Maximum Credible Earthquake Research Based On Stochastic Finite Fault Method

In order to prevent major water conservancy projects from causing secondary disasters under the action of strong earthquakes,the relevant state departments have put forward requirements for seismic check of their seismic safety for High Dam.The Code for Seismic Design of Hydraulic Structures for Hydropower Projects(NB35047-2015)stipulates that for major hydraulic structures,in addition to the seismic design of the designed ground motion peak acceleration,it should also be no leakage of reservoir water out of control when the dam subjected to the most credible earthquake at the project site.A series of major hydropower projects are distributed in the strong earthquake zones of the west China.In the absence of earthquake damage examples and referenced seismic design parameters,adopting effective methods to ensure the safety of earthquake-proofing of these major water conservancy projects is the focus of research on seismic engineering.The stochastic finite fault method is recommended method to calculate maximum rredible earthquake to replace the conventional method based on the ground motion attenuation relationship.For such major hydropower project as Xiluodu Hydropower Station,the geological structure of the location is very complex,there is more than one potential source area that affects it,and the geological structure of the source area is not clear.For this complex problem,a calculation method that can fully consider various uncertainties needs to be established.Based on the consideration of uncertain parameters,this work provides a set of calculation schemes for the Maximum Credible Earthquake,and provides reliable ground motion parameters for seismic fortification of major hydraulic projects located in complex geological structures.The stochastic finite fault method is based on stochastic vibration theory and finite fault model,which can take into account the influence of fault rupture on the near-field field.The stochastic finite fault method has a clear physical meaning and better simulation result in the high-frequency part,which meets the self-vibration period of hydraulic structure.In this study,we analyze the principles and problems of stochastic finite fault method.We improve the mothod from only simulate rectangular faults to simulate complex faults.From the simulation of the Kumamoto earthquake,we compare the imporved method.The result shows that imporved method is more in line with the actual rupture process.When simulating using the stochastic finite fault method,it is necessary to determine the value of the input parameters.The different parameter values will have a greater impact for the simulated result.In order to study the influence degree of input parameters,we analyze the uncertainty of parameters including seismic source,transmission and site by using local sensitivity analysis and global sensitivity analysis method.In the near field,the effect of stress-drop and high frequency attenuation is the most affected parameters,and the influence of quality factor can be ignored.The effect of stress-drop is the same for different fault distance,and the effect of high frequency attenuation increases with the increase of fault distance.The stress drop is the most influential parameter in the stochastic finite fault method.In this study,we use general inverse method to analyze more than 800 ground motion records in Sichuan-Yunnan region.The results show that the stress drop has a positive correlation with the earthquake intensity.The distribution of stress drop satisfies the lognormal distribution with a mean of 4.5MPa.By comparison with research by others,it is considered that the maximum range of the stress drop in the Sichuan-Yunnan region can be selected from 3.0MPa to 8.8MPa.For calculating the Maximum Credible Earthquake of major hydropower projects under complex geological structures,it is necessary to consider the influence of different factors such as potential source area,fault distribution and rupture mode based on regional structural data.This study uses logic tree and statistical methods to analyze various uncertain parameters.Using the results in sensitivity analysis to weight the uncertain parameters,we establish different schemes through the logic tree method.And we built a set of schemes to calculate Maximum Credible Earthquake.In this study we take the Xiluodu Hydropower Station as an example to calculate the Maximum Credible Earthquake.Considering the maximum potential earthquakes in three different potential source areas and the uncertainty of the parameters,we calculate the Maximum Credible Earthquake.The result is very close to the calibration ground motion parameters calculated by the probabilistic method.It indicates that the calculation using this set of schemes is reliable.