A Study on Impulsive Strong Ground Motion and its Observation

1995年的神户地震是日本近代历史上最严重的灾难之一，不仅因为总伤亡人数，而且因为地震发生在一个高度城市化的地区，许多建筑物、住房和基础设施都集中在那里。结构破坏的原因一般用大水平地震动的作用来解释。但对于桥墩、钢框架、基础结构、各种刚体等所观察到的所有异常现象都没有弄清楚。从作为脉冲载荷的垂直地震动的角度进行讨论仍然是不够的。但事实上，地震仪并没有捕捉到这种垂直分量的脉冲加速度。本文首先对强地震动对结构的损伤进行了调查，并研究了损伤与输入地震动的关系，以确定脉冲波的存在及其作用。对伺服型和换能器两种加速度计进行了比较，检验了高频元件的灵敏度。采用断层破裂模型进行有限元分析，研究了非线性单元对垂直地震动的影响。主要结果如下：(1)根据1955年神户地震中观测到的许多异常现象，不能否认脉冲强作用力的存在。(2)当断层的临界位移较小且考虑断层周围土体的非线性时，地震的上下分量振幅变大。(3)当记录中存在大于20- 50hz的高频时，伺服式加速度计无法检测到该成分。(4)我们的观测点尚未观测到高频分量。

The 1995 Kobe earthquake is one of the worst disasters in recent Japan history, because of not only total number of casualties but also the fact that the earthquake struck a highly urbanized area in which many buildings, housings and infrastructures were concentrating. The cause of damage to the structures is generally explained by the action of large horizontal ground motion. But all unusual phenomena observed for bridge piers, steel frames, foundation structures, various rigid bodies and so on have not been made clear. Discussion from the view point of vertical ground motion which is applied as an impulsive load is not still sufficient. It is a fact, however, that the seismographs did not catch such impulsive accelerations of vertical component. In this paper, damage to structures due to the strong ground motion is surveyed first and the relation between damage and input ground motion is investigated in order to find the existence and the effect of the impulsive wave. Two accelerographs of a servo type and a transducer type are compared to check the sensitivity of high frequency component. FEM analysis by using a fault rupture model is carried out and the effect of nonlinear elements on the vertical ground motion is studied.Main results obtained here is as follows :(1) The existence of impulsive strong force can not denied according to many unusual phenomena observed in the 1955 Kobe earthquake.(2) The amplitude of the up-down component of the earthquake becomes larger when the critical displacement of the fault is small and nonlinearity of the surrounding soil of the fault is considered.(3) When the high frequency more than 20-50 Hz exists in the record, the servo-type accelerograph can not detect such component.(4) The high frequency components have not observed in our observation site yet.