Abstract: The paper selected the observation data of Radon and Water quality ion in Dapingzi spring, which located near the epicenter of Ludian M_S6.5 earthquake, and compared the anomalous characteristic before and after the earthquake. The results show that moderate-strong seismic activity is clustered before the earthquake. Radon and water quality anomaly lasted for a long time and had a large amplitude. It presents a phenomenon of breaking annual variation. While after the earthquake, weak activity of small earthquake swarmed, the abnormal amplitude is relatively small, and the duration is shorter, and there are many simultaneous twists and turns, which is the precursor anomaly of the medium weakening region. Based on the results of water sample test, GNSS displacement observation data, GPS time series curve and earthquake activity, this paper analyzed the precursor mechanism of the Radon and Water quality ion. We think that the group anomalies are related to the regional deformation activities. Local stress is continuously accumulated in Northeastern Yunnan, so that deep underground material activities and tectonic activities continue to be intensified. The recent synchronic transition phenomenon belongs to the precursor anomaly in the stress adjustment of the medium weakening area.

Key words: Ludian M_S6.5 earthquake, Precursory anomaly, Seismic weak activity, Medium weakening area, Northeastern Yunnan