Research On Stochastic Simulation Method For High Frequency Of Ground Motions

It is important to setup a reasonable simulation method for the estimation of ground motion earthquake motion input,in particular in region without adequate strong earthquake records.It is a hot spot of geotechnical earthquake engineering research.Because natural periods of most structures are below 3.0s,they are more sensitive to the high frequency of ground motions.This thesis focus on the simulation of high frequency ground motions,utilizing the commonly used stochastic simulation method.The research history and present situation of this method is summarized systematically.Then the principle and features are introduced including the point model and finite fault model.On the base of it,following aspects are researched deeply and the main research contents and achievement are as follows:1.The relationship is constructed between corner frequency and rupture velocity.The expression of corner frequency is deduced,and the hypothesis and scope of application of each step is pointed out.It is also shown that corner frequency has different expression under different hypothesis.The results show that corner frequency is related to rupture velocity directly in both point model and finite fault model.The expression of corner frequency and rupture velocity is given,inside which the theoretical contradiction is avoided and faulty of stochastic simulation procedure of Boore and Motazedian.It supplement and improve the calculation of corner frequency.2.Distribution law of high frequency decay parameter kappa is analyzed for Wenchuan earthquake.The results show that the distribution of kappa is affected by earthquake magnitude,epicentral distance,site conditions and selected frequency range used in statistics,and the distribution of kappa shows great randomness.It can be obtained from the result that the horizontal kappa of strong motion records is greater than the vertical ones.It also shows that some of vertical strong motion records may amplify along with the increasing of the high frequency.If epicentral distance or earthquake magnitude is small,the horizontal strong motion records may also amplify when the high frequency increase.According to site classification method of seismic design code,the distribution law of kappa and the corresponding fitting formula are given along with earthquake magnitude and epicentral distance for station located on class Ⅰ,class Ⅱ and class Ⅲ sites.3.A method is provided for the setup of input parameters in the stochastic simulation method,and the effects of parameters on main character of simulated ground motions are analyzed,such as the effect on the simulated peak ground acceleration,response spectrum and dynamical amplification coefficient.The results show all parameters have substantial effect on PGA.The intensity of sensitivity is stress drop,high frequency decay parameter,quality factor Q0,density,rupture velocity,quality factor index and shear wave velocity.Kappa is the most sensitive parameter to dynamical amplification coefficient β,while the other parameters affect β a little.The effect on response spectrum mainly come from the effect to PGA.According to the effects of all the input parameters,suggestion for the setup of input parameter has been given given.If there are no inversion results,the value of less sensitive parameters can be chosen from the empirical scope.The optimal value of more sensitive parameters is searched that can make the errors between simulations and observed records smallest.This method can be used as reference to calculate the valves of input parameters.4.The ground motion records of bedrock are simulated for Ms6.5 Ludian earthquake by the stochastic finite fault simulation method coupling with an engineered method is given to ground motion simulation.The inversion results of Ludian earthquake are analyzed and summarized,then the acceleration records are simulated.The results show that the PGA and response spectrum of simulations fit closely to those of the observed records.For site without observed records,an engineered method is given that divides the simulations of response spectrum to simulations of PGA and envelope line of amplification coefficient.The scope of PGA is given by stochastic simulation method and attenuation relationship.The envelope line of amplification coefficient is gotten by the stochastic simulation method because it uses different phase spectrum to simulate ground motions.This method can use different phase spectrum to simulate ground motions,while it also use the research of attenuation relationship,then the simulation accuracy is more higher.At last,the envelope line of response spectrum can be obtained to simulate ground motions with no observed sites.