Abstract: Daily variation ratio of geomagnetic vertical component (Z) is important in the study of earthquake precursor analysis. The variation magnetic field makes an induced magnetic field because of the earth conductivity, and the induced magnetic field includes information from outer magnetic field and underground electrical conductivity, so the geomagnetic field Z component reflects more changes of underground activity than other components. We collected data of Jiangsu area since 1984 and got 29 events of anomalies with day by day ratio, and then carried out a statistical analysis on the relation between them by combining geomagnetic storm and the earthquake after the abnormal events. The study has found that the anomaly event of day by day ratio and earthquake have a good correlation without geomagnetic storm, with coefficient of 0.77, while the corresponding relationship drops to 0.25 with geomagnetic storm. Combining all of the abnormal events, we consider that, after the abnormal event of day by day ratio, the possibility of the abnormal event of day by day ratio being an earthquake precursor will be 0.8 if there is no geomagnetic storm on that day or the Dst index is smaller than that on the second day. Otherwise, the abnormality maybe be caused by geomagnetic storm rather than the influence of underground activity.

Key words: Daily ratio of Geomagnetic vertical component, Electrical conductivity, Geomagnetic precursor, Geomagnetic storm