Characteristics of precursor anomalies of earth resistivity at Yushu and Siping seismic stations of Jilin Province

This paper studies the anomalous changes of earth resistivity before 1999 Wangqing M_S7.0 earthquake and 2006 Qian’an-Qianguo M_S5.0 earthquake observed at Yushu and Siping stations of Jilin Province. The results show that, at the Yushu and Siping stations a long-trend decreasing or short-impending anomaly of earth resistivity appeared before the two earthquakes. Monthly mean curves of earth resistivity displayed two-year decrease anomalies at Yushu and Siping stations before the Wangqing earthquake. Before Wangqing and Qian’an-Qianguo earthquakes, normalized variation rates of earth resistivity are larger than anomaly index at Yushu and Siping stations. Before Qian’an-Qianguo earthquake, earth resistivity anisotropy degree showed obviously low value anomaly at Siping station, and at Yushu station it broke its annual variation value. Possible reasons for the anomalous changes maybe are related to the maximum principal compressive stress orientation’s (or close to the range) compressing NWwards in the source region and its vicinity resulting from NWwards extrusion of Pacific Plate, which makes the conductive fluid in the media quickly enter or redistribute, so that the earth resistivity shows a long-trend decrease and anisotropic variation.