ON THE CORRELATION OF VARIATION OF EARTH-RESISTIVITY AND UNDERGROUND WATER LEVEL AT SHALLOW DEPTHS OF AROUND THE EPICENT BEFORE AND AFTER THE 1976 TANGSHAN EARTHQUAKE

This paper gives data for the variation of earth-resistivity at a number of observation stations around the epicentral area since the 1976 Tangshan Earthquake. It also provides data for the change of water level and temperature in some water wells within the same region previous to the earthquake. From these observed facts, one can see that 2-3 years prior to the earthquake, lowering of earth-resistivity value at the 9 stations around Tangshan is an only significant and unique phenomenon since their establishment in 1970. For other time periods, say 1970-1973 and 1977-1979, the average variation of earth-resistivity value has been very slight, essentially unaltered, while after the earthquake it kept at a lower value without recovering.Since the variation of earth-resistivity of the ground is usually considered as caused by the variation of water content in rock layers beneath the ground being investigated and their stress condition and temperature, the observed facts that before the Tangshan Earthquake, underground water level was lowered and the temperature kept almost constant seem to indicate that variation of earth-resistivity is independent of the water content and temperature of the rock medium beneath, but dependent chiefly on its stress condition.