Motivation & Objectives
The uncertainties affecting ground motion models remain the highest source of uncertainty in the results of seismic hazard studies.
Despite the progress already done, it is important to obtain a prediction of ground motion adapted to rock conditions (as an input for site response simulation), and adapted to various types of sites.
The main scope of WP3 is to improve the understanding and description of ground motion, in order to provide a more realistic assessment of seismic motion for seismic hazard assessment.
The work axes include:
- Improving empirical ground motion estimates for both generic and site-specific hazard assessment studies
- Improving the quantification and the physical understanding behind seismic wave attenuation
- Ground Motion in low seismicity regions
- Ground Motion modeling through simulations
Paola TRAVERSA (EDF) and Philippe RENAULT (SwissNuclear)
- In site-specific seismic hazard assessment studies, the soft-rock ground-motion predictions from existing ground motion models are commonly adapted for hard-rock site conditions using the “kappa” scaling. This model has been the standard approach for computing hard-rock ground motions for the last 20 years in the central and eastern US (CEUS), and has recently become the standard practice in Europe and other regions of the world as well. This approach leads to a large increase in the high frequency (> 10 Hz) ground motions as compared to soft-rock (“standard” rock) ground motions, e.g. in the western US (WUS), Italy, Turkey.
- An initial evaluation performed on empirical data indicates a weaker dependence on kappa than found using the standard approach (Kishida, et al, 2016; Ktenidou & Abrahamson, 2016). This result, however, requires further evaluation to make sure that it is robust and is not the result of trade-offs with other parameters in the data sets.
- The amount of available ground motion data on hard-rock sites has greatly expanded worldwide during the last years. However, the dependence of the high-frequency ground motion on kappa has not yet explicitly been addressed.
Funding member: EDF
Type: Scientific Partnership (4 years).
Collaboration: University of California - Berkeley, EDF.
Status: In progress (start date: 2017).
In the framework of site-specific ground motion assessment, one of the major challenges consists in determining the seismic bedrock ground motion (reference ground motion) to be used as input for site response analyses. Before the beginning of SIGMA2, with the exception of a prototype model developed using Japanese ground motion (Laurendeau et al., 2017), no ground motion model was suitable for use in estimating reference ground motion at hard rock horizons, particularly in low seismicity regions.
Program of research
The aim of this work is to develop ground motion models specifically referred to hard rock horizons, obtained using “corrected” ground motions. Ground motion recorded at the surface at stiff soils and rock sites is corrected from its site response in order to obtain reference hard rock ground motion. Site response with respect to the reference condition is evaluated both, by 1D simulations and by Generalized Inversion Techniques (GIT) of Ground Motion.
Funding member: CEA
Collaboration: CEA-Isterre, Université de Grenoble
Status: in progress
To assess site-specific ground motion it is common practice to calculate seismic hazard at bedrock and then perform 1D / 2D site response analyses. For this reason, the ground motion at bedrock should be free from amplification phenomena and its site response flat (the so-called “reference ground motion”). However, Ground Motion Prediction Equations (GMPE) generally associate this behavior to the generic rock condition, usually identified only through the VS30 exceeding a given threshold. This assumption does not imply that the ground-motion recorded at these sites is completely unaffected by amplification, which might be caused by peculiar morphological/stratigraphic features. Indeed it has been recently shown that several recording stations belonging to the EC8-A soil category are affected by relevant site effects.
Then the assumption that GMPE-based rock site predictions are unaffected by amplification phenomena may cause inaccurate prediction of the expected motion when the hazard is evaluated including site effects, due to the amplified response of the rock motion.
The identification of reference rock sites, where the amplification response is expected to be negligible, would be of great help to avoid this ambiguity in the predictions.
The main objectives of this research task are:
- to propose a methodology to identify reference rock sites;
- to define geological, geotechnical and seismological proxies for recognizing reference rock sites;
- to compare the reference rock sites characteristics in active regions (Italy) and moderate activity regions (France);
- to calibrate ground motion models (GMPEs) for reference rock sites, possibly for response and Fourier spectra.
Funding member: ORANO
Type: Post-doc (2,5 years)
Collaboration: INGV-Milan (Italy), EDF (France)
Status: In progress (starting date : June 2018)
Estimation of the Site-term for site-specific seismic hazard assessments in low-to-moderate seismicity regions
Collaboration: GFZ Potsdam
- Generalized inversion techniques (GIT) have become popular techniques for determining ground motion parameters, particularly in low-to-moderate seismicity regions. Indeed, it has been shown that GIT can potentially provide reliable site response estimates, even at sites where few recordings are available, as well as valuable information about source features and regional attenuation characteristics.
- Recent advances have been made in GIT, different approaches and alternative basic hypothesis are followed by different research groups, such as the application of “non-parametric” and “parametric” inversion schemes. In this context, some scientific questions rise: depending on the final scope of GIT, what can be the optimal dataset configuration and the best inversion strategy? What is the impact of the different assumptions and implementations on the reliability of the results? What is the dependence of the results on the chosen reference conditions? How to quantify epistemic uncertainties?
The aim of this research task is to better understand how to use the GIT schemes in different applications. The objective will be fulfilled by performing a methodological benchmark of different generalized spectral inversion methods and dataset configurations. The benchmark will be run as a project open to worldwide volunteer teams: the GITEC (Generalized Inversion TEchniques Comparisons) project.
The participating groups will perform GIT on all the provided datasets following different strategies and hypothesis. The results will be shared and discussed during workshops, at the end of which conclusions and recommendations will be drawn.
One relatively dense national dataset from Japan;
A very dense local dataset from central Italy;
A sparse dataset from France.
Type: Benchmark project based on volunteer basis (1 year)
Collaboration: Organizing team composed of GFZ, ISTerre, CEA, EDF and participant teams worldwide
Status: In progress (currently in the Data Preparation and Discussion phase; computations are planned to start end of May 2018).
Estimation and mapping of the intrinsic component of seismic wave attenuation in Europe as a complement of the absorption component.
Collaboration: IRAP Toulouse