Aftershock sequence can last a long time in the vicinity of historical strong earthquakes,especially in the mid-continental regions where deformation rate is low.The earthquake-sequence-association (ESA) problem,that is,whether an earthquake in these regions,long after the mainshock,can be associated with the aftershock sequence,or it is a ‘normal' background event,is important in the assessment of time-dependent seismic hazard.For investigating this problem,a candidate tool is the space-time epidemic-type aftershock sequence (ETAS) model,which separates seismicity into ‘background' and ‘clustering' represented by probability.Taking the 1976 Tangshan Ms7.8 earthquake sequence as an example,we applied the space-time ETAS model to the seismicity in the Tangshan region since 1970,with magnitude no less than ML4.0,and discussed the ESA problem of three earthquakes with magnitudes above Ms4.0 occurring since 2010.The ETAS model gives relatively high background probabilities of 0.72,0.88 and 0.76,for these three earthquakes,indicating the probabilities that they are triggered by previous events,including the 1976 Tangshan Ms7.8 mainshock,are small.