Character of geoelectric intermediate term precursory anomaly and judgment for intermediate term tendency of strong earthquake

Based on systematic processing to long-term geoelectric observation data, we mainly studied the character of earth resistivity tendency anomaly for M _S>7.0 strong earthquakes and its relation with seismogenic process. The character of quick descending at early stage and then slow development with steady shape in geoelectricity is a basic feature of geoelectric tendency anomaly for strong earthquake. Usually the ratio of average time between the early quick descend stage and the post slow development stage is about 1:4. In addition, for Tangshan M _S=7.8 and Songpan M _S=7.2 earthquake, we studied the diffusion character of the geoelectric tendency anomaly development. For Tangshan earthquake, we also studied the time-space-strength dynamic evolution character of its geoelectric precursory information quantity. The results showed that diffusion velocity of geoelectric precursory tendency anomaly for the two earthquakes is respectively 90 km/a and 100 km/a. The variation character of geoelectric tendency anomaly presents some indexes for judgment of strong earthquake tendency background and intermediate-term prediction.