Comment on se-2021-141 by Grosset et al.

doi:10.5194/se-2021-141-rc1

Abstract

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.

Highlights

Sive stress perturbation in the upper half of the elastic lithosphere beneath the load
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
Because Plate Tectonics cannot be the main source of Stable Continental Regions (SCR) seismicity, the transient stress perturbations of Glacial Isostatic Adjustment (GIA) are commonly considered to explain the recent seismic activity and fault surface ruptures following the Last Glacial Maximum in regions such as Fennoscandia or northern North America (e.g., Hetzel and Hampel, 2005; Steffen et al, 2014; Wu and Hasegawa, 1996)

Summary

Postglacial strain rate - stress paradox for intracontinental regions

The origins and characteristics of seismicity in Stable Continental Regions (SCR) are an ongoing conundrum in Earth science 15 studies (e.g., Calais et al, 2016; Talwani, 2017). Because Plate Tectonics cannot be the main source of SCR seismicity, the transient stress perturbations of Glacial Isostatic Adjustment (GIA) are commonly considered to explain the recent seismic activity and fault surface ruptures following the Last Glacial Maximum in regions such as Fennoscandia or northern North America (e.g., Hetzel and Hampel, 2005; Steffen et al, 2014; Wu and Hasegawa, 1996). 55 The present-day Alpine tectonics are primarily controlled by the counter-clockwise rotation of the Adria microplate relative to Eurasia, with a rotation pole directly east of the Western Alps near Turin, northwestern Italy (Fig. 2a; Battaglia et al, 2004; D’Agostino et al, 2008) This kinematics is incompatible with the observed seismicity and geodetic horizontal deformation patterns (Fig. 2). We test the compatibility of GIA models with present-day deformation of the Western Alps

GIA models

GIA stress perturbations on active faults

GIA explains present-day Western Alps GNSS deformation but not seismicity

Findings

Horizontal Velocity field