GEOELECTRIC RESISTIVITY ANOMALIES BEFORE EARTHQUAKES

Abstract

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 premonitory slip on the fault. (ⅳ) The slippage on the fault during or after an earthquake may sometimes cause post-earthquake effect in the variation of ρ_x.