Abstract:Six major earthquakes have occurred in the Tibetan Plateau during the past few decades：the 1997 Mani earthquake of magnitude 7.5 in Tibet,the 2001 Kunlun Mountain Pass earthquake of magnitude 8.1 in Qinghai,the 2008 Yutian earthquake of magnitude 7.3 in Xinjiang,the 2008 Wenchuan earthquake of magnitude 8.0 in Sichuan,the 2010 Yushu earthquake of magnitude 7.1 in Qinghai,and the 2013 Lushan earthquake of magnitude 7.0 in Sichuan.All of these occurred around the boundaries of the Bayan Har block of the Tibetan block.A series of strong earthquakes have occurred at the southwestern,northwestern,western,eastern,and southeastern boundaries of the Bayan Har block,which have attracted a great deal of attention from geologists.Some research has indicated that the adjacent zone and the boundary of the Bayan Har block are earthquake-prone areas,where strong earthquakes might occur in future.Earthquakes frequently occur in the north-south seismic belt,which has also been the focus of research in recent years.The southeastern area of Gansu is located in the northern section of the north-south seismic belt,and adjacent to the northeastern boundary of the Bayan Har block.We therefore believe that the southeastern part of Gansu Province is an area at risk of disastrous earthquakes in future.The Minxian earthquake of magnitude Ms 6.6 occurred on July 22,2013 in this area.In this study,we discussed the effect of strong earthquakes on the seismic situation and studied data on underground fluids in southeast Gansu Province.From analysis of underground fluid data from the southeastern part of Gansu Province,we concluded that underground fluid data from 300 km from the epicenter showed obvious mid-term anomalies,shortterm anomalies,impending earthquake anomalies,and co-seismic response.We analyzed and discussed data on the water radon level,water temperature,water level,and outflow data to elucidate the medium-term anomalies,short-term anomalies,impending earthquake anomalies,and co-seismic responses.The medium-term anomalies before the Minxian earthquake were extracted by combining qualitative and quantitative methods.The data on water radon and water level changes in the southeast of Gansu provinces were analyzed according to the idea of ＂identity＂ and the method of subordinate function method.The preliminary conclusion is that it is obvious that the anomaly trend in the underground fluid appeared before the Minxian earthquake,and the subordinate function method can be successfully used to extract the medium-term anomaly of water radon.Based on the data for water radon （from Fujianchang spring,No.1 spring,and No.2 spring at Pingliang station,Huaniu spring at Tianshui station,No.1 spring at Wushan station,Diangou spring at Longnan station）,water level （from Wenquan Well at Qingshui station）,water temperature （from Wenquan Well and Ligou Well at Qingshui station）,flow data （from Ligou Well at Qingshui station） for the Minxian earthquake,there were marked short-term precursor anomalies in southeastern Gansu.From one record of water radon,four records of water temperature （Liuhu Well at Pingliang station,Liangshui Well at Longnan station,No.1 spring at Wushan station,and Diangou Well and Chengxian Well at Longnan station）,and two records of water levels （Huating Well and Liuhu Well at Pingliang station）,we detected obvious coseismic effects caused by the earthquake.From the statistical analysis,water temperature and water level data sampled at a high rate showed more coseismic response,and flow data and water radon data sampled at a low rate did not show coseismic response.
