Abstract: The Longtan reservoir is located in Tian'e County, Guangxi Zhuang Autonomous Region, southwestern China on the upper reaches of Hongshui River, the main stream of the Pearl River. The dam of the reservoir is 200m high, and the maximum water depth can be up to 194m as the water level reaches 400m. The reservoir storage capacity is 27.3 billion cubic meters, so it is a typical high-dam reservoir with large storage capacity. Terrain of the reservoir is high in the west and low in the east. The reservoir is located at the confluence of the Hongshui River, Buliu River, Nanpan River, Beipan River, Mengjiang River and Caodu River. The construction of Longtan hydropower station officially started in July 2001, and the reservoir impoundment was on September 30, 2006. The power station is equipped with 9 sets of 700 000kW water turbine generator units, with a total installed capacity of 6.3 million kW and an average annual generating capacity of 18.7 billion kW·h. So its storage and hydropower capacity rank third only to the world-famous Three Gorges hydropower project and the ultra-large hydropower project in Xiluodu of Jinsha River in China. Seismicity enhanced rapidly in the reservoir area after the impoundment. More the 5 000 earthquakes have been recorded so far, with the maximum magnitude of M_L4.8, which occurred on September 18, 2010. The earthquakes are mainly concentrated in the deep water area where fault zones run through. Assuming the seismogenic fault can be simulated by a plane and most small earthquakes occur nearby the fault plane, the information of seismogenic fault can be obtained by the hypocenter location parameters of small earthquakes.

Key words: fault plane solution, small earthquake clustering, focal mechanism, stress field, Longtan reservoir

摘要： 文中基于2006-2016年龙滩水库数字地震台网记录的地震波形和震相资料, 采用结合波形互相关技术的双差定位方法对4 017次地震进行重新定位, 求解得到库区126个中小地震的震源机制解, 并反演各地震丛所在区域的应力场。 将库区地震划分为11个地震密集带, 依据成丛小地震发生在活动断裂面及其附近的原则, 结合模拟退火算法和高斯-牛顿算法反演了11个地震密集带相应的断层及断层面参数, 包括断层面的走向、 倾向、 倾角、 长度、 深度和地理位置, 并依据局部区域应力场参数计算了各断层面的滑动角。 结果显示: 11条断层的走向、 倾向、 倾角及运动性质与地质调查给出的穿过各地震丛所在区域的主要断裂信息基本一致, 研究成果可作为龙滩库区主要断裂带资料的重要补充。 龙滩库区最大主应力优势方向为NW-SE或NWW-SEE, 最大和最小主应力轴倾角近水平, 中间主应力轴倾角很陡, 蓄水前、 后区域应力场主压应力方向未发生明显变化, 与华南块体的构造应力场方向基本一致。 蓄水后库区的R值逐渐增大, 且不同地震丛所在区域的R值变化存在差异。 八茂、 拉浪和布柳河地震丛所在区域R值增幅显著, 表明中间应力轴压应力状态减弱并逐渐转为张应力状态。 精定位和断层面参数计算结果显示, 地震丛集活动与穿过地震丛的主要断裂带密切相关。 这些断裂的倾角普遍很陡, 均与库区主要河流交会, 利于流体向断层深部渗透。 推测库区蓄水主要影响库区应力场主应力的相对大小, 对主应力方向的影响有限。 水对库区介质的润滑作用及附加应力场与原地应力的叠加可能是导致库区主应力相对大小发生变化的主要因素。 龙滩库区蓄水后的张剪性应力环境有利于库区断层失稳滑动, 可较好地解释蓄水后库区地震密集活跃现象。 量化的断裂信息可为龙滩库区地震危险性分析、 地震成因的数值模拟、 地震预测等方面的深入研究提供支持。

关键词: 断层面解, 小震丛集, 震源机制解, 应力场, 龙滩水库