震源過程と伝播体地盤を考慮したランダム地震動モデル

Abstract

In order to evaluate the reliable earthquake response characteristics of structural systems, it is required to investigate the correspondence between the irregular geological properties of transmission path and the unspecified faulting process of source region and such important physical quantities as peak ground motions, spectral characteristics, duration times and so forth. In this paper, the idealized earthquake ground motion models represened by convolution of the average Green's function of a random medium and the random dislocation time function are analyzed to obtain some indication of the influence these factors may have on aseismic design of structural systems. The vertical, radial and cross-radial components of random earthquake time histories are calculated for Mach numbers M_n=0.6, 0.7, 0.8 and 0.9, rise time t_r=0.03 and the ratios of epicentral distance to focal depth R/H=1 and 5. The observation points of vertical and radial components are assumed to be in the direction of acceleration and deceleration of the fault rupture. The peak ground motions and duration times of vertical and radial components show significant variation with up-and down-Doppler effect associated with the fault movement. They become large and short when the fault propagates toward the observation points of vertical and radial components. On the contrary, they become small and long when the fault propagates in the opposite direction for the observation points. The peak ground motions and duration times of cross-radial component of the earthquake models in the perpendicular direction of the fault movement is insensitive to the variation of Mach number. Then, there is a strong dependence of peak ground motions and duration times at the site in the near field on the rupture direction of dislocation and the relative position between the focus of an earthquake and the observation point. There being some differences in the propagation velocities of P, SP, SV and Rayleigh wave motions, the duration times of earthquake ground motions become inevitably long with the increase of propagation distance unless the fault rupture occurs in the finite source region. The wave form functions, maximum values and their generation times of the earthquake models vary with the ratio of epicentral distance to focal depth since the energies of SV wave motions are transformed into those of SP and Rayleigh wave motions with the increase of the ratio. On the other hand, the wave form functions of cross-radial component of the earthquake models are varied a little with the ratio because there is no such energy transformation for SH wave motion of the component. Since the peak accelerations of random earthquake ground motion models become less sensitive to Mach number with the increase of propagation distance, the stochastic characteristics of them tend to be gradually stationary as the seismic wave motion propagates into the far-field. From the above discussion, it is concluded that Mach number, the ratio of epicentral distance to focal depth and the relative position between the causative fault and the observation point are important parameter and physical quantities for aseismic design of structural systems, which would describe the maximum values, frequency characteristics and duration times of earthquake ground motion.