SHORT DURATION RESISTIVITY VARIATIONS BEFORE AND AFTER THE M = 7.8 TANGSHAN EARTHQUAKE
ZHAO YU-LIN QIAN FU-YE(State Seismologieal Bureau)
Before and after the M = 7.8 Tangshan Earthquake of July 28, 1976, within a period of time of from July 12 to October 15, 1976, the Tsangli resistivity station located in the epicentral area, recorded a series of short duration resistivity drops, 31 in number (called "Negative pulses"). Among them, nine appeared about half month before the main shock. They superimposed on the relatively rapid descending trend of the resistivity values, composing a rather typical form of variation. This is the first observation of such a peculiar phenomena at this station since its establishment in 1969.The chief characteristics of the resistivity pulses are: their amplitudes exceed the standard errors of measurements by 3-20 folds; they are negative relative to the background value of the descending resistivity; the duration of each pulse lasts to as long as .13 hours; the time of appearance of their minimum values delays day by day, this delay agrees with that of the tidal force. It indicates that between negative pulses and tidal forces, there exists a close relationship in their causes. Meanwhile, the time of occurrence of the stronger shocks of the Tangshan Earthquake sequence corres-ponds closely with the variation of the tioal force. It is assumed that the appearance of negative pulses of the resistivity values may involve the change of mechanical property of the medium at the focal region of the earthquake sequence.