Study On Characteristics Of Focal Mechanisms And Stress Field In The Downstream Reservoir Area Of Jinsha River

The downstream reservoir area of the Jinsha River is located at the eastern edge of the Sichuan-Yunnan block.Due to the eastward extrusion of the Tibetan Plateau and the resistance of the South China block,the crustal deformation and tectonic movement are very strong.The four cascade hydropower stations in the region are huge in scale,and seismicity in the Xiangjiaba and Xiluodu dams has increased significantly after impoundment.Under the strong tectonic setting and effect of impoundment,we need to explore dynamic change characteristics in reservoir areas by long-term monitoring in reservoirs and adjacent areas before and after impoundment,especially the monitoring and research of micro-seismicity,so that lay a foundation for understanding the mechanism and risk of reservoir-induced earthquakes.We select Xiluodu-Wudongde section in the downstream reservoir area of Jinsha River(26~29?N 102~104.5?E)as the study area.Firstly,we use CAP and GPAT method to invert focal mechanisms for earthquakes with magnitude of M_L?2.0 from2016 to July 2018 recorded by the reservoir dense network to obtain the spatial distribution image of focal mechanism solutions in each reservoir and adjacent areas.Secondly,based on 542 focal mechanism solutions,we use iterative joint inversion method(IJIM)to invert stress field,and then compare the stress field in each zone,and thus reveal the current stress environment and the effect of impoundment.Finally,we calculate the focal mechanism consistency parameters of the typical earthquake sequence-Ludian earthquake's aftershocks area,to analyze variation law of stress concentration strength and provide a basis for the assessment of medium and long-term seismic hazard.Based on the above research work,we have got the following cognitions:(1)The focal mechanism solutions in Xiluodu dam and adjacent areas are dominated by thrust and strike-slip faultings which are consistent with those before impoundment.Since impoundment in 2013,focal mechanism solutions for earthquakes with magnitude of M_L>4.0 are dominated by strike-slip faultings,which has no obvious relationship with the change of water level.The strike of one node plane is consistent with the distribution characteristics of the EBian-JinYang fault zone,and some M_L>4.0 earthquakes near it may be controlled by regional structures.Small earthquakes are continuously active in the region,and the nodal planes are,which are inconsistent with local geological structure characteristics near NS distribution,indicating that the regional stress field is relatively complex.This may be related to the increase of pore pressure and interaction between the planes of small faults due to the infiltration and diffusion of reservoir water.(2)Focal mechanism solutions in unimpounded Baihetan and Wudongde dams and adjacent areas are mainly strike-slip faultings with few thrust and normal faultings.Left-lateral strike-slip faultings are consistent with the nature of Xiaojiang fault zone and Puduhe fault zone,the strike angles of which are both near NS direction.The focal mechanism solutions and nodal planes in these two regions distribute discretely,reflecting complex structural features in the region.(3)Until July 2018,focal mechanism solutions in the Ludian earthquake aftershock area are mainly thrust and strike-slip types,and the strikes of nodal planes are distributed in two dominant directions:near EW and NE direction but the NW direction are relatively less.There may be a buried structure in the EW direction,and different kinds of faults controlled the seismicity.(4)The centroid depths in each region are concentrated in the range of 5-15km,indicating that the seismogenic layer in the study area is within the middle and upper crust of 5-15km deep.(5)The maximum principal stress axes in the study area are roughly in NW-SE direction,and the minimum principal stress axes are roughly in NE-SW direction.They rotate clockwise from north to south.The azimuth,plunge and R value in each area are different from one another.The stress environment in Xiluodu dam and its adjacent area has gradually recovered to unimpoundment,and the azimuth of principal stress axes deflected,and the inclination of the minimum principal stress axis is more vertical,which is significantly different from the spatial distribution of the near-horizontal stress at the initial stage of impoundment.The R value is much higher than that before impoundment,indicating that the medium stress state in the region changed from compression to tension.Combined with characteristics of focal mechanism solutions in this region,the regional stress is still dominated by horizontal compression.The change of medium stress state may be related to additional stress caused by impoundment,the cause of which needs to be further discussed.In other regions,the maximum and minimum principal stress axes are near horizontal,and the R value is concentrated in the range of 0.61~0.67,indicating that the tension of medium stress is relatively weak.(6)The focal mechanisms in the Ludian earthquake's aftershock area has strong consistency,and the stress concentration is characterized by“strong-weak-strong”variation with a period greater than six months.The stress concentration has a strengthen process before several large earthquakes occurred.Before the M_L5.3earthquake,the intensity of stress concentration varied from strong to weak,indicating that the stress concentration intensity before some large earthquake does not necessarily increase continuously,and the stress will gradually adjust during accumulation.Nowadays,the intensity of stress concentration in this area may continue to increase,and there is a risk of moderate-strong earthquakes in the future.