向家坝水库及邻区地震活动分析

Analysis of Seismicity in the Xiangjiaba Reservoir and Adjacent Areas

摘要:

向家坝水电站是金沙江梯级开发规划中最下游的一级巨型电站,目前水库蓄水年限已达12年。观测资料显示,库区多次发生3级以上地震,小微地震密集。地震精定位与地震活动分析表明,库尾段蓄水初期,地震频次显著升高,地震大多分布在库区河道附近,具有条带状展布特征; 随着蓄水时间增长,河道附近地震活动频次逐渐降低,玛瑙断裂中段附近的地震活动有所增强; 库尾段高角度断层发育,河道附近地震活动的震源深度浅于玛瑙断裂附近。b值计算结果显示,库尾段b值随深度增加而减小,其数值在4～6km深度范围内变化显著; b值空间分布不均匀,玛瑙断裂中段附近b值分布为0.7～0.8,玛瑙断裂北段、猰子坝断裂以及库区河道附近b值为0.9～1.2,这与不同地区应力状态差异及断层构造作用有关。

Abstract:

The Xiangjiaba Hydroelectric Power Station,the most downstream project in the Jinsha River cascade development plan,has maintained reservoir impoundment for 12 years. During this period,multiple earthquakes of magnitude 3.0 and above have occurred in the reservoir area,accompanied by a high density of small and micro-earthquakes. Seismic relocation and seismicity analyses reveal a marked increase in earthquake frequency at the reservoir's tail section following initial impoundment,with most events distributed along the river channel in a linear pattern. Over time,the frequency of earthquakes near the river channel declines,while seismic activity intensifies near the central segment of the Manao fault. This region is characterized by the development of high-angle faults,and the focal depths of earthquakes near the river channel are generally shallower than those associated with the Manao Fault. Analysis of the b-value indicates a depth-dependent decrease in the reservoir's tail section,with notable variations occurring between 4km and 6km. Spatially,b-values are heterogeneously distributed,ranging from 0.7 to 0.8 near the central Manao fault,and from 0.9 to 1.2 near the northern Manao Fault,the Qieziba fault,and the river channel. These variations reflect differences in stress states and the tectonic influence of fault structures.