三峡水库蓄水后小微震事件的震源机制及成因分析

利用三峡工程地震监测台网记录的2003~2014年地震振幅比和P波初动数据，对三峡水库重点监视区蓄水后地震的震源机制进行解析。结果表明，这些地震震源的基本特征是大仰角的主压应力轴和近于正断的震源错动。在库水的循环加卸载及其水岩的理化作用下，浅表岩层软弱结构面（体）失稳，是蓄水后小微地震活动的主要诱因。 值得注意的是，2010年以后，逆冲走滑类型地震的比例显著增多，显示了区域构造应力场的控制作用。

Possible Mechanisms of the Earthquakes that Occurred in the Key Monitoring Area of the Three Gorges Reservoir after the First Impoundment

Using P/S amplitude ratio and P-wave first motion data obtained by the digital telemetered seismic network of induced earthquake monitoring system in the Three Gorges area, we analyze the focal mechanisms of earthquakes occurring in the key monitoring area of the Three Gorges reservoir. The main features are near vertical P axes and pure normal faulting. Water loading and its fluctuation in the reservoir, together with the chemical effects of water, might cause the unstable planes to lose balance, resulting in small earthquakes. This is supported by the focal mechanisms of the induced earthquakes that show oblique P axes, which are different from the regional stress fields and focal mechanisms of the earthquakes that occurred before the first impoundment.