In this paper the aftershock activity of shallow strong shocks(M≥5) in China continent is analysed, it shows that the duration of aftershock sequence has some regularity. There are four periods of aftershock activity which consist of initial stage, middle stage, late stage and extremely late stage.
According to certain characteristic of seismic activity of the north China, we suggest some estimated method of strong aftershocks and late strong aftershocks. Some corresponding relation exists between the adscissa intercept in b value of the earthquake sequence and the maxium magnitude of aftershock as well as between the divergeney point of b value curve and the frequency of the strong aftershocks. The possible places of strong aftershocks are the extend rupture direction of strong earthquake, the seismic gap in aftershock sequence and the edge of the aftershock region. However, there are some corresponding relations between frequency change of earthquake sequence in aftershock region and the duration of strong aftershocks.
In this paper mechanism of strong aftershocks is discussed in theory. The strong aftershocks of initial and middle stage associate with the surplus strain energy and the stress distribution in seismic region. The late and extremely late strong aftershocks associate with the region stress increased and its propagating process in seismic zone.