Analysis of Small Earthquake Activities before Medium-strong Earthquakes in Northeast China

13 earthquakes with M≥5.0 have been recorded in the northeast region of China since1970,among which eight medium-strong earthquakes are independent in space and time.Studies of seismicity before these eight medium-strong earthquakes show that small earthquake activity was enhanced before the occurrences.Though seismicity increase is a common phenomenon in the northeast China region,we have difficulty in predicting the medium-strong earthquakes by this phenomenon alone.In order to predict medium-strong earthquakes through seismicity increase,this paper tries to propose a new method that calculates small earthquake frequency through the changing pattern of small earthquake activities based on the characteristics of small earthquake activity in the northeast China region.The results show that we can get the obvious anomaly frequency of small earthquakes before medium-strong earthquakes through the new method,and can obtain a medium to short term anomaly index for the northeast China region.