Using Ambient Noise To Study The Seismic Velocity Changes Of The Crustal Media Before And After Impoundment In The Zipingpu Reservoir Region

China has the most reservoirs in the world. In spite of playing an important role, reservoirs can lead to a series of geological disasters. China is also one of the reservoir-induced earthquake-prone countries in the world. Reservoir-induced earthquakes may not only damage the reservoir dams directly, but also cause serious secondary disasters. Especially in recent years, our country has constructed many cascade hydropower stations with high dams and large capacities in the Sichuan-Yunnan region where earthquakes often happen. As the reservoir-induced earthquakes occurred, seismologists pay great attentions to the large reservoirs. After the Wenchuan earthquake, Zipingpu reservoir attracted widespread attention because of the special space-time relationship.We use the continuous waveform data of the Zipingpu seismic network and the YZP station from January 1, 2005 to January 1, 2008 to study the underground medium in Zipingpu reservoir area. The study period includes the process of three main impoundment and two large scale of disemboguement. The detailed research method is: using the cross-correlation of seismic ambient noise to extract the empirical green functions(CCFs) between station-pairs; To obtain the reference green functions(REF) by stack CCFs for a long time; To measure the relative velocity change of medium by comparing the empirical green functions(CCFs) and the reference green functions(REF). According to the results, we analyze the process of three main impoundment and two large scale of disemboguement respectively, and discuss the main factors influencing the relative velocity change in the different stages, and also track the infiltration process of water in medium of different depth. At the same time, we analyze the relationship between the relative velocity changes of the underground medium of the reservoir area and the earthquake frequency and depth. The conclusions of this research are as follows:1、Before the first time of main impoundment, there was no obvious correlation between the relative velocity change and the water level change. After that, however, a significant negative correlation was shown during the loading/unloading process, ie, the relative velocity reduced while the water level rose, and vice versa. Through the analysis, we think the pressure was the main factor leading to the relative velocity changes during the first impoundment and disemboguement, and osmosis only affected the shallow medium within 2 km. During the last two impoundment and disemboguement, the relative velocity change in the medium is attributed to both the pressure and osmosis. When the water level rose to the maximum value during the second impoundment, osmosis has affected the fault of about 4 km. And when the water level fell to the minimum value during the second disemboguement, osmosis has already affected the fault of about 8 km. During the last time of impoundment, the relative velocity change of three period were very consistent with the water level change, showing high cross correlation coefficients. By this time, the water has infiltrated the fault deeper than 8 km.2、Analyzing the relationship between the relative velocity changes and the earthquake frequency, we think that during the three months of the rapid rising of the water level, the earthquake frequency increased correspondingly. At the same time, the relative velocity of the medium fell rapidly from nearly zero.3、According to the analysis of the relationship between the relative velocity changes and the earthquake depth, we think that most of the small earthquakes’ focal depth obviously concentrated in about 5 km after the impoundment, which is consistent with the analysis of the osmosis process.The research conclusions of this paper are helpful for deeply understanding the underground structure and the process of state and dynamics. At the same time, through the analysis with earthquake frequency and depth, we find that the study of relative velocity changes may be useful for the judging and predicting of reservoir induced earthquakes, which is of great significance.