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 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.

Regional centroid MT inversion of small to moderate earthquakes in the Alps using the dense AlpArray seismic network: challenges and seismotectonic insights

“…10b, features f-h). This observation is in accordance with Mathey et al (2020), who recently studied the stress field of the western Alps based on an new extensive dataset of P-wave first motion polarity focal mechanisms.…”

Unprecedented seismic swarm in the Maurienne valley (2017–2019) observed by the SISmalp Alpine seismic network: operational monitoring and management

“…The nonlinearity of this tectonic structure (due to the geological and tectonic heritage) is responsible for the existence of relays that accommodate movements between two more linear zones. This regional hypothesis is in accordance with regional tectonic and regional strain [Delacou et al, 2004] and with present days geodynamics reconstruct, with earthquake mechanisms, since 1989 [Mathey et al, 2020]. Furthermore, these fault segments identified on the field (which is in accordance with seismic data and focal mechanisms calculated) should be added to the BDFA hazard assessment.…”

“…This assumption implies that BBF have no more like with the Belledonne Middle Fault (BMF) system. The global kinematic of the area [Delacou et al, 2004] and recent studies of global geodynamics of the western Alps [Mathey et al, 2020] disapprove this hypothesis. This fault segment (in red in Figure 1), where seismicity and geological structure match is now recognized as potential seismogenic fault [Mazzotti et al, 2020] and is referenced in the French active fault database (BDFA) for seismic hazard assessment [Jomard et al, 2017].…”

Lithospheric transdimensional ambient-noise tomography of W-Europe: implications for crustal-scale geometry of the W-Alps

“…To the east of the IF, the subduction wedge is backthrusted onto the Adria crust and the Adria lithosphere is significantly shortened (Mathey et al, 2020). In contrast, to the west of the IF, extensional deformation dominates in a crustal compartment bounded by the IF and the PFT (Mathey et al, 2020), as a result of vertical indentation. This latter thrust was reactivated since at least 3 4 Ma, and it is still currently undergoing extensional deformation (Bilau et al, 2021).…”