场地放大效应的估计

Estimation of absolute site amplification

本文利用唐山地区９个台站得到的１３次近场地震记录分析了场地放大效应。其中一些台站位于废弃的地下煤矿坑道内，最深的一个台站在地下 ８２２ ｍ处。首先检验确定了地面反射波对地下场地的入射波没有很大影响，然后利用地下基岩场地台站（－ ８２２ ｍ）作参考场地，用线性反演法来同时分离震源、传播途径和局部场地效应。结果发现：在 １至 １０Ｈｚ的频段上Ｓ波品质因子Ｑｓ基本上与频率成正比；与以地面基岩场地作参考场地所得的结果相比较，地面基岩场地的反应并不是一常数，它在大于 ６ Ｈｚ的高频段上有明显的放大作用，在本文的研究事例中，在６至 １０ Ｈｚ的频段上这个放大系数约为２至４。这表明由于地面基岩场地本身的放大作用，将导致在作场地地震动预测时会低估地震作用。最后采用一种新的非参考场地法，即遗传算法，用ｗ２震源模型来估计绝对场地效应。在本文的研究结果中，对大多数台站场地来说，所得的绝对场地效应与以地下基岩台站作参考场地所得的相对场地效应十分一致。这表明在作场地地震动分析时，当仔细考虑了反射波对入射波的影响后，地下或井下基岩场地是比地面基岩场地更合适的参考场地．

Site amplification was analyzed for the Tangshan area using the strong motion data ob tained by 9 stations from 13 events.The strong motion array was deployed in Tangshan area and several stations located in the underground tunnel of an abandoned coal-mine. The deepest one was about 822m below the surface. Firstly, the effect of the waves reflected from surface to the underground site was examined and no existence of destructive interference between the input and reflected waves was confirmed.Secondly,an underground rock site station (-822m) was used as a reference site to separate the source,propagation path and local site effects simultaneously by means of a linear inversion method.lt is found that the S-wave quality factor, Qs, is closely proportional to frequencies in the range of 1.0 to 10Hz, and the surface rock site has a significant amplification at high frequencies more than 6 Hz,which means that an underestimation of seismic hazard prediction will be caused when a surface rock site is used as reference site. Finally, a new non-reference-site method,Genetic Algorithms,was used for estimating the absolute site effect using omega-square source model,and the results agree well with the relative site effect where the underground station at -822m was used as the reference site. It shows that an underground or down-hole bedrock site can be considered as a better reference site than a surface rock site for seismic hazard analysis provided the destructive interference of the reflective wave is carefully taken into account.