Abstract:On July 22,2013,an MS6.6 earthquake occurred at a depth of 20 km in the border region of Minxian and Zhangxian （34.5°N,104.2°E）.The earthquake was on the Lintan-Tanchang dislocation of the north-south seismic belt.The north-south seismic belt became active after the 2008 Wenchuan MS8.0 earthquake in Sichuan Province.The 2010 Yushu MS7.1 earthquake in Qinghai Province and the 2013 Lushan MS7.0 earthquake in Sichuan Province show that activity has continued unabated.Pingliang Station （35.55°N,106.68°E） was built in December 2007,and is located in the eastern part of the north Qilianshan fold belt and the southern part of the Helan-Liupan Mountain inland organic belt.The system for observing the geo-electric field runs well,the electromagnetic environment is good,and there is no remarkable interference around the station.The observation device makes observations in three directions and has a total of six channels,with a sampling interval of one minute.This paper studied changes in the geo-electric field for the Minxian-Zhangxian MS6.6 earthquake in Gansu Province in 2013 using data from Pingliang Station.The data used were from November 1,2012 to July 31,2013,which covers a period of 163 days before and nine days after the Minxian-Zhangxian earthquake.The results showed：（1） significant self-potential changes were recorded at Pingliang Station,situated on the northeastern edge of the Qingzang-Tibet Plateau,before the Minxian-Zhangxian MS6.6 earthquake of 2013 in Gansu Province and the Lushan MS7.0 earthquake of 2013 in Sichuan Province.These changes were more prominent and lasted longer before the Minxian-Zhangxian MS6.6 earthquake than before the Lushan MS7.0 earthquake.The anomalies began on April 7,2013,about 100 days before the Minxian-Zhangxian MS6.6 earthquake.（2） An increased power spectrum density （PSD） of the geo-electric field （in particular,PSD anomalies in the low-frequency components,under 5× 10-3 Hz） was recorded at Pingliang Station before the Lushan MS7.0 earthquake,and lasted until the end of the Minxian-Zhangxian MS6.6 earthquake.Finally,the mechanism of the extremely low frequency phenomenon was discussed.The observed electric field at Pingliang station is in the southern part of the Haiyuan-Liupan Mountain dislocation,and so the medium under the station is weak.The station is close to the epicenter of the Minxian-Zhangxian earthquake,and it is easy to change the physical environment of the underground medium and thus cause this significant electromagnetic phenomenon.From our preliminary analysis,we consider that tectonic movement associated with the source region caused changes in the physical conditions of the medium under the station because of the unusual tectonic setting.During non-linear development of numerous micro-cracks and their directional arrangement in the media of the focal region before the earthquake,influencing factors such as underground water migration and electro-mechanical transformation function induced geo-electromagnetic anomalies.The focal process of the earthquake and the local media action under these geo-electric stations led to the spatially and temporally complicated evolution of geo-electric anomalies.
