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

Abstract

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, Ra(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 Rp(t) are studied. The results show that: 0) 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; ® with the shortening of countdown before earthquake, the Ra(t ) value in epicentral region increases gradually (ground resistivity value decreases at an increasing rate); ® the Rt,(t) value has the epicentral area as a center and its contour lines propagate towards surrounding areas with the shortening of countdown before earthquake; ® after the Rp(t) value in epicentral region has descended at increasing rate to reach an extremity [R;(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 Rp(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.Key words: short-impending process of earthquake ground resistivity precursor front dynamic evolution physical process •