Preprints
https://doi.org/10.5194/se-2021-141
https://doi.org/10.5194/se-2021-141

  14 Dec 2021

14 Dec 2021

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

Postglacial strain rate – stress paradox, example of the Western Alps active faults

Juliette Grosset, Stéphane Mazzotti, and Philippe Vernant Juliette Grosset et al.
  • Géosciences Montpellier, CNRS, Université de Montpellier, Montpellier, 34000, France

Abstract. The understanding of the origins of seismicity in intraplate regions is crucial to better characterize seismic hazards. In formerly glaciated regions such as Fennoscandia North America or the Western Alps, stress perturbations from Glacial Isostatic Adjustment (GIA) have been proposed as a major cause of large earthquakes. In this study, we focus on the Western Alps case using numerical modeling of lithosphere response to the Last Glacial Maximum icecap. We show that the flexural response to GIA induces present-day stress perturbations of ca. 1–2 MPa, associated with horizontal extension rates up to ca. 2.5 × 10−9 yr−1. The latter is in good agreement with extension rates of ca. 2 × 10−9 yr−1 derived from high-resolution geodetic (GNSS) data and with the overall seismicity deformation pattern. In the majority of simulations, stress perturbations induced by GIA promote fault reactivation in the internal massifs and in the foreland regions (i.e., positive Coulomb Failure Stress perturbation), but with predicted rakes systematically incompatible with those from earthquake focal mechanisms. Thus, although GIA explains a major part of the GNSS strain rates, it tends to inhibit the observed seismicity in the Western Alps. A direct corollary of this result is that, in cases of significant GIA effect, GNSS strain rate measurements cannot be directly integrated in seismic hazard computations, but instead require detailed modeling of the GIA transient impact.

Juliette Grosset et al.

Status: open (until 10 Feb 2022)

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Juliette Grosset et al.

Juliette Grosset et al.

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Short summary
Glacial Isostatic Adjustment is considered as a major process of seismicity in intraplate regions such as Scandinavia and eastern North America. We show that GIA associated with the alpine icecap induces a present-day response in vertical motion and horizontal deformation seen in GNSS strain rate field. We show that GIA induced stress is opposite to strain rate, with the paradoxical consequence that postglacial rebound in the Western Alps can explain the strain rate field but not the seismicity.