The authors investigated the seismic capacities of an immersed tunnel that has been used for about thirty years. Level-2 input ground motions were defined by the seismic investigation of historical earthquakes and active faults around this site. Seismic capacities of the tunnel were evaluated through earthquake response analyses for the surrounding ground and earthquake response analyses for the tunnel structures against a cross section and a longitudinal section.
The results of the seismic capacity evaluation are as follows.
1) Some structural parts of the underground shaft were not ensured strength against shear forces.
2) Joints between the tunnel and the underground shaft were not ensured capacity against watertight.