OBSERVATION OF CHANGES WITH TIME OF APPARENT RESISTIVITY WITHIN THE DEPTHS OF EARTHQUAKE FOCI AND THE OCCURRENCE OF EARTHQUAKES

This paper deals with test results of repeated observations of the apparent resistivity at the Ebo site,Qilian,Qinghai Province and at Huocheng,Shandan,Gansu Province from July 1979 to October 1986 by the MT method. The noise background is low and seismicity is high around the observation sites. The errors of observations are analysed and discussed. It is found that in comparison with initial curves,systematic changes of 15-30% in the apparent resistivity in part of the frequency range of two repeated observations in 1984 at the Ebo site were associated with the occurrences of earthquakes of M5 around the sites; then returning to initial values when measurements were repeated in 1985 at the Ebo site with no earthquakes of M5 occurring around the sites. Systematic changes of 40-70% in the apparent resistivity in the entire frequency range repeatedly observed in 1986 at the Ebo site and at the Huocheng site were closely associated with the occurrences of earthquakes of M=6.7,5.7,5.0 at Menyuan,Qinghai Province. The epicentral distances of the two observation sites were about 60 km.