Numerical Simulation Study Of The Changes In Stress Fields Caused By Reservoir Storage And Their Influence On Tectonic Activities

China has rich hydraulic resources,and there are more than 98,000 reservoirs that have been built in the country,with a water storage capacity of 900 million cubic meters,ranking the highest in the world.In addition to bringing a large amount of fresh water resources and clean energy,the construction of the reservoir has also caused many earthquakes,which are called reservoir induced earthquakes（RIS）.Current researches show that the interaction between the reservoir and the tectonic activity is very complex.The water load of the reservoir,the diffusion of pore pressure,the infiltration and softening of the fault material by groundwater,etc.,all play a certain role in inducing earthquakes,but which one of these roles play a leading role is not clear yet.In this study,the pore pressure obtained by the diffusion equation and the stress obtained by the pore elastic equation are used to study the mechanism of earthquake induced by the reservoir.The Dagangshan reservoir in Sichuan and the Fushun west open pit in Liaoning were selected as examples to carry out the study respectively.For Dagangshan Reservoir in Sichuan Province,based on the actual structural geological characteristics of the area and high-precision DEM data,a three-dimensional numerical model of poreelasticity in the area was established,and the effect of additional stress caused by the reservoir storage on the fault in the area was calculated by finite element method.The calculation results show that the elastic load effect of the reservoir water storage causes the activity of the Moxi Fault to be more active and inhibits the activity of the Dadu River Fault.The superimposed effect of gravity on the reservoir water and the increase in pore pressure caused by water seepage makes the change in coulomb stress in the area west of the Moxi fault positive,and the maximum increase of 5km depth is 45 k Pa;the elastic effect of the reservoir water makes the change in the coulomb stress in the area east of the Dadu River negative and the maximum decrease is about-12 k Pa.This is consistent with the seismic activity change before and after the storage of water in this area.Over time,the increase in pore pressure caused by infiltration will make the volume of stress changes in the entire storage area of Coulomb stress,which leads to an overall enhancement of earthquake activity in the area.For the proposed Liaoning Fushun Mine Pumped Storage Power Station,because its rapid water storage process will be accompanied by a strong undrained effect,we further improved the calculation model,in the case of considering the undrained effect,the effect of different water storage schemes on the coulomb stress caused by the nearby fault and reservoir area was analyzed.Furthermore,a reasonable water storage scheme was proposed.The results show that the rapid water storage process,such as a rapid water storage in a year,would be accompanied by strong undrained effect.A close reference point observed an increase of about 2 k Pa in pore pressure compared to the drainage model,which is more pronounced at a distance from the storage area.Therefore,extending the water storage time or grading water storage can effectively reduce the impact of the undrained effect.At the same time,considering that the large active faults in this area directly pass through the mine pit,it is necessary to conduct anti-seepage treatment on the bottom of the reservoir.