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

摘要： 分析了晋冀蒙交界地区3次6级以上地震前的电场变化,发现各台异常具有如下相似特征:1)异常形态绝大部分为阶变类型,即突升(或突降)-转平-突降(或突升)型;2)异常具有非常明显的短临特征,统计的10次异常中有9次在震前1个月内存在异常;3)异常的持续性,即地震发生后异常持续一段时间就突然恢复到原来的水平,同震结束的现象较少;4)单测向异常,即强震前一般只在台站的1个测向出现异常,文中研究的异常信息多出现在NW或EW方向,与震中方位有关;5)异常幅度与震级及震中距密切相关。最后,通过与水位资料进行对比分析,认为这类异常与地下流体的运移关系密切。

关键词: 自然电位, 大同地震, 包头地震, 张北地震, 阶变异常