10 YEARS QUASI CYCLE OF GROUNDWATER LEVEL IN NORTHERN PART OF NORTH CHINA AND ITS RELATIONSHIP WITH SEISMIC ACTIVITY

16 over M5.0 earthquakes occurred in the northern part of North China since 1966, and they are can be divided into 5 earthquake clusters.Eliminated the tendency interference that caused by overdraft, groundwater levels of many wells show a 10 years quasi cycle variation synchronously.The earthquake clusters occurred in the high value period yearly cycle, and no medium and strong earthquakes occurred in low value period.The cycle can be divided into four segments, which is lower, rising, higher and dropping segment.The probability that earthquakes occur in each segment is 0, 0.25, 0.5 and 0.25 respectively.