Tsunami simulation taking into account dynamic seabed displacement due to seismic faulting

本研究旨在开发一种更高精度地模拟海啸发生和传播的数值技术，并评估地震断层引起的海床动态位移对海啸的影响。与传统技术相比，我们开发的新技术既考虑了海床动态位移，又考虑了声波。原则上，本模拟由两个步骤组成。第一种是地震地面运动模拟，采用三维边界元方法(BEM)；第二种是海啸模拟，采用三维有限差分方法。这是基于海床-海水系统可视为弱耦合系统的假设。基于这一假设，首先考虑地震断裂的破裂机制，模拟包括静力位移在内的海床动态位移。然后，在…的底部输入与海床位移相关的速度再加上海水，通过求解N-S方程来模拟海水扰动。两步模拟的结果不仅以快照的形式表示，而且还以动画图形的形式呈现，以便于评估模拟的有效性。由于海床的动态位移和静态位移的叠加效应，动态海床位移和声波使水波高度显著增加，特别是在近断层区域。此外，本模拟还证明了短周期水扰动是由沿海床以远高于海啸的速度传播的瑞利波引起的。最后，将本方法应用于最近地震引起的实际3次海啸。对1998年三陆地震(M6.3级)的模拟表明，如果给出高精度的断层参数，本方法能够模拟与之相关的海啸，达到令人满意的精度水平。用地震测得的断层参数对1983年日本中部(7.7.)和1993年北海道-南岸(7.8.)地震进行了模拟，结果表明潮汐记录在波高和到达时间上有相当大的差异，这主要是由于断层参数S和数值模型的粗糙所致。较少

The present study aims to develop a numerical technique to simulate generation and propagation of tsunamis with higher accuracy, and to evaluate effects of dynamic seabed displacement due to seismic faulting on tsunamis.In contrast with the conventional one, the new technique we have developed takes into account the dynamic seabed displacement as well as acoustic water waves. In principle, the present simulation is made up of two steps. The first is earthquake ground motion simulation for which the 3-D boundary element method (BEM) is used, and the second is tsunami simulation for which the 3-D finite difference method (FDM) is used. This is based on an assumption that the seabed-sea water system can be regarded as a weakly coupled system. Due to this assumption, the dynamic displacement of the seabed that includes the staticdisplacement is simulated first, considering rupture mechanismof seismic faulting. Then, velocity associated with the seabed displacement is input at the bottom of … More the sea water, and sea water disturbance is simulated by solving the Navier-Stokes equation. Results of the two-step simulation are represented in terms of not only snapshots but also animated figures, in order to facilitate evaluation of validity of the simtulation.The dynamic seabed displacement and the acoustic wave are found to give a remarkable increase in the water wave height especially in the near-fault area, as a result of superposed effects of the dynamic and static seabed displacements. In addition, the present simulation has demonstrated that short-period water disturbance is induced by Rayleigh waves that travels along the seabed with a velocity much higher than tsunamis.Finally, the present technique is applied to actual three tsunamis caused by recent earthquakes. From simulation for the 1998 Sanriku-oki earthquake (M6.3 ), the present technique proves to be able to simulate the associated tsunami to a satisfactory level of accuracy if the fault parameters are given with high accuracy. Simulation for the 1983 Nihonkai-Chubu (7.7) and the 1993 Hokkaido-Nansei-oki (7.8) earthquakes, is conduced by using seismically determined fault parameters and shows considerable difference between tidal records in the wave height and arrival time, mainly because of roughness of the fault parameters s and numerical models. Less