Seismic fragility analysis of deep underground structures based on fault-to-structure earthquake simulation

There is a significant difference in the seismic response of deep underground structures subjected to near-field and far-field seismic motions. To accurately assess the fragility of structures in near-fault sites, a three-dimensional regional geophysics model considering the fault-to-structure physics process was constructed by the finite difference method software SW4 to perform the seismic wavefield calculations. Taking the calculated seismic motions as input, the domain reduction method (DRM) model considering the ground loss was established and the probabilistic seismic demand model of the structure was computed by the OpenSees-based visualization pre/post-processing software STKO. The pushover analysis considering the effect of bi-directional ground motions was also conducted to quantify the seismic performance levels of the structure. The seismic fragility curves for deep underground structures were eventually developed, and the influence of site condition and burial depth on them was analyzed. The results show that the deriving method of structural fragility curves proposed in this study, accounting for the whole physical process of fault-to-structure, could provide an effective assessment for the structural seismic fragility. It is essential to consider the variation of seismic capacity at different burial depths as the bearing capacity and ductility would significantly decrease as the burial depth increases. The structural fragility increases with the burial depth at both site C and site D, while remaining consistently lower at site C than at site D. Furthermore, the effect of site condition on structural fragility curves would diminish as the burial depth increases.