Preliminary study on regional geo-resistivity anomaly before the Wenchuan M_S8.0 earthquake

Geo-resistivity data recorded at 17 stations around the epicenter of the Wenchuan M_S8.0 earthquake were processed. Taken the geo-resistivity data in 2007 as background values, the absolute values of relative variations of monthly average value in NS and EW directions in 2008 were calculated, and then the larger one was chosen as the relative variation of each month. At last the variation images of geo-resistivity anomaly for the 5 months before the Wenchuan earthquake were obtaind. The results show that, as the occurrence time of the earthquake drew near, the spatial variation of geo-resistivity anomaly shrinked gradually and finally concentrated in the area near the epicenter. In the period just before the earthquake (March and April, 2008), the major axis of geo-resistivity anomaly was almost perpendicular to the rupture direction of the Wenchuan M_S8.0 earthquake and the strike of Longmenshan fault or the major axis of the distribution of seismic intensity, but nearly parallel to the principal compressive stress axis from focal mechanism solution.