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An Earthquake Focal Mechanism Database for Seismotectonic FMHex20: An earthquake focal mechanism database for seismotectonic analyses in metropolitan France and bordering regions Stephane Mazzotti, Clémence Aubagnac, Laurent Bollinger, Karla Coca Oscanoa, Bertrand Delouis, Denis Do Paco, Cécile Doubre, Maxime Godano, Hervé Jomard, Christophe Larroque, et al. To cite this version: Stephane Mazzotti, Clémence Aubagnac, Laurent Bollinger, Karla Coca Oscanoa, Bertrand Delouis, et al.. FMHex20: An earthquake focal mechanism database for seismotectonic analyses in metropolitan France and bordering regions. Bulletin de la Société Géologique de France, Société géologique de France, 2021, 192, pp.10. 10.1051/bsgf/2020049. hal-03278811 HAL Id: hal-03278811 https://hal.archives-ouvertes.fr/hal-03278811 Submitted on 8 Jul 2021 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Distributed under a Creative Commons Attribution| 4.0 International License BSGF - Earth Sciences Bulletin 2021, 192, 10 © S. Mazzotti et al., Published by EDP Sciences 2021 https://doi.org/10.1051/bsgf/2020049 Available online at: www.bsgf.fr FMHex20: An earthquake focal mechanism database for seismotectonic analyses in metropolitan France and bordering regions Stephane Mazzotti1,*, Clémence Aubagnac1, Laurent Bollinger2, Karla Coca Oscanoa1, Bertrand Delouis3, Denis Do Paco2, Cécile Doubre4, Maxime Godano3, Hervé Jomard5, Christophe Larroque3, Aurore Laurendeau5, Frédéric Masson4, Matthieu Sylvander6 and Aurélie Trilla2 1 Géosciences Montpellier, CNRS, Université de Montpellier, Université des Antilles, Montpellier, France 2 CEA DAM Ile-de-France, Bruyères-le-Châtel, France 3 Géoazur, Université Côte d’Azur, CNRS, Observatoire de la Côte d’Azur, IRD, Valbonne, France 4 Institut Terre Environnement Strasbourg, École et Observatoire des Sciences de la Terre, CNRS, Université de Strasbourg, Strasbourg, France 5 Bureau d’Evaluation des Risques Sismiques pour la Sûreté des Installations, Institut de Radioprotection et Sûreté Nucléaire, Fontenay-aux-Roses, France 6 Institut de Recherche en Astrophysique et Planétologie, CNRS, UPS, CNES, Université de Toulouse, Toulouse, France Received: 27 April 2020 / Accepted: 21 December 2016 Abstract – We present a compilation of over 1700 focal mechanisms for nearly 1300 earthquakes in metropolitan France and bordering regions of Western Europe. It is based on both published and unpublished sources (articles, reports, observatory websites) for which the focal mechanism solutions have been verified for internal consistency, corrected in cases of minor errors and rejected in cases of major inconsistencies between the parameters. The database, labeled FMHex20, is a first version and should be regularly updated in the future as part of an ongoing effort within the Seismicity Transverse Action of the French Résif research infrastructure. We also present first-order seismotectonic analyses for the whole metropolitan France and for two regions (Western France and Northern Alps-Jura-Vosges) to illustrate how the FMHex20 database can serve as a basis for geodynamic or seismic hazard zonation studies. Combined with complementary datasets, it can improve our understanding of the kinematics of potentially active faults, including in very-low-strain-rate regions as is the case for most of France. Keywords: Focal mechanism / earthquake / seismotectonics / metropolitan France 1 Introduction the construction of seismotectonic models for seismic hazard assessments (e.g., Cushing et al., 2007; Vanneste et al., 2013). In this article, we present a database of 1733 focal The characterization of earthquake sources using focal mechanisms for 1290 individual earthquakes in metropolitan mechanisms – also called fault plane solutions – is concomitant France and bordering regions of Western Europe based on the with the development of modern seismology in the 1950s and compilation of solutions from published articles, unpublished the installation in the 1960s of the World-Wide Standardized reports and online publications of seismological agencies. The Seismograph Network that allowed computations of focal database, labeled FMHex20 (Focal Mechanisms of Hexagon mechanisms for large earthquakes everywhere on the globe 2020), represents the first realization of the compilation, with (e.g., Byerly and Stauder, 1958; Stauder, 1962). These proved an objective of regular updates within the Seismicity an important dataset in the development of the Plate Tectonics Transverse Action of the French Résif (“Réseau Sismologique framework (Sykes, 1967). Since then, earthquake focal et Géodésique Français”) research infrastructure. mechanisms are used in most studies of active tectonics, We first present in Section 2 a short review of previous from individual fault mechanics to regional dynamics (e.g., studies that illustrate the current knowledge on focal Hardebeck and Michael, 2004; Stich et al., 2006), including in mechanisms and state of stress in France, with a description of the main limitations on focal mechanism estimations. The FMHex20 database is described in Section 3, including the *Corresponding author: [email protected] presentation of the sources, database format and processing. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. S. Mazzotti et al.: BSGF 2021, 192, 10 In particular, we present the procedures used for identification deformation w (s3) horizontal and oriented roughly NE- and correction of erroneous data, and for estimating the SW (Amorèse et al., 2000; Delouis et al., 1993; Mazabraud variability between multiple mechanism solutions for the same et al., 2005; Perrot et al., 2005; Veinante-Delahaye and earthquake. Finally, a short discussion of the French national Santoire, 1980). and regional seismotectonics is presented in Section 4 using – Rhenish Massif and Lower Rhine Graben: Mix of first-order analyses of the new database. This last section extension and strike-slip deformation, with a minimum illustrates the importance of focal mechanisms to better principal stress (s3) horizontal and oriented NNE-SSW understand the complex patterns and processes related to (Delouis et al., 1993; Hinzen, 2003; Nicolas et al., 1990). active deformation, seismotectonics and seismic hazard in – Jura and Upper Rhine Graben: Predominantly strike-slip various regions of metropolitan France. deformation with a maximum principal stress (s1) oriented Hereafter, earthquake magnitudes are noted Mw when NW-SE (Delouis et al., 1993; Kastrup et al., 2004; Nicolas referring to moment magnitudes and M for all other undifferen- et al., 1990; Fréchet et al., 2011; Rabin et al., 2018). tiated magnitudes (ML,MS,mb, etc.). For detailed information on – Alps: Overall orogen-normal extension in the inner (high focal mechanism computation, the reader is referred to method topography) region and a mix of strike-slip and shortening publications such as Dziewonski et al. (1981), Reasenberg and deformation in the French and Swiss foreland. The Oppenheimer (1985) or Hardebeck and Shearer (2002). minimum (resp. maximum) horizontal stress rotates with the Alps curvature in the extension (resp. shortening) 2 Previous studies and limitations domains (Delacou et al., 2004; Kastrup et al., 2004; Sue et al., 1999). Early studies of earthquake focal mechanisms in France – Southwestern Alps and Ligurian Sea: Complex deforma- date from the late 1970s–early 1980s, with applications to the tion pattern with short spatial variations including Alps, Pyrenees and Western France (Fréchet and Pavoni, 1979; extension in the inner (highest topography) region and Godefroy, 1980; Veinante-Delahaye and Santoire, 1980; Dorel shortening in the Provence and Italian forelands and the et al., 1983). Most of these and of the following studies Liguria and Corsica-Sardinia margins (Baroux et al., 2001; estimate focal mechanisms from the polarities of direct and Eva et al., 1997; Larroque et al., 2009; Larroque et al., refracted P waves (Pg and Pn) at local and regional distances 2016). (< 200–300 km). These so-called first-motion solutions pro- – Pyrenees: Overall orogen-normal extension with lateral vide information on the double-couple component of the variations from transtensive deformation in the East to a mechanism (azimuths and dips of the nodal planes, directions mix of extension and local shortening in the West. The of the slip vectors, azimuths and dips of the compression and minimum principal stress (s3) is horizontal and oriented in tension axes). They are computed for earthquakes of the NE-SW quadrant (Chevrot et al., 2011; Rigo et al., magnitude typically M ≥ 2.0–3.0, depending primarily on 2015; Sylvander et al., 2008). the density and azimuthal distribution of the regional seismometer network. Several limitations are associated with regional focal In parallel, since the 1980s–1990s several global and regional mechanism studies. The most
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