We document a moderate earthquake in the French Alps (26 February 2012 M w 4.1) that has been much more distinctly felt south of the event than north of it. This discrepancy was especially clear in the two large cities of Nice and Grenoble, both situated at 100 km from the epicenter. This observation was confirmed by ground-motion measurements that were eight times larger in one city than in the other one, for the same site conditions. Using a time-domain deconvolution between the broadband recordings of the mainshock and an aftershock used as empirical Green's functions, we show that the rupture process of this event had a strong directivity effect toward a direction of N155° 5 on an ∼2 km long fault, detectable only at frequencies higher than 1 Hz. The fault size and direction are in good accordance with the location of the aftershocks. Despite the various possible contributions leading to highfrequency amplification, we show here that this simple directivity effect controlled the intensity felt by the population and the acceleration measured in the cities.
International audienceAbstract Comparison between accelerometric and macroseismic observations is made for three Mw~4.5 earthquakes which occurred in north-eastern France and south-western Germany in 2003 and 2004. Scalar and spectral instrumental parameters are processed from the accelerometric data recorded by nine accelerometric stations located between 29 km and 180 km from the epicentres. Macroseismic data are based on French Internet reports. In addition to the single questionnaire intensity, analysis of the internal correlation between the encoded answers highlights four predominant fields of questions bearing different physical meanings: 1) "Vibratory Motions of small objects", 2) "Displacement and Fall of Objects", 3) "Acoustic Noise", and 4) "Personal Feelings". Best correlations between macroseismic and instrumental observations are obtained when the macroseismic parameters are averaged over 10 km-radius circles around each station. Macroseismic intensities predicted by published PGV-intensity relationships agree with our observed intensities, contrary to those based on PGA. Correlation between the macroseismic and instrumental data for intensities between II and V (EMS-98) is better for PGV than for PGA. Correlation with the response spectra exhibits clear frequency dependence for all macroseismic parameters. Horizontal and vertical components are significantly correlated with the macroseismic parameters between 1 and 10 Hz, a range corresponding to both natural frequencies of most buildings and high energy content in the seismic ground motion. Between 10 and 25 Hz, a clear lack of correlation between macroseismic and instrumental observations exists. It could be due to a combination of the decrease of the energy signal above 10 Hz, a high level of anthropogenic noise, and an increase of variability in soil conditions. Above 25 Hz the correlation coefficients between the acceleration response spectra and the macroseismic parameters are close to the PGA correlation level
In recent years, the French seismological, geodetic, and gravimetric community has been structured within Réseau Sismologique et géodésique Français (RESIF) (French seismological and geodetic network). In addition to instrumental developments, RESIF has structured the work on French seismicity (metropolitan and overseas) within the RESIF transverse seismicity action (ATS). The purpose of this article is to present the ATS and the way it is structured to propose to the community different products: seismicity bulletin and catalog, historical and instrumental macroseismicity data, and ShakeMaps. The places where these products can be found are indicated, as well as the way they are realized and the improvements in progress for a better realization and availability. The link with European plate observing system is also underlined.
On November 11, 2019, a Mw 4.9 earthquake hit the region close to Montelimar (lower Rhône Valley, France), on the eastern margin of the Massif Central close to the external part of the Alps. Occuring in a moderate seismicity area, this earthquake is remarkable for its very shallow focal depth (between 1 and 3 km), its magnitude, and the moderate to large damages it produced in several villages. InSAR interferograms indicated a shallow rupture about 4 km long reaching the surface and the reactivation of the ancient NE-SW La Rouviere normal fault in reverse faulting in agreement with the present-day E-W compressional tectonics. The peculiarity of this earthquake together with a poor coverage of the epicentral region by permanent seismological and geodetic stations triggered the mobilisation of the French post-seismic unit and the broad French scientific community from various institutions, with the deployment of geophysical instruments (seismological and geodesic stations), geological field surveys, and field evaluation of the intensity of the earthquake. Within 7 days after the mainshock, 47 seismological stations were deployed in the epicentral area to improve the Le Teil aftershocks locations relative to the French permanent seismological network (RESIF), monitor the temporal and spatial evolution of microearthquakes close to the fault plane and temporal evolution of the seismic response of 3 damaged historical buildings, and to study suspected site effects and their influence in the distribution of seismic damage. This seismological dataset, completed by data owned by different institutions, was integrated in a homogeneous archive and distributed through FDSN web services by the RESIF data center. This dataset, together with observations of surface rupture evidences, geologic, geodetic and satellite data, will help to unravel the causes and rupture mechanism of this earthquake, and contribute to account in seismic hazard assessment for earthquakes along the major regional Cévenne fault system in a context of present-day compressional tectonics.
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