Following a new train of thinking, this paper has explored first the potential information in the ground resistivity data observed by the existing geoelectric observation system, investigated and proposed a new dimensionless geoelectric precursor factor, the degree of ground resistivity anisotropy, S, and studied the characteristics of dynamic evolution pattern of S during the seismogenic process. The results show that, during the seismogenic process, the degree of ground resistivity anisotropy (S) displays a process of "normal" -~ "abnormal strengthening (amplitude, range)" ~ "abnormal weakening"~ "earthquake occurrence" ~ "normal". The earthquake would occur at the time when the S value has entered the late stage of strengthening and turns to weaken and in the gradient belt on the margin of S anomaly region. The dynamic evolution pattern of S reflects the changes of the tectonic stress field during the seismogenic process. Therefore, it would be possible to trace the process of earthquake generation and occurrence from the dynamic evolution pattern of S so as to service earthquake prediction.Key words: degree of ground resistivity anisotropy dynamic evolution pattern seismogenic process

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 •