STUDY ON THE PRECURSORY ANOMALOUS CHARACTERISTICS OF GEOMAGNETIC FIELD IN NORTH CHINA

Analysing North Chinas precursory geomagnetic anomalies of moderately strong earthquakes in the last 30 years using of different geomagnetic methods,it is found that the build-up processes of earthquaks have relation to the variation of geomagnetic field.In the area length of 200 kilometers around the epicentre,there are many anomalous variations occurred in some seismostations before a moderately strong earthquake happens, and the anomalous duration is less than a year.The anomalies ies often shows a increasing or decreasing tendency.