Influence Of Three Gorges Reservoir On Seismic Activity Near The Northern Part Of The Xiannv Mountain Fault

After the impoundment of the Three Gorges Reservoir,a large number of small and medium earthquakes occurred near the northern segment of the Xian nushan fault.Based on the seismic monitoring data recorded by the induced ea rthquake monitoring network and mobile stations of the Three Gorges Reservoi r from May 2013 to November 2015,the northern segment of the Xiannushan fault was taken as the research object,and the precise location was carried out by using the double-difference location method combined with waveform cross-correlation technology.By comparing the two-difference locations using catalogue phases only wit h those using waveform cross-correlation technique,it is shown that the epicent er of double-difference locations combining waveform cross-correlation techniqu e is more tightly bunched in the northern segment of Xiannushan fault,with s hallower average focal depth and smaller vertical and horizontal errors.After r epositioning,the focal depths are mostly concentrated in the 0-2km and 4-10 k m regions,and there is a blank area of focal distribution in the 2-4km region.0-2km earthquakes should be mainly caused by superficial collapse,while 4-10 km earthquakes are related to structures such as Xiannushan fault.Seismic acti vity generally shows the characteristics of migration to the southwest and along the strike of Xiannushan fault.The depth profile along the vertical fault direct ion shows a south-west trend,which is consistent with the Xiannushan fault tr end.Based on the precise location of the earthquake and the impoundment pro cess of the Three Gorges Reservoir during this period,the relationship between the impoundment of the Three Gorges Reservoir and the seismicity of the nor thern segment of the Xiannushan fault is analyzed.It is found that during the impoundment period,the frequency of earthquakes increases obviously.In the t wo drainage sections,the seismic activity belongs to the reservoir-induced earth quake of "fast response type",and both of them show a negative correlation.Based on the calculation data of waveform correlation,this paper regards the pairs of earthquakes with waveform correlation coefficients of at least three stations not less than 0.8 and magnitude difference(+0.2)as repetitive seismic pairs,and obtains 230 pairs of repetitive earthquakes with average distance of event pairs of 2.08 km.Two groups of multiple earthquakes are further classifi ed according to focal depth.The slip rates of 0-2 km group are 2.43 mm/a an d 3.83 mm/a respectively,and those below 4 km are 2.30 mm/a and 3.17 mm/a respectively.The slip rates before February 2014 and after February 2015 are estimated,and the results are basically consistent with the leveling measureme nts.The overall recurrence interval of the four groups of multiple earthquakes increases,and the earlier recurrence interval is shorter.The increase of the ove rall recurrence interval may be due to the repeated rupture of concave-convex bodies surrounded by stable sliding areas.The short time interval of the earlier recurrence may be due to the acceleration of the slip rate before the occurren ce of two larger earthquakes at the end of March 2014 and the triggering effe ct of earthquakes on the region after the occurrence.