Methodology and main results of seismic source characterization for the PEGASOS Project, Switzerland

Abstract

Under the direction of National Cooperative for the Disposal of Radioactive Waste (NAGRA), a probabilistic seismic hazard analysis was conducted for the Swiss nuclear power plant sites. The study has become known under the name “PEGASOS Project.” This is the first of a group of papers in this volume that describes the seismic source characterization methodology and the main results of the project. A formal expert elicitation process was used, including dissemination of a comprehensive database, multiple workshops for identification and discussion of alternative models and interpretations, elicitation interviews, feedback to provide the experts with the implications of their preliminary assessments, and full documentation of the assessments. A number of innovative approaches to the seismic source characterization methodology were developed by four expert groups and implemented in the study. The identification of epistemic uncertainties and treatment using logic trees were important elements of the assessments. Relative to the assessment of the seismotectonic framework, the four expert teams identified similar main seismotectonic elements: the Rhine Graben, the Jura / Molasse regions, Helvetic and crystalline subdivisions of the Alps, and the southern Germany region. In defining seismic sources, the expert teams used a variety of approaches. These range from large regional source zones having spatially-smoothed seismicity to smaller local zones, to account for spatial variations in observed seismicity. All of the teams discussed the issue of identification of feature-specific seismic sources (i.e. individual mapped faults) as well as the potential reactivation of the boundary faults of the Permo-Carboniferous grabens. Other important seismic source definition elements are the specification of earthquake rupture dimensions and the earthquake depth distribution. Maximum earthquake magnitudes were assessed for each seismic source using approaches that consider the magnitudes of observed earthquakes within analogous tectonic regions. All four expert teams used the PEGASOS earthquake catalogue for estimating earthquake recurrence parameters. This catalogue was developed by the Swiss Seismological Service and provided all historical and instrumental events in a uniform moment magnitude. The teams evaluated alternative declustering approaches and used available historical data to assess catalog completeness as a function of location, magnitude, and time period.