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2. SEISMIC ACTIVITY There are total of 8 seismic stations within a 30km radius of Shanxi reservoir. And this earthquake observation network is putting into operation since 2010[2]. In this study, a seismic catalogue contains 5908 records are used to study the seismic activity of Shanxi reservoir region (27°60’～27°80’N, 119°80’～120°10 ’E), which is recorded from July 2002 to July 2016. 2.1. SEISMIC FREQUENCY Shanxi reservoir is located at weak seismic region where there is no earthquake recorded before the construction of reservoir. As showed in Table.1 and Fig.1, the frequency of seismic activities in Shanxi reservoir region increases significantly after the impoundment. The seismic activities in this area could be recognized as 4 clusters: ① from July to September 2002, the maximum earthquake is ML3.9; ② from February to October 2006, the maximum earthquake is ML4.6; ③ from March to November 2010, the maximum earthquake is ML2.1; ④ from August 2014 to May 2015, the maximum earthquake is ML4.4. Therefore, we could get a conclusion that the frequency of seismic activity in Shanxi reservoir will increase prominently in every 4 years. Furthermore, we also found that the seismic events are always accompany with water level decrease, and the latency is relatively obvious. This phenomenon could be observed easily in Fig.2. We suppose the possible mechanism of the seismic activity is that the infiltration of water into the causative fault will increase accompany with water level rising. Thereupon, the original balance of the fault plane was broken under the effect of water erode, and the fault plane will be soften and sliming. In case the water level decreases dramatically, the accumulated energy will release immediately and then cause earthquake. Fig. 1 Monthly variations of seismic frequency (bottom bar) Blue curve: water level 457