Abstract: The frequency fluctuation change of aftershocks of Datong-Yanggao 6. 1 moderately strong earthquake swarm is analyzed. It is considered that the frequency change of this sequence well corresponds to the greater earthquake in northern North China, and the frequency and intensity feature is also helpful to the judgement of future epicenters, which can be reg arded as the“ window” for monitoring the seismic activity in northern North China.

Key words: Seismic frequency, Seismic window, Post-earthquake, Datong-yanggao earthquake