地震前地电阻率的异常变化

自1967年开展地电阻率(ρ_x)观测以来,发现地震前有以下几个现象:(1)唐山震前2—3年ρ_x异常约以100公里/年的视速度从震中向外扩展,扩展半径约150公里;(2)震前ρ_x多为下降异常,也记到一些上升变化,ρ_x的升降取决于台址下介质的受力状态并与震源机制有关,其相对变化约比应变放大10~3倍;(3)震前ρ_x异常有两类:长趋势异常,它反映孕震区应力加速积累过程,其持续时间、分布范围及幅度与震级成对数关系;临震异常,它是断层预滑的一种表现;(4)断层在震时或震后滑动有时可引起ρ_x震后效应。     

GEOELECTRIC RESISTIVITY ANOMALIES BEFORE EARTHQUAKES

Since geoelectric resistivity (ρ_x) measurements were started in 1967, the following phenomena have been observed before earthquakes: ⅰ) During 2-3 years before the great Tangshan earthquake, the anomaly of ρ_x propagated outwards from the epicenter with an apparent velocity of 100 km/year, and the radius of propagation reached about 150 km. (ⅱ) Before most of the earthquakes, the resistivity drops, but before a few earthquakes it may rise; all these are controlled by the stress conditions under the geoelectric station and the source mechanism of the earthquake. The relative variation of ρ_x is a magnified (up to about 1000 times) manifestation of rock strain. (ⅲ) Before an earthquake, geoelectric resistivity may exhibit both long-term and close-upon anomalies. The long-term anomaly reflects the accelerated accumulation of stress in the seismo-genotic region; its amplitude, duration and extent are logarithmically related to the earthquake magnitude. The close-upon anomaly may be a manifestation of