The crustal stress field during glaciation and its relation to fault stability: Application to the Scandinavian endglacial faults
|Category||Tectonic & Seismotectonic|
|Location||International Geological Congress,oslo 2008|
|Author||Lund, Bjorn۱; Zoback, Mark۲|
|Holding Date||03 September 2008|
At the end of the last glaciation, northern Scandinavia experienced a dozen or so very large earthquakes, reaching magnitude 8. These reverse faulting events ruptured the surface in throws of 10 to 15 m, leaving faults scarps (some more than 100 km in length) that are still visible today. Although the deglaciation process is widely accepted as the cause of the earthquakes, little is known about the mechanics of the process and why they only occurred in northern Scandinavia, but not southern Scandinavia nor northern North America, for example. In this study we use 2D finite element modeling to investigate the effects of rheology and the tectonic state of stress on faulting potential during a glaciation. We use both a simple parabolic ice sheet model and a northwest-southeast profile through a three-dimensional model of the entire Weichselian glaciation. The earth models are based on the concept of an elastic plate overriding a viscoelastic half-space. We study how the response of the elastic lithosphere changes as we introduce layering and an increasing elastic thickness, from the thin oceanic lithosphere off the coast of Norway to the very thick cratonic lithosphere under Finland. Using established earth models derived from different data sets (glacial isostatic rebound, seismology and gravity) we see surprisingly small differences in the stability of faults at depths down to approximately 20 km. The pre-existing tectonic state of stress is critical when assessing fault stability. We show how a reverse and a strike-slip state of stress, respectively, influence the time history of fault stability during a glacial cycle. As the current tectonic stress state has likely not changed markedly in the last 100,000 years, we compare the modeled stress states to current stress estimates from inversion of earthquake focal mechanisms and deep boreholes. Hydrology at the base of ice sheets is a complex topic, but because pore pressure is very important in fault mechanics, we explore the effects of simple static end-member models of possible pore pressures in the shallow crust during glaciation on fault stability.