Abstract: As a Mesozoic decratonization craton, seismicity is intense in North China. Since the 1960s, several major earthquakes occurred in North China, including the 1966 Xingtai M7.2 earthquake, the 1975 Haicheng M7.3 earthquake, and the 1976 Tangshan M7.8 earthquake etc. Recently, a small earthquake sequence occurred in the hypocenter region of the 1976 Tangshan M7.8 earthquake. The sequence has caused widespread concerns and heated debate. In the science community, the debate is whether the recent Tangshan earthquakes are the aftershocks of the 1976 earthquake or are harbingers of a new period of active seismicity in the area and the rest of North China, where seismic activity seems to fluctuate between highs and lows over periods of a few decades. Seismic activity in the Xingtai, Haicheng and Tangshan regions is studies based on the small earthquake catalog in the past 40 years. The results show that seismic activities in the three regions are more frequent than not only regional seismicity before major earthquakes, but also the background seismicity in North China. Despite the earthquake activity, present-day crustal strain rates in the Xingtai, Haicheng and Tangshan regions obtained from GPS survey are also obviously higher than the background strain rates in North China. Both the small earthquake activity and the crustal strain rate indicate that the recent seismicity in the Xingtai, Haicheng and Tangshan regions should be aftershock sequences of last major events in the regions, respectively. The aftershock activity should last at least tens of years in North China. The crustal stress relaxation after major earthquake may be one of the major reasons for the long aftershock sequence in North China.

Key words: Long aftershock sequence, Crustal deformation, Major earthquakes of North China