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The French Institute for Radiation Protection and Nuclear Safety (IRSN), with the support of the Ministry of Environment, compiled a database (BDFA) to define and characterize known potentially active faults of metropolitan France. The general structure of BDFA is presented in this paper. BDFA reports to date 136 faults and represents a first step toward the implementation of seismic source models that would be used for both deterministic and probabilistic seismic hazard calculations. A robustness index was introduced, highlighting that less than 15 % of the database is controlled by reasonably complete data sets. An example of transposing BDFA into a fault source model for PSHA (probabilistic seismic hazard analysis) calculation is presented for the Upper Rhine Graben (eastern France) and exploited in the companion paper (Chartier et al., 2017, hereafter Part 2) in order to illustrate ongoing challenges for probabilistic fault-based seismic hazard calculations.

The so-called site effects caused by superficial geological layers may be responsible for strong ground motion amplification in certain configurations. We focus here on the industrialized Tricastin area, in the French Rhône valley, where a nuclear site is located. This area lies above an ancient Rhône Canyon whose lithology and geometry make it prone to site effects. This study presents preliminary measurements to investigate the local seismic amplification. We deployed three seismic stations in the area for several months: two stations were located above the canyon, the third one was located on a nearby reference rock site. The recorded seismicity was analysed using the Standard Spectral Ratio technique (SSR). The estimated amplification from weak motions reaches a value of 6 for some frequencies. These first results confirm the possibility of estimating seismic amplification using earthquakes recorded for less than one year, in this highly anthropogenic and industrialized environment, despite the local low-to-moderate level of seismicity. Noise-based SSR, that presents an obvious interest in such seismic context, shows also promising results in the area. To complement this empirical approach, we estimated the amplification using 1D wave propagation modelling. This numerical estimate is based on shear wave velocity profiles resulting from geophysical characterization campaigns. Comparison of the two approaches at low frequency, where numerical estimate is considered as the most representative, tends to suggest that edge-generated surface waves may have a strong influence in the local seismic response. This interpretation will be further investigated in the future.

The western part of the Argentera–Mercantour massif (French Alps) hosts very large currently active landslides responsible of many disorders and risks to the highly touristic valleys of the Mercantour National Park and skiing resorts. A regional scale mapping of gravitational deformations has been compared to the main geo-structures of the massif. A relative chronology of the events has been established and locally compared to absolute 10 Be dating obtained from previous studies. Two types of large slope destabilisations were identified as follows: deep-seated landslides (DSL) that correspond to rock volumes bounded by a failure surface, and deep-seated gravitational slope deformations (DSGSD) defined as large sagging zones including gravitation landforms such as trenches and scarps or counterscarps. Gravitational landforms are mainly collinear to major N140°E and N020°E tectonic faults, and the most developed DSGSD are located in areas where the slope direction is comparable to the orientation of faults. DSL are mostly included within DSGSD zones and located at the slopes foot. Most of DSL followed a similar failure evolution process according to postglacial over steepened topographies and resulting from a progressive failure growing from the foot to the top of the DSGSD that lasts over a 10 ky time period. This massif-scale approach shows that large-scale DSGSD had a peak of activity from the end of the last deglaciation, to approximately 7000 years bp . Both morphologic and tectonic controls can be invoked to explain the gravitational behaviour of the massif slopes.

The 2019-11-1, Mw4.9 Le Teil earthquake occurred within the NE termination of the Cévennes faults system (CFS) in southern France, along the La Rouvière fault (LRF), an Oligocene normal fault which was not known to be potentially active. This shallow moderate magnitude reversefaulting event produced a 5 km-long surface rupture and strong ground shaking. No evidence of previous quaternary activity was observed in the morphology, raising the question whether the fault had been reactivated for the first time since the Oligocene or had broken the surface in the past without being detected in the morphology. To address this issue, we carried out paleoseismological investigations to analyze and characterize evidences of paleo-ruptures in Quaternary deposits. We discovered that at least one event prior 2019, occurred between 13.5 and 3.3 ka within the central part of the fault segment that broke in 2019, and that a possible earlier surfacerupturing event occurred within the northern part of this segment during the 16th century. Further investigations coupling sub-surface geophysical investigations and trenching are now carried out within the southern and northern segments of the LRF as well as along the other fault segments of the CFS.

Subduction zones pose a considerable challenge within the realm of seismotectonics, owing to their fault and structure interactions. The Lesser Antilles arc is a good example of how these complexities impact seismic hazard studies with strong along-strike variations in tectonic, seismic, and volcanic activities. While these activities have generated significant damage, the 1839 and 1843 event characteristics (locations, depths, mechanisms, magnitudes) along with their potential implications for megathrust seismicity remain a subject of debate, in particular in the frame of low interseismic coupling. This study is grounded in the compilation of instrumental and historical seismicity and fault catalogs, complemented by analyses of focal mechanisms and rupture types as well as geodetic velocities and strain rates. The resulting seismotectonic zoning model of the Lesser Antilles encompasses the upper plate, subducting oceanic plate, subduction interface, mantle wedge, and volcanoes. We propose a better depth resolution, resulting from recent studies on slab top and upper-plate bottom geometries; a specific area source for the Marie-Galante graben; new propositions for mantle wedge and volcanic zoning; and fully revised area sources for the subduction interface. Our study highlights specific needs for a better seismic hazard assessment in this region.

