关键词: 自然电位, 大同地震, 包头地震, 张北地震, 阶变异常

Abstract: With the continuous observation of earth resistivity,there are more than 30 years'observation records of geoelectric potential in many stations,but analysis on them is relatively lacking. In this paper,we analyzed the variations of geoelectric potential before three earthquakes above 6.0 and one earthquake above 5.0,which are located at the joint areas of Shanxi Province,Hebei Province and Inner Mongolia Autonomous Region. Some similar features are found in these anomalies recorded at 5 stations in a distance of about 300km: (1)The majority of anomalies shows a step-variation type,which means rising quickly (or dropping quickly) at first,then turning to change slowly,and finally dropping quickly (or rising suddenly),and then the earthquake occurs. Sometimes it shows the type of quick turning from sudden rise or drop,and the earthquake occurs around the high point or the low point. (2)The anomalies have apparent imminent characteristic. In statistics,9 anomalies out of 10 presented themselves one month before earthquakes,and 7 anomalies just began to appear at one month before earthquake. (3)The anomalies have a lasting feature,which indicates that they will not end with the occurrence of earthquake,but continue for a period of time after the earthquake. They scarcely cease with earthquake occurrence. (4)Usually,anomaly only occurs at one measuring direction,on each station before strong earthquakes,and there is no synchronous variation on the other measuring directions. The anomalies in our study mainly appeared at northwest or east-west directions,which was related to the location of epicenters. (5) The amplitude of anomalies is correlated to the magnitude of earthquake and the epicentral distance. The bigger the magnitude is and the smaller the epicentral distance is,the relatively bigger amplitude of anomaly recorded by stations will be. Finally,compared with the groundwater table data,it is considered that this type of anomaly is related to the electrokinetic effect induced by movement of underground fluid to epicenter area during the earthquake preparation,when pore pressure of rocks is enlarged and fractures increased in epicentral areas under the action of regional stress field.

Key words: geoelectric potential, Datong earthquake, Baotou earthquake, Zhangbei earthquake, step-variation anomaly