Georesistivity precursors to the Tangshan earthquake of 1976

Abstract

Georesistivity precursors and corresponding coseismic effects to the Tangshan earthquake of 1976 are given as follows: 1) resistivity measurements with accuracies of 0.5% or better for over 20 years show that resistivity decreases of several percent, which began approximately 3 years prior to the Tangshan earthquake, were larger than the background fluctuations and hence statistically significant. An outstanding example of an intermediate-term resistivity precursor is given. 2) Georesistivity decreases of several percent observed simultaneously at 9 stations beginning 2-3 years prior to the 1976 Tangshan earthquake are such a pervasive phenomenon that the mean decrease, in percent, can be contoured on a map of the Beijing-Tianjin-Tangshan region. This shows the maximum decrease centered over the epicenter. 3) Corresponding coseismic resistivity changes, ∆ρc/ρc, during the M 7.8 Tangshan earthquake were observed at all 16 stations within 240 km from the epicentre. These observed ∆ρc/ρc are opposite in sense but similar in spatial distribution to corresponding georesistivity precursors. This observation suggests that the Tangshan earthquake is a rebound process. Calculation indicates that these georesistivity precursors could be represented by a virtual dislocation, of opposite sign to the real dislocation produced at the time of the Tangshan earthquake. These reported ∆ρc/ρc offer very convincing evidence for accepting corresponding anomalies prior to the earthquake as its precursors. 4) It is inferred from observed anisotropic decreases in georesistivity that before the Tangshan earthquake the crust was compressed and that the angle between the maximum principal stress σ1 and the earthquake fault was about 80° before the earthquake i.e., the fault was locked by the σ1 which is almost normal to the fault.