Seismic performance of tunnels and verification of available seismic risk models for the 2023 Kahramanmaraş earthquakes

The Kahramanmaraş earthquake sequence of February 6, 2023, has provided an opportunity to investigate the seismic performance of tunnels under very strong ground motions. The field survey conducted following the events revealed limited damages to the tunnels in the affected region despite the very high recorded ground motions. It is therefore important first to examine the reasons for these limited damages and then to investigate whether the available state of the art methods and tools for the seismic risk assessment of tunnels could have predicted this behaviour. The paper examines the response of 87 tunnels of various typologies and periods of construction, located in the provinces most affected by the two main events of the seismic sequence, with emphasis on the influencing factors like their structural and geotechnical characteristics, as well as their location with respect to the faults and the amplitudes of the ground motion. To assess and investigate the repeatability of the experienced PGA values of the two events at the tunnel sites, the Probabilistic Seismic Hazard Analysis (PSHA) was employed. A significant degree of variability is observed compared to the recorded motions, which is attributed to local site conditions, near-field and rupture directivity effects, parameters which cannot be captured ideally in all cases. Then, scenario-type risk analyses were performed using the USGS ShakeMaps of the two earthquakes and various established fragility models to describe the vulnerability of the tunnels. The estimated damage levels are compared with the actually observed damages, enhancing an extensive discussion on the capability of the existing models to provide reliable assessments of the expected damages in case of strong ground motions. The study underscores the complexity of assessing the seismic performance and risk of spatially extended underground structures and the importance of robust modelling approaches in enhancing the understanding of tunnel vulnerability and resilience in seismic events.