Near-field Strong Ground Motion Simulation In Broadband Frequency Based On Fault Rupture Decomposition Method

In the intercrosss area of engineering and earth science, study on broadband ground motions generated by strong or great earthquake is a key research field because these waves is the main reason to cause the damage and collapse of buildings and other engineering structures. A good and efficient estimation method of the ground motion is usful to reduce the damage of future strong earthquake and help to do the resisting earthquake design and probabilistic seismic hazard analysis for the major projects. Earthquake researchers and earthquake workers have proposed several methods for the near fault ground motion synthesis. This article introduces the mainly three methods such as the deterministric method(including theoretical and empirical green function), the stochastic method and the hybrid broadband frequency strong ground motion simulation technique. The theoretical fundament of the methods have been explained and compared their characteristic and advantage. For now, the theoretical green function is mainly used to simulate the low frequencies (0.1-1Hz) due to the computation conditions. The high frequencies(1.0-10Hz) can be simulated by empirical green function method or stochastic method. The empirical green function method needs some some earthquake records which have high signal-to-noise ratio the same source mechanism as the simulated target earthquake. The stochastic, bandlimited, white-noise synthetics have not considered the propagation in the earth materials and the accuracy of long period earthquake waves.In this paper, a discreting model of source function based on kinematics is adopted for predicting near field strong ground motion in broadband frequencies (0.1-10Hz) using theoretical elastic-wave-propagation sythetics in multi-layered half-space media. The high frequency components can be calculated by keeping the high frequency signal of the source functions when we discrete the fault plane in second order and disintegrate the rise time of each fault element. Theoretical elastic-wave-propagation sythetics introduce proper seismic-phase arrivals of body waves and surface waves and reduce the dependence with the earthquake records'richness and accuracy.In order to illustrate the reliability and praeticability of the changed source function theoretical green function method developed by this paper,2000 Mw5.0 Yountville, California, Earthquak is taken as an example. Firstly, according to the relation between the rupture area, the longth and width of the fault, average slip and the earthquake moment magnitude, we determine the fault model. Then we calculate 10 stations ground motion near the fault (the distance from station to epicenter<50km). Acceleration time histries and the frequency spectra were compared with those records observed at the same stations during the shock. The results show that the acceleration frequency spectra from the discreted source model of the paper with theoretical green function synthesis can be predict well. And also some difference in shape of acceleration time history envelops.As another test of the application for the great earthquake near fault strong ground motion simulation, we simulate ground motions of Wenchuan earthquake in 8 selected stations located near the fault. The results show overall agreement on the acceleration histries, duration within±50% tolerance between the simulated and observed acceleration. The simulated Fourier spectrum has the components as high as 10Hz. The simulated Fourier spectrums at the stations in the Sichuan basin attenuate faster than the observed because the method has not considered site effect. The strong ground motion simulation should consider all the source, propagation path and site effect in the complicated topography. The results indicate that the deterministic method changing input source functions can be applied to simulate the near-fault strong ground motion in the broadband frequencies.Then Calculate the velocity histries on two survey line 20km from the fault line to learn more about the characteristic of the near fault ground motion. These results show the effects of hanging wall and the forward directivity. Futhermore, the PGA distribution on the areas within 200km away from the fault displays the generally influence regions.