Seismic fragility and risk assessment of transportation tunnels in Marmara and Aegean regions of Türkiye

Turkiye is located in the Alpine-Himalayan belt, one of the most active seismic zones in the world. In addition to these natural processes, the Marmara and Aegean regions of Türkiye have a very high population density. As a result of this situation, the transportation network in both regions is well developed, and there are many railway and roadway tunnels. However, the general seismic fragility of these tunnels has not yet been investigated. For this reason, the purpose of the study is to analyze the seismic fragility of the transportation tunnels in the Marmara and the Aegean regions of Türkiye. For the purpose of the study, the necessary information of the 184 tunnels was compiled and analyzed with the three well-established methodologies. The results were also checked by the April 23, 2025 Marmara earthquake parameters. In this study, Peak Ground Acceleration (PGA) was used as intensity measure and these values were selected based on the coordinates of each tunnel, using the interactive seismic hazard map developed by AFAD (Disaster and Emergency Management Authority of Türkiye) for 475-year return period. To select the categorized median and dispersion factor values for each of the three methods, tunnel-specific parameters such as construction type, construction condition and overburden depth were taken into account. Based on these inputs, fragility analyses were performed to estimate the expected post-earthquake damage states of tunnels under different assessment approaches. The findings of this study may be used to support the identification of tunnels that are most likely to be affected by earthquakes and the prioritization of mitigation measures for those tunnels determined at higher seismic risk.