Abstract:The earth resistivity observation station in Tianshui,China,is located in the northern region of the Qinling latitudinal tectonic belt,which is the western settlement zone of the Neocathaysian structural system and includes the southward-insertion portion of Qilianshan-Luliangshan-Helanshan front arc structural system and the southern edge of the Weihe fault.This area exhibits a clear diamond structure with an N-E,N-W,and E-W configuration,in which seismic activity is frequent and epicenter location varies.The station has an altitude of 1 150 m,average humidity of approximately 63％,and annual average temperature of 18 ℃.The polar region of this area includes Ⅰ-and Ⅱ-grade terraces of the Yongchuan River.During the past five years,two larger earthquakes have occurred in this region including the Lushan M7.0 earthquake on April 20,2013,and the Minxian-Zhangxian M6.6 earthquake on July 22,2013.The epicenters of these two earthquakes were 550 km and 200 km,respectively,from Tianshui station.Prior to their occurrences,the earth resistivity observation data of Tianshui station showed obvious short-term anomalies that differed in morphology and amplitude.Eleven days prior to the Lushan earthquake,the N-S,E-W,and N45°W channels of the deep well observing system showed clear synchronous abnormal changes,which remained for 11 days following the earthquake.Forty days prior to the Minxian-Zhangxin earthquake,the 7.05±0.01 normal value of the E-W channel showed intermittent fluctuation anomalies of approximately ± ±0.02.Although both earthquakes occurred in the S-N seismic belt,the Lushan earthquake occurred in the Longmenshan fault zone,and the Minxian-Zhangxian earthquake occurred in the Lintan-Tanchang fault zone.
