Slip-rate variations on active faults caused by glacial-interglacial changes in ice and water volumes on Earth’s surface
|Category||Tectonic & Seismotectonic|
|Location||International Geological Congress,oslo 2008|
|Author||Hampel, Andrea۱; Hetzel, Ralf۲; Turpeinen, Heidi۱; Karow, Tobias۱; Maniatis, Georgios۱; Densmore, Alexander L.۳|
|Holding Date||03 September 2008|
Palaeoseismologic data show that many active faults in different tectonic settings experienced an increase in their slip rate after the last glacial period. Examples include the Wasatch and Teton normal faults in the Basin and Range Province (Byrd et al., JGR, 1994; Friedrich et al., JGR, 2003) and the Pنrvie and Lansjنrv reverse faults in northern Scandinavia (Lagerbنck, J. Geol. Soc. London, 1992; Mِrner, Tectonophysics, 2005). Here we use finite-element models including a fault embedded in a rheologically layered lithosphere to demonstrate that the slip rate variations can be explained as a response to glacial-interglacial changes in ice and water volumes on Earth’s surface. The model results show that glacial loading and postglacial unloading cause flexure and rebound of the lithosphere, respectively, and hence alter the stress field in the crust. As a consequence, the slip of the model fault varies through time. In most cases, both normal and reverse faults experience a slip rate decrease during glacial loading and a strong slip rate increase during postglacial unloading. Parameter studies further reveal that the magnitude of the slip rate variations is mainly controlled by the spatial dimensions of the load and the rheology of the crust and mantle lithosphere, whereas the rate of unloading and the thickness of the lithosphere play an only minor role (Hampel and Hetzel, JGR, 2006).
We apply our normal fault models to the Wasatch and Teton faults in the eastern Basin-and-Range Province to show that the slip rate increase on the Wasatch fault resulted from regression of the huge Pleistocene Lake Bonneville (Hetzel and Hampel, Nature, 2005), whereas the enhanced postglacial slip on the Teton fault was caused by melting of the Yellowstone ice cap and the glaciers in the Teton Range (Hampel et al., Geology, 2007). We hypothesize that the rebound caused by the Lake Bonneville and Yellowstone ice cap has also triggered earthquakes on other normal faults in the eastern Basin-and-Range Province. The slip acceleration in our thrust fault models agrees well with the offset and timing of large earthquakes in the Lapland Fault Province, which occurred at ~9 ka after the melting of the Fennoscandian ice sheet.
In general, our findings imply that postglacial slip on faults in glaciated regions may not be uniform through time. Rather, a significant fraction of slip may have accumulated within a few thousand years after the last glaciation. Furthermore, our model results support the idea that the low level of seismicity in currently glaciated re¬gions like Greenland and Antarctica is caused by the presence of the ice sheets. Based on our models we expect that the decay of the Greenland and Antarctica ice sheets in the course of global warming will ultimately lead to an increase in earthquake frequency in these regions.