It is well-known that superficial geological layers can strongly modify the surface ground motion induced by an earthquake, that can lead to significant amplifications. These so-called site effects are highly variable from one site to another. Site-specific studies are therefore necessary to take into account this phenomenon in all its complexity at a given site. However site-specific studies have historically been considered difficult to carry out in low-to-moderate seismicity regions.
The DARE project proposes to exploit data from 2 complementary dense seismic experiments carried out on the Messinian canyon of the Rhône river (South-East France) where some critical infrastructures are located. These data will be used to investigate the contribution and interest of innovative methods, combining in particular dense array processing and the use of seismic ambient noise for site effect studies. Using dense arrays will help to improve the spatial resolution of imaging studies and to catch the ground motion variability and site effect features related to the complex geometry of the canyon. Seismic ambient noise will be used as alternative seismic data of obvious interest in low-to-moderate seismicity regions where moderate to large earthquakes have long return periods and require recordings over long duration. <br />This project aims to study the contribution of these methodologies for the implementation of site-specific studies using relatively short temporary experiments in low-to-moderate seismicity regions. Beyond the study of this specific canyon, the results of the project will more generally contribute to improve seismic site effect estimation in terms of 1) methodological developments, 2) understanding of physical processes leading to seismic amplifications observed in complex geological structures and 3) observations on deep western European sedimentary valleys. <br />Besides, site-specific results obtained in this project will be confronted to estimates based on ergodic approaches (i.e. using data from global databases) that are commonly used in seismic hazard assessment studies especially in low-to-moderate seismicity regions. In the future, the repetition of this type of study will help understand the conditions of applications and limitations of the use of such ergodic approaches in seismic site effect estimations.
In order to characterize the geological medium and its impact on the seismic motion, data from 2 complementary seismic campaigns will be exploited. The first campaign, carried out by IRSN prior to the DARE project, consisted in deploying 400 all-in-one seismic nodes over a 10 km x 10 km area for one month. This campaign targets the recording of seismic ambient noise generated by both global and local sources. To complement this first acquisition, a second seismic campaign will be carried out within the framework of the project. This second campaign consists in deploying about 50 broadband stations over the same target area for nine months and aims to record the seismicity (including teleseismic events, local and regional seismicity). These 2 campaigns with complementary time scales will allow us to compare the results obtained from the analysis of seismic ambient noise and seismicity.
In addition, several approaches will be adopted to estimate the site effects. The first approach relies on the empirical (or experimental) estimation of seismic local effects through the analysis of seismic motion recordings acquired during the 2 campaigns mentioned above. The second approach is based on the simulation of surface ground motion using numerical modelling of seismic wave propagation in a model of the Earth’s sub-soil.
This strategy combining different seismic observations (ambient noise & seismicity) and approaches (numerical & empirical) will allow us to propose and confront alternative methods; evaluate their own interests, uncertainties and limitations. The application of this integrated procedure in the Rhône Valley will also lead to a robust site effect estimation in our target area.
This French-German collaboration (budget of 700 840 euros funded 53.9% by ANR and 47.1% by the German agency) brings together experts in the various fields of seismology and geology covered by the project.
Mouvements sismiques : amplification sous surveillance (Pour La Science -French popular science magazine - June 2021) -https://www.pourlascience.fr/sr/article-partenaire/mouvements-sismiques-amplification-sous-surveillance-21924.php
Seismic hazard represents a major worldwide scientific issue in view of potential catastrophic consequences experienced by people and facilities. Methodological developments to improve our ability to evaluate the seismic hazard are then of particular importance. We focus here on the so-called site effects that correspond to the modification of the seismic motion by the local geological configuration and that can lead to dramatic seismic amplifications. By being related to local conditions, site effects are highly variable from one site to another. That is why site-specific studies can greatly contribute to improve the hazard prediction at a specific site in comparison to ergodic estimates based on data from global databases. However site-specific studies have historically been considered difficult to carry out in low-to-moderate areas (such as France and Germany) where moderate to large earthquakes have long return periods. The DARE project proposes to exploit data from 2 complimentary dense passive experiments (100s of captors) that will be acquired on one specific km-scale sedimentary basin in the French Rhône valley. These data will be used to investigate the contribution and interest of innovative methods combining dense array processing and the use of seismic noise to offer possibilities to perform site-specific studies using relatively short temporary experiments in low-to-moderate areas. The density of instruments proposed will help to 1) improve the spatial resolution of imaging studies, allowing for a better characterization of the basin and 2) to catch the variability and multi-dimensional features of the site effects. We propose to adopt a multi-approach estimation of site effects using different seismic observations (noise & seismicity) and approaches (numerical & empirical). This strategy will allow us to propose and confront alternative methods; evaluate their own interests, uncertainties and limitations. The application of this integrated procedure in the Rhône Valley will lead to a robust estimation of site effects in an area where many critical infrastructures are located. Beyond improving our knowledge of this specific basin, the results of the project DARE will more generally contribute to improve seismic site effect estimation in terms of 1) methodological developments, 2) understanding of physical processes leading to seismic amplifications and 3) observations on km-scale western European sedimentary basins. Site-specific results obtained in this project will be confronted to estimates based on ergodic approaches that are commonly used in seismic hazard assessment studies especially in low-to-moderate areas. This will help understand the conditions of applications and limitations of the use of such ergodic approaches in seismic site effect estimations.
Madame Bérénice Froment (Pôle Santé Environnement - Direction Environnement)
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
UP University of Potsdam / Institute of Earth and Environmental Science
GFZ German Research Centre for Geosciences / Section 2.6 (Seismic Hazard and Risk Dynamics)
PSE-ENV Pôle Santé Environnement - Direction Environnement
ISTERRE Institut des Sciences de la Terre
Help of the ANR 370,440 euros
Beginning and duration of the scientific project: March 2020 - 36 Months