Currently, the average value of seismic response spectrum is taken＇as the fortification level, however, the probability of earthquake fortification level can not be clearly defined. The 368 seismic records of the soft soil site are analysed in the paper and the three-dimensional distributive probability plots of periods, variety intervals and distributive probability are established. It is shown that the ratio between every seismic response spectrum value and the average value are not around 1. So, the mean response spectrum can＇t reflect the unified fortification level. Thus, the three-dimensional cumulative probability plots are established and the response spectrum with different probability reliability can be obtained. Finally, the spectrum＇s expression between the provided fortification level and period is fitted.%目前反应谱都是以平均谱值作为设防水准，无法明确设防水准上的概率水平．选取软土场地上的368条地震记录进行分析，建立了关于周期、变化区间和分布概率的三维分布概率图，发现各条地震反应谱值与平均谱值的比值难以集中在1附近．．说明平均谱值反应谱不能体现统一的设防水准．因此建立了三维的累积概率分布图，求得不同概率保证率下的反应谱．最后还拟合出规定设防水准的反应谱和周期之间的关系表达式．
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