Stochastic Seismic Response Analysis of Soil-Fault-Tunnel Systems Using the Modified Domain Reduction Method

Underground tunnels are vital parts of modern infrastructure, serving as crucial lifelines for transportation, utilities, and emergency services worldwide. As urbanization increases, maintaining the safety and functionality of these structures during seismic events has become important. Past earthquakes, such as the 2008 Wenchuan (China) and 2011 Tohoku (Japan) quakes, have shown how vulnerable underground tunnels are to fault-induced damage, highlighting the need for reliable seismic analysis tools. This study introduces a comprehensive numerical framework for analyzing the seismic response of soil-fault-tunnel (SFT) systems. The simulation includes realistic models of the seismic source, wave travel paths, local site effects, and spatial variations in soil properties, as recommended by the National Earthquake Hazards Reduction Program (NEHRP, 2001). Fault rupture modeling requires extending the computational domain over several kilometers, which makes this approach computationally inefficient. To address this, an efficient modeling algorithm called the Domain Reduction Method (DRM) was incorporated for analyzing the SFT system. DRM involves a two-step process where free-field ground motions are first calculated and then applied as effective forces to a reduced domain containing the tunnel and surrounding soil with absorbing boundaries. A circular tunnel cross-section subjected to inclined fault excitation is analyzed using the three-layer DRM framework. To account for material uncertainties, 210 Monte Carlo simulations are performed. All analyses are carried out using MASTODON, an open-source finite element software based on the MOOSE framework, capable of handling dynamic multiphysics problems. The results provide insights into the effects of fault inclination, surface wave amplification (especially when the incident angle exceeds the critical angle), and local site responses such as frequency shifting and wave attenuation or amplification. These findings offer valuable guidance for tunnel seismic design and contribute to developing advanced materials and strategies to improve the resilience of underground infrastructure. Keywords: Underground tunnel, Longitudinal deformation, Numerical modeling, Local site effects.