2003年民乐—山丹地震前山丹地电中短期—短期前兆变化和无临震显示问题

THE MEDIUM-SHORT AND SHORT TERM ANOMALIES OF EARTH RESISTIVITY AND THE REASON FOR ON IMPENDING PRECURSOR BEFORE MINLE—SHANDAN EARTHQUAKES IN 2003 AT SHANDAN STATION

山丹地电阻率台站距离2003年甘肃民乐—山丹MS6.1地震震中仅43km,在震前1年尺度—短期阶段前兆性变化突出,属于在强地震孕震晚期阶段震源区及附近的典型前兆性变化。应用"异常—震源地点—活动断层—发震机制"之间的特定关系,分析震前山丹地电未检测到明显临震信息的原因,可能是由本次地震的发震应力场分布和山丹地电相对于本次地震的位置决定的。

Shandan geo-electric station is only 43 km from the epicentre of Minle—Shandan M_S6.1 earthquake in 2003. The electric variations precursor is obviously recorded at the station during the one year and short-term period before the earthquake, which is the typical precursory electric variation in or near the focal region in the late preparation period of strong earthquake. But the anomalously electric change does not appear in impending period. The main reason is probably the direction of pressure stress in seismic stress field and the station orientation to the event, according to the special relation ship among the imminent anomaly, focal site, active fault and the causative mechanism.