Study On Site Effects Of Stochastic Simulation Of Earthquake Ground Motions

Stochastic simulation method is a kind of semi-theoritical and semi-experiential method based on seismic observation data. The Fourier amplitude spectrum used in a seismological model can be expressed as a product of source spectrum, propagation path factor and site effect in frequency domain. One advantage of this approach is that it can reflect the characteristics from source, propagation path and site effects in selected area.It is usually accepted that the path effect and the soft rock site effect determined from small-moderate earthquakes are fit for strong earthquakes in the same region.This paper aims at the study of the effect of propagation path and site in selected area based on observation data from small-moderate earthquakes in case of lack of strong motion records. The results of research will serve the stochastic simulation of strong ground motions.The source spectrum of small-moderate earthquake can be represented by theoretical "Brune" model, the geometric attenuation factor of whole path can be represented by three-segment geometric model. But it's very difficult to divide the effects of anelastic attenuation of the whole path and the effect of the soft rock site, the key problem of the paper is to separate the anelastic attenuation and the combined effect of site(i.e. the combied effect of site amplification and attenuation ).Through studying the physical meanings of every factors in prediction equation of stochastic simulation, according to the physical nature that anelastic attenuation factor is a function of source distance and site effects is almost independent of source distance(Boore,1983, 2003), the anelastic attenuation and site effects can be separated and be respectively confirmed within 1-10Hz by analyzing large of seismic data from different distance.Two research regions are chosen, i.e. to study the combined effects of soft rock site and the combined effects of thick soil cover site repectively.In the first research region lied in Northern Tian-shan of Xinjiang, there are large of digital seismic data. The combined effects of soft rock sites have been estimated using inversion method of stochastic simulation prediction equation. The average site effects of soft rock reach 1.5 or so in 2-4Hz, and can be directly applied to stochastic modelingground motions in the region. The S wave quality factor is obtained as QS = 278f 0.346,which reflects objectively the anelastic attenuation in the region. On the basis of researches mentioned above, the acceleration spectrum of moderate-strong earthquakes on soft rock sites has been simulated according to source spectrum of singnal corner frequency using stochastic method.On the other hand, the amplification characteristic is also studied using H/V spectral ratio method, the results show: (1) The H/V spectral ratios of observation data are independent of source distance, and reflect the amplification characteristic of soft rock sites. (2)The ratios of soft rock sites are greater than 1, because the ratios not only reflects the site effect themselves, but also the topographic effect of some stations.In another region in Jiashi-Bachu area, there are a lot of digital seismic data and...