Abstract
Abstract. Due to the low to moderate seismicity of the European Western Alps, few focal mechanisms are available in this region to this day, and the corresponding current seismic stress and strain fields remain partly elusive. The development of dense seismic networks in past decades now provides a substantial number of seismic records in the 0–5 magnitude range. 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 between 1989 and 2013 and more than 2200 newly computed focal mechanisms in a consistent manner. The global distribution of P and T axis plunges confirms a majority of transcurrent focal mechanisms in the entire western 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 to be clustered, instead of continuous, along the backbone of the belt. Robust indications for compression are only observed at the boundary between the Adriatic and Eurasian plates. Short-wavelength spatial variations of the seismic deformation are consistent with surface horizontal Global Navigation Satellite System (GNSS) measurements, as well as with deep lithospheric structures, thereby providing new elements with which 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 anticlockwise rotation of Adria with respect to Europe – and buoyancy forces in the core of the belt, which together explain the short-wavelength patches of extension and marginal compression overprinted on an overall transcurrent tectonic regime.
Highlights
The European Alps are characterized by a complex orogenic history
To better explore the current tectonic regime in each subarea, we investigated the distribution of stress orientations using focal mechanism inversions
The exceptional density of seismic data acquired within the Western Alps for the past 25 years allowed us to derive focal mechanisms for lowmagnitude events that were, to date, unavailable
Summary
The European Alps are characterized by a complex orogenic history. The Alpine belt results from the collision between the African and European plates during the Cenozoic The anticlockwise rotation of the Adriatic plate with respect to the European plate prevails in the tectonic history of the Western Alps (e.g. D’Agostino et al, 2008; Nocquet and Calais, 2004; Serpelloni et al, 2007). Mathey et al.: Present-day geodynamics of the Western Alps networks in the Western Alps (e.g. Sismalp, Thouvenot et al, 1990, 2013; CIFALPS, Zhao et al, 2013) improved the catalogues by decreasing both the magnitude of completeness and the minimum magnitude from which focal mechanisms could be computed These updated catalogues offer the opportunity to reappraise the disputed current dynamics of the Western Alps These updated catalogues offer the opportunity to reappraise the disputed current dynamics of the Western Alps (e.g. Nocquet et al, 2016; Sternai et al, 2019; Champagnac et al, 2012; Mazzotti et al, 2020)
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