Abstract: Spatio-temporal evolution characteristics of medium-term anomaly in North China from 1986-1999, as w ell as the stag e-by-stage evolutional processes of precursory anomaly before and after the Datong earthquake with M 6. 1 in 1989 and Zhangbei earthquake with M 6. 2 in 1998 have been studied in this paper. And the relationship between the precursory anomaly evolution in group and group activity of moderately strong earthquakes is also studied. The results show that, in North China, the precursory anomaly is mainly distributed in the main areas of earthquake group activity and along the relevant active faults. Ratio of the anomaly observing it ems to all observing items in this area increases about 10% from normal level about 3 or 4 year before the strong earthquakes, maybe this is the obvious sign from long-term stag e into middle-term stage. The ratio m ay increase again and this means from medium-term stag e into the short-term stag e. Crustal deformation is the major type of precursory anomaly and distributed in the area of 100-250 km range around the epicenter. There are few anomalies near the epicentral area, indicating the medium strengthening in seismic source area. It is helpful for judging the emphatic area of medium-term earthquake risk by synthetical analysis of seismic activity and the features of precursory anomaly evolution in group.

Key words: Group activity of strong ea rthquakes, Precursory anomaly in group, Medium-term stage, Evolution, North China