锦屏一级水电站库区中小地震震源机制解与局部应力场特征

利用四川及邻区数字地震台网2013—2018年的波形资料, 基于CAP和HASH方法反演得到锦屏一级水电站(以下简称锦屏水库)库区及周边M_L≥2.5地震的震源机制解, 根据DRSSI方法计算得到库区局部应力场信息, 结合该区域的地质资料, 从震源机制参数和局部应力场变化等方面讨论了水库加卸载对该区域地震活动的影响.取得的主要认识如下: (1)库区地震活动类型以走滑型为主, 不同加卸载阶段震源机制类型存在一定差异, 库区蓄水前期震源机制类型呈现多样性, 蓄水后期与区域背景场趋于一致; (2)库区应力场最大主应力方向以NNW-SSE向为主, 分时段结果显示, 蓄水后期局部应力场最大主应力方向由NNW向逐渐发生偏转至NW向; (3)库区周边的地质构造和岩性条件有利于库水的渗透, 蓄水后重力载荷和流体渗透的共同作用导致岩体断层面库仑应力的变化, 从而呈现出库区蓄水前后中小地震活跃程度的显著差异特征.

Characteristics of focal mechanism solution and local stress field of small to moderate earthquakes in Jinping hydropower station reservoir area

In this paper, CAP waveform inversion method and HASH method are used to calculate the new focal mechanism of the ML≥2.5 earthquake at Jinping Hydropower Station and its surrounding areas based on broadband waveforms from the Sichuan Regional Seismic Network between 2013 to 2018, and DRSSI method is applied to inversion of the stress field in the study area. Combined with the region's geological structure and hydrogeological environment background, the focal mechanism and the various characteristics of the stress field in the study area are analyzed, and the influence of reservoir loading and unloading on the seismic activity in the region is further discussed. The results are as follows: (1) The majority of the focal mechanisms represent strike-slip mechanisms, but there are some differences in focal mechanism solutions at different loading and unloading stages. The focal mechanism solutions are diversified at the early stage of reservoir impoundment and tend to be consistent with the region background at the later stage. (2) The direction of the maximum principal stress of the tectonic stress field in the study area is mainly the NNW-SSE direction, which is different from the NW-SW direction of the regional stress field. The variation results of the local stress field at different periods show that the direction of the maximum principal stress of the local stress field gradually shifts from the NNW direction to the NW direction at the later stage of impoundments. (3) Jinping hydropower station reservoir distribution around the marble, metamorphic sand plate has created favorable conditions for reservoir water infiltration, in the process of rapid water, fluid seepage, and reservoir water stress on the surface of the joint action of gravity load causing the fault, the fault enhanced dielectric strength weak, small earthquakes in this area was significantly increased at the same time, it also played a triggering role in moderate earthquakes. It is worth noting that there are certain differences in the factors affecting seismic activity in different stages.