Abstract:The MS6.6 earthquake occurred at the junction of Minxian County and Zhangxian County.The earthquake focal depth was 20 km.It was the largest event since the Shandan MS7.25 earthquake of 1954.The epicentral area was already known to be earthquake-prone,as there have been 25 earthquakes with magnitudes greater than five in recorded history within about 200 km of the epicenter.This earthquake occurred on the northeastern edge of the Qinghai-Tibet Plateau,which is formed by the subduction of the Indian plate in the north of China.Therefore,the earthquake was caused by the push from the Indian plate.The north-south seismic belt in which this earthquake happened has entered a new active period since May 12,2008,after the Wenchuan earthquake.The Wenchuan earthquake,the Yushu earthquake,and the Lushan earthquake show that its activity has not decreased.There are nine apparent resistivity stations within 400 km of from the epicenter.These are at Tongwei,Hanwang,Tianshui,Lanzhou,and Linxia in Gansu Province,Guyuan in the Ningxia Hui Autonomous Region,and Baoji,Zhouzhi,and Qianli in Shanxi Province.The Tianshui seismic station in Gansu uses the deep bore-hole technique,and the electrode depth is 100 m.This station is 156 km from the epicenter of the earthquake.This paper analyzed apparent resistivity observation data from these stations for 2002 to 2013 using the Fourier slide method and the normalized variation rate method.The Fourier slide method is used to exclude annual variation in geo-electrical resistivity observation data.The normalized variation rate method avoids the difficulties and arbitrariness of using human judgment to determine the abnormal changes in the base value.At the same time,this method retains the abnormal change patterns of the original curve.The results show,（1） for the geo-electrical resistivity anomaly phenomena before the earthquake,the relation between the abnormity and the earthquake is significant in time.The normalized variation rate method is better for the extraction of mid-term and short-term anomaly effects,and there were obvious abnormal changes.（2） The mediumterm anomalies mainly decreased,and the amplitude of the anomalies increased with distance from the epicenter.The Tongwei,Lanzhou,Tianshui,and Zhouzhi stations recorded sustained mediumterm anomalies for months or about a year.However,examination of the observation environment of the stations with abnormal changes showed that the recorded anomalies at some stations require further study.（3） Although the geo-electrical resistivity stations had environmental interference and other effects in the area surrounding the epicenter,abnormal changes in the geo-electrical resistivity were quite clear before the earthquake.There were large-amplitude seismic anomalies before the earthquake,as well as a decline in the main anomaly.Finally,we discussed the preliminary causes of the apparent resistivity anomalies.During the gestation of the earthquake,geo-electrical resistivity anomaly changes in the source area were associated with the accumulation of stress and ultimately result in progressive failure of rock.When the stress had accumulated up to the critical strength of the medium,micro-fractures started to form,this time because of the stress load in the later part of the preparation period.The accelerated change in this medium produced prominent geo-electrical resistivity anomalies.Decreasing anomalies are credible precursors in earthquake preparation processes.
