The　rock　mass　surrounding　a　subsea　tunnel　often　contains　seawater,　and　the　coupled　effects　of　an　earthquake　and　seepage　introduce　more　uncertainty　into　analyses　of　tunnel　safety.　However,　research　on　this　topic　is　still　lacking.　In　the　present　study,　models　of　structure　and　fluid　fields　were　established　by　considering　the　effect　of　a　two-dimensional　viscoelastic　boundary　condition,　and　the　stability　of　a　cross-sea　tunnel　structure　under　seepage　and　a　bidirectional　earthquake　was　studied.　Using　a　static　strength-reduction　method　on　the　basis　of　dynamic　finite　elements,　a　stability-safety　coefficient　was　obtained.　In　addition,　the　influences　of　sea　depth,　overlying　rock　thickness,　and　permeability　coefficient　on　the　seismic　dynamic　stability-safety　coefficient　and　varying　characteristics　of　the　plastic　zone　in　a　cross-sea　tunnel　structure　under　seepage　and　a　bidirectional　earthquake　were　studied　through　numerical　examples.　The　results　showed　that　under　this　type　of　earthquake,　plastic　zones　first　appeared　on　both　sides　of　the　tunnel　arch　feet　and　in　the　peripheral　areas　of　the　arch　shoulders;　by　contrast,　the　vault　was　secure,　and　no　plastic　zone　appeared　in　this　area.　When　all　factors　were　considered,　the　plastic　zones　extended　farther　when　the　overlying　rock　thickness　and　sea　depth　were　greater.　When　the　overlying　rock　was　thick　and　the　rock　mass　surrounding　the　tunnel　was　only　slightly　weathered,　the　rock　mass　was　close　to　intact,　and　as　a　consequence,　the　permeability　coefficient　did　not　significantly　affect　the　development　of　the　plastic　zone　or　the　stability-safety　factor.　The　influences　of　these　three　parameters　on　the　safety　of　a　subsea　tunnel,　as　obtained　in　this　paper,　can　provide　a　theoretical　basis　for　the　reasonable　design　of　submarine　tunnels.　(C)　2017　American　Society　of　Civil　Engineers.