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Abstract

The Izmit earthquake of August 1999 with a magnitude 7.4 is the latest of an earthquake sequence that started in 1939 in Eastern Turkey. This sequence brought the plate boundary between the Anatolian plate and the Eurasian plate successively from East to West to fail. Thus, the next event in this series is expected to take place West of Izmit, i.e. South of Istanbul under the Marmara Sea. This segment is a so-called seismic gap – the stresses resulting from the plate motion have not been released since the last major earthquake in 1766. One important parameter for earthquake hazard assessment is the slip rate along plate boundaries. However, the precise estimation of these fault slip rates is difficult to determine in this area for two reasons. The plate boundary under the Marmara Sea is split into several splays with complex geometries. Furthermore, the main part of this fault system is below the Marmara Sea and classical observation methods cannot be applied. In order to estimate the fault slip rates a computer simulation is necessary. The simulation results show that the slip rate along the main fault is 10 to 45 % smaller than previously assumed and that it varies along the main fault by 40 %. The latter indicates that the accumulated stresses at this fault will potentially not be released with one strong earthquake, but in several events with smaller magnitude. However, it is important to emphasise that Istanbul, situated at a short distance to the main fault (15 km), still faces a very high seismic hazard.