上海软土场地的地震反应特征分析

近年来的宏观震害资料和强震观测记录已经证明：场地条件不仅对地震动的峰值加速度和频谱形状具有重要的影响,而且与不同类型工程结构的地震反应和震害机理也具有相当密切的关系.上海地区的第四纪沉积土层呈水平层状分布,基岩埋深可达300m左右,且浅部普遍发育有滨海沼泽相软土.因此,上海地区上覆深厚、软弱的沉积土层,对场地的地震反应特性无疑具有重要的影响.文章基于一维场地地震反应的等效线性化频域分析方法,建立了上海软土场地的动力分析模型,以El Centro地震波为例,重点分析了上海地区场地土的地震反应加速度反应和频谱特征.研究表明,在7度抗震设防即场地地面峰值加速度（PGA）等于0.1 g的情况下,上海软土场地的地表峰值加速度具有放大特性,地表加速度的频谱组成具有低频放大,高频过滤的特征,地面运动加速度反应谱的卓越周期也具有向长周期方向移动的趋势.

Characteristic Analysis for Seismic Ground Response of Soft Soils in Shanghai

From both macro-data of recent macro earthquake damage surveys and observational records of strong motions,it has been proved that the ground conditions affect not only the peak ground acceleration(PGA) and spectral distribution but also the seismic response and damage mechanism of various kinds of engineering structures.The Quaternary deposits in Shanghai are horizontal soil layers with bedrock depth up to 300 m or so and shallow soft soils of littoral and fenny facies.Therefore,the soil deposits of Shanghai are deep and soft and have important influences upon seismic ground response of soil layers.Based on the equivalent linear approach for one-dimensional seismic ground response in frequency domain, a seismic analysis model is developed to describe the characters of deep and soft soil deposits in Shanghai.Then the seismic ground response of Shanghai soils,which is subjected to El Centro base excitations,is studied on acceleration amplification and spectral characteristics particularly.The research result shows that ground acceleration will be amplified when PGA equals 0.1 g,e.g.the seismic fortification intensity of 7.The soil deposit filters out a significant portion of the high frequency content of the bedrock acceleration and amplify the portion of the low frequency content.Spectral predominant period of the ground motion also has a tendency moving to the long-period direction.