Experimental observation and preliminary dataanalysis of vertical geo－electric field

Electromagnetic anomalies before earthquakes have been observed many times on ground and by satellite. At present, there are more than one hundred stations for horizontal geo-electric field observation in China. However, there had been none for vertical geo-electric observation before this study. Since April 2009 we have set up three vertical geo-electric field observation sites at Songshan, Huangyangchuan and Hongshawan in Tianzhu region, Gansu province. It is the first time to do this kind of study in China. For one year's measurement the vertical geo-electric data display a long-term fluctuation and a strong diurnal variation. The diurnal variation amplitude is about 16 mV/km in Songshan and Huangyangchuan, and 8 mV/km in Hongshawan. By spectrum analysis we found that the amplitude of the 24 h period component is the largest and the 12 h period ones is secondary in vertical geo-electric diurnal variation.