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

Abstract

Abstract. In regions formerly glaciated during the Last Glacial Maximum (LGM), Glacial Isostatic Adjustment (GIA) explains most of the measured uplift and deformation rates. GIA is also proposed as a key process contributing to fault activity and seismicity shortly after the LGM and potentially up to present-day. Here, we study the impact of GIA on present-day fault activity and seismicity in the Western Alps. We show that, in the upper crust, GIA induces horizontal compressive stress perturbations associated with horizontal extension rates. The latter agree with the observed geodetic strain rates and with the seismicity deformation patterns. Yet, in nearly all cases, the GIA stress perturbations tend either to inhibit fault slip, or to promote fault slip with the wrong mechanism compared to the seismicity deformation style. Thus, although GIA from the LGM explains a major part of the geodetic strain rates, it does not drive nor promote the observed seismicity (which must be driven by other processes). This apparent strain rate - stress paradox results from the gradual diminution over time of the finite shortening induced in the upper crust by the LGM icecap. A direct corollary of our results is that seismicity and seismic hazard studies in the Western Alps cannot directly integrate geodetic velocities and strain rates, but instead require detailed modeling of the GIA transient impact.