We perform a fault-based probabilistic seismic hazard assessment (PSHA) exercise in the Upper Rhine Graben to quantify the relative influence of fault parameters on the hazard at the Fessenheim nuclear power plant site. Specifically, we show that the potentially active faults described in the companion paper (Jomard et al., 2017, hereafter Part 1) are the dominant factor in hazard estimates at the low annual probability of exceedance relevant for the safety assessment of nuclear installations. Geological information documenting the activity of the faults in this region, however, remains sparse, controversial and affected by a high degree of uncertainty. A logic tree approach is thus implemented to explore the epistemic uncertainty and quantify its impact on the seismic hazard estimates. Disaggregation of the peak ground acceleration (PGA) hazard at a 10 000-year return period shows that the Rhine River fault is the main seismic source controlling the hazard level at the site. Sensitivity tests show that the uncertainty on the slip rate of the Rhine River fault is the dominant factor controlling the variability of the seismic hazard level, greater than the epistemic uncertainty due to ground motion prediction equations (GMPEs). Uncertainty on slip rate estimates from 0.04 to 0.1 mm yr−1 results in a 40 to 50 % increase in hazard levels at the 10 000-year target return period. Reducing epistemic uncertainty in future fault-based PSHA studies at this site will thus require (1) performing in-depth field studies to better characterize the seismic potential of the Rhine River fault; (2) complementing GMPEs with more physics-based modelling approaches to better account for the near-field effects of ground motion and (3) improving the modelling of the background seismicity. Indeed, in this exercise, we assume that background earthquakes can only host M  <  6. 0 earthquakes. However, this assumption is debatable, since faults that can host M  >  6. 0 earthquakes have been recently identified at depth within the Upper Rhine Graben (see Part 1) but are not accounted for in this exercise since their potential activity has not yet been described.

Earthquake hazard analyses rely on seismogenic source models. These are designed in various fashions, such as point sources or area sources, but the most effective is the three-dimensional representation of geological faults. We here refer to such models as fault sources. This study presents the European Fault-Source Model 2020 (EFSM20), which was one of the primary input datasets of the recently released European Seismic Hazard Model 2020. The EFSM20 compilation was entirely based on reusable data from existing active fault regional compilations that were first blended and harmonized and then augmented by a set of derived parameters. These additional parameters were devised to enable users to formulate earthquake rate forecasts based on a seismic-moment balancing approach. EFSM20 considers two main categories of seismogenic faults: crustal faults and subduction systems, which include the subduction interface and intraslab faults. The compiled dataset covers an area from the Mid-Atlantic Ridge to the Caucasus and from northern Africa to Iceland. It includes 1248 crustal faults spanning a total length of ∼95 100 km and four subduction systems, namely the Gibraltar, Calabrian, Hellenic, and Cyprus arcs, for a total length of ∼2120 km. The model focuses on an area encompassing a buffer of 300 km around all European countries (except for Overseas Countries and Territories) and a maximum of 300 km depth for the subducting slabs. All the parameters required to develop a seismic source model for earthquake hazard analysis were determined for crustal faults and subduction systems. A statistical distribution of relevant seismotectonic parameters, such as faulting mechanisms, slip rates, moment rates, and prospective maximum magnitudes, is presented and discussed to address unsettled points in view of future updates and improvements. The dataset, identified by the DOI https://doi.org/10.13127/efsm20 (Basili et al., 2022), is distributed as machine-readable files using open standards (Open Geospatial Consortium).

We present a 1:25,000 scale map of the coseismic surface ruptures following the 30 October 2016 M w 6.5 Norcia normal-faulting earthquake, central Italy. Detailed rupture mapping is based on almost 11,000 oblique photographs taken from helicopter flights, that has been verified and integrated with field data (>7000 measurements). Thanks to the common efforts of the Open EMERGEO Working Group (130 people, 25 research institutions and universities from Europe), we were able to document a complex surface faulting pattern with a dominant strike of N135°-160° (SW-dipping) and a subordinate strike of N320°-345° (NE-dipping) along about 28 km of the active Mt. Vettore-Mt. Bove fault system. Geometric and kinematic characteristics of the rupture were observed and recorded along closely spaced, parallel or subparallel, overlapping or step-like synthetic and antithetic fault splays of the activated fault systems, comprising a total surface rupture length of approximately 46 km when all ruptures were considered.

After a few years of research, the observation and the analysis of the deep-seated landslides suggest that these are mainly controlled by tectonic structures, which play a dominant role in the deformation of massif slopes. The La Clapière deep-seated landslide (Argentera Mercantour massif) is embedded in a deep-seated gravitational slope deformation affecting the entire slope, and characterized by specific landforms (trenches, scarps…). Onsite, the tangential displacement direction of the trenches and the scarps are controlled by the tectonic structures. The reactivation of the inherited fault in gravitational faults create a gouge material exposed to an additional mechanical and chemical weathering as well as an increased of leaching. The displacement of these reactivated faults gets increasingly important around the area of the La Clapière landslide and this since 3.6 ka BP. In this study, mechanical analysis and grain size distributions were performed and these data were analysed according to their proximity the La Clapiere landslide and times of initiation of the landslide by 10 Be dating. Triaxial test results show that the effective cohesion decreases and the effective angle of internal friction increases from the unweathered area to the weathered area. The whole distribution of the grain size indicates that the further the shear zone is open or developed, the further the residual material loses its finest particles. This paper suggests that the mechanical evolution along the reactivated fault is influenced by the leaching processes. For the first time, we can extract from these data temporal behaviour of the two main mechanical parameters (cohesion and angle of internal friction) from the beginning of the La Clapiere landslide initiation (3.6 ka BP) to now.