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  02 Dec 2020

02 Dec 2020

Review status: this preprint is currently under review for the journal SE.

Present-day geodynamics of the Western Alps: new insights from earthquake mechanisms

Marguerite Mathey1, Christian Sue1,2, Colin Pagani3, Stéphane Baize4, Andrea Walpersdorf1, Thomas Bodin3, Laurent Husson1, Estelle Hannouz1, and Bertrand Potin5 Marguerite Mathey et al.
  • 1University Grenoble Alpes, University Savoie Mont Blanc, CNRS, IRD, IFSTTAR, ISTerre, Grenoble, 38000, France
  • 2Chrono-Environnement Besançon, OSU THETA, University Bourgogne-Franche-Comté, Besançon, 25000, France
  • 3Univ Lyon, Université Lyon 1, Ens de Lyon, CNRS, Lyon, 69000, France
  • 4IRSN,PSE-ENV/SCAN/BERSSIN, BP 17, Fontenay-aux-roses, F-92262, France
  • 5Departamento de Geofìsica, Universidad de Chile, Blanco Encalada 2002, Santiago, 8320000, Chile

Abstract. Due to the low to moderate seismicity of the European Western Alps, few focal mechanisms are available to this day in this region, and the corresponding current seismic stress and strain fields remain partly elusive. The development of dense seismic networks in the past decades now provides a substantial amount of seismic records down to low magnitudes. The corresponding data, while challenging to handle due to their amount and relative noise, represent a new opportunity to increase the spatial resolution of seismic deformation fields. The aim of this paper is to quantitatively assess the current seismic stress and strain fields within the Western Alps, from a probabilistic standpoint, using new seismotectonic data. The dataset comprises more than 30,000 earthquakes recorded by dense seismic networks since 1989 and more than 2200 focal mechanisms newly computed in a consistent manner. The global distribution of P and T axes plunges confirms a majority of transcurrent focal mechanisms in the overall alpine realm, combined with pure extension localized in the core of the belt. We inverted this new set of focal mechanisms through several strategies, including a seismotectonic zoning scheme and grid procedure, revealing extensional axes oriented obliquely to the strike of the belt. The Bayesian inversion of this new dataset of focal mechanisms provides a probabilistic continuous map of the style of seismic deformation in the Western Alps. Extension is found clustered, instead of continuous along the backbone of the belt. Compression is robustly retrieved only in the Po plain, which lays at the limit between the Adriatic and Eurasian plates. High frequency spatial variations of the seismic deformation are consistent with surface horizontal GNSS measurements as well as with deep lithospheric structures, thereby providing new elements to understand the current 3D dynamics of the belt. We interpret the ongoing seismotectonic and kinematic regimes as being controlled by the joint effects of far-field forces –imposed by the counterclockwise rotation of Adria with respect to Europe- and of buoyancy forces in the core of the belt, which together explain the high frequency patches of extension and of marginal compression overprinted on an overall transcurrent tectonic regime.

Marguerite Mathey et al.

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Marguerite Mathey et al.

Marguerite Mathey et al.


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