Abstract: Using the seismic waveform data of Xiaowan seismic network and Yunnan seismic network, we determined the focal mechanisms of 36 earthquakes(M_L ≥ 3.0)from Jun. 2005 to Dec. 2008 and 51 earthquakes(M_L ≥ 2.5)from Jan. 2009 to Dec. 2015 by generalized polarity and amplitude technique.
We inverted tectonic stress field of the Xiaowan reservoir before impounding, using the focal mechanisms of 36 earthquakes(M_L ≥ 3.0)from Jun. 2005 to Dec. 2008 and CAP solutions of 58 earthquakes(M_L ≥ 4.0)collected and the solutions in the Global Centroid Moment Tensor(GCMT)catalog; We inverted local stress field of the reservoir-triggered earthquake clustering area, using 51 earthquakes(M_L ≥ 2.5)from Jan. 2009 to Dec. 2015.
Focal mechanisms statistics show that, the Weixi-Qiaohou Fault is the seismic fault. Focal mechanisms were strike-slip type in initial stage, but normal fault type in later stage. Focal depths statistics of 51 earthquakes(M_L ≥ 2.5)show that, the average value of focal depths in period Ⅰ, period Ⅱ and period Ⅲ are 8.2km, 7.3km and 7.8km respectively and the standard deviations are 4.3km, 3.5km and 6.0km respectively. The average value of focal depths is basically stable in different period, only the standard deviation is slightly different. Therefore, there is not positive connection between focal depth and deviation of focal mechanisms. What's more, there are 2 earthquakes(number 46 and number 47 in Fig.5 and Table 3)with almost the same magnitude, epicenter and focal depth, but they have different faulting types as normal and strike-slip. The focal mechanism of event No.46 is strike:302°, dip:40° and rake:-97° for plane Ⅰ, however, the focal mechanism of event No.47 is strike:292°, dip:82° and rake:140° for plane Ⅰ. Likewise, earthquake of number 3 and number 18 have similar characteristic. Therefore, the obvious focal mechanism difference of similar earthquake pair indicates the complexity of Weixi-Qiaohou Fault.
Considering the quiet-active character of reservoir-triggered earthquakes, we discussed the change of local stress field in different period. The σ₁ of tectonic stress field was in the near-south direction, with a dip angle of 14° before the impoundment, however, the direction of σ₁ of local stress field changed continuously, with the dip angle getting larger after the impoundment. The direction of σ₁ of local stress field of reservoir-triggered earthquake clustering area is close to the strike of Weixi-Qiaohou Fault, and reservoir impoundment increased the shear stress in the fault, so the weakening of fault was beneficial to trigger earthquakes. Comprehensive analysis suggests that fluid permeation and pore pressure diffusion caused by the water impounding, and the weakening of fault caused by local stress field are the key factors to trigger earthquake in the Xiaowan reservoir.

Key words: Xiaowan reservoir, triggered earthquake, generalized polarity and amplitude technique, stress field

摘要： 选取小湾水库地震台网和云南地震台网的波形资料，采用广义极性振幅技术（GPAT）反演了小湾库区及附近2005年6月—2008年12月的36次M_L≥3.0地震和2009—2015年库区的51次M_L≥2.5地震的震源机制解。利用2008年12月前的36次M_L≥3.0地震，联合搜集到的58次M_L≥4.0地震的CAP计算结果和哈佛大学历史地震震源机制解，反演了小湾水库库区蓄水前的构造应力场；利用2009—2015年触发地震丛集区的51次M_L≥2.5地震的震源机制解获得了该区的局部应力场。库区蓄水后，触发地震丛集区的小震震源机制节面统计显示，发震断层的特征与维西-乔后南支断裂一致，确认其为发震断层。小震震源机制解初期以走滑为主，后期以正断为主，结合触发地震丛集区小震密集—平静的活跃特征，分时段讨论了局部应力场的变化。小湾水库库区蓄水前构造应力场的σ₁近S向、倾角较低（14°），蓄水后触发地震丛集区局部应力场的σ₁在时段Ⅱ和时段Ⅲ的方位角连续偏转，倾角逐渐加大。局部应力场的σ₁与断裂性质相近，表明水库蓄水载荷导致断层面上剪应力增大、断层介质强度变弱，有利于触发地震。分析认为，库水渗透作用及孔隙压扩散、局部应力场导致的断层变弱为触发地震丛集区发生地震的主要控制因素。

关键词: 小湾水库, 触发地震, 广义极性振幅技术, 应力场