ランダム地震動モデルの非定常スペクトル特性

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This paper investigates the formation of nonstationary spectral density function and energy envelope function of actual earthquake by making use of random earthquake ground motion model with consideration of unspecified source movement and characteristics of transmission path from source-to-site. The earthquake ground motion model is represented by convolution of three dimensional average Green's function of a random medium and dislocation time function which is simply idealized by the dynamic response to the random subevents propagating along a fault line. The nonstationary spectral density functions of vertical, radial and cross-radial components of the earthquake model are evaluated along the rupture time of dislocation for Mach numbers M_n=0.6, 0.7, 0.8, and 0.9, nondimensional rise time t_r=0.03 and the ratio of epicentral distance to focal depth R/H=1 and 5. The peak frequencies of the spectral density functions are related to the specific time intervals between nucleation and stopping of rupture process, and the physical process occuring in source region should be considered as random time function which is governed with the frictional stress level of fault plane and the fracture stress level of soil, in relation to the problem of generation of high frequency components in earthquake ground motion. The nonstationary spectral density functions of three components are different each other for the activity of one earthquake. Thus, there is a strong dependence of spectral density function of the earthquake model on the rupture direction of dislocation and the relative position between the focus of an earthquake and the observation point. The peak frequencies of the spectral density function move to higher frequency range when the rupture propagates toward the observation point. On the contrary, they move to the lower frequency range when the rupture propagates in the opposite direction to the observation point. The spectral density functions show generally a little variation along rupture time under uniform distribution of subevents occuring on a fault line. However, if the nonstationary distribution for the occurence of them is assumed, it will vary significantly along rupture time. From the fact that spectral density for R/H=5 are less sensitive to M_n than those for R/H=1, the stochastic characteristics of them become gradually stationary as seismic wave motion propagates into the far-field, and R/H is important physical quantity which is related to determine the nonstationarity of characteristics of earthquake ground motion. The energy envelope function of the earthquake model show significant variations with time in spite of the assumption of the uniform distribution for the occurence of the subevents. Therefore, the nonstationarity concerning with the energy envelope function is basically produced by the spatial formation of the propagating seismic wave motions which is described by the relative position between the earthquake focus and the observation point. The effectiveness of the random earthquake ground motion model is presented by the fact that the energy envelope function of the earthquake model is similar to the ones which are idealized from the actual earthquake ground motions.
社団法人日本建築学会の論文
1984-04-30

ランダム地震動モデルの非定常スペクトル特性

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