Research On Ground Motion Simulation With Empirical Green's Function Method

With the continued construction of major projects and frequent large earthquakes, the experiential statistic mehod for setting sesmic parameters has been unable to satisfy the anti-sesmic design. Anti-sesmic design criterion of construction requires that important buildings must be analysed with the Time History Analysis method in anti-sesmic design, which can simulate the real sesmic affection. But there is always a lack of ethquake time-histories in the regions where buildings are constructed,which makes ground motion simulation is one of the most important problem in earthquake engineering domain. The most commonly used methods of ground motion simulation, the Empirical Green Function method and the Stochastic method, have their own advantages and disadvantages.This paper conducts systematic reserch on the two methods above, comes up with some new viewpoints, and obtains the achievements below:The Empirical Green Function method takes the foreshock or aftershock as empirical green functions to synthesize the main shock. At present, there is a lack of consideration about the dislocation's unevenness on large earthquake faults. Slip velocities and dislocations on earthquake faults, especially on large earthquake faults, are usually uneven, and the unevenness of slip distributions is an important reason for high frequency emission.This paper starts with assuming both large and small earthquakes have inequable slip velocities, then comes up with the Improved Empirical Green Function method. Because the formula has taken the unevenness of the fault slip into account, ground motion simulation with this method accords well with record.The number of small earthquakes that synthesize the big earthquake depends on the ratio of length, width and dislocation. The ratios is usually not integer, but the number of small earthquakes should be integer. So it is either overestimating or underestimating the ground motion. And the method to set the integer just depends on experience, which leads less rigorous of this method. This paper tends to round up the ratio of length, width and dislocation. The amplitude of small earthquakes of the integral part remain as it is when synthesizing big earthquake, while the amplitude of the decimal part decreases properly in accordance with the decimal value. This method makes an uniform standard for the confirmation of the small earthquake number.At present, many researchers think the Empirical Green Function method must satisfy "similarity conditions" and "quasi- similarity conditions", which limits the application ranges of this method. Luo Qifeng popularizes the Empirical Green Function method to the situation that rectangular slip fault doesn't satisfy similar conditions. On the basis of Luo's theory, this paper extends Empirical Green Function method to the situation that dip fault and dip fault with slip component don't satisfy similar conditions. Because there are both slip and dip components in wenchuan earthquake fault, the generalization has important meanings to the study of wenchuan earthquake.Futhermore, the paper theoretically generalizes the Empirical Green Function method to the situation that the ratio of length,width and stress drop about big and small earthquakes is known but the ratio of dislocation is unknown.The paper introduces the conception of "strong motion generation area"(by Miyake) into the Empirical Green Function method. Only part fault which is important to ground motion is used to simulate ground motion. This method solves the problem that the ground motion amplitude increases infinitely when the epicenter distance decreases. This paper has simulated the ground motion near Wenchuan fault with this method, and the efficiency of this method is proved.This paper has simulated Tangshan, Lulong and Wenchuan Earthquakes with the Improved Empirical Green Function method. The simulation results show that the response spectrums accord with the records very well, and the efficiency of this method is proved.This paper takes the examples of Tangshan Earthquake, Lulong Earthquake and Wenchuan Earthquake to compare the two methods of ground motion simulation--the Empirical Green Function method and the Stochastic method. The results show: When using the Empirical Green Function method to simulate ground motion, response spectrums accord with the records perfectly. It suggests that the Empirical Green Function method is the most effective way to simulate ground motion. When using the Stochastic method to simulate ground motion, the response spectrum of Lulong earthquake(as an example of middle intensity earthquake) does not accord with the record very well, yet the differences are not very big. Thus, it is acceptable to simulate middle intensity ground motion of Chinese mainland with the Stochastic method. Just because of that, this paper synthesizes small earthquake time-histories of Dujiangyan region with the Stochastic method. Together with the finite records of ambulatory stations, this paper set the small earthquake time-histories as Empirical green functions to simulate the main shock time-history. The work above provides basic information to the construction's anti-seismic design in Dujiangyan.