3-D P-wave Velocity Structure And Relocation Of Earthquakes In Xiangjiaba&Xiluodu Reservoir Region

Xiluodu hydropower station and xiangjiaba hydropower station are the last two rungs of the downstream section of jinsha river hydropower base,and they are the second and third largest hydropower stations in China.Xiangjiaba and xiluodu hydropower stations are located in the unstable geological structure of southwest China,with strong seismicity.There have been more than 20 large earthquakes of magnitude 7 and more than 50 earthquakes of magnitude 6?7 in the region.Since the xiangjiaba and xiluodu hydropower stations were impounded until June 30,2017,the seismic network of the reservoir recorded more than 20,000 earthquakes in the reservoir area,the largest earthquake with a magnitude of ML5.3.The velocity structure is the basis for accurate seismic location,detection of buried faults,and analysis of the relationship between seismicity and seismic wave velocity.Based on the abundant seismic observation data of the reservoir seismic monitoring system for cascade hydropower stations in the lower reaches of jinsha river,this study used Loc3dSB location method and the three-dimensional velocity model of the sichuan-yunnan region to obtain the absolute location results and phase timing data of 1491 ML?1.5 earthquakes.The conventional location results show that the average horizontal error is 248m,and the average depth error is 486m.Based on the Kissling method,the minimum 1-D velocity model required for the inversion of the 3-D velocity model in xiangjiaba and xiluodu reservoir areas was calculated.The residual error of the seismic location based on this model was significantly lower than that of the original model,and there was a good correlation between the station corrections and the shallow geological structure of the reservoir area.The 3-D P-wave velocity structure and locations in the area of xiangjiaba and xiluodu reservoir were obtained by using the method of double difference tomography.The results are as follows:1.Based on precise results of conventional location,we using DD tomography method significantly improves the locating precision of the xiangjiaba and xiluodu reservoir earthquake,locating residual RMS average value dropped to 0.021 s,generally near to picking errors of Pg wave.The Source location measurement error:average error in longitude direction is 30.4m,average error in latitude direction is 27.1m,average error in depth direction is 36.6m.2.The baisheng earthquake series was distributed along the N W direction.From the source depth distribution,most of the earthquake sources were concentrated in the depth of 2-4km.According to the location results of aftershocks and the transverse velocity distribution,it was speculated that there were nearly vertical steep dip faults in the transition zone between high speed and low speed in the underground.3.The wuji earthquakes series was also distributed in the NW direction.From the source depth,there are significant differences between the wuji earthquake clusters and the baisheng earthquake clusters.Before the magnitude 5.0 earthquake,the focal depth of the wuji sequence was between 2 and 5km.After the magnitude 5.0 earthquake,the focal depth was extended below 10km.The fault dip Angle fitted according to aftershocks(2km<depth<5km)was significantly larger than the fault dip Angle fitted according to aftershocks(5km<depth<10km).4.The velocity structure of the baisheng-wuji source area showed obvious lateral non-uniformity.The two earthquakes of magnitude ?5 both occurred in the high and low velocity transition zone,and most of the earthquakes were located on the side of the high-speed area.There is an obvious low-speed abnormal area in the east of baisheng.The study suggests that the possible reasons for the small number of aftershocks in the baisheng 5.3 earthquake are as follows:(1)the focal depth is relatively shallow and the stress accumulation level is relatively low;(2)the low-velocity body on the east side of the source is of strong toughness,which is not conducive to the accumulation of strong strain energy and brittle fracture.However,the high-speed body may show the characteristics of brittleness,and the high-speed body with the characteristics of brittleness is more prone to fracture after the main earthquake,and then aftershocks occur,which may be the reason why the aftershocks of the wuji 5.0 earthquake are significantly higher than those of the baisheng 5.3 earthquake.5.Most of the earthquakes in the last segment of Xiangjiaba reservoir occurred at a depth of more than 2km,mainly on the manao fault and the yiziba fault.The velocity structure of the underground medium clearly reflects that the manao fault and the yiziba fault are blind faults in this section,and both of them have a steep dip(to the west),but the dip Angle of the manao fault is slightly lower.6.The focal depth in the head region of Xiluodu Reservoir was 0?3km,and the seismogenic layer at the dam site was slightly deeper than the seismogenic layer in the doushaxi-huangjiaobu region.There is no obvious correlation between the earthquake in the head region of the Xiluodu Reservoir and the discontinuous surface of underground medium velocity structure,which is significantly different from baisheng-wuji region and the last region of xiangjiaba reservoir,which may be related to the small scale of karst development,which cannot significantly change the underground medium velocity.