简介：Recentstudiesonassessmentofaverylowannualprobabilityofexceeding(APE)groundmotions,10-4orless,havehighlightedtheimportanceoftheupperboundofgroundmotionswhenverylowprobabilityresultsareacquired.Thetruncationleveladoptedinprobabilisticseismichazardanalysis(PSHA)shouldbedeterminedbyanaleatoryuncertaintymodel(i.e.,distributionmodel)ofgroundmotionsandthepossiblemaximumandminimumgroundmotionvaluesofaspecificearthquake.However,atthepresenttime,itisimpossibletoestablishtheupperboundmodelforgroundmotionsbasedonthesourcecharacteristicsand/orgroundmotionpropagation.McGuiresuggestedatruncationlevelbefixedatanumberof=6,orthedistributionofresidualsbetruncatedinsuchamannerthatsiteintensitycannotbegreaterthantheepicenterintensity.ThisstudyaimstofindareasonableandfeasibletruncationleveltobeusedinPSHAwhenthephysicalmechanismisnotavailabletofindtheextremegroundmotion.AmathematicalanalysisoftheinfluenceofthetruncationlevelonPSHA,casestudiesofsitesindifferentseismotectonicsettings,andadistributionanalysisofgroundmotionresidualsareconductedinthisstudy.Itisconcludedthat=4istheminimumacceptablevalueforengineeringapplicationsforAPEswithin0.002to10-4,andforlowAPEs,suchas10-5and10-6,thevalueofshouldbenolessthan5inmostregionsofChina.