Dense ARray for seismic site effect Estimation – DARE

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.

To come

To come

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.