基于伪谱和有限差分混合方法的兰州盆地强地面运动二维数值模拟

2D Simulation of the Strong Ground Motion in Lanzhou Basin with Hybrid PSM/FDM Method

摘要:

利用伪谱和有限差分混合方法对兰州盆地进行二维强地面运动模拟。假设3个不同的震源深度5、10、20km的同一地震下,以一个5层的二维剖面作为模型,以此来研究地震波传播过程并分析复杂的二维非均质路径效应,以及局部地下速度构造对强地面运动振幅加乘的影响,从而了解路径效应对强地面运动最大峰值位移的影响程度。研究结果显示:伪谱和有限差分混合方法模拟地震波场结合了有限差分法和伪谱法的优点,弥补了二者的不足,能较好地处理介质不连续面的计算,同时保证了和伪谱法相当的计算精度。通过地震波场模拟可知沉积盆地的强地面运动较基岩相比具有放大效应,震源深度的不同对兰州盆地垂直分量地面最大峰值位移影响较大,在震源深度为5km时最小,为0.06cm;震源深度10km时在水平剖面30～55km范围内峰值位移最大,达到0.14cm;但对水平分量的峰值位移影响较小。

Abstract:

In this study,we simulate two-dimensional(2D) numerical data related to a strong ground motion in the Lanzhou basin using a hybrid scheme based on the pseudospectral method(PSM) and the finite difference method(FDM).The simulation is based on a 10-km-deep focal point,and a 2D profile of five layers is used as a model for analyzing the site response and the peak displacement of the strong ground motion.The results show that the hybrid PSM/FDM method for the seismic wavefield simulation combines the advantage of PSM and FDM and compensates for the disadvantages of these methods.Therefore,the proposed method can betterprocess the calculation of the discontinuous medium surface.The calculation accuracy of this method is similar to that of PSM.First,we simulate the theoretical seismograms and find that the peak ground displacement(PGD)of the vertical is obvious at the basin edge.Then,the wavefield snapshots of the profile at t=3,6,9,12,15,18,21,24,27,and 30sexplain the propagation of the seismic wave,and it is ensured that the amplification of P waves and Sv waves is obvious in the inner basin.The results of the wavefield simulation reveal that the sedimentary basin can amplify a strong ground motion compared with the bedrock,and that the PGD of the vertical in the inner basin is 0.1cm,which is larger by 0.03cm than the PGD near the seismic source.Further,the PGD at the east edge of the basin is larger by 0.02cm than that in the inner basin.However,the PGD of the horizontal in the inner basin is 0.025cm,which is smaller by 0.2cm than the PGD near the seismic source.Therefore,it is known that the PGD of the vertical is larger than that of the horizontal in the inner basin,and the results are opposite near a seismic source,which proves that the influence of a distant earthquake on a tall building is significant.Further,the wavefield simulation revealed a positive correlation between the site conditions and the PGD:if the site conditions are more complex,the PGD will be larger for the same earthquake.Otherwise,the influence of the surface wave of the vertical is more obvious than that of the horizontal at the basin edge,which provides the theoretical evidence for future quakeproof research.