Source Parameters And B-value Characteristics Of Xiluodu Reservoir Earthquake

Reservoir induced earthquake refers to the occurrence of earthquakes in the earthquake monitoring area or around the reservoir after the reservoir is filled with water.Compared with nature tectonic earthquakes,this kind of earthquake has the characteristics of low magnitude,shallow focal depth and high intensity.And the earthquake usually presents a cluster swarm.The construction of large reservoirs plays an Positive role in the improvement of the local economic development,water use conditions and farmland irrigation.Therefore,the prediction and judgment of reservoir induced earthquake is an important premise to ensure the safety of reservoir construction and operation.Many researchers have done a lot of research on it for many years.Xiluodu hydropower station is the third hydropower station in the construction project of cascade hydropower stations along the downstream of Jinsha River.Before the construction of Xiluodu dam.This area is an area with high earthquake incidence.After impoundment,earthquakes continue,especially with a magnitude of ML5.6.Due to its shallow focal depth and high intensity,This kind of earthquake have impact and property loss on the safe operation of hydropower station and surrounding residents and buildings to a certain extent.Therefore,it is of great significance to study the earthquake after the impoundment of Xiluodu Reservoir to identify whether it is a reservoir induced earthquake for the stable operation of hydropower station and the safety of life and property of surrounding residents.In order to fully understand the source parameters and seismicity parameters of reservoir induced earthquakes and preliminarily identify reservoir induced earthquakes based on this,this paper compares the source parameters and seismicity parameters of earthquakes before and after the impoundment of Xiluodu Reservoir,and gives the method for preliminary identification of reservoir induced earthquakes according to the calculation results.The main work of this paper is as follows:(1)Firstly,the generalized inversion method based on reference stations is used to inverse the source spectra of 9 strong earthquakes(ML4.2-ML5.6)in Xiluodu Reservoir in the upper reaches of Jinsha River since the impoundment in 2015.9 source spectra are obtained,and the source parameters are calculated,in which the corner frequency fc variate range between 0.64-3.96Hz;ω variate range between 2.2-2.4,and the seismic moment M0 variate range between2.399～33.11×1022dyne.cm.Source rupture radius R variate between 284.65～724.71m,and the stress drop Δσ variate range between 1.66～3.81Mpa,the average stress drop Δσm=2.27Mpa,the variation range of energy moment ratio e is 0.934～4.540×10-11,the variation range of seismic wave radiation energy ER is 0.394～8.650×1012J,The variation range of apparent stressσapp is 0.280～1.361Mpa,and the average apparent stress σapp-m=0.605Mpa.By comparing the stress drop obtained from the 9 source spectra with the ordinary tectonic earthquake in Ludian,Yunnan Province,which occurred in the area close to the reservoir,it is found that in the study area,the stress drop changes with the magnitude,and the relationship between the stress drop and the magnitude of the swarm earthquake in Xiluodu Reservoir area is Δσ1=0.13301ML2.1358,and the relationship between the stress drop of Ludian aftershock and the magnitude isΔσ=0.13495ML2.70275,In contrast,the stress drop of reservoir induced earthquake is smaller than the same magnitude as that of ordinary tectonic earthquake in the same area.(2)At the same time,the b value of Xiluodu Reservoir area before impoundment is calculated based on the maximum likelihood estimation method,in which the calculation result of b value is bbefore impoundment=0.3682.At the same time,the seismicity of Xiluodu Reservoir area after impoundment is calculated by the same method,and the calculation result of b value is bafter impoundment=0.8246,in contrast,the value of b after impoundment is greater than that before impoundment.(3)Combined with the comparison of focal parameters and value b,We find that the main reason for the difference of these parameters is that the reservoir water seepage and the overall gravity of dam body reservoir water have an impact on the stress background in the study area.The reason for the change of background stress is analyzed based on Moore Coulomb fracture criterion.(4)At last,based on the results of previous sequence calculation,an identification method of reservoir induced earthquake is given:recognition based on stress drop and seismicity analysis,that is,recognition from the change of background stress.In this way,there is a very important premise:The natural tectonic earthquake for comparison shall be in the same area as the research earthquake.If the research earthquake meet two condition:the calculated stress drop Δσ When it is less than the seismic stress drop of natural structure and the b value of research seismicity parameter is greater than the calculated b value of natural tectonic earthquake,it can be preliminarily judged as reservoir induced earthquake.