The growing and melting of continental ice sheets during a glacial cycle is accompanied by stress changes and reactivation of faults. To better understand the relationship between stress changes, fault activation time, fault parameters, and fault slip magnitude, a new physics‐based two‐dimensional numerical model is used. In this study, tectonic background stress magnitudes and fault parameters are tested as well as the angle of the fault and the fault locations relative to the ice sheet. Our results show that fault slip magnitude for all faults is mainly affected by the coefficient of friction within the crust and along the fault and also by the depth of the fault tip and angle of the fault. Within a compressional stress regime, we find that steeply dipping faults (∼75°) can be activated after glacial unloading, and fault activity continues thereafter. Furthermore, our results indicate that low‐angle faults (dipping at 30°) may slip up to 63m, equivalent to an earthquake with a minimum moment magnitude of 7.0. Finally, our results imply that the crust beneath formerly glaciated regions was close to a critically stressed state, in order to enable activation of faults by small changes in stress during a glacial cycle.

Stresses induced in the crust and mantle by continental-scale ice sheets during glaciation has triggered earthquakes along pre-existing faults, commencing near the end of the deglaciation. In order to get a better understanding of the relationship between glacial loading/unloading and fault movement due to the spatio-temporal evolution of stresses, a commonly used model for glacial isostatic adjustment (GIA) is extended by including a fault structure. Solving this problem is enabled by development of a workflow involving three cascaded finite-element simulations. Each step has identical lithospheric and mantle structure and properties, but evolving stress conditions along the fault.The purpose of the first simulation is to compute the spatio-temporal evolution of rebound stress when the fault is tied together. An ice load with a parabolic profile and simple ice history is applied to represent glacial loading of the Laurentide Ice Sheet. The results of the first step describes the evolution of the stress and displacement induced by the rebound process. The second step in the procedure augments the results of the first, by computing the spatio-temporal evolution of total stress (i. e. rebound stress plus tectonic background stress and overburden pressure) and displacement with reaction forces that can hold the model in equilibrium. The background stress is estimated by assuming that the fault is in frictional equilibrium before glaciation. The third steps simulates fault movement induced by the spatio-temporal evolution of total stress by evaluating fault stability in a subroutine. If the fault remains stable, no movement occurs; in case of fault instability, the fault displacement is computed.We show an example of fault motion along a 45 • -dipping fault at the ice-sheet centre for a two-dimensional model. Stable conditions along the fault are found during glaciation and the initial part of deglaciation. Before deglaciation ends, the fault starts to move, and fault offsets of up to 22 m are obtained. A fault scarp at the surface of 19.74 m is determined.The fault is stable in the following time steps with a high stress accumulation at the fault tip. Along the upper part of the fault, GIA stresses are released in one earthquake.

There is growing evidence that climate-induced melting of large ice sheets has been able to trigger fault reactivation and earthquakes around the migrating ice limit. Even today, the stress due to glacial isostatic adjustment can continue to induce seismicity within the onceglaciated region. Northern Central Europe lies outside the former ice margin and is regarded as a low-seismicity area. However, several historic earthquakes with intensities of up to VII occurred in this region during the past 1200 years. Here we show with numerical simulations that the seismicity can potentially be explained by the decay of the Scandinavian ice sheet after the Weichselian glaciation. Combination of historic earthquake epicenters with fault maps relates historic seismicity to major reverse faults of Late Cretaceous age. Mesozoic normal faults remained inactive in historic times. We suggest that many faults in northern Central Europe are active during postglacial times. This is a novelty that sheds new light on the distribution of postglacial faulting and seismicity. In addition, we present the first consistent model that can explain both the occurrence of deglaciation seismicity and the historic earthquakes in northern Central Europe.

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