Seismic source characterization of the Alpine foreland in the context of a probabilistic seismic hazard analysis by PEGASOS Expert Group 1 (EG1a)

Seismic source characterization is performed as part of the PEGASOS project for the assessment of the seismic hazard at the 4 sites of the Swiss Nuclear Power Plants. The analysis is performed according to the Level 4 procedures for expert elicitation defined in the guidelines of the US Nuclear Regulatory Committee whereby the quantification of uncertainties plays a crucial role. According to our analysis, which is one amongst four that were performed in the frame of PEGASOS, the most important epistemic uncertainty is related to the question as to weather basement-rooted faults at the margins of pre-existing Permo-Carboniferous troughs are prone for compressive or transpressive reactivation under the present-day stress field or not. The question after the present-day style of deformation in the Alpine foreland (thick-skinned versus thin-skinned) is closely related to this key question. Together with the consideration of uncertainties regarding the mapping of seismogenic zones and/or line sources alternative zonations are presented in form of a logic tree with 21 branches. Area sources play a predominant role in the working area located at the margin of a diffuse plate boundary. Earthquake recurrence relationships are discussed by taking into account a series of uncertainties. These concern the evaluation of b-values and the evaluation of a-values once the b-values were fixed. Both parameters in the Gutenberg-Richter law are based on non-perfect and incomplete catalogue data that were carefully analysed beforehand. Since PEGASOS demanded an analysis of annual probabilities down to one event in 10(7) years, the question after the value of the maximum possible earthquake magnitude M-max and related error in M-max estimates plays a crucial role. We estimate M-max by using geological as well as statistical methods. M-max = 6.9 cannot be excluded in most areas, in the Basel area M-max = 7.3 is possible. Uncertainties in a, b and M-max are again discussed in form of a logic tree, this time with 18 branches. Hence the final logic tree has 378 branches and represents the seismic source characterization input into PSHA that takes account of all uncertainties we are aware of.