Features and physical process of the dynamic evolution pattern of ground resistivity precursor front

In order to quantitatively describe the geoelectric precursor anomaly in the short-impending process of earthquakes, a new geoelectric precursor index — (monthly) relative change rate of ground resistivity, R _ρ(t), is designed. Using this index and choosing the internationally accepted ground resistivity data before the Tangshan M=7.8 earthquake of July 28, 1976, the features of dynamic evolution pattern of R _ρ(t) are studied. The results show that: (1) about 10–9 months before earthquake, the ground resistivity in a certain range around the epicentral region begins to display the anomaly of accelerating descent, and the rate of descent is higher in the epicentral region than in surrounding areas; (2) with the shortening of countdown before earthquake, the R _ρ(t) value in epicentral region increases gradually (ground resistivity value decreases at an increasing rate); (3) the R _ρ(t) value has the epicentral area as a center and its contour lines propagate towards surrounding areas with the shortening of countdown before earthquake; (4) after the R _ρ(t) value in epicentral region has descended at increasing rate to reach an extremity R _ρ(t) = −7.0], it turns to descend at decreasing rate (2–3 months) and earthquake occurs when it accelerates again. Meanwhile, earthquake occurs when the contour lines of R _ρ(t) stop propagating towards surrounding areas and turn to shrink back (2–3 months later). Its physical process can be explained by the “swollen hypothesis” of Prof. Fu and the theory of “Slip-weakening and rockmass instability” of Mei, Niu, et al.. Contribution No. 98A01009, Institute of Geophysics, China Seismological Bureau, China.