Abstract:

Xiangjiaba Reservoir is currently China's third largest reservoir and began impounding water at the end of 2012. After the impoundment, the water level rose to 71m, while seismic activity near the dam was not significantly increased. At the end of June 2013, the reservoir began impounding water again, the water level continued to rise and flooded the tail region of the reservoir. In the reservoir area and the Xiluodu reservoir area in the upstream, a reservoir seismic network including 35 seismic stations was set up which can roundly record earthquakes in this area. According to the records of the reservoir seismic network from September 2007 to June 2013, only 38 earthquakes with M_L≥1.0 occurred, 0.66 times a month on average, while in July-September 2013, 186 earthquakes with M_L≥1.0 occurred, with an average of 62 events a month, nearly 100 times of that in the past. So, most of the earthquakes are induced earthquakes. At the same time 553 earthquakes with M_L≤1.0 were also recorded in this area. A large number of small earthquakes occurring in the strong earthquake background area have caused a big stir. The source location of these earthquakes are rechecked based on 3D velocity model, 94% of the rechecked focal depth is less than 5km. Based on observations of the reservoir seismic network and vertical P- and S-wave's maximum amplitude ratio method, we inversed 9 focal mechanisms before the impoundment and 69 focal mechanisms after the impoundment in the tail region of the reservoir. Using these focal mechanisms, the stress field of the northern part and southern part of the study area is calculated in order to analyze the characteristic and cause of the induced earthquakes. The results indicate that most of the 69 focal mechanisms are strike-slip type, there is more transitional type, and less normal type and thrust type. The focal mechanisms spatial orientation is complex, fracture types are diverse, which may indicate that the stress state is uneven and the control of regional stress field to small earthquakes is weak. The stress field in the south and north is quite different and not consistent with regional stress field. The north shows compressive stress state while the south shows a state of weak extension. The Yaziba Fault, which passes through the tail region of reservoir, is an active fault, but does not control the induced seismicity, which may indicate that the reservoir storage inhibits the reverse fault activity. Carbonate rocks, limestone and karst cave are developed in the tail region. Analysis believes that reservoir water flows into the caves, penetrates into cracks and joints, leading to increased pore pressure, reducing the frictional strength and fracture strength and increasing reservoir water load which cause elastic deformation, so, it is believed that the combined action of all the above factors is the cause for the induced earthquakes.

Key words: Jinshajiang-Xiangjiaba reservoir, induced earthquake, focal mechanism, stress field, genesis of earthquake

摘要：

金沙江向家坝水库是目前中国第三大水库, 2012年底水库开始蓄水, 水位升高71m; 2013年6月底再次蓄水, 升高的水位淹没到库尾段。该水库以及上游的溪洛渡水库布设了地震台网, 有35台仪器连续记录, 能够很好地控制库区及周边的地震。水库地震台网在2007年9月—2013年6月记录了库尾一带1级以上地震共计38次, 平均每月发生0.66次。而2013年7—9月则记录M_L≥1.0地震186次, 平均水平达到每月62次, 已经接近以往月均值的100倍, 可判别为诱发地震。同期还记录1级以下地震553次, 在强震背景区发生大量小地震已经引起巨大反响。震源位置采用3维速度模型进行重新修订, 发现这些地震有94%发生在0~5km深度范围。利用水库地震台网的观测资料, 取垂直向记录到直达P、S波的最大振幅, 得到库尾一带蓄水之前9个、蓄水后69个小地震的震源机制, 并进一步利用这些震源机制反演了南、北2个区的应力场, 以期通过分析震源机制和应力场变化, 探索水库诱发地震发生的特点及成因。结果显示, 在蓄水后69个震源机制中走向滑动类型占最多, 存在较多过渡类型, 倾向滑动的正断层、逆冲断层数目较少, 震源机制空间取向复杂、破裂类型多样, 显示区域应力场对小震的控制较弱。利用这些震源机制反演得到南、北2个区的应力场差别较大, 状态不均匀, 北区呈现挤压应力状态, 而南区呈现弱拉张状态, 均与区域应力场不一致。穿过库尾的猰子坝断裂是活动断裂, 却没有控制诱发地震, 有可能表明水库蓄水抑制了逆断层的活动。库尾区分布碳酸盐岩、灰岩, 存在溶洞。分析认为库水涌入溶洞, 渗透到裂隙、节理, 导致孔隙压力增加, 摩擦强度、岩石破裂强度降低以及库水载荷加大造成弹性变形等共同作用是诱发地震的成因。

关键词: 金沙江向家坝水库, 诱发地震, 震源机制, 应力场, 地震成因