Abstract:Since April 2013,the water radon concentration of Huaniu Spring has shown abnormal changes that are very similar to the precursory anomaly of the Wenchuan MS8.0 earthquake.From the beginning,it was tracked as important anomaly.Although this change began three months prior to the Minxian-Zhangxian MS6.6 earthquake,we could not conclude that it was a precursor to this earthquake it.Further research is needed to determine a reasonable relationship between the anomaly of water radon concentration and the Minxian-Zhangxian MS6.6 earthquake.In the present study,we analyze the tendency of annual changes in radon concentration of Huaniu Spring,Tianshui,China,which began in April 2013 and terminated after the Minxian-Zhangxian MS6.6 earthquake.The results show a decline tendency with a fixed rate.The annual change is high in summer and low in winter,and the radon concentration is higher than that recorded during the corresponding period of the previous year.In addition,we analyze the main factors that influence the radon concentration,and we determine that the annual change is caused by atmospheric temperature.Abnormal changes in radon concentration occur when the rainfall reaches a particular value; however,the duration is short.Analysis of the reliability of the radon concentration anomaly indicates that although the result is shown to be dependable,it is not consistent with the influencing factors.Moreover,we analyze the relationship of the radon concentration in Huaniu Spring and the earth resistivity and water temperature in Tianshui.The results show that their anomalies began at the same time; however,their change directions differed.The water temperature and earth resistivity showed high anomalies,and the radon concentration anomaly was low.We also analyze the reproducibility of the radon concentration anomaly.The anomaly characteristics of the radon concentration beginning in April 2013 were very similar to the precursory anomaly of the Wenchuan MS8.0 earthquake.Finally,we examine the temporal and spatial relationship between the radon concentration anomaly and the Minxian-Zhangxian earthquake.We conclude that the radon concentration anomaly of Huaniu Spring beginning in April 2013 is a credible and reliable precursory anomaly of the Minxian-Zhangxian MS6.6 earthquake.