Study Of The Characteristic Changes In Seismicity And Stress Field Of Baihetan Reservoir Zone Before And After Impoundment

Baihetan Reservoir is located in the lower reaches of the Jinsha River crossing the intersection area of multiple large faults,including Xiaojiang Fault,Zemuhe Fault,and Daliangshan Fault,with high seismicity in history.On April 6,2021,the impoundment of the reservoir began.How the impoundment will affect the seismicity and stress field in the reservoir zone and its surrounding areas has become one of the important issues.In this study,the seismic data recorded in years 2020 and 2021 by a high-density seismic array(the Qiaojia Seismograph Array)deployed in the northern and central section of the Xiaojiang fault zone are used to quantitatively analyze the changes in seismicity,focal mechanism solution and regional stress field before and after the impoundment using techniques of seismic location,focal mechanism inversion and stress field inversion.With the relocation results of 11477 earthquake events obtained obtained by the Double-difference location method,seismicity in multi-areas is analyzed.The analysis shows that the upper reaches of the reservoir was characterized with increasement in occurrence frequency,decreasement in magnitude as well as becoming shallower in focal depth after the impoundment started,and the occurence frequency appear to be closely related to the water level of the reservoir,with more earthquakes following while the water level rising,but less events appearing as the water level unchanging or falling.By the generalized polarity and amplitude technique,the focal mechanism solutions are inverted for 654 earthquakes with magnitudes ML≥1.5.The comparison of the focal mechanism solutions reveals that strike-slip type is domainated before impounding,with a ratio of 46.5%,and normal-slip types obviously increased after impounding,with a ratio larger than 50%.Based on the focal mechanism solutions inverted,the changes in stress field are investigated for three sub-areas by the stress field damping inversion method.Spatially,before the impounding,the stress field in the main part of the reservoir area(sub region ①)is mainly characterized by strike slip type,which means that the middle principal stress axis is nearly vertical;However,after the impounding,it becomes characterized mainly with normal-slip type,which means that the main compressive stress axis is nearly vertical.No distinct change in the stress field direction is found in the eastern area away from the reservoir(sub area②)before and after the impounding,with dominant strike-slip type.The stress field in the southern section of the reservoir(sub area ③)is dominated by normal-slip type all the time,but the main compressive stress axis is changed from 78° to 56° by the impounding.Temporally,the principal dilatational axis has little observable variation in azimuth and plunge while the principal compressive stress axis has great fluctuation in both azimuth and plunge during the observation period.The R-values describing the stress situation vary in the three sub-areas,but no obvious correspondence is found to the impounding process.In general,the impoundment of the Baihetan Reservoir has impact on both the seismic activity and the stress field direction in the reservoir zone and its surroundings.The closer to the reservoir,the greater the impact;reversely,the farther away from the reservoir,the smaller the impact.