Seismological hazard assessment for a site in Northern Germany, an area of low seismicity

Abstract

Potential sites of nuclear waste deposits in the Federal Republic of Germany are situated in areas of low seismicity. Nevertheless, seismic hazard assessment has to be performed for a very long time period in order to prove the facilities of the repositories able to withstand seismic induced loads; even though there is a considerable debate whether or not it is possible to quantify the seismic risk in such an area. A combination of deterministic and probabilistic methods is used to assess the seismic hazard for a site in Northern Germany, fulfilling the standards of the German building code for nuclear power plants. As an example, the site of the former iron ore mine Konrad is investigated. The deterministic method is based on the assumption that the strongest earthquakes inside a tectonic region can happen everywhere there, also near the selected site. For the probabilistic method, several models describing the seismicity in an area of 200 km around the site are used to show the influence of the variability in input parameters, like the maximum intensity of each source region on the exceeding probability of the site intensity. It can be shown that the seismic hazard of a site in an area of low seismicity is mainly caused by the effects of distant but strong source regions and the background seismicity for very low probabilities. Probabilistic evaluation has the advantage of quantifying the seismic risk. But deterministic and probabilistic methods together seem a practical tool for mutual control of the results and to overcome the weakness of each approach alone. The historical German earthquake catalog with an observation period of about 1200 years is the basis for the input data for a probabilistic model. From a deep knowledge of geological development and structural geology, the time history of the surrounding faults is developed. Indications were found that the nearest and most important fault was active at least 5 Ma ago. The combination of both seismicity and tectonics provides the basis for a long term prognostic with probabilities of exceedance in the order of 10 −5 per year. For the investigated site the following parameters were derived: site intensity as a function of exceeding probability; site acceleration; strong motion duration; site dependent response spectra for the surface and the underground inside the mine.