Space-time distribution of near field seismic ground motions

本文研究了由内陆活动断层引起的近场地震动。由于断层附近地震地震动的空间分布受断层破裂机制的影响，采用有限元断层模型模拟近场地震动。断层模型为二维逆断层。通过有限元模拟，验证了断层附近存在脉冲地震动的可能性。收集了1995年神户地震中有关冲击运动的旧文献描述，并对冲击运动造成的破坏进行了研究。研究结果如下：(1)有许多证据表明，在神户地震中，建筑物受到了冲击波的破坏。(2)有许多描述支持脉冲波的存在。(3)应力降、临界位移等断层参数对上下分量加速度幅值影响较大。(4)上下分量的大振幅是由于破裂的迅速停止而产生的。(5)断层周围土体的非线性效应有时会放大地表地面运动的振幅。(6)当表层横波速度较小时，出现长持续时间脉冲。(7)即使地震波中含有大振幅、高频分量，这些分量也不能被伺服式加速度仪记录下来。

We here investigated near field ground motion induced by inland active faults. As the spatial distribution of earthquake ground motion near the fault is affected by the fault rupture mechanisms, FEM fault model is used to simulate near field ground motion. The fault model is two dimensional inverse fault. The possibility of existence of impulsive ground motion near the fault is also checked by using the FEM simulation. The descriptions of old manuscripts about the impulsive motions are collected and damage by the impulsive motions in 1995 Kobe earthquake is investigated. The results obtained by the study are as follows.(1) There are many evidences that the structures are destroyed by the impulsive waves in Kobe earthquake.(2) There are many descriptions that support the existence of the impulsive waves.(3) The fault parameters such as stress drop and critical displacement have a strong effect to the acceleration amplitude of the up-down component.(4) The large amplitude of the up-down component occurs by the rapid stop of the rupture.(5) The nonlinear effect of the surrounded soil of the fault sometimes amplifies the amplitude of the surface ground motion.(6) In case that the S-wave velocity of the surface layer is smaller, long duration pulse is appeared.(7) Even if the seismic wave contains large amplitude, high-frequency components, the components can not be recorded by the servo-type accelerograph.