Abstract: In this paper,we analysed and studied moderate-strong seismic characteristics in the 3rd and 4th seismically active periods in North China and proposed that: ① Seven active episodes in nine of the 3rd and 4th seismically active periods in North China have the same character that moderate-strong earthquakes occurred three years before the first strong shock and two years before for the 1st to 4th active episodes of the 4th seismically active period espically, so it will be more propably to have moderate-strong earthquake as prelude before the first strong shock in the coming 5th active episode; ② The moderate-strong seismicities in the later stage of seismic quiescence episodes in North China have certain indication in time and location to the first strong shock in the following active episode. It may proceed in a new strong-shock active episode in North China, so this study will be useful for strong earthquake trendency prediction in North China.

Key words: North China, Later stage of seismic quiescence episode, Moderate-strong seismicity, Active episode, First occurring strong earthquake, Strong shock prediction