Seismic Activity And Shear-wave Splitting Before And After Impoundment In Nuozhadu Reservoir

In this paper Nuozhadu Reservoir Network from November 2009 to September 2014 earthquake records were double difference, the use of relocation result, the use of shear-wave splitting analysis method SAM system, obtained Nuozhadu Reservoir 8 units shear wave splitting station. We discussed in the reservoir area of seismic activity, and shear-wave splitting characteristics before and after the impoundment, the following conclusions:1. seismogram 7817 M≥1.0 be double difference, the results show that after the relocation of the epicenter of the earthquake distribution is more concentrated, the overall distribution is more convergence. Depth distribution source location after more continuous, the focal depth of the vast majority of seismic events distributed in 0-20km. Positioning results were analyzed, compared to the original report given reservoir Network, precision double difference relocation results slightly improved.2. The relocation results discussed seismic activity inside and outside the reservoir area before and after the impoundment. Seismic activity outside the reservoir area before and after the impoundment, from earthquakes to the maximum frequency of earthquake magnitude, were no major changes, indicating that impoundment does not affect the seismic activity outside the reservoir area, outside the reservoir area of the earthquake occurred in the region belong to normal tectonic earthquake activity. Library area, seismic activity before and after impoundment significant change, earthquake frequency and magnitude of earthquakes largest display, the more water before the earthquake activity has increased significantly after impounding, impoundment month after the earthquake frequency is 4.44 times the water before largest earthquake rose from 2.5 to 4.5, change the reservoir area after impoundment water level of seismic activity and has a good correlation.3. Increase from Reservoir seismic activity more significant A, B two regions before and after impoundment focal depth analysis of seismic change, water activities less before the earthquake, the depth distribution is divergent, after the earthquake focal depth of water in the two regions 0～10km distribution of a significant increase in earthquake-prone in the shallow crust, but the earthquake 15km inside more, show a strong influence in the impoundment of 10km, which is similar to results of previous studies, and because of the presence of faults seismic activity due to enhanced reservoir storage, will enhance the fault of seismic activity, and the presence of deeper earthquakes. From the time course of earthquake focal depth of the A zone WTZ nearby, the focal depth of the process of gradually increasing,reflecting the gradual process of progressive approach to the role of water.4. The result of the relocation, the selection meet record shear wave waveform window using SAM method, were shear-wave splitting analysis, a total of 129 stations in eight groups shear wave splitting.5. Analysis of shear-wave splitting parameters believe that the advantages of fast shear wave polarization direction of each station to some extent reflects the stress field station below, but close to fracture by impact, fast polarization direction of advantage will be reflected. Overall consistent with the direction of the main advantages of compressive stress in the region. Slow shear wave time delay average of (3.34±0.18), two more seismic stations are located Gorges dam and reservoir tail region, the two regions is caused by crustal stress Reservoir largest regional differences in changes.6. The shear wave and wave time delay variation exhibited good relationship with the reservoir level. Delay time and with all the seismic stations recorded seismic Mangnuo time delay, are in line with this feature.