Abstract:Using the primary components analysis method,six main parameters for indicating seismicity strenth such as frequency N(ML≥3.0),b value,η value,A(b) value,Mf value,Ac-value are selected to analyse.The parameters are simplified effectively and a new synthetic index W for indicating seismicity strenth is obtained.It is found that the index W appeared obvious anomalies before Haicheng MS7.3 earthquake in 1975 and Xiuyan MS5.4 earthquake in Liaoning province,and returned after events.It is shown that the synthetic index W can reflect anomalous characterisitic of seiemicity well.