Influence of tunnel-fault intersection angle on the seismic deformation mode of a large-span cross-fault subsea tunnel

Cross-fault subsea tunnels are subjected to spatially non-uniform seismic motion and fault-induced permanent displacement, but the role of the tunnel-fault intersection angle remains difficult to evaluate when fault rupture is represented only by prescribed offsets. Taking the Qingdao Jiaozhou Bay Second Subsea Tunnel crossing the Cangkou Fault as an engineering case, this study combines deterministic ground-motion simulation with modified domain reduction method (MDRM) seismic input to investigate the seismic response of a large-span cross-fault subsea tunnel. The simulated cross-fault seismic field shows that the surrounding rock at tunnel depth is affected by both fault-parallel and fault-normal displacement components; the latter is smaller but non-negligible. Under the actual intersection angle of 27 degrees, the tunnel displacement varies smoothly along the alignment because the tunnel passes above the fault core, with a cumulative lateral displacement of about 6 cm and a longitudinal compression of about 11 cm. Across the six discrete alignment cases, the peak normal and shear strain components exhibit different, non-monotonic patterns. The hoop and longitudinal normal strains are relatively large and the shear strain is smallest at 42°, whereas the shear component is larger at 12°, 72° and 87°; the maximum equivalent strain occurs at 57°, where both normal and shear components remain appreciable. These results indicate that changing the tunnel alignment within the prescribed earthquake-source and regional-site model, together with the associated change in spatial ground-motion sampling, primarily modifies the normal–shear strain combination rather than monotonically amplifying the overall strain level.