Abstract: The anomaly discriminant method of hydrochemical parameters before " the Nine-Five" project was summed up, and the method, which fits the medium- and short- term anomaly discrimination was optimally selected. Authors proposed the feature marks: the acceleration index of anomaly, the dynamic change index and t rend backg round index of medium- and short-term anomaly. At the same time, authors gave the discriminant method and distinguishing index to the medium- and shot-term anomaly related to their characters. Using the observational data from 275 stations, the authors computed R -value for all kinds of ways. The results show that because the strong earthquake-generation lies in different structural surrounding, their anomaly characteristics are different, so R -value is different. The result regarding the effect of the discriminant method on anomaly show that the anomaly acceleration index is the important mark that earthquake-generation entered the medium- and short-term. The main methods to describe anomaly acceleration index include the amplitude acceleration method and the high frequency anomalous frequency acceleration method. The acceleration anomalies are mainly concentrated in adjusting unit in earthquake -genera ting area or active segment of the fault. Descriptive dynamic change index mainly uses the relative ratio. This method has strong ability to discriminate the anomalies for observation station with annual stable change.The important index of M_S ≥ 7. 0 earthquake is that at least 20 percent anomaly stations have t rend anomaly lasting more than six months in earthquake-generatingarea.

Key words: Hydrochemistry, Medium- and short-term anomaly, Discriminant method, Anomaly index