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 M-L >= 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.