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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ML≥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。
Xiangjiaba Reservoir in Jinsha River is currently the third largest reservoir in China. By the end of 2012, the reservoir began to store water and the water level increased by 71m. In the end of June 2013, the reservoir was re-impounded and the rising water level was submerged to the end of the reservoir. The reservoir and the upstream Xiluodu reservoir seismic network set up, 35 consecutive records of equipment, can well control the reservoir area and the surrounding earthquakes. In September 2007-June 2013, the reservoir seismic network recorded a total of 38 earthquakes of magnitude 1 or above in the vicinity of Kuwei, taking an average of 0.66 times per month. In July-September 2013, ML ≥1.0 earthquakes were recorded 186 times with an average of 62 times per month, which is close to 100 times of the previous monthly average value and can be judged as induced earthquake. In the same period, 553 earthquakes below level 1 were also recorded. A large number of small earthquakes in the background area of strong earthquakes have caused great repercussions. The source location was revised with 3-D velocity model and found that 94% of these earthquakes occurred in the depth of 0-5 km. Using the observation data of the reservoir seismograph network, the maximum amplitudes of the direct P and S waves are recorded perpendicularly, and the focal mechanisms of nine small earthquakes after 9 impoundments and the water impounding are obtained. Further, by using these focal mechanisms In this paper, the stress fields in the two areas of South and North are inversed to explore the characteristics and causes of reservoir-induced earthquakes by analyzing focal mechanism and stress field changes. The results show that in the 69 focal mechanisms after impoundment, there are a large number of strike-slip types, there are more transitional types, normal-slip faults, fewer thrust faults, complex spatial orientation of focal mechanisms and diverse types of ruptures, indicating regional stress Field control of small earthquakes is weak. Using these focal mechanisms, the stress fields in the two northern and southern regions are greatly different and inhomogeneous. Compressive stress state appears in the north and weak tension in the south, which are inconsistent with the regional stress field. The Ruzi dam fault that passes through the tail of the reservoir is an active fault but does not control the induced earthquake. It is possible that the reservoir water suppresses the activity of the thrust fault. Karst area distribution of carbonate rocks, limestone, there are caves. It is considered that the combined action of reservoir water influx into caves, infiltration into fissures and joints, resulting in the increase of pore pressure, friction strength, rock burst strength and elastic deformation caused by the increase of reservoir water load are the causes of earthquake induced.