Adaptation of the paleoseismological approach to local tectonic regime: comparative study of the intraplate Basel-Reinach fault, Switzerland and the interplate North Anatolian fault, Turkey.

Abstract

The assessment of seismic risk in a given region relies on the proper estimate of related seismic hazard, i.e. the probability that an earthquake of said magnitude occurs within a said time window. Calculating that probability requires a precise knowledge of local seismotectonic parameters such as the frequency of damaging earthquakes, their magnitude and the annual deformation rate. These parameters may only be properly determined when observed over a representative time window. As return periods for strong earthquakes vary between a few decades and several millennia, instrumental and historical seismological datasets often prove to be insufficient. To improve the completeness of available seismicity catalogues, we applied the paleoseismological approach to two different regions: the Basel region (southern Upper Rhine Graben) which was struck by a destructive earthquake in 1356 and the Izmit region (east of the Marmara Sea), partly destroyed in August 1999 by a magnitude 7.4 tremor. A full integrated study involving geomorphology, geophysics and trenching in the Basel region led to the identification of the seismogenic fault responsible for the last strong earthquake and to deciphering its seismic history for the Holocene. Thus, we demonstrate that the newly identified Basel-Reinach fault is the locus for a magnitude 6.4-6.7 earthquake every ~2600 years, on average. Furthermore, we give strong evidence for the probable extension of the fault through the city of Basel to the north and the folded Jura to the south. These results have a direct impact on the level of seismic risk to the region as it was consequently lowered by one degree on the basis of our original work. We applied a similar, though adapted, approach to the North Anatolian fault around the Marmara Sea. An extensive trenching effort combined with ground-penetrating radar profiles carried out on the Ganos fault to the west and the Izmit-Sapanca segment to the east evidence several recent events and yield measurements of pertaining co-seismic offsets. They clearly show characteristic behavior on both sides of the Marmara Sea, however with varying offsets: 4 to 5 m for the Ganos fault and 2 to 2.5 m for the Izmit-Sapanca segment. This brings new constrains to better characterize faulting behavior in the region and consequent seismic risk to the city of Istanbul.