Seismic Response Of The Zhangzhou Basin

The Zhangzhou basin is the largest faulting-subsided basin on land in Fujian Province. The Jiulongjiang blind fault is just beneath the northern part of the basin, which is potential to cause a strong earthquake. Strong ground motion in Zhangzhou basin and its vicinity from an earthquake with magnitude 6 on the Jiulongjiang fault is forecasted in this dissertation. Distribution of ground motion field at Zhangzhou City is calculated, the spatial characteristics and pattern is summarized.Study on strong ground motion insights into the generation and propagation characteristics and pattern of surface shaking during earthquake and deals with a series of basic problems about the source and source model, the propagation medium effects and the local site effects. Near fault ground motion is much more complex due to the significant effects of the source. Effect of large sediment basin on ground motion and earthquake damage become a highlight topic in earthquake engineering world wide these decades, which involves the amplifying effect of soft soil, the focusing effect of buried rock surface and the basin edge effect.A finite fault source model (FFM) is adopted to describe the Jiulongjiang fault for the forecasting. Global parameters such as strike, dip, dip angle, length and width of the fault, and the average slip on the fault are determined from the fault exploration data and a set of semi-empirical scaling laws. The slip distribution on the fault plane and then the slip at each sub-source are determined from a hybrid source model, which combines asperity model and k square model. 30 source models are generated taking the randomness into account on global and local parameters from the uncertainty of the semi-empirical scaling laws. The rupture start point is located at the southeast end of the Jiulongjiang fault and the rupture velocity is taken as 0.85 of the shear wave velocity in source area.High frequency ( f > 1Hz) ground motion is synthesized by a stochastic procedure developed by Atkinson, and the dynamic corner frequency is applied in source spectra. The rock site ground motions at 5 selected points around and in the basin from 30 source models are firstly calculated, and one of the models is chosen from the best fit to the mean response spectra. Acceleration time histories and spectra of motions at 5796 ground points on 200 km×200 km grids that covers the whole region, are calculated one by one. For the points located in basin, the motions at a depth 50 m (the double of the largest soil depth in the basin) are synthesized at first, and then their Fourier spectra are multiplied by the corresponding transfer function, the acceleration time histories are finally obtained from the inverse Fourier transformation. The transfer function is calculated for each zone by means of 1D wave propagation procedure with equivalent linearization, from the velocity structure of representative borehole in the zone. Furthermore, the peak ground acceleration distribution and zoning maps are plotted from the values at 5796 points. The whole region is divided into three zones. Each of them covers two sub-zones, in-basin and out-basin. The average acceleration response spectrum, the normalized response spectrum and corresponding parameters for each sub-zone are presented.Low frequency ( f < 1Hz) ground motion is calculated by a simplified Numerical Green Function approach. The 3D velocity structure model of Zhangzhou basin is worked out from the data provided by the China Center for Earthquake Disaster Emergency and SAR. The displacement field at the top of crystal rock from the finite fault source is calculated by analytical Green Function method and is taken as the input for 3D wave propagation finite element analysis. Acceleration time histories at the 5796 points are calculated by finite element method.The acceleration time history of each point is obtained from the superimposition of high and low frequency ground motions after high and low-pass filtering respectively, in time domain. The representative time history of each zone is taken from the point whose spectrum fits the average spectrum of the zone best, for inputs of time history analysis in seismic design.