Abstract: This paper, by using the method of fuzzy recognition of earthquake precursors, the eighty thousands' sets of hydrochemical data, such as Rn, He, H2, C02, Hg, etc. of 50 wells before three strong earthquakes (Bohai, Haicheng and Tangshan earthquakes) and its strong aftershocks as well as three moderately strong earthquakes (Datong, western Baotou and Zhangbei earthquake) in North China during the period 1969~1998 have been analysed. The temporal and spatial distribution characteristics of precursory variations of Radon content, ionic and gaseous components in groundwater before strong earthquakes have been studied. The medium- and short term anomaly indexes of hydrochemical compo­nents before strong and moderately strong earthquakes in this region are suggested. Final­ly, the capability to predict earthquake for the fuzzy recognition method is also discussed.

Key words: Hydrochemistry, Fuzzy recognition, Prediction capability, North China