Abstract: 26 M_S≥5.0 earthquakes were recorded in the northeast margin of Qinghai-Tibet block since 1980, in which 22 medium-strong earthquakes are independent in space and time. Study of seismicity increasing before the 22 medium-strong earthquakes shows that small earthquakes increasing occurred before 17 medium-strong earthquakes. Though seismicity increasing is a common phenomenon in the northeast margin of Qinghai-Tibet block, we have difficulty in predicting the medium-strong earthquakes by this phenomenon alone. In order to predict the medium-strong earthquakes through seismicity increasing, this paper tries to propose a new method which calculate small earthquake frequency through the changing pattern of small earthquake activities based on the character of small earthquake activity in the northeast Qinghai-Tibet block and then make primary application, the result showed that we can get obvious anomaly frequency of small earthquake before medium strong earthquake through the new method, and it is affected very little from the range of area where small earthquakes occurred. We can get medium to short term anomaly index in the northeast margin of Qinghai-Tibet block .

Key words: Medium-strong earthquake, Qinghai-Tibet block, Small earthquake activity