have developed within Résif over the years. The cross-disciplinary seismicity action has also enabled the rapid mobilization of a multidisciplinary team thanks to the collaborations initiated in recent years, including with scientists from outside Résif.
Editor: Véronique Bertrand (coordinator) and the actors of this mobilization
The field photos and the preliminary report of the BCSF are available in HD format on the HAL-Résif open archive.