Precise　seismic　fragility　analysis　holds　significant　importance　in　the　evaluation　of　seismic　resilience　for　underground　structures.　However,　the　conventional　seismic　fragility　analysis　of　underground　structures　usually　ignores　the　hybrid　epistemic　uncertainties　caused　by　the　limited　samples　of　both　seismic　demand　and　thresholds　and　deterministic　boundaries　between　different　limit　states,　which　can　inevitably　cause　errors　in　the　seismic　resilience　evaluation　of　underground　structures.　Thus,　focusing　on　the　quantification　of　epistemic　uncertainties,　this　paper　aims　to　propose　an　approach　to　seismic　fragility　evaluation　of　underground　structures　considering　hybrid　epistemic　uncertainties　of　both　seismic　demand　and　capacity.　In　this　approach,　the　analytical　formulation　of　seismic　fragility　considering　the　fuzziness　of　limit　states　is　firstly　derived　via　adopting　the　entropy　equivalence　method.　Then,　the　non-parametric　Bootstrap　method　and　maximum　entropy　principle,　as　well　as　the　Copula　theory　are　combined　to　establish　a　probability　model　characterizing　the　statistical　uncertainties　of　both　seismic　demand　and　capacity.　On　this　basis,　the　variability　of　failure　probability　of　underground　structures　is　quantified,　and　the　envelope　fuzzy　seismic　fragility　is　obtained.　Moreover,　the　influences　of　the　coupling　effect　of　statistical　uncertainty　and　fuzziness　on　the　seismic　fragility　of　underground　structures　are　also　analyzed　in　this　paper.　The　results　show　that　the　seismic　fragility　of　underground　structures　based　on　limited　samples　of　both　seismic　demand　and　thresholds　and　deterministic　boundaries　between　different　limit　states　has　significant　variability,　and　the　variability　degree　of　fragility　is　various　under　different　ground　motion　intensities.　Besides,　the　envelope　fuzzy　seismic　fragility　curves　can　effectively　reflect　the　coupling　effect　of　statistical　uncertainty　and　fuzziness　and　adequately　characterize　the　variability　of　estimated　seismic　fragility,　which　can　provide　a　more　accurate　basis　for　seismic　resilience　evaluation　of　underground　structures.　©　2024