利用国家测震台网数据备份中心提供的近震数字波形记录,采用基于P波初动和S/P地动位移振幅比的HASH方法,计算了溪洛渡水库近场区2009年1月—2018年12月ML≥2.0地震的震源机制解; 从中选取质量可靠的震源机制解,采用阻尼区域应力反演方法获得了该区的应力场图像; 基于震源机制解和应力场的时空分布,探讨了水库蓄水对当地应力环境的影响。结果 表明:蓄水前地震类型以过渡型为主,走滑和逆冲型地震次之,正断型最少; 蓄水后,过渡型地震明显减少,其它类型地震均不同程度增多,逆冲型地震数量与蓄水前相比增加近一倍。蓄水前后区域应力场发生了一定变化,具体表现为最小压应力轴的仰角增大,变化非常显著。这可能表明,蓄水造成的应力加载与地壳中应力量级可以比拟,对当地的应力环境提供了重要约束。
The impoundment of water reservoirs may induce significant seismicity and increase regional seismic hazard,thus has attracted more attention in recent years. Here we focused on the Xiluodu Reservoir,one of the biggest hydropower stations worldwide. We collected digital waveform data during 01/2009 ～ 01/2018 recorded by regional seismic network from the Data Management Centre of China National Seismic Network at Institute of Geophysics,China Earthquake Administration. We first adopted the HASH method to determine the focal mechanism solutions of ML≥2.0 seismic events using their P wave first-motion polarities and S/P amplitude-ratios. Comparisons between different methods or agencies confirmed these solutions were well constrained. Then the source mechanism solutions with high quality were inverted to obtain the local stress field utilizing the spatial and temporal stress inversion(SATSI)method. The effect of reservoir impoundment on the changes in local stress was assessed by analyzing these focal mechanism solutions and the spatio-temporal distribution of the stress field. Most of earthquakes showed oblique focal mechanisms prior to the impoundment of reservoir while were dominated by the thrust or strike-slip focal mechanisms after that. In addition,the inversion of local stress field revealed relatively significant change in the plunge angle of the minimum stress axis after the impoundment. We suggest that the stress changes due to the impoundment of reservoir may have affected the stress state in the crust and therefore provide some constraints on the local stress field.
