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

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.

Bayesian analysis of azimuthal anisotropy in the Alpine lithosphere from beamforming of ambient noise cross-correlations

“…The present-day deformation and stress fields in the crust have been studied in the Alps using analysis of GNSS data and earthquake focal mechanisms (e.g. Delacou et al, 2004;Heidbach et al, 2018;Mathey et al, 2020). Seismic anisotropy provides complementary information on the history of deformation as stress and deformation directly affect the alignment of seismically anisotropic minerals, as well as lithospheric structures such as faults, fluids, layering, etc.…”

Reply on RC1

“…4b). This faulting style is opposite to that observed in the focal mechanisms along the IMNF (Delacou et al, 2004;Mathey et al, 2021;Sue et al, 1999). For the Belledonne Fault, the GIA stress perturbation favors reverse faulting (r = 60-130) while the focal mechanisms show right-lateral strike-slip (r = 180 ± 20; Thouvenot et al, 2003).…”

Glacial isostatic adjustment strain rate – stress paradox in the Western Alps, impact on active faults and seismicity

“…70 5-10 km, and a smaller eastern cluster, with deeper focal depths ca. 10-20 km (Mathey et al, 2021). The surrounding lower https://doi.org/10.5194/egusphere-2023-538 altitude and foreland regions show a mix of strike-slip and reverse faulting with a general arc-perpendicular compression (Fig.…”

“…The surrounding lower https://doi.org/10.5194/egusphere-2023-538 altitude and foreland regions show a mix of strike-slip and reverse faulting with a general arc-perpendicular compression (Fig. 2b; Delacou et al, 2004;Mathey et al, 2021;Mazzotti et al, 2021;. In general, only a few known fault structures can be associated with observed seismicity, e.g.…”

“…15-20 km depth east of the French-Italian border (cf. Section 2; Delacou et al, 2004;Mathey et al, 2021). In order to match our GIA model stress predictions to fault and seismicity depths, we consider the following points: (1)…”