CHARACTERS OF GRAVITY CHANGE IN NORTH CHINA SURVEYING NETWORK AND ITS CORRESPONDENCE WITH MEDIUM AND STRONG EARTHQUAKE

The repeated surveying network of gravity in Norty China(transported by train), which has 45 points in three loops, was established in 1972 to 1973.Since the application of precision LCR-G gravimeter in 1986(the surveying points rdduced from 45 to 15 in the three loops), it has accumulated 17observations data. The variation of gravity on each point in North China network can be divided into three types:(1)abnormal points, (2)relative stable points(3)there are some change in gravity, but not caused by deep materials.If the anomalies of gravity lasted 3 to 5years, it may have relations with a mid-stronger earthquake.Prediction practice shows that repeated survey of gravity is an effective approach in earthquake prediction.