Near-Fault Broadband Ground Motion Simulations Using Empirical Green’s Functions: Application to the Upper Rhine Graben (France–Germany) Case Study

Abstract

Seismic hazard estimation relies classically on data based ground motion prediction equations (GMPEs) giving the expected motion level as a function of several parameters characterizing the source and the sites of interest. However, records of moderate to large earthquakes at short distances from the faults are still rare. For this reason, it is difficult to obtain a reliable ground motion prediction for such a class of events and distances where also the largest amount of damage is usually observed. A possible strategy to fill this lack of information is to generate synthetic accelerograms based on an accurate modeling of both extended fault rupture and wave propagation process.The development of such modelling strategies is essential for estimating seismic hazard close to faults in moderate seismic activity zones, where data are even scarcer. For that we reason we selected a target site in Upper Rhine Graben (URG), at the French-German border. URG is a region where faults producing micro-seismic activity are very close to the sites of interest (e.g. critical infrastructures like supply lines, nuclear power plants, etc.) needing a careful investigation of seismic hazard.In this work we demonstrate the feasibility of performing near-fault broadband ground motion numerical simulations in a moderate seismic activity region such as Upper Rhine Graben (URG) and discuss some of the challenges related to such an application. The modelling strategy is to couple the multi-Empirical Green's Function technique (multi-EGFt) with a k -2 kinematic source model. One of the advantages of the multi-EGFt is that it does not require a detailed knowledge of the propagation medium since the records of small events are used as the medium transfer function, if, at the target site, records of small earthquakes located on the target fault are available. 3The selection of suitable events to be used as multi-EGF is detailed and discussed in our specific situation where little number of events are available. We then showed the impact that each source parameter characterizing the k -2 model has on ground motion amplitude. Finally we performed ground motion simulations showing results for different probable earthquake scenarios in the URG.Dependency of ground motions and of their variability are analyzed at different frequencies in respect of rupture velocity, roughness degree of slip distribution (stress drop), and hypocenter location. In near source conditions, ground motion variability is shown to be mostly governed by the uncertainty on source parameters. In our specific configuration (magnitude, distance), the directivity effect is only observed in a limited frequency range. Rather, broad-band ground motions are shown to be sensitive to both average rupture velocity and its possible variability, and to slip roughness. Ending up with a comparison of simulation results and GMPEs, we conclude that source parameters and their variability should be set up carefully in order to obtain reliable broadband ground motion estimations. In particula...

Transposing an active fault database into a fault-based seismic hazard assessment for Nuclear facilities. Part B: Impact of fault parameter uncertainties on a site-specific PSHA exercise in the Upper Rhine Graben, Eastern France

“…A study is presently ongoing in the Upper Rhine Graben to better constrain the recent activity of the fault system (Baize et al, 2016) based on geophysical and palaeoseismological investigations. An additional study (Del Gaudio et al, 2016) is modelling ground motion in the Upper Rhine Graben with the help of the empirical green function approach (Del Gaudio et al, 2015). It is hoped that these studies will provide new insights for reducing epistemic uncertainty in faultbased PSHA of the Upper Rhine Graben.…”

Empirical Green’s Function Simulations Toward Site-Specific Ground Motion Prediction for Kopili Fault of NER India

“…The primary goal of this work is to identify the Strong Motion Ground Acceleration (SMGA) that most accurately mimics the broadband ground motions at a certain distance. In order to forecast broadband ground-motion time histories, we employed the Extended k-2 Source model (Causse et al 2009;Dujardin et al 2016Dujardin et al , 2018Del Gaudio et al 2018) and combined it with the EGF (Hartzell, 1978) approach, based on a number of small events by taking radiation pattern adjustments into consideration. Numerous publications have indicated that this method is effective at replicating ground motion for various places (Causse et al 2021;Castro-cruz et al 2021;Dujardin et al 2019;Del Gaudio et al 2018;Del Gaudio et al 2015).…”