Epistemic uncertainties in probabilitic earthquake hazard models and their effects on the results :|the case of Marmara region

Abstract

Throughout the history and also in the not too distant past, Marmara region has been a center that hosted several of the most destructive earthquakes around the World. Considering the density of population and building stock and the concentration of economic activities, the performance of comprehensive earthquake hazard assessment studies is one of the essential steps towards the mitigation of the seismic risk in the Marmara region. The seismicity and the earthquake characteristics of this region have been studied extensively in the last decades, and various hazard maps have been created. In the light of the increasing amount and quality of data and new studies on seismotectonic and the developments in the earthquake hazard calculation methods, the need for a regular updating of the earthquake hazard estimates for regions with high seismic activity arises. Therefore, new earthquake hazard maps for specific regions or regions covering many countries are being generated continually. In today’s practice, the use of the probabilistic earthquake hazard assessment method has become a common implementation in the preparation of earthquake hazard maps. However, as opposed to site-specific assessments, large scale regional studies usually investigate the effects of epistemic uncertainties only in a limited way, and in most cases, only the mean hazard outputs are reported. Nonetheless, analysis of epistemic uncertainties in the hazard assessment and reporting of the uncertainty ranges associated with the ground motion estimations can provide valuable insights towards a better understanding of the seismic hazard and consequently of the seismic risk. Starting from this point of view, developing an earthquake hazard assessment model specific to the Marmara region, dealing specifically with the uncertainties associated with the modelling approaches, is quite meaningful when the earthquake history of the region is also considered. In this thesis, in order to examine the effects of uncertainties on probabilistic earthquake hazard analysis results specific to the Marmara region, the Turkish Seismic Hazard Map developed within the scope of the “Update of seismic hazard maps of Turkey (UDAP-Ç-13-06)” project was evaluated. Alternative models were included in the earthquake hazard calculations by obtaining uncertainties related to the data and modelling parameters and combining them in a logic tree structure. Accordingly, hazard maps are obtained for PGA and 5 % damped Spectral Accelerations at T = 0.2 s and 1.0 s for 475 and 2475 years return periods, and uncertainty ranges for the computed ground motion parameters are presented. The sensitivity of the results to the uncertainties associated with different modelling parameters is investigated. The results indicate that, even for the Marmara region, which is one of the best-studied regions in terms of seismic activity and geological structure, the seismic hazard models can be associated with large uncertainties.