Scandinavian postglacial faults and their physical connection to present day seismicity

Friday, 18 December 2015
Poster Hall (Moscone South)
Ronald Arvidsson, Geological Survey of Sweden, Uppsala, Sweden
In Scandinavia large earthquakes up to M~8.2 occurred 9500 yBP due to rapid deglaciation leaving fault scarps with lengths up to 160km and vertical offsets of at least 10 m. Today a lion share of local earthquakes are located to the vicinity of the faults. I show here from Coulomb failure stress modeling a physical connection between clustering of recent earthquakes and high Coulomb failure stresses around the faults. This can be interpreted In such a fashion that the location of the current earthquakes resembles locations of aftershock sequences. The explanation is that when these faults where formed it was due to state of stress in the crust at time of deglaciation, different from today's conditions. The crust was heavily depressed at deglaciation about 250 m in the region and due of the receding icesheet the crust was subjected to high stresses resulting in fault motion. This fault motion occurred in order to minimize state of stress at deglaciation. However, this state of stress has since changed with the regional postglacial uplift and thus today these faults remain as perturbations in the crust with concentrations of high stresses. I elaborate on this mechanism. I also advocate that this correlation between high stressed fault areas and locations of earthquakes indicates that seismicity within stable continental regions like Scandinavia might be caused by previous crustal disturbances that show local perturbations of the stress field. Therefore if faults are favorably oriented in the present stress field they can be released by brittle earthquake faulting . Thus past transient tectonic events can explain part of the stable continental region's seismicity. This may be of large importance to assessment of seismic hazard within stable continental regions particularly for critical structures like e.g., nuclear waste deposits and hydroelectric dams.