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我国大陆地震多发生在１０～２５ｋｍ深处，在此部位的电性前兆异常，按穿透深度规律传输至地表已相当微弱。当同时考虑场兆的作用时（周硕愚，１９９３），由于电性异常区域的扩展，可大大增强大地电磁法监测地震活动的能力。一维和二维模拟计算结果与震前观测到视电阻率下降的事实相吻合（刘国栋等，１９８４），为此提供了极好的佐证。相应的物理机制是，在地震孕育过程中，因应力作用，岩石扩容，被流体充填，提高了异常区的导电性。
The earthquakes in China occurred mostly in the depths of 10 ~ 25km. The anomalies of electrical precursors in this area were quite weakly transmitted to the earth surface according to the law of penetration depth. When considering the role of the field mech- anism (Zhou Shuoyu, 1993), the ability of the MT method to monitor seismic activity can be greatly enhanced due to the extension of the electrical anomalous region. The one-dimensional and two-dimensional simulation results coincide with the observed decrease in apparent resistivity before the earthquake (Liu et al., 1984), providing excellent evidence for this. The corresponding physical mechanism is that in the process of earthquake gestation, due to the stress effect, the rock is dilated and filled with the fluid, which improves the conductivity of the abnormal area